1 libpng-manual.txt - A description on how to use and modify libpng 2 3 libpng version 1.6.25 - September 1, 2016 4 Updated and distributed by Glenn Randers-Pehrson 5 <glennrp at users.sourceforge.net> 6 Copyright (c) 1998-2016 Glenn Randers-Pehrson 7 8 This document is released under the libpng license. 9 For conditions of distribution and use, see the disclaimer 10 and license in png.h 11 12 Based on: 13 14 libpng versions 0.97, January 1998, through 1.6.25 - September 1, 2016 15 Updated and distributed by Glenn Randers-Pehrson 16 Copyright (c) 1998-2016 Glenn Randers-Pehrson 17 18 libpng 1.0 beta 6 - version 0.96 - May 28, 1997 19 Updated and distributed by Andreas Dilger 20 Copyright (c) 1996, 1997 Andreas Dilger 21 22 libpng 1.0 beta 2 - version 0.88 - January 26, 1996 23 For conditions of distribution and use, see copyright 24 notice in png.h. Copyright (c) 1995, 1996 Guy Eric 25 Schalnat, Group 42, Inc. 26 27 Updated/rewritten per request in the libpng FAQ 28 Copyright (c) 1995, 1996 Frank J. T. Wojcik 29 December 18, 1995 & January 20, 1996 30 31 TABLE OF CONTENTS 32 33 I. Introduction 34 II. Structures 35 III. Reading 36 IV. Writing 37 V. Simplified API 38 VI. Modifying/Customizing libpng 39 VII. MNG support 40 VIII. Changes to Libpng from version 0.88 41 IX. Changes to Libpng from version 1.0.x to 1.2.x 42 X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x 43 XI. Changes to Libpng from version 1.4.x to 1.5.x 44 XII. Changes to Libpng from version 1.5.x to 1.6.x 45 XIII. Detecting libpng 46 XIV. Source code repository 47 XV. Coding style 48 XVI. Y2K Compliance in libpng 49 50 I. Introduction 51 52 This file describes how to use and modify the PNG reference library 53 (known as libpng) for your own use. In addition to this 54 file, example.c is a good starting point for using the library, as 55 it is heavily commented and should include everything most people 56 will need. We assume that libpng is already installed; see the 57 INSTALL file for instructions on how to configure and install libpng. 58 59 For examples of libpng usage, see the files "example.c", "pngtest.c", 60 and the files in the "contrib" directory, all of which are included in 61 the libpng distribution. 62 63 Libpng was written as a companion to the PNG specification, as a way 64 of reducing the amount of time and effort it takes to support the PNG 65 file format in application programs. 66 67 The PNG specification (second edition), November 2003, is available as 68 a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at 69 <http://www.w3.org/TR/2003/REC-PNG-20031110/ 70 The W3C and ISO documents have identical technical content. 71 72 The PNG-1.2 specification is available at 73 <http://png-mng.sourceforge.net/pub/png/spec/1.2/>. 74 It is technically equivalent 75 to the PNG specification (second edition) but has some additional material. 76 77 The PNG-1.0 specification is available as RFC 2083 78 <http://png-mng.sourceforge.net/pub/png/spec/1.0/> and as a 79 W3C Recommendation <http://www.w3.org/TR/REC-png-961001>. 80 81 Some additional chunks are described in the special-purpose public chunks 82 documents at <http://www.libpng.org/pub/png/spec/register/> 83 84 Other information 85 about PNG, and the latest version of libpng, can be found at the PNG home 86 page, <http://www.libpng.org/pub/png/>. 87 88 Most users will not have to modify the library significantly; advanced 89 users may want to modify it more. All attempts were made to make it as 90 complete as possible, while keeping the code easy to understand. 91 Currently, this library only supports C. Support for other languages 92 is being considered. 93 94 Libpng has been designed to handle multiple sessions at one time, 95 to be easily modifiable, to be portable to the vast majority of 96 machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy 97 to use. The ultimate goal of libpng is to promote the acceptance of 98 the PNG file format in whatever way possible. While there is still 99 work to be done (see the TODO file), libpng should cover the 100 majority of the needs of its users. 101 102 Libpng uses zlib for its compression and decompression of PNG files. 103 Further information about zlib, and the latest version of zlib, can 104 be found at the zlib home page, <http://zlib.net/>. 105 The zlib compression utility is a general purpose utility that is 106 useful for more than PNG files, and can be used without libpng. 107 See the documentation delivered with zlib for more details. 108 You can usually find the source files for the zlib utility wherever you 109 find the libpng source files. 110 111 Libpng is thread safe, provided the threads are using different 112 instances of the structures. Each thread should have its own 113 png_struct and png_info instances, and thus its own image. 114 Libpng does not protect itself against two threads using the 115 same instance of a structure. 116 117 II. Structures 118 119 There are two main structures that are important to libpng, png_struct 120 and png_info. Both are internal structures that are no longer exposed 121 in the libpng interface (as of libpng 1.5.0). 122 123 The png_info structure is designed to provide information about the 124 PNG file. At one time, the fields of png_info were intended to be 125 directly accessible to the user. However, this tended to cause problems 126 with applications using dynamically loaded libraries, and as a result 127 a set of interface functions for png_info (the png_get_*() and png_set_*() 128 functions) was developed, and direct access to the png_info fields was 129 deprecated.. 130 131 The png_struct structure is the object used by the library to decode a 132 single image. As of 1.5.0 this structure is also not exposed. 133 134 Almost all libpng APIs require a pointer to a png_struct as the first argument. 135 Many (in particular the png_set and png_get APIs) also require a pointer 136 to png_info as the second argument. Some application visible macros 137 defined in png.h designed for basic data access (reading and writing 138 integers in the PNG format) don't take a png_info pointer, but it's almost 139 always safe to assume that a (png_struct*) has to be passed to call an API 140 function. 141 142 You can have more than one png_info structure associated with an image, 143 as illustrated in pngtest.c, one for information valid prior to the 144 IDAT chunks and another (called "end_info" below) for things after them. 145 146 The png.h header file is an invaluable reference for programming with libpng. 147 And while I'm on the topic, make sure you include the libpng header file: 148 149 #include <png.h> 150 151 and also (as of libpng-1.5.0) the zlib header file, if you need it: 152 153 #include <zlib.h> 154 155 Types 156 157 The png.h header file defines a number of integral types used by the 158 APIs. Most of these are fairly obvious; for example types corresponding 159 to integers of particular sizes and types for passing color values. 160 161 One exception is how non-integral numbers are handled. For application 162 convenience most APIs that take such numbers have C (double) arguments; 163 however, internally PNG, and libpng, use 32 bit signed integers and encode 164 the value by multiplying by 100,000. As of libpng 1.5.0 a convenience 165 macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point) 166 which is simply (png_int_32). 167 168 All APIs that take (double) arguments also have a matching API that 169 takes the corresponding fixed point integer arguments. The fixed point 170 API has the same name as the floating point one with "_fixed" appended. 171 The actual range of values permitted in the APIs is frequently less than 172 the full range of (png_fixed_point) (-21474 to +21474). When APIs require 173 a non-negative argument the type is recorded as png_uint_32 above. Consult 174 the header file and the text below for more information. 175 176 Special care must be take with sCAL chunk handling because the chunk itself 177 uses non-integral values encoded as strings containing decimal floating point 178 numbers. See the comments in the header file. 179 180 Configuration 181 182 The main header file function declarations are frequently protected by C 183 preprocessing directives of the form: 184 185 #ifdef PNG_feature_SUPPORTED 186 declare-function 187 #endif 188 ... 189 #ifdef PNG_feature_SUPPORTED 190 use-function 191 #endif 192 193 The library can be built without support for these APIs, although a 194 standard build will have all implemented APIs. Application programs 195 should check the feature macros before using an API for maximum 196 portability. From libpng 1.5.0 the feature macros set during the build 197 of libpng are recorded in the header file "pnglibconf.h" and this file 198 is always included by png.h. 199 200 If you don't need to change the library configuration from the default, skip to 201 the next section ("Reading"). 202 203 Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all 204 of the build project files in the 'projects' directory simply copy 205 scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build 206 systems do not permit easy auto-configuration of the library - they only 207 support the default configuration. 208 209 The easiest way to make minor changes to the libpng configuration when 210 auto-configuration is supported is to add definitions to the command line 211 using (typically) CPPFLAGS. For example: 212 213 CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC 214 215 will change the internal libpng math implementation for gamma correction and 216 other arithmetic calculations to fixed point, avoiding the need for fast 217 floating point support. The result can be seen in the generated pnglibconf.h - 218 make sure it contains the changed feature macro setting. 219 220 If you need to make more extensive configuration changes - more than one or two 221 feature macro settings - you can either add -DPNG_USER_CONFIG to the build 222 command line and put a list of feature macro settings in pngusr.h or you can set 223 DFA_XTRA (a makefile variable) to a file containing the same information in the 224 form of 'option' settings. 225 226 A. Changing pnglibconf.h 227 228 A variety of methods exist to build libpng. Not all of these support 229 reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be 230 rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand. 231 232 Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to 233 pnglibconf.h and changing the lines defining the supported features, paying 234 very close attention to the 'option' information in scripts/pnglibconf.dfa 235 that describes those features and their requirements. This is easy to get 236 wrong. 237 238 B. Configuration using DFA_XTRA 239 240 Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later 241 variant such as 'nawk' or 'gawk', is available. The configure build will 242 automatically find an appropriate awk and build pnglibconf.h. 243 The scripts/pnglibconf.mak file contains a set of make rules for doing the 244 same thing if configure is not used, and many of the makefiles in the scripts 245 directory use this approach. 246 247 When rebuilding simply write a new file containing changed options and set 248 DFA_XTRA to the name of this file. This causes the build to append the new file 249 to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines 250 of the following forms: 251 252 everything = off 253 254 This turns all optional features off. Include it at the start of pngusr.dfa to 255 make it easier to build a minimal configuration. You will need to turn at least 256 some features on afterward to enable either reading or writing code, or both. 257 258 option feature on 259 option feature off 260 261 Enable or disable a single feature. This will automatically enable other 262 features required by a feature that is turned on or disable other features that 263 require a feature which is turned off. Conflicting settings will cause an error 264 message to be emitted by awk. 265 266 setting feature default value 267 268 Changes the default value of setting 'feature' to 'value'. There are a small 269 number of settings listed at the top of pnglibconf.h, they are documented in the 270 source code. Most of these values have performance implications for the library 271 but most of them have no visible effect on the API. Some can also be overridden 272 from the API. 273 274 This method of building a customized pnglibconf.h is illustrated in 275 contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and 276 pngusr.dfa in these directories. 277 278 C. Configuration using PNG_USER_CONFIG 279 280 If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built, 281 the file pngusr.h will automatically be included before the options in 282 scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only 283 macro definitions turning features on or off or setting settings. 284 285 Apart from the global setting "everything = off" all the options listed above 286 can be set using macros in pngusr.h: 287 288 #define PNG_feature_SUPPORTED 289 290 is equivalent to: 291 292 option feature on 293 294 #define PNG_NO_feature 295 296 is equivalent to: 297 298 option feature off 299 300 #define PNG_feature value 301 302 is equivalent to: 303 304 setting feature default value 305 306 Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the 307 pngusr file you supply override the contents of scripts/pnglibconf.dfa 308 309 If confusing or incomprehensible behavior results it is possible to 310 examine the intermediate file pnglibconf.dfn to find the full set of 311 dependency information for each setting and option. Simply locate the 312 feature in the file and read the C comments that precede it. 313 314 This method is also illustrated in the contrib/pngminim/* makefiles and 315 pngusr.h. 316 317 III. Reading 318 319 We'll now walk you through the possible functions to call when reading 320 in a PNG file sequentially, briefly explaining the syntax and purpose 321 of each one. See example.c and png.h for more detail. While 322 progressive reading is covered in the next section, you will still 323 need some of the functions discussed in this section to read a PNG 324 file. 325 326 Setup 327 328 You will want to do the I/O initialization(*) before you get into libpng, 329 so if it doesn't work, you don't have much to undo. Of course, you 330 will also want to insure that you are, in fact, dealing with a PNG 331 file. Libpng provides a simple check to see if a file is a PNG file. 332 To use it, pass in the first 1 to 8 bytes of the file to the function 333 png_sig_cmp(), and it will return 0 (false) if the bytes match the 334 corresponding bytes of the PNG signature, or nonzero (true) otherwise. 335 Of course, the more bytes you pass in, the greater the accuracy of the 336 prediction. 337 338 If you are intending to keep the file pointer open for use in libpng, 339 you must ensure you don't read more than 8 bytes from the beginning 340 of the file, and you also have to make a call to png_set_sig_bytes() 341 with the number of bytes you read from the beginning. Libpng will 342 then only check the bytes (if any) that your program didn't read. 343 344 (*): If you are not using the standard I/O functions, you will need 345 to replace them with custom functions. See the discussion under 346 Customizing libpng. 347 348 FILE *fp = fopen(file_name, "rb"); 349 if (!fp) 350 { 351 return (ERROR); 352 } 353 354 if (fread(header, 1, number, fp) != number) 355 { 356 return (ERROR); 357 } 358 359 is_png = !png_sig_cmp(header, 0, number); 360 if (!is_png) 361 { 362 return (NOT_PNG); 363 } 364 365 Next, png_struct and png_info need to be allocated and initialized. In 366 order to ensure that the size of these structures is correct even with a 367 dynamically linked libpng, there are functions to initialize and 368 allocate the structures. We also pass the library version, optional 369 pointers to error handling functions, and a pointer to a data struct for 370 use by the error functions, if necessary (the pointer and functions can 371 be NULL if the default error handlers are to be used). See the section 372 on Changes to Libpng below regarding the old initialization functions. 373 The structure allocation functions quietly return NULL if they fail to 374 create the structure, so your application should check for that. 375 376 png_structp png_ptr = png_create_read_struct 377 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 378 user_error_fn, user_warning_fn); 379 380 if (!png_ptr) 381 return (ERROR); 382 383 png_infop info_ptr = png_create_info_struct(png_ptr); 384 385 if (!info_ptr) 386 { 387 png_destroy_read_struct(&png_ptr, 388 (png_infopp)NULL, (png_infopp)NULL); 389 return (ERROR); 390 } 391 392 If you want to use your own memory allocation routines, 393 use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use 394 png_create_read_struct_2() instead of png_create_read_struct(): 395 396 png_structp png_ptr = png_create_read_struct_2 397 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 398 user_error_fn, user_warning_fn, (png_voidp) 399 user_mem_ptr, user_malloc_fn, user_free_fn); 400 401 The error handling routines passed to png_create_read_struct() 402 and the memory alloc/free routines passed to png_create_struct_2() 403 are only necessary if you are not using the libpng supplied error 404 handling and memory alloc/free functions. 405 406 When libpng encounters an error, it expects to longjmp back 407 to your routine. Therefore, you will need to call setjmp and pass 408 your png_jmpbuf(png_ptr). If you read the file from different 409 routines, you will need to update the longjmp buffer every time you enter 410 a new routine that will call a png_*() function. 411 412 See your documentation of setjmp/longjmp for your compiler for more 413 information on setjmp/longjmp. See the discussion on libpng error 414 handling in the Customizing Libpng section below for more information 415 on the libpng error handling. If an error occurs, and libpng longjmp's 416 back to your setjmp, you will want to call png_destroy_read_struct() to 417 free any memory. 418 419 if (setjmp(png_jmpbuf(png_ptr))) 420 { 421 png_destroy_read_struct(&png_ptr, &info_ptr, 422 &end_info); 423 fclose(fp); 424 return (ERROR); 425 } 426 427 Pass (png_infopp)NULL instead of &end_info if you didn't create 428 an end_info structure. 429 430 If you would rather avoid the complexity of setjmp/longjmp issues, 431 you can compile libpng with PNG_NO_SETJMP, in which case 432 errors will result in a call to PNG_ABORT() which defaults to abort(). 433 434 You can #define PNG_ABORT() to a function that does something 435 more useful than abort(), as long as your function does not 436 return. 437 438 Now you need to set up the input code. The default for libpng is to 439 use the C function fread(). If you use this, you will need to pass a 440 valid FILE * in the function png_init_io(). Be sure that the file is 441 opened in binary mode. If you wish to handle reading data in another 442 way, you need not call the png_init_io() function, but you must then 443 implement the libpng I/O methods discussed in the Customizing Libpng 444 section below. 445 446 png_init_io(png_ptr, fp); 447 448 If you had previously opened the file and read any of the signature from 449 the beginning in order to see if this was a PNG file, you need to let 450 libpng know that there are some bytes missing from the start of the file. 451 452 png_set_sig_bytes(png_ptr, number); 453 454 You can change the zlib compression buffer size to be used while 455 reading compressed data with 456 457 png_set_compression_buffer_size(png_ptr, buffer_size); 458 459 where the default size is 8192 bytes. Note that the buffer size 460 is changed immediately and the buffer is reallocated immediately, 461 instead of setting a flag to be acted upon later. 462 463 If you want CRC errors to be handled in a different manner than 464 the default, use 465 466 png_set_crc_action(png_ptr, crit_action, ancil_action); 467 468 The values for png_set_crc_action() say how libpng is to handle CRC errors in 469 ancillary and critical chunks, and whether to use the data contained 470 therein. Note that it is impossible to "discard" data in a critical 471 chunk. 472 473 Choices for (int) crit_action are 474 PNG_CRC_DEFAULT 0 error/quit 475 PNG_CRC_ERROR_QUIT 1 error/quit 476 PNG_CRC_WARN_USE 3 warn/use data 477 PNG_CRC_QUIET_USE 4 quiet/use data 478 PNG_CRC_NO_CHANGE 5 use the current value 479 480 Choices for (int) ancil_action are 481 PNG_CRC_DEFAULT 0 error/quit 482 PNG_CRC_ERROR_QUIT 1 error/quit 483 PNG_CRC_WARN_DISCARD 2 warn/discard data 484 PNG_CRC_WARN_USE 3 warn/use data 485 PNG_CRC_QUIET_USE 4 quiet/use data 486 PNG_CRC_NO_CHANGE 5 use the current value 487 488 Setting up callback code 489 490 You can set up a callback function to handle any unknown chunks in the 491 input stream. You must supply the function 492 493 read_chunk_callback(png_structp png_ptr, 494 png_unknown_chunkp chunk); 495 { 496 /* The unknown chunk structure contains your 497 chunk data, along with similar data for any other 498 unknown chunks: */ 499 500 png_byte name[5]; 501 png_byte *data; 502 png_size_t size; 503 504 /* Note that libpng has already taken care of 505 the CRC handling */ 506 507 /* put your code here. Search for your chunk in the 508 unknown chunk structure, process it, and return one 509 of the following: */ 510 511 return (-n); /* chunk had an error */ 512 return (0); /* did not recognize */ 513 return (n); /* success */ 514 } 515 516 (You can give your function another name that you like instead of 517 "read_chunk_callback") 518 519 To inform libpng about your function, use 520 521 png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr, 522 read_chunk_callback); 523 524 This names not only the callback function, but also a user pointer that 525 you can retrieve with 526 527 png_get_user_chunk_ptr(png_ptr); 528 529 If you call the png_set_read_user_chunk_fn() function, then all unknown 530 chunks which the callback does not handle will be saved when read. You can 531 cause them to be discarded by returning '1' ("handled") instead of '0'. This 532 behavior will change in libpng 1.7 and the default handling set by the 533 png_set_keep_unknown_chunks() function, described below, will be used when the 534 callback returns 0. If you want the existing behavior you should set the global 535 default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current 536 versions of libpng and with 1.7. Libpng 1.6 issues a warning if you keep the 537 default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0. 538 539 At this point, you can set up a callback function that will be 540 called after each row has been read, which you can use to control 541 a progress meter or the like. It's demonstrated in pngtest.c. 542 You must supply a function 543 544 void read_row_callback(png_structp png_ptr, 545 png_uint_32 row, int pass); 546 { 547 /* put your code here */ 548 } 549 550 (You can give it another name that you like instead of "read_row_callback") 551 552 To inform libpng about your function, use 553 554 png_set_read_status_fn(png_ptr, read_row_callback); 555 556 When this function is called the row has already been completely processed and 557 the 'row' and 'pass' refer to the next row to be handled. For the 558 non-interlaced case the row that was just handled is simply one less than the 559 passed in row number, and pass will always be 0. For the interlaced case the 560 same applies unless the row value is 0, in which case the row just handled was 561 the last one from one of the preceding passes. Because interlacing may skip a 562 pass you cannot be sure that the preceding pass is just 'pass-1'; if you really 563 need to know what the last pass is record (row,pass) from the callback and use 564 the last recorded value each time. 565 566 As with the user transform you can find the output row using the 567 PNG_ROW_FROM_PASS_ROW macro. 568 569 Unknown-chunk handling 570 571 Now you get to set the way the library processes unknown chunks in the 572 input PNG stream. Both known and unknown chunks will be read. Normal 573 behavior is that known chunks will be parsed into information in 574 various info_ptr members while unknown chunks will be discarded. This 575 behavior can be wasteful if your application will never use some known 576 chunk types. To change this, you can call: 577 578 png_set_keep_unknown_chunks(png_ptr, keep, 579 chunk_list, num_chunks); 580 581 keep - 0: default unknown chunk handling 582 1: ignore; do not keep 583 2: keep only if safe-to-copy 584 3: keep even if unsafe-to-copy 585 586 You can use these definitions: 587 PNG_HANDLE_CHUNK_AS_DEFAULT 0 588 PNG_HANDLE_CHUNK_NEVER 1 589 PNG_HANDLE_CHUNK_IF_SAFE 2 590 PNG_HANDLE_CHUNK_ALWAYS 3 591 592 chunk_list - list of chunks affected (a byte string, 593 five bytes per chunk, NULL or '\0' if 594 num_chunks is positive; ignored if 595 numchunks <= 0). 596 597 num_chunks - number of chunks affected; if 0, all 598 unknown chunks are affected. If positive, 599 only the chunks in the list are affected, 600 and if negative all unknown chunks and 601 all known chunks except for the IHDR, 602 PLTE, tRNS, IDAT, and IEND chunks are 603 affected. 604 605 Unknown chunks declared in this way will be saved as raw data onto a 606 list of png_unknown_chunk structures. If a chunk that is normally 607 known to libpng is named in the list, it will be handled as unknown, 608 according to the "keep" directive. If a chunk is named in successive 609 instances of png_set_keep_unknown_chunks(), the final instance will 610 take precedence. The IHDR and IEND chunks should not be named in 611 chunk_list; if they are, libpng will process them normally anyway. 612 If you know that your application will never make use of some particular 613 chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below. 614 615 Here is an example of the usage of png_set_keep_unknown_chunks(), 616 where the private "vpAg" chunk will later be processed by a user chunk 617 callback function: 618 619 png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'}; 620 621 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) 622 png_byte unused_chunks[]= 623 { 624 104, 73, 83, 84, (png_byte) '\0', /* hIST */ 625 105, 84, 88, 116, (png_byte) '\0', /* iTXt */ 626 112, 67, 65, 76, (png_byte) '\0', /* pCAL */ 627 115, 67, 65, 76, (png_byte) '\0', /* sCAL */ 628 115, 80, 76, 84, (png_byte) '\0', /* sPLT */ 629 116, 73, 77, 69, (png_byte) '\0', /* tIME */ 630 }; 631 #endif 632 633 ... 634 635 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) 636 /* ignore all unknown chunks 637 * (use global setting "2" for libpng16 and earlier): 638 */ 639 png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0); 640 641 /* except for vpAg: */ 642 png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1); 643 644 /* also ignore unused known chunks: */ 645 png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks, 646 (int)(sizeof unused_chunks)/5); 647 #endif 648 649 User limits 650 651 The PNG specification allows the width and height of an image to be as 652 large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns. 653 For safety, libpng imposes a default limit of 1 million rows and columns. 654 Larger images will be rejected immediately with a png_error() call. If 655 you wish to change these limits, you can use 656 657 png_set_user_limits(png_ptr, width_max, height_max); 658 659 to set your own limits (libpng may reject some very wide images 660 anyway because of potential buffer overflow conditions). 661 662 You should put this statement after you create the PNG structure and 663 before calling png_read_info(), png_read_png(), or png_process_data(). 664 665 When writing a PNG datastream, put this statement before calling 666 png_write_info() or png_write_png(). 667 668 If you need to retrieve the limits that are being applied, use 669 670 width_max = png_get_user_width_max(png_ptr); 671 height_max = png_get_user_height_max(png_ptr); 672 673 The PNG specification sets no limit on the number of ancillary chunks 674 allowed in a PNG datastream. By default, libpng imposes a limit of 675 a total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored. 676 If you have set up both info_ptr and end_info_ptr, the limit applies 677 separately to each. You can change the limit on the total number of such 678 chunks that will be stored, with 679 680 png_set_chunk_cache_max(png_ptr, user_chunk_cache_max); 681 682 where 0x7fffffffL means unlimited. You can retrieve this limit with 683 684 chunk_cache_max = png_get_chunk_cache_max(png_ptr); 685 686 Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount of 687 memory that a compressed chunk other than IDAT can occupy, when decompressed. 688 You can change this limit with 689 690 png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max); 691 692 and you can retrieve the limit with 693 694 chunk_malloc_max = png_get_chunk_malloc_max(png_ptr); 695 696 Any chunks that would cause either of these limits to be exceeded will 697 be ignored. 698 699 Information about your system 700 701 If you intend to display the PNG or to incorporate it in other image data you 702 need to tell libpng information about your display or drawing surface so that 703 libpng can convert the values in the image to match the display. 704 705 From libpng-1.5.4 this information can be set before reading the PNG file 706 header. In earlier versions png_set_gamma() existed but behaved incorrectly if 707 called before the PNG file header had been read and png_set_alpha_mode() did not 708 exist. 709 710 If you need to support versions prior to libpng-1.5.4 test the version number 711 as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures 712 described in the appropriate manual page. 713 714 You give libpng the encoding expected by your system expressed as a 'gamma' 715 value. You can also specify a default encoding for the PNG file in 716 case the required information is missing from the file. By default libpng 717 assumes that the PNG data matches your system, to keep this default call: 718 719 png_set_gamma(png_ptr, screen_gamma, output_gamma); 720 721 or you can use the fixed point equivalent: 722 723 png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, 724 PNG_FP_1*output_gamma); 725 726 If you don't know the gamma for your system it is probably 2.2 - a good 727 approximation to the IEC standard for display systems (sRGB). If images are 728 too contrasty or washed out you got the value wrong - check your system 729 documentation! 730 731 Many systems permit the system gamma to be changed via a lookup table in the 732 display driver, a few systems, including older Macs, change the response by 733 default. As of 1.5.4 three special values are available to handle common 734 situations: 735 736 PNG_DEFAULT_sRGB: Indicates that the system conforms to the 737 IEC 61966-2-1 standard. This matches almost 738 all systems. 739 PNG_GAMMA_MAC_18: Indicates that the system is an older 740 (pre Mac OS 10.6) Apple Macintosh system with 741 the default settings. 742 PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates 743 that the system expects data with no gamma 744 encoding. 745 746 You would use the linear (unencoded) value if you need to process the pixel 747 values further because this avoids the need to decode and re-encode each 748 component value whenever arithmetic is performed. A lot of graphics software 749 uses linear values for this reason, often with higher precision component values 750 to preserve overall accuracy. 751 752 753 The output_gamma value expresses how to decode the output values, not how 754 they are encoded. The values used correspond to the normal numbers used to 755 describe the overall gamma of a computer display system; for example 2.2 for 756 an sRGB conformant system. The values are scaled by 100000 in the _fixed 757 version of the API (so 220000 for sRGB.) 758 759 The inverse of the value is always used to provide a default for the PNG file 760 encoding if it has no gAMA chunk and if png_set_gamma() has not been called 761 to override the PNG gamma information. 762 763 When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode 764 opaque pixels however pixels with lower alpha values are not encoded, 765 regardless of the output gamma setting. 766 767 When the standard Porter Duff handling is requested with mode 1 the output 768 encoding is set to be linear and the output_gamma value is only relevant 769 as a default for input data that has no gamma information. The linear output 770 encoding will be overridden if png_set_gamma() is called - the results may be 771 highly unexpected! 772 773 The following numbers are derived from the sRGB standard and the research 774 behind it. sRGB is defined to be approximated by a PNG gAMA chunk value of 775 0.45455 (1/2.2) for PNG. The value implicitly includes any viewing 776 correction required to take account of any differences in the color 777 environment of the original scene and the intended display environment; the 778 value expresses how to *decode* the image for display, not how the original 779 data was *encoded*. 780 781 sRGB provides a peg for the PNG standard by defining a viewing environment. 782 sRGB itself, and earlier TV standards, actually use a more complex transform 783 (a linear portion then a gamma 2.4 power law) than PNG can express. (PNG is 784 limited to simple power laws.) By saying that an image for direct display on 785 an sRGB conformant system should be stored with a gAMA chunk value of 45455 786 (11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification 787 makes it possible to derive values for other display systems and 788 environments. 789 790 The Mac value is deduced from the sRGB based on an assumption that the actual 791 extra viewing correction used in early Mac display systems was implemented as 792 a power 1.45 lookup table. 793 794 Any system where a programmable lookup table is used or where the behavior of 795 the final display device characteristics can be changed requires system 796 specific code to obtain the current characteristic. However this can be 797 difficult and most PNG gamma correction only requires an approximate value. 798 799 By default, if png_set_alpha_mode() is not called, libpng assumes that all 800 values are unencoded, linear, values and that the output device also has a 801 linear characteristic. This is only very rarely correct - it is invariably 802 better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the 803 default if you don't know what the right answer is! 804 805 The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS 806 10.6) which used a correction table to implement a somewhat lower gamma on an 807 otherwise sRGB system. 808 809 Both these values are reserved (not simple gamma values) in order to allow 810 more precise correction internally in the future. 811 812 NOTE: the values can be passed to either the fixed or floating 813 point APIs, but the floating point API will also accept floating point 814 values. 815 816 The second thing you may need to tell libpng about is how your system handles 817 alpha channel information. Some, but not all, PNG files contain an alpha 818 channel. To display these files correctly you need to compose the data onto a 819 suitable background, as described in the PNG specification. 820 821 Libpng only supports composing onto a single color (using png_set_background; 822 see below). Otherwise you must do the composition yourself and, in this case, 823 you may need to call png_set_alpha_mode: 824 825 #if PNG_LIBPNG_VER >= 10504 826 png_set_alpha_mode(png_ptr, mode, screen_gamma); 827 #else 828 png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma); 829 #endif 830 831 The screen_gamma value is the same as the argument to png_set_gamma; however, 832 how it affects the output depends on the mode. png_set_alpha_mode() sets the 833 file gamma default to 1/screen_gamma, so normally you don't need to call 834 png_set_gamma. If you need different defaults call png_set_gamma() before 835 png_set_alpha_mode() - if you call it after it will override the settings made 836 by png_set_alpha_mode(). 837 838 The mode is as follows: 839 840 PNG_ALPHA_PNG: The data is encoded according to the PNG 841 specification. Red, green and blue, or gray, components are 842 gamma encoded color values and are not premultiplied by the 843 alpha value. The alpha value is a linear measure of the 844 contribution of the pixel to the corresponding final output pixel. 845 846 You should normally use this format if you intend to perform 847 color correction on the color values; most, maybe all, color 848 correction software has no handling for the alpha channel and, 849 anyway, the math to handle pre-multiplied component values is 850 unnecessarily complex. 851 852 Before you do any arithmetic on the component values you need 853 to remove the gamma encoding and multiply out the alpha 854 channel. See the PNG specification for more detail. It is 855 important to note that when an image with an alpha channel is 856 scaled, linear encoded, pre-multiplied component values must 857 be used! 858 859 The remaining modes assume you don't need to do any further color correction or 860 that if you do, your color correction software knows all about alpha (it 861 probably doesn't!). They 'associate' the alpha with the color information by 862 storing color channel values that have been scaled by the alpha. The 863 advantage is that the color channels can be resampled (the image can be 864 scaled) in this form. The disadvantage is that normal practice is to store 865 linear, not (gamma) encoded, values and this requires 16-bit channels for 866 still images rather than the 8-bit channels that are just about sufficient if 867 gamma encoding is used. In addition all non-transparent pixel values, 868 including completely opaque ones, must be gamma encoded to produce the final 869 image. These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes 870 described below (the latter being the two common names for associated alpha 871 color channels). Note that PNG files always contain non-associated color 872 channels; png_set_alpha_mode() with one of the modes causes the decoder to 873 convert the pixels to an associated form before returning them to your 874 application. 875 876 Since it is not necessary to perform arithmetic on opaque color values so 877 long as they are not to be resampled and are in the final color space it is 878 possible to optimize the handling of alpha by storing the opaque pixels in 879 the PNG format (adjusted for the output color space) while storing partially 880 opaque pixels in the standard, linear, format. The accuracy required for 881 standard alpha composition is relatively low, because the pixels are 882 isolated, therefore typically the accuracy loss in storing 8-bit linear 883 values is acceptable. (This is not true if the alpha channel is used to 884 simulate transparency over large areas - use 16 bits or the PNG mode in 885 this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is 886 treated as opaque only if the alpha value is equal to the maximum value. 887 888 PNG_ALPHA_STANDARD: The data libpng produces is encoded in the 889 standard way assumed by most correctly written graphics software. 890 The gamma encoding will be removed by libpng and the 891 linear component values will be pre-multiplied by the 892 alpha channel. 893 894 With this format the final image must be re-encoded to 895 match the display gamma before the image is displayed. 896 If your system doesn't do that, yet still seems to 897 perform arithmetic on the pixels without decoding them, 898 it is broken - check out the modes below. 899 900 With PNG_ALPHA_STANDARD libpng always produces linear 901 component values, whatever screen_gamma you supply. The 902 screen_gamma value is, however, used as a default for 903 the file gamma if the PNG file has no gamma information. 904 905 If you call png_set_gamma() after png_set_alpha_mode() you 906 will override the linear encoding. Instead the 907 pre-multiplied pixel values will be gamma encoded but 908 the alpha channel will still be linear. This may 909 actually match the requirements of some broken software, 910 but it is unlikely. 911 912 While linear 8-bit data is often used it has 913 insufficient precision for any image with a reasonable 914 dynamic range. To avoid problems, and if your software 915 supports it, use png_set_expand_16() to force all 916 components to 16 bits. 917 918 PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD 919 except that completely opaque pixels are gamma encoded according to 920 the screen_gamma value. Pixels with alpha less than 1.0 921 will still have linear components. 922 923 Use this format if you have control over your 924 compositing software and so don't do other arithmetic 925 (such as scaling) on the data you get from libpng. Your 926 compositing software can simply copy opaque pixels to 927 the output but still has linear values for the 928 non-opaque pixels. 929 930 In normal compositing, where the alpha channel encodes 931 partial pixel coverage (as opposed to broad area 932 translucency), the inaccuracies of the 8-bit 933 representation of non-opaque pixels are irrelevant. 934 935 You can also try this format if your software is broken; 936 it might look better. 937 938 PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component 939 values, including the alpha channel are gamma encoded. This is 940 broken because, in practice, no implementation that uses this choice 941 correctly undoes the encoding before handling alpha composition. Use this 942 choice only if other serious errors in the software or hardware you use 943 mandate it. In most cases of broken software or hardware the bug in the 944 final display manifests as a subtle halo around composited parts of the 945 image. You may not even perceive this as a halo; the composited part of 946 the image may simply appear separate from the background, as though it had 947 been cut out of paper and pasted on afterward. 948 949 If you don't have to deal with bugs in software or hardware, or if you can fix 950 them, there are three recommended ways of using png_set_alpha_mode(): 951 952 png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG, 953 screen_gamma); 954 955 You can do color correction on the result (libpng does not currently 956 support color correction internally). When you handle the alpha channel 957 you need to undo the gamma encoding and multiply out the alpha. 958 959 png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD, 960 screen_gamma); 961 png_set_expand_16(png_ptr); 962 963 If you are using the high level interface, don't call png_set_expand_16(); 964 instead pass PNG_TRANSFORM_EXPAND_16 to the interface. 965 966 With this mode you can't do color correction, but you can do arithmetic, 967 including composition and scaling, on the data without further processing. 968 969 png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED, 970 screen_gamma); 971 972 You can avoid the expansion to 16-bit components with this mode, but you 973 lose the ability to scale the image or perform other linear arithmetic. 974 All you can do is compose the result onto a matching output. Since this 975 mode is libpng-specific you also need to write your own composition 976 software. 977 978 The following are examples of calls to png_set_alpha_mode to achieve the 979 required overall gamma correction and, where necessary, alpha 980 premultiplication. 981 982 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB); 983 984 This is the default libpng handling of the alpha channel - it is not 985 pre-multiplied into the color components. In addition the call states 986 that the output is for a sRGB system and causes all PNG files without gAMA 987 chunks to be assumed to be encoded using sRGB. 988 989 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC); 990 991 In this case the output is assumed to be something like an sRGB conformant 992 display preceeded by a power-law lookup table of power 1.45. This is how 993 early Mac systems behaved. 994 995 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR); 996 997 This is the classic Jim Blinn approach and will work in academic 998 environments where everything is done by the book. It has the shortcoming 999 of assuming that input PNG data with no gamma information is linear - this 1000 is unlikely to be correct unless the PNG files where generated locally. 1001 Most of the time the output precision will be so low as to show 1002 significant banding in dark areas of the image. 1003 1004 png_set_expand_16(pp); 1005 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB); 1006 1007 This is a somewhat more realistic Jim Blinn inspired approach. PNG files 1008 are assumed to have the sRGB encoding if not marked with a gamma value and 1009 the output is always 16 bits per component. This permits accurate scaling 1010 and processing of the data. If you know that your input PNG files were 1011 generated locally you might need to replace PNG_DEFAULT_sRGB with the 1012 correct value for your system. 1013 1014 png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB); 1015 1016 If you just need to composite the PNG image onto an existing background 1017 and if you control the code that does this you can use the optimization 1018 setting. In this case you just copy completely opaque pixels to the 1019 output. For pixels that are not completely transparent (you just skip 1020 those) you do the composition math using png_composite or png_composite_16 1021 below then encode the resultant 8-bit or 16-bit values to match the output 1022 encoding. 1023 1024 Other cases 1025 1026 If neither the PNG nor the standard linear encoding work for you because 1027 of the software or hardware you use then you have a big problem. The PNG 1028 case will probably result in halos around the image. The linear encoding 1029 will probably result in a washed out, too bright, image (it's actually too 1030 contrasty.) Try the ALPHA_OPTIMIZED mode above - this will probably 1031 substantially reduce the halos. Alternatively try: 1032 1033 png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB); 1034 1035 This option will also reduce the halos, but there will be slight dark 1036 halos round the opaque parts of the image where the background is light. 1037 In the OPTIMIZED mode the halos will be light halos where the background 1038 is dark. Take your pick - the halos are unavoidable unless you can get 1039 your hardware/software fixed! (The OPTIMIZED approach is slightly 1040 faster.) 1041 1042 When the default gamma of PNG files doesn't match the output gamma. 1043 If you have PNG files with no gamma information png_set_alpha_mode allows 1044 you to provide a default gamma, but it also sets the ouput gamma to the 1045 matching value. If you know your PNG files have a gamma that doesn't 1046 match the output you can take advantage of the fact that 1047 png_set_alpha_mode always sets the output gamma but only sets the PNG 1048 default if it is not already set: 1049 1050 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB); 1051 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC); 1052 1053 The first call sets both the default and the output gamma values, the 1054 second call overrides the output gamma without changing the default. This 1055 is easier than achieving the same effect with png_set_gamma. You must use 1056 PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will 1057 fire if more than one call to png_set_alpha_mode and png_set_background is 1058 made in the same read operation, however multiple calls with PNG_ALPHA_PNG 1059 are ignored. 1060 1061 If you don't need, or can't handle, the alpha channel you can call 1062 png_set_background() to remove it by compositing against a fixed color. Don't 1063 call png_set_strip_alpha() to do this - it will leave spurious pixel values in 1064 transparent parts of this image. 1065 1066 png_set_background(png_ptr, &background_color, 1067 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1); 1068 1069 The background_color is an RGB or grayscale value according to the data format 1070 libpng will produce for you. Because you don't yet know the format of the PNG 1071 file, if you call png_set_background at this point you must arrange for the 1072 format produced by libpng to always have 8-bit or 16-bit components and then 1073 store the color as an 8-bit or 16-bit color as appropriate. The color contains 1074 separate gray and RGB component values, so you can let libpng produce gray or 1075 RGB output according to the input format, but low bit depth grayscale images 1076 must always be converted to at least 8-bit format. (Even though low bit depth 1077 grayscale images can't have an alpha channel they can have a transparent 1078 color!) 1079 1080 You set the transforms you need later, either as flags to the high level 1081 interface or libpng API calls for the low level interface. For reference the 1082 settings and API calls required are: 1083 1084 8-bit values: 1085 PNG_TRANSFORM_SCALE_16 | PNG_EXPAND 1086 png_set_expand(png_ptr); png_set_scale_16(png_ptr); 1087 1088 If you must get exactly the same inaccurate results 1089 produced by default in versions prior to libpng-1.5.4, 1090 use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr) 1091 instead. 1092 1093 16-bit values: 1094 PNG_TRANSFORM_EXPAND_16 1095 png_set_expand_16(png_ptr); 1096 1097 In either case palette image data will be expanded to RGB. If you just want 1098 color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr) 1099 to the list. 1100 1101 Calling png_set_background before the PNG file header is read will not work 1102 prior to libpng-1.5.4. Because the failure may result in unexpected warnings or 1103 errors it is therefore much safer to call png_set_background after the head has 1104 been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be 1105 used with the high level interface. 1106 1107 The high-level read interface 1108 1109 At this point there are two ways to proceed; through the high-level 1110 read interface, or through a sequence of low-level read operations. 1111 You can use the high-level interface if (a) you are willing to read 1112 the entire image into memory, and (b) the input transformations 1113 you want to do are limited to the following set: 1114 1115 PNG_TRANSFORM_IDENTITY No transformation 1116 PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to 1117 8-bit accurately 1118 PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to 1119 8-bit less accurately 1120 PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel 1121 PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit 1122 samples to bytes 1123 PNG_TRANSFORM_PACKSWAP Change order of packed 1124 pixels to LSB first 1125 PNG_TRANSFORM_EXPAND Perform set_expand() 1126 PNG_TRANSFORM_INVERT_MONO Invert monochrome images 1127 PNG_TRANSFORM_SHIFT Normalize pixels to the 1128 sBIT depth 1129 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA 1130 to BGRA 1131 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA 1132 to AG 1133 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity 1134 to transparency 1135 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples 1136 PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples 1137 to RGB (or GA to RGBA) 1138 PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits 1139 1140 (This excludes setting a background color, doing gamma transformation, 1141 quantizing, and setting filler.) If this is the case, simply do this: 1142 1143 png_read_png(png_ptr, info_ptr, png_transforms, NULL) 1144 1145 where png_transforms is an integer containing the bitwise OR of some 1146 set of transformation flags. This call is equivalent to png_read_info(), 1147 followed the set of transformations indicated by the transform mask, 1148 then png_read_image(), and finally png_read_end(). 1149 1150 (The final parameter of this call is not yet used. Someday it might point 1151 to transformation parameters required by some future input transform.) 1152 1153 You must use png_transforms and not call any png_set_transform() functions 1154 when you use png_read_png(). 1155 1156 After you have called png_read_png(), you can retrieve the image data 1157 with 1158 1159 row_pointers = png_get_rows(png_ptr, info_ptr); 1160 1161 where row_pointers is an array of pointers to the pixel data for each row: 1162 1163 png_bytep row_pointers[height]; 1164 1165 If you know your image size and pixel size ahead of time, you can allocate 1166 row_pointers prior to calling png_read_png() with 1167 1168 if (height > PNG_UINT_32_MAX/(sizeof (png_byte))) 1169 png_error (png_ptr, 1170 "Image is too tall to process in memory"); 1171 1172 if (width > PNG_UINT_32_MAX/pixel_size) 1173 png_error (png_ptr, 1174 "Image is too wide to process in memory"); 1175 1176 row_pointers = png_malloc(png_ptr, 1177 height*(sizeof (png_bytep))); 1178 1179 for (int i=0; i<height, i++) 1180 row_pointers[i]=NULL; /* security precaution */ 1181 1182 for (int i=0; i<height, i++) 1183 row_pointers[i]=png_malloc(png_ptr, 1184 width*pixel_size); 1185 1186 png_set_rows(png_ptr, info_ptr, &row_pointers); 1187 1188 Alternatively you could allocate your image in one big block and define 1189 row_pointers[i] to point into the proper places in your block. 1190 1191 If you use png_set_rows(), the application is responsible for freeing 1192 row_pointers (and row_pointers[i], if they were separately allocated). 1193 1194 If you don't allocate row_pointers ahead of time, png_read_png() will 1195 do it, and it'll be free'ed by libpng when you call png_destroy_*(). 1196 1197 The low-level read interface 1198 1199 If you are going the low-level route, you are now ready to read all 1200 the file information up to the actual image data. You do this with a 1201 call to png_read_info(). 1202 1203 png_read_info(png_ptr, info_ptr); 1204 1205 This will process all chunks up to but not including the image data. 1206 1207 This also copies some of the data from the PNG file into the decode structure 1208 for use in later transformations. Important information copied in is: 1209 1210 1) The PNG file gamma from the gAMA chunk. This overwrites the default value 1211 provided by an earlier call to png_set_gamma or png_set_alpha_mode. 1212 1213 2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This 1214 damages the information provided by an earlier call to png_set_background 1215 resulting in unexpected behavior. Libpng-1.5.4 no longer does this. 1216 1217 3) The number of significant bits in each component value. Libpng uses this to 1218 optimize gamma handling by reducing the internal lookup table sizes. 1219 1220 4) The transparent color information from a tRNS chunk. This can be modified by 1221 a later call to png_set_tRNS. 1222 1223 Querying the info structure 1224 1225 Functions are used to get the information from the info_ptr once it 1226 has been read. Note that these fields may not be completely filled 1227 in until png_read_end() has read the chunk data following the image. 1228 1229 png_get_IHDR(png_ptr, info_ptr, &width, &height, 1230 &bit_depth, &color_type, &interlace_type, 1231 &compression_type, &filter_method); 1232 1233 width - holds the width of the image 1234 in pixels (up to 2^31). 1235 1236 height - holds the height of the image 1237 in pixels (up to 2^31). 1238 1239 bit_depth - holds the bit depth of one of the 1240 image channels. (valid values are 1241 1, 2, 4, 8, 16 and depend also on 1242 the color_type. See also 1243 significant bits (sBIT) below). 1244 1245 color_type - describes which color/alpha channels 1246 are present. 1247 PNG_COLOR_TYPE_GRAY 1248 (bit depths 1, 2, 4, 8, 16) 1249 PNG_COLOR_TYPE_GRAY_ALPHA 1250 (bit depths 8, 16) 1251 PNG_COLOR_TYPE_PALETTE 1252 (bit depths 1, 2, 4, 8) 1253 PNG_COLOR_TYPE_RGB 1254 (bit_depths 8, 16) 1255 PNG_COLOR_TYPE_RGB_ALPHA 1256 (bit_depths 8, 16) 1257 1258 PNG_COLOR_MASK_PALETTE 1259 PNG_COLOR_MASK_COLOR 1260 PNG_COLOR_MASK_ALPHA 1261 1262 interlace_type - (PNG_INTERLACE_NONE or 1263 PNG_INTERLACE_ADAM7) 1264 1265 compression_type - (must be PNG_COMPRESSION_TYPE_BASE 1266 for PNG 1.0) 1267 1268 filter_method - (must be PNG_FILTER_TYPE_BASE 1269 for PNG 1.0, and can also be 1270 PNG_INTRAPIXEL_DIFFERENCING if 1271 the PNG datastream is embedded in 1272 a MNG-1.0 datastream) 1273 1274 Any of width, height, color_type, bit_depth, 1275 interlace_type, compression_type, or filter_method can 1276 be NULL if you are not interested in their values. 1277 1278 Note that png_get_IHDR() returns 32-bit data into 1279 the application's width and height variables. 1280 This is an unsafe situation if these are not png_uint_32 1281 variables. In such situations, the 1282 png_get_image_width() and png_get_image_height() 1283 functions described below are safer. 1284 1285 width = png_get_image_width(png_ptr, 1286 info_ptr); 1287 1288 height = png_get_image_height(png_ptr, 1289 info_ptr); 1290 1291 bit_depth = png_get_bit_depth(png_ptr, 1292 info_ptr); 1293 1294 color_type = png_get_color_type(png_ptr, 1295 info_ptr); 1296 1297 interlace_type = png_get_interlace_type(png_ptr, 1298 info_ptr); 1299 1300 compression_type = png_get_compression_type(png_ptr, 1301 info_ptr); 1302 1303 filter_method = png_get_filter_type(png_ptr, 1304 info_ptr); 1305 1306 channels = png_get_channels(png_ptr, info_ptr); 1307 1308 channels - number of channels of info for the 1309 color type (valid values are 1 (GRAY, 1310 PALETTE), 2 (GRAY_ALPHA), 3 (RGB), 1311 4 (RGB_ALPHA or RGB + filler byte)) 1312 1313 rowbytes = png_get_rowbytes(png_ptr, info_ptr); 1314 1315 rowbytes - number of bytes needed to hold a row 1316 1317 signature = png_get_signature(png_ptr, info_ptr); 1318 1319 signature - holds the signature read from the 1320 file (if any). The data is kept in 1321 the same offset it would be if the 1322 whole signature were read (i.e. if an 1323 application had already read in 4 1324 bytes of signature before starting 1325 libpng, the remaining 4 bytes would 1326 be in signature[4] through signature[7] 1327 (see png_set_sig_bytes())). 1328 1329 These are also important, but their validity depends on whether the chunk 1330 has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and 1331 png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the 1332 data has been read, or zero if it is missing. The parameters to the 1333 png_get_<chunk> are set directly if they are simple data types, or a 1334 pointer into the info_ptr is returned for any complex types. 1335 1336 The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks 1337 is simply returned to give the application information about how the 1338 image was encoded. Libpng itself only does transformations using the file 1339 gamma when combining semitransparent pixels with the background color, and, 1340 since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels 1341 within the simplified API. Libpng also uses the file gamma when converting 1342 RGB to gray, beginning with libpng-1.0.5, if the application calls 1343 png_set_rgb_to_gray()). 1344 1345 png_get_PLTE(png_ptr, info_ptr, &palette, 1346 &num_palette); 1347 1348 palette - the palette for the file 1349 (array of png_color) 1350 1351 num_palette - number of entries in the palette 1352 1353 png_get_gAMA(png_ptr, info_ptr, &file_gamma); 1354 png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma); 1355 1356 file_gamma - the gamma at which the file is 1357 written (PNG_INFO_gAMA) 1358 1359 int_file_gamma - 100,000 times the gamma at which the 1360 file is written 1361 1362 png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, 1363 &red_y, &green_x, &green_y, &blue_x, &blue_y) 1364 png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, 1365 &green_X, &green_Y, &green_Z, &blue_X, &blue_Y, 1366 &blue_Z) 1367 png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, 1368 &int_white_y, &int_red_x, &int_red_y, 1369 &int_green_x, &int_green_y, &int_blue_x, 1370 &int_blue_y) 1371 png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y, 1372 &int_red_Z, &int_green_X, &int_green_Y, 1373 &int_green_Z, &int_blue_X, &int_blue_Y, 1374 &int_blue_Z) 1375 1376 {white,red,green,blue}_{x,y} 1377 A color space encoding specified using the 1378 chromaticities of the end points and the 1379 white point. (PNG_INFO_cHRM) 1380 1381 {red,green,blue}_{X,Y,Z} 1382 A color space encoding specified using the 1383 encoding end points - the CIE tristimulus 1384 specification of the intended color of the red, 1385 green and blue channels in the PNG RGB data. 1386 The white point is simply the sum of the three 1387 end points. (PNG_INFO_cHRM) 1388 1389 png_get_sRGB(png_ptr, info_ptr, &srgb_intent); 1390 1391 srgb_intent - the rendering intent (PNG_INFO_sRGB) 1392 The presence of the sRGB chunk 1393 means that the pixel data is in the 1394 sRGB color space. This chunk also 1395 implies specific values of gAMA and 1396 cHRM. 1397 1398 png_get_iCCP(png_ptr, info_ptr, &name, 1399 &compression_type, &profile, &proflen); 1400 1401 name - The profile name. 1402 1403 compression_type - The compression type; always 1404 PNG_COMPRESSION_TYPE_BASE for PNG 1.0. 1405 You may give NULL to this argument to 1406 ignore it. 1407 1408 profile - International Color Consortium color 1409 profile data. May contain NULs. 1410 1411 proflen - length of profile data in bytes. 1412 1413 png_get_sBIT(png_ptr, info_ptr, &sig_bit); 1414 1415 sig_bit - the number of significant bits for 1416 (PNG_INFO_sBIT) each of the gray, 1417 red, green, and blue channels, 1418 whichever are appropriate for the 1419 given color type (png_color_16) 1420 1421 png_get_tRNS(png_ptr, info_ptr, &trans_alpha, 1422 &num_trans, &trans_color); 1423 1424 trans_alpha - array of alpha (transparency) 1425 entries for palette (PNG_INFO_tRNS) 1426 1427 num_trans - number of transparent entries 1428 (PNG_INFO_tRNS) 1429 1430 trans_color - graylevel or color sample values of 1431 the single transparent color for 1432 non-paletted images (PNG_INFO_tRNS) 1433 1434 png_get_hIST(png_ptr, info_ptr, &hist); 1435 (PNG_INFO_hIST) 1436 1437 hist - histogram of palette (array of 1438 png_uint_16) 1439 1440 png_get_tIME(png_ptr, info_ptr, &mod_time); 1441 1442 mod_time - time image was last modified 1443 (PNG_VALID_tIME) 1444 1445 png_get_bKGD(png_ptr, info_ptr, &background); 1446 1447 background - background color (of type 1448 png_color_16p) (PNG_VALID_bKGD) 1449 valid 16-bit red, green and blue 1450 values, regardless of color_type 1451 1452 num_comments = png_get_text(png_ptr, info_ptr, 1453 &text_ptr, &num_text); 1454 1455 num_comments - number of comments 1456 1457 text_ptr - array of png_text holding image 1458 comments 1459 1460 text_ptr[i].compression - type of compression used 1461 on "text" PNG_TEXT_COMPRESSION_NONE 1462 PNG_TEXT_COMPRESSION_zTXt 1463 PNG_ITXT_COMPRESSION_NONE 1464 PNG_ITXT_COMPRESSION_zTXt 1465 1466 text_ptr[i].key - keyword for comment. Must contain 1467 1-79 characters. 1468 1469 text_ptr[i].text - text comments for current 1470 keyword. Can be empty. 1471 1472 text_ptr[i].text_length - length of text string, 1473 after decompression, 0 for iTXt 1474 1475 text_ptr[i].itxt_length - length of itxt string, 1476 after decompression, 0 for tEXt/zTXt 1477 1478 text_ptr[i].lang - language of comment (empty 1479 string for unknown). 1480 1481 text_ptr[i].lang_key - keyword in UTF-8 1482 (empty string for unknown). 1483 1484 Note that the itxt_length, lang, and lang_key 1485 members of the text_ptr structure only exist when the 1486 library is built with iTXt chunk support. Prior to 1487 libpng-1.4.0 the library was built by default without 1488 iTXt support. Also note that when iTXt is supported, 1489 they contain NULL pointers when the "compression" 1490 field contains PNG_TEXT_COMPRESSION_NONE or 1491 PNG_TEXT_COMPRESSION_zTXt. 1492 1493 num_text - number of comments (same as 1494 num_comments; you can put NULL here 1495 to avoid the duplication) 1496 1497 Note while png_set_text() will accept text, language, 1498 and translated keywords that can be NULL pointers, the 1499 structure returned by png_get_text will always contain 1500 regular zero-terminated C strings. They might be 1501 empty strings but they will never be NULL pointers. 1502 1503 num_spalettes = png_get_sPLT(png_ptr, info_ptr, 1504 &palette_ptr); 1505 1506 num_spalettes - number of sPLT chunks read. 1507 1508 palette_ptr - array of palette structures holding 1509 contents of one or more sPLT chunks 1510 read. 1511 1512 png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, 1513 &unit_type); 1514 1515 offset_x - positive offset from the left edge 1516 of the screen (can be negative) 1517 1518 offset_y - positive offset from the top edge 1519 of the screen (can be negative) 1520 1521 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER 1522 1523 png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, 1524 &unit_type); 1525 1526 res_x - pixels/unit physical resolution in 1527 x direction 1528 1529 res_y - pixels/unit physical resolution in 1530 x direction 1531 1532 unit_type - PNG_RESOLUTION_UNKNOWN, 1533 PNG_RESOLUTION_METER 1534 1535 png_get_sCAL(png_ptr, info_ptr, &unit, &width, 1536 &height) 1537 1538 unit - physical scale units (an integer) 1539 1540 width - width of a pixel in physical scale units 1541 1542 height - height of a pixel in physical scale units 1543 (width and height are doubles) 1544 1545 png_get_sCAL_s(png_ptr, info_ptr, &unit, &width, 1546 &height) 1547 1548 unit - physical scale units (an integer) 1549 1550 width - width of a pixel in physical scale units 1551 (expressed as a string) 1552 1553 height - height of a pixel in physical scale units 1554 (width and height are strings like "2.54") 1555 1556 num_unknown_chunks = png_get_unknown_chunks(png_ptr, 1557 info_ptr, &unknowns) 1558 1559 unknowns - array of png_unknown_chunk 1560 structures holding unknown chunks 1561 1562 unknowns[i].name - name of unknown chunk 1563 1564 unknowns[i].data - data of unknown chunk 1565 1566 unknowns[i].size - size of unknown chunk's data 1567 1568 unknowns[i].location - position of chunk in file 1569 1570 The value of "i" corresponds to the order in which the 1571 chunks were read from the PNG file or inserted with the 1572 png_set_unknown_chunks() function. 1573 1574 The value of "location" is a bitwise "or" of 1575 1576 PNG_HAVE_IHDR (0x01) 1577 PNG_HAVE_PLTE (0x02) 1578 PNG_AFTER_IDAT (0x08) 1579 1580 The data from the pHYs chunk can be retrieved in several convenient 1581 forms: 1582 1583 res_x = png_get_x_pixels_per_meter(png_ptr, 1584 info_ptr) 1585 1586 res_y = png_get_y_pixels_per_meter(png_ptr, 1587 info_ptr) 1588 1589 res_x_and_y = png_get_pixels_per_meter(png_ptr, 1590 info_ptr) 1591 1592 res_x = png_get_x_pixels_per_inch(png_ptr, 1593 info_ptr) 1594 1595 res_y = png_get_y_pixels_per_inch(png_ptr, 1596 info_ptr) 1597 1598 res_x_and_y = png_get_pixels_per_inch(png_ptr, 1599 info_ptr) 1600 1601 aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, 1602 info_ptr) 1603 1604 Each of these returns 0 [signifying "unknown"] if 1605 the data is not present or if res_x is 0; 1606 res_x_and_y is 0 if res_x != res_y 1607 1608 Note that because of the way the resolutions are 1609 stored internally, the inch conversions won't 1610 come out to exactly even number. For example, 1611 72 dpi is stored as 0.28346 pixels/meter, and 1612 when this is retrieved it is 71.9988 dpi, so 1613 be sure to round the returned value appropriately 1614 if you want to display a reasonable-looking result. 1615 1616 The data from the oFFs chunk can be retrieved in several convenient 1617 forms: 1618 1619 x_offset = png_get_x_offset_microns(png_ptr, info_ptr); 1620 1621 y_offset = png_get_y_offset_microns(png_ptr, info_ptr); 1622 1623 x_offset = png_get_x_offset_inches(png_ptr, info_ptr); 1624 1625 y_offset = png_get_y_offset_inches(png_ptr, info_ptr); 1626 1627 Each of these returns 0 [signifying "unknown" if both 1628 x and y are 0] if the data is not present or if the 1629 chunk is present but the unit is the pixel. The 1630 remark about inexact inch conversions applies here 1631 as well, because a value in inches can't always be 1632 converted to microns and back without some loss 1633 of precision. 1634 1635 For more information, see the 1636 PNG specification for chunk contents. Be careful with trusting 1637 rowbytes, as some of the transformations could increase the space 1638 needed to hold a row (expand, filler, gray_to_rgb, etc.). 1639 See png_read_update_info(), below. 1640 1641 A quick word about text_ptr and num_text. PNG stores comments in 1642 keyword/text pairs, one pair per chunk, with no limit on the number 1643 of text chunks, and a 2^31 byte limit on their size. While there are 1644 suggested keywords, there is no requirement to restrict the use to these 1645 strings. It is strongly suggested that keywords and text be sensible 1646 to humans (that's the point), so don't use abbreviations. Non-printing 1647 symbols are not allowed. See the PNG specification for more details. 1648 There is also no requirement to have text after the keyword. 1649 1650 Keywords should be limited to 79 Latin-1 characters without leading or 1651 trailing spaces, but non-consecutive spaces are allowed within the 1652 keyword. It is possible to have the same keyword any number of times. 1653 The text_ptr is an array of png_text structures, each holding a 1654 pointer to a language string, a pointer to a keyword and a pointer to 1655 a text string. The text string, language code, and translated 1656 keyword may be empty or NULL pointers. The keyword/text 1657 pairs are put into the array in the order that they are received. 1658 However, some or all of the text chunks may be after the image, so, to 1659 make sure you have read all the text chunks, don't mess with these 1660 until after you read the stuff after the image. This will be 1661 mentioned again below in the discussion that goes with png_read_end(). 1662 1663 Input transformations 1664 1665 After you've read the header information, you can set up the library 1666 to handle any special transformations of the image data. The various 1667 ways to transform the data will be described in the order that they 1668 should occur. This is important, as some of these change the color 1669 type and/or bit depth of the data, and some others only work on 1670 certain color types and bit depths. 1671 1672 Transformations you request are ignored if they don't have any meaning for a 1673 particular input data format. However some transformations can have an effect 1674 as a result of a previous transformation. If you specify a contradictory set of 1675 transformations, for example both adding and removing the alpha channel, you 1676 cannot predict the final result. 1677 1678 The color used for the transparency values should be supplied in the same 1679 format/depth as the current image data. It is stored in the same format/depth 1680 as the image data in a tRNS chunk, so this is what libpng expects for this data. 1681 1682 The color used for the background value depends on the need_expand argument as 1683 described below. 1684 1685 Data will be decoded into the supplied row buffers packed into bytes 1686 unless the library has been told to transform it into another format. 1687 For example, 4 bit/pixel paletted or grayscale data will be returned 1688 2 pixels/byte with the leftmost pixel in the high-order bits of the byte, 1689 unless png_set_packing() is called. 8-bit RGB data will be stored 1690 in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha() 1691 is called to insert filler bytes, either before or after each RGB triplet. 1692 1693 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant 1694 byte of the color value first, unless png_set_scale_16() is called to 1695 transform it to regular RGB RGB triplets, or png_set_filler() or 1696 png_set_add alpha() is called to insert two filler bytes, either before 1697 or after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can 1698 be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(), 1699 or png_set_scale_16(). 1700 1701 The following code transforms grayscale images of less than 8 to 8 bits, 1702 changes paletted images to RGB, and adds a full alpha channel if there is 1703 transparency information in a tRNS chunk. This is most useful on 1704 grayscale images with bit depths of 2 or 4 or if there is a multiple-image 1705 viewing application that wishes to treat all images in the same way. 1706 1707 if (color_type == PNG_COLOR_TYPE_PALETTE) 1708 png_set_palette_to_rgb(png_ptr); 1709 1710 if (png_get_valid(png_ptr, info_ptr, 1711 PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr); 1712 1713 if (color_type == PNG_COLOR_TYPE_GRAY && 1714 bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr); 1715 1716 The first two functions are actually aliases for png_set_expand(), added 1717 in libpng version 1.0.4, with the function names expanded to improve code 1718 readability. In some future version they may actually do different 1719 things. 1720 1721 As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was 1722 added. It expands the sample depth without changing tRNS to alpha. 1723 1724 As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as 1725 png_set_expand(); however, the resultant channels have 16 bits rather than 8. 1726 Use this when the output color or gray channels are made linear to avoid fairly 1727 severe accuracy loss. 1728 1729 if (bit_depth < 16) 1730 png_set_expand_16(png_ptr); 1731 1732 PNG can have files with 16 bits per channel. If you only can handle 1733 8 bits per channel, this will strip the pixels down to 8-bit. 1734 1735 if (bit_depth == 16) 1736 #if PNG_LIBPNG_VER >= 10504 1737 png_set_scale_16(png_ptr); 1738 #else 1739 png_set_strip_16(png_ptr); 1740 #endif 1741 1742 (The more accurate "png_set_scale_16()" API became available in libpng version 1743 1.5.4). 1744 1745 If you need to process the alpha channel on the image separately from the image 1746 data (for example if you convert it to a bitmap mask) it is possible to have 1747 libpng strip the channel leaving just RGB or gray data: 1748 1749 if (color_type & PNG_COLOR_MASK_ALPHA) 1750 png_set_strip_alpha(png_ptr); 1751 1752 If you strip the alpha channel you need to find some other way of dealing with 1753 the information. If, instead, you want to convert the image to an opaque 1754 version with no alpha channel use png_set_background; see below. 1755 1756 As of libpng version 1.5.2, almost all useful expansions are supported, the 1757 major ommissions are conversion of grayscale to indexed images (which can be 1758 done trivially in the application) and conversion of indexed to grayscale (which 1759 can be done by a trivial manipulation of the palette.) 1760 1761 In the following table, the 01 means grayscale with depth<8, 31 means 1762 indexed with depth<8, other numerals represent the color type, "T" means 1763 the tRNS chunk is present, A means an alpha channel is present, and O 1764 means tRNS or alpha is present but all pixels in the image are opaque. 1765 1766 FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O 1767 TO 1768 01 - [G] - - - - - - - - - - - - - 1769 31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q 1770 0 1 G + . . G G G G G G B B GB GB 1771 0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt 1772 0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt 1773 2 C P C C C + . . C - - CB CB B B 1774 2T Ct - Ct C C t + t - - - CBt CBt Bt Bt 1775 2O Ct - Ct C C t t + - - - CBt CBt Bt Bt 1776 3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q 1777 3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt 1778 3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt 1779 4A lA G A T T GA GT GT GA GT GT + BA G GBA 1780 4O lA GBA A T T GA GT GT GA GT GT BA + GBA G 1781 6A CA PA CA C C A T tT PA P P C CBA + BA 1782 6O CA PBA CA C C A tT T PA P P CBA C BA + 1783 1784 Within the matrix, 1785 "+" identifies entries where 'from' and 'to' are the same. 1786 "-" means the transformation is not supported. 1787 "." means nothing is necessary (a tRNS chunk can just be ignored). 1788 "t" means the transformation is obtained by png_set_tRNS. 1789 "A" means the transformation is obtained by png_set_add_alpha(). 1790 "X" means the transformation is obtained by png_set_expand(). 1791 "1" means the transformation is obtained by 1792 png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() 1793 if there is no transparency in the original or the final 1794 format). 1795 "C" means the transformation is obtained by png_set_gray_to_rgb(). 1796 "G" means the transformation is obtained by png_set_rgb_to_gray(). 1797 "P" means the transformation is obtained by 1798 png_set_expand_palette_to_rgb(). 1799 "p" means the transformation is obtained by png_set_packing(). 1800 "Q" means the transformation is obtained by png_set_quantize(). 1801 "T" means the transformation is obtained by 1802 png_set_tRNS_to_alpha(). 1803 "B" means the transformation is obtained by 1804 png_set_background(), or png_strip_alpha(). 1805 1806 When an entry has multiple transforms listed all are required to cause the 1807 right overall transformation. When two transforms are separated by a comma 1808 either will do the job. When transforms are enclosed in [] the transform should 1809 do the job but this is currently unimplemented - a different format will result 1810 if the suggested transformations are used. 1811 1812 In PNG files, the alpha channel in an image 1813 is the level of opacity. If you need the alpha channel in an image to 1814 be the level of transparency instead of opacity, you can invert the 1815 alpha channel (or the tRNS chunk data) after it's read, so that 0 is 1816 fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit 1817 images) is fully transparent, with 1818 1819 png_set_invert_alpha(png_ptr); 1820 1821 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as 1822 they can, resulting in, for example, 8 pixels per byte for 1 bit 1823 files. This code expands to 1 pixel per byte without changing the 1824 values of the pixels: 1825 1826 if (bit_depth < 8) 1827 png_set_packing(png_ptr); 1828 1829 PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels 1830 stored in a PNG image have been "scaled" or "shifted" up to the next 1831 higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] 1832 to 8 bits/sample in the range [0, 255]). However, it is also possible 1833 to convert the PNG pixel data back to the original bit depth of the 1834 image. This call reduces the pixels back down to the original bit depth: 1835 1836 png_color_8p sig_bit; 1837 1838 if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) 1839 png_set_shift(png_ptr, sig_bit); 1840 1841 PNG files store 3-color pixels in red, green, blue order. This code 1842 changes the storage of the pixels to blue, green, red: 1843 1844 if (color_type == PNG_COLOR_TYPE_RGB || 1845 color_type == PNG_COLOR_TYPE_RGB_ALPHA) 1846 png_set_bgr(png_ptr); 1847 1848 PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them 1849 into 4 or 8 bytes for windowing systems that need them in this format: 1850 1851 if (color_type == PNG_COLOR_TYPE_RGB) 1852 png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE); 1853 1854 where "filler" is the 8-bit or 16-bit number to fill with, and the location 1855 is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether 1856 you want the filler before the RGB or after. When filling an 8-bit pixel, 1857 the least significant 8 bits of the number are used, if a 16-bit number is 1858 supplied. This transformation does not affect images that already have full 1859 alpha channels. To add an opaque alpha channel, use filler=0xffff and 1860 PNG_FILLER_AFTER which will generate RGBA pixels. 1861 1862 Note that png_set_filler() does not change the color type. If you want 1863 to do that, you can add a true alpha channel with 1864 1865 if (color_type == PNG_COLOR_TYPE_RGB || 1866 color_type == PNG_COLOR_TYPE_GRAY) 1867 png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER); 1868 1869 where "filler" contains the alpha value to assign to each pixel. 1870 The png_set_add_alpha() function was added in libpng-1.2.7. 1871 1872 If you are reading an image with an alpha channel, and you need the 1873 data as ARGB instead of the normal PNG format RGBA: 1874 1875 if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) 1876 png_set_swap_alpha(png_ptr); 1877 1878 For some uses, you may want a grayscale image to be represented as 1879 RGB. This code will do that conversion: 1880 1881 if (color_type == PNG_COLOR_TYPE_GRAY || 1882 color_type == PNG_COLOR_TYPE_GRAY_ALPHA) 1883 png_set_gray_to_rgb(png_ptr); 1884 1885 Conversely, you can convert an RGB or RGBA image to grayscale or grayscale 1886 with alpha. 1887 1888 if (color_type == PNG_COLOR_TYPE_RGB || 1889 color_type == PNG_COLOR_TYPE_RGB_ALPHA) 1890 png_set_rgb_to_gray(png_ptr, error_action, 1891 double red_weight, double green_weight); 1892 1893 error_action = 1: silently do the conversion 1894 1895 error_action = 2: issue a warning if the original 1896 image has any pixel where 1897 red != green or red != blue 1898 1899 error_action = 3: issue an error and abort the 1900 conversion if the original 1901 image has any pixel where 1902 red != green or red != blue 1903 1904 red_weight: weight of red component 1905 1906 green_weight: weight of green component 1907 If either weight is negative, default 1908 weights are used. 1909 1910 In the corresponding fixed point API the red_weight and green_weight values are 1911 simply scaled by 100,000: 1912 1913 png_set_rgb_to_gray(png_ptr, error_action, 1914 png_fixed_point red_weight, 1915 png_fixed_point green_weight); 1916 1917 If you have set error_action = 1 or 2, you can 1918 later check whether the image really was gray, after processing 1919 the image rows, with the png_get_rgb_to_gray_status(png_ptr) function. 1920 It will return a png_byte that is zero if the image was gray or 1921 1 if there were any non-gray pixels. Background and sBIT data 1922 will be silently converted to grayscale, using the green channel 1923 data for sBIT, regardless of the error_action setting. 1924 1925 The default values come from the PNG file cHRM chunk if present; otherwise, the 1926 defaults correspond to the ITU-R recommendation 709, and also the sRGB color 1927 space, as recommended in the Charles Poynton's Colour FAQ, 1928 Copyright (c) 2006-11-28 Charles Poynton, in section 9: 1929 1930 <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9> 1931 1932 Y = 0.2126 * R + 0.7152 * G + 0.0722 * B 1933 1934 Previous versions of this document, 1998 through 2002, recommended a slightly 1935 different formula: 1936 1937 Y = 0.212671 * R + 0.715160 * G + 0.072169 * B 1938 1939 Libpng uses an integer approximation: 1940 1941 Y = (6968 * R + 23434 * G + 2366 * B)/32768 1942 1943 The calculation is done in a linear colorspace, if the image gamma 1944 can be determined. 1945 1946 The png_set_background() function has been described already; it tells libpng to 1947 composite images with alpha or simple transparency against the supplied 1948 background color. For compatibility with versions of libpng earlier than 1949 libpng-1.5.4 it is recommended that you call the function after reading the file 1950 header, even if you don't want to use the color in a bKGD chunk, if one exists. 1951 1952 If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), 1953 you may use this color, or supply another color more suitable for 1954 the current display (e.g., the background color from a web page). You 1955 need to tell libpng how the color is represented, both the format of the 1956 component values in the color (the number of bits) and the gamma encoding of the 1957 color. The function takes two arguments, background_gamma_mode and need_expand 1958 to convey this information; however, only two combinations are likely to be 1959 useful: 1960 1961 png_color_16 my_background; 1962 png_color_16p image_background; 1963 1964 if (png_get_bKGD(png_ptr, info_ptr, &image_background)) 1965 png_set_background(png_ptr, image_background, 1966 PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1); 1967 else 1968 png_set_background(png_ptr, &my_background, 1969 PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1); 1970 1971 The second call was described above - my_background is in the format of the 1972 final, display, output produced by libpng. Because you now know the format of 1973 the PNG it is possible to avoid the need to choose either 8-bit or 16-bit 1974 output and to retain palette images (the palette colors will be modified 1975 appropriately and the tRNS chunk removed.) However, if you are doing this, 1976 take great care not to ask for transformations without checking first that 1977 they apply! 1978 1979 In the first call the background color has the original bit depth and color type 1980 of the PNG file. So, for palette images the color is supplied as a palette 1981 index and for low bit greyscale images the color is a reduced bit value in 1982 image_background->gray. 1983 1984 If you didn't call png_set_gamma() before reading the file header, for example 1985 if you need your code to remain compatible with older versions of libpng prior 1986 to libpng-1.5.4, this is the place to call it. 1987 1988 Do not call it if you called png_set_alpha_mode(); doing so will damage the 1989 settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is 1990 supported then you can certainly do png_set_gamma() before reading the PNG 1991 header.) 1992 1993 This API unconditionally sets the screen and file gamma values, so it will 1994 override the value in the PNG file unless it is called before the PNG file 1995 reading starts. For this reason you must always call it with the PNG file 1996 value when you call it in this position: 1997 1998 if (png_get_gAMA(png_ptr, info_ptr, &file_gamma)) 1999 png_set_gamma(png_ptr, screen_gamma, file_gamma); 2000 2001 else 2002 png_set_gamma(png_ptr, screen_gamma, 0.45455); 2003 2004 If you need to reduce an RGB file to a paletted file, or if a paletted 2005 file has more entries than will fit on your screen, png_set_quantize() 2006 will do that. Note that this is a simple match quantization that merely 2007 finds the closest color available. This should work fairly well with 2008 optimized palettes, but fairly badly with linear color cubes. If you 2009 pass a palette that is larger than maximum_colors, the file will 2010 reduce the number of colors in the palette so it will fit into 2011 maximum_colors. If there is a histogram, libpng will use it to make 2012 more intelligent choices when reducing the palette. If there is no 2013 histogram, it may not do as good a job. 2014 2015 if (color_type & PNG_COLOR_MASK_COLOR) 2016 { 2017 if (png_get_valid(png_ptr, info_ptr, 2018 PNG_INFO_PLTE)) 2019 { 2020 png_uint_16p histogram = NULL; 2021 2022 png_get_hIST(png_ptr, info_ptr, 2023 &histogram); 2024 png_set_quantize(png_ptr, palette, num_palette, 2025 max_screen_colors, histogram, 1); 2026 } 2027 2028 else 2029 { 2030 png_color std_color_cube[MAX_SCREEN_COLORS] = 2031 { ... colors ... }; 2032 2033 png_set_quantize(png_ptr, std_color_cube, 2034 MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, 2035 NULL,0); 2036 } 2037 } 2038 2039 PNG files describe monochrome as black being zero and white being one. 2040 The following code will reverse this (make black be one and white be 2041 zero): 2042 2043 if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY) 2044 png_set_invert_mono(png_ptr); 2045 2046 This function can also be used to invert grayscale and gray-alpha images: 2047 2048 if (color_type == PNG_COLOR_TYPE_GRAY || 2049 color_type == PNG_COLOR_TYPE_GRAY_ALPHA) 2050 png_set_invert_mono(png_ptr); 2051 2052 PNG files store 16-bit pixels in network byte order (big-endian, 2053 ie. most significant bits first). This code changes the storage to the 2054 other way (little-endian, i.e. least significant bits first, the 2055 way PCs store them): 2056 2057 if (bit_depth == 16) 2058 png_set_swap(png_ptr); 2059 2060 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you 2061 need to change the order the pixels are packed into bytes, you can use: 2062 2063 if (bit_depth < 8) 2064 png_set_packswap(png_ptr); 2065 2066 Finally, you can write your own transformation function if none of 2067 the existing ones meets your needs. This is done by setting a callback 2068 with 2069 2070 png_set_read_user_transform_fn(png_ptr, 2071 read_transform_fn); 2072 2073 You must supply the function 2074 2075 void read_transform_fn(png_structp png_ptr, png_row_infop 2076 row_info, png_bytep data) 2077 2078 See pngtest.c for a working example. Your function will be called 2079 after all of the other transformations have been processed. Take care with 2080 interlaced images if you do the interlace yourself - the width of the row is the 2081 width in 'row_info', not the overall image width. 2082 2083 If supported, libpng provides two information routines that you can use to find 2084 where you are in processing the image: 2085 2086 png_get_current_pass_number(png_structp png_ptr); 2087 png_get_current_row_number(png_structp png_ptr); 2088 2089 Don't try using these outside a transform callback - firstly they are only 2090 supported if user transforms are supported, secondly they may well return 2091 unexpected results unless the row is actually being processed at the moment they 2092 are called. 2093 2094 With interlaced 2095 images the value returned is the row in the input sub-image image. Use 2096 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to 2097 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). 2098 2099 The discussion of interlace handling above contains more information on how to 2100 use these values. 2101 2102 You can also set up a pointer to a user structure for use by your 2103 callback function, and you can inform libpng that your transform 2104 function will change the number of channels or bit depth with the 2105 function 2106 2107 png_set_user_transform_info(png_ptr, user_ptr, 2108 user_depth, user_channels); 2109 2110 The user's application, not libpng, is responsible for allocating and 2111 freeing any memory required for the user structure. 2112 2113 You can retrieve the pointer via the function 2114 png_get_user_transform_ptr(). For example: 2115 2116 voidp read_user_transform_ptr = 2117 png_get_user_transform_ptr(png_ptr); 2118 2119 The last thing to handle is interlacing; this is covered in detail below, 2120 but you must call the function here if you want libpng to handle expansion 2121 of the interlaced image. 2122 2123 number_of_passes = png_set_interlace_handling(png_ptr); 2124 2125 After setting the transformations, libpng can update your png_info 2126 structure to reflect any transformations you've requested with this 2127 call. 2128 2129 png_read_update_info(png_ptr, info_ptr); 2130 2131 This is most useful to update the info structure's rowbytes 2132 field so you can use it to allocate your image memory. This function 2133 will also update your palette with the correct screen_gamma and 2134 background if these have been given with the calls above. You may 2135 only call png_read_update_info() once with a particular info_ptr. 2136 2137 After you call png_read_update_info(), you can allocate any 2138 memory you need to hold the image. The row data is simply 2139 raw byte data for all forms of images. As the actual allocation 2140 varies among applications, no example will be given. If you 2141 are allocating one large chunk, you will need to build an 2142 array of pointers to each row, as it will be needed for some 2143 of the functions below. 2144 2145 Remember: Before you call png_read_update_info(), the png_get_*() 2146 functions return the values corresponding to the original PNG image. 2147 After you call png_read_update_info the values refer to the image 2148 that libpng will output. Consequently you must call all the png_set_ 2149 functions before you call png_read_update_info(). This is particularly 2150 important for png_set_interlace_handling() - if you are going to call 2151 png_read_update_info() you must call png_set_interlace_handling() before 2152 it unless you want to receive interlaced output. 2153 2154 Reading image data 2155 2156 After you've allocated memory, you can read the image data. 2157 The simplest way to do this is in one function call. If you are 2158 allocating enough memory to hold the whole image, you can just 2159 call png_read_image() and libpng will read in all the image data 2160 and put it in the memory area supplied. You will need to pass in 2161 an array of pointers to each row. 2162 2163 This function automatically handles interlacing, so you don't 2164 need to call png_set_interlace_handling() (unless you call 2165 png_read_update_info()) or call this function multiple times, or any 2166 of that other stuff necessary with png_read_rows(). 2167 2168 png_read_image(png_ptr, row_pointers); 2169 2170 where row_pointers is: 2171 2172 png_bytep row_pointers[height]; 2173 2174 You can point to void or char or whatever you use for pixels. 2175 2176 If you don't want to read in the whole image at once, you can 2177 use png_read_rows() instead. If there is no interlacing (check 2178 interlace_type == PNG_INTERLACE_NONE), this is simple: 2179 2180 png_read_rows(png_ptr, row_pointers, NULL, 2181 number_of_rows); 2182 2183 where row_pointers is the same as in the png_read_image() call. 2184 2185 If you are doing this just one row at a time, you can do this with 2186 a single row_pointer instead of an array of row_pointers: 2187 2188 png_bytep row_pointer = row; 2189 png_read_row(png_ptr, row_pointer, NULL); 2190 2191 If the file is interlaced (interlace_type != 0 in the IHDR chunk), things 2192 get somewhat harder. The only current (PNG Specification version 1.2) 2193 interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7); 2194 a somewhat complicated 2D interlace scheme, known as Adam7, that 2195 breaks down an image into seven smaller images of varying size, based 2196 on an 8x8 grid. This number is defined (from libpng 1.5) as 2197 PNG_INTERLACE_ADAM7_PASSES in png.h 2198 2199 libpng can fill out those images or it can give them to you "as is". 2200 It is almost always better to have libpng handle the interlacing for you. 2201 If you want the images filled out, there are two ways to do that. The one 2202 mentioned in the PNG specification is to expand each pixel to cover 2203 those pixels that have not been read yet (the "rectangle" method). 2204 This results in a blocky image for the first pass, which gradually 2205 smooths out as more pixels are read. The other method is the "sparkle" 2206 method, where pixels are drawn only in their final locations, with the 2207 rest of the image remaining whatever colors they were initialized to 2208 before the start of the read. The first method usually looks better, 2209 but tends to be slower, as there are more pixels to put in the rows. 2210 2211 If, as is likely, you want libpng to expand the images, call this before 2212 calling png_start_read_image() or png_read_update_info(): 2213 2214 if (interlace_type == PNG_INTERLACE_ADAM7) 2215 number_of_passes 2216 = png_set_interlace_handling(png_ptr); 2217 2218 This will return the number of passes needed. Currently, this is seven, 2219 but may change if another interlace type is added. This function can be 2220 called even if the file is not interlaced, where it will return one pass. 2221 You then need to read the whole image 'number_of_passes' times. Each time 2222 will distribute the pixels from the current pass to the correct place in 2223 the output image, so you need to supply the same rows to png_read_rows in 2224 each pass. 2225 2226 If you are not going to display the image after each pass, but are 2227 going to wait until the entire image is read in, use the sparkle 2228 effect. This effect is faster and the end result of either method 2229 is exactly the same. If you are planning on displaying the image 2230 after each pass, the "rectangle" effect is generally considered the 2231 better looking one. 2232 2233 If you only want the "sparkle" effect, just call png_read_row() or 2234 png_read_rows() as 2235 normal, with the third parameter NULL. Make sure you make pass over 2236 the image number_of_passes times, and you don't change the data in the 2237 rows between calls. You can change the locations of the data, just 2238 not the data. Each pass only writes the pixels appropriate for that 2239 pass, and assumes the data from previous passes is still valid. 2240 2241 png_read_rows(png_ptr, row_pointers, NULL, 2242 number_of_rows); 2243 or 2244 png_read_row(png_ptr, row_pointers, NULL); 2245 2246 If you only want the first effect (the rectangles), do the same as 2247 before except pass the row buffer in the third parameter, and leave 2248 the second parameter NULL. 2249 2250 png_read_rows(png_ptr, NULL, row_pointers, 2251 number_of_rows); 2252 or 2253 png_read_row(png_ptr, NULL, row_pointers); 2254 2255 If you don't want libpng to handle the interlacing details, just call 2256 png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images. 2257 Each of the images is a valid image by itself; however, you will almost 2258 certainly need to distribute the pixels from each sub-image to the 2259 correct place. This is where everything gets very tricky. 2260 2261 If you want to retrieve the separate images you must pass the correct 2262 number of rows to each successive call of png_read_rows(). The calculation 2263 gets pretty complicated for small images, where some sub-images may 2264 not even exist because either their width or height ends up zero. 2265 libpng provides two macros to help you in 1.5 and later versions: 2266 2267 png_uint_32 width = PNG_PASS_COLS(image_width, pass_number); 2268 png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number); 2269 2270 Respectively these tell you the width and height of the sub-image 2271 corresponding to the numbered pass. 'pass' is in in the range 0 to 6 - 2272 this can be confusing because the specification refers to the same passes 2273 as 1 to 7! Be careful, you must check both the width and height before 2274 calling png_read_rows() and not call it for that pass if either is zero. 2275 2276 You can, of course, read each sub-image row by row. If you want to 2277 produce optimal code to make a pixel-by-pixel transformation of an 2278 interlaced image this is the best approach; read each row of each pass, 2279 transform it, and write it out to a new interlaced image. 2280 2281 If you want to de-interlace the image yourself libpng provides further 2282 macros to help that tell you where to place the pixels in the output image. 2283 Because the interlacing scheme is rectangular - sub-image pixels are always 2284 arranged on a rectangular grid - all you need to know for each pass is the 2285 starting column and row in the output image of the first pixel plus the 2286 spacing between each pixel. As of libpng 1.5 there are four macros to 2287 retrieve this information: 2288 2289 png_uint_32 x = PNG_PASS_START_COL(pass); 2290 png_uint_32 y = PNG_PASS_START_ROW(pass); 2291 png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass); 2292 png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass); 2293 2294 These allow you to write the obvious loop: 2295 2296 png_uint_32 input_y = 0; 2297 png_uint_32 output_y = PNG_PASS_START_ROW(pass); 2298 2299 while (output_y < output_image_height) 2300 { 2301 png_uint_32 input_x = 0; 2302 png_uint_32 output_x = PNG_PASS_START_COL(pass); 2303 2304 while (output_x < output_image_width) 2305 { 2306 image[output_y][output_x] = 2307 subimage[pass][input_y][input_x++]; 2308 2309 output_x += xStep; 2310 } 2311 2312 ++input_y; 2313 output_y += yStep; 2314 } 2315 2316 Notice that the steps between successive output rows and columns are 2317 returned as shifts. This is possible because the pixels in the subimages 2318 are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original 2319 image. In practice you may need to directly calculate the output coordinate 2320 given an input coordinate. libpng provides two further macros for this 2321 purpose: 2322 2323 png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass); 2324 png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass); 2325 2326 Finally a pair of macros are provided to tell you if a particular image 2327 row or column appears in a given pass: 2328 2329 int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass); 2330 int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass); 2331 2332 Bear in mind that you will probably also need to check the width and height 2333 of the pass in addition to the above to be sure the pass even exists! 2334 2335 With any luck you are convinced by now that you don't want to do your own 2336 interlace handling. In reality normally the only good reason for doing this 2337 is if you are processing PNG files on a pixel-by-pixel basis and don't want 2338 to load the whole file into memory when it is interlaced. 2339 2340 libpng includes a test program, pngvalid, that illustrates reading and 2341 writing of interlaced images. If you can't get interlacing to work in your 2342 code and don't want to leave it to libpng (the recommended approach), see 2343 how pngvalid.c does it. 2344 2345 Finishing a sequential read 2346 2347 After you are finished reading the image through the 2348 low-level interface, you can finish reading the file. 2349 2350 If you want to use a different crc action for handling CRC errors in 2351 chunks after the image data, you can call png_set_crc_action() 2352 again at this point. 2353 2354 If you are interested in comments or time, which may be stored either 2355 before or after the image data, you should pass the separate png_info 2356 struct if you want to keep the comments from before and after the image 2357 separate. 2358 2359 png_infop end_info = png_create_info_struct(png_ptr); 2360 2361 if (!end_info) 2362 { 2363 png_destroy_read_struct(&png_ptr, &info_ptr, 2364 (png_infopp)NULL); 2365 return (ERROR); 2366 } 2367 2368 png_read_end(png_ptr, end_info); 2369 2370 If you are not interested, you should still call png_read_end() 2371 but you can pass NULL, avoiding the need to create an end_info structure. 2372 If you do this, libpng will not process any chunks after IDAT other than 2373 skipping over them and perhaps (depending on whether you have called 2374 png_set_crc_action) checking their CRCs while looking for the IEND chunk. 2375 2376 png_read_end(png_ptr, (png_infop)NULL); 2377 2378 If you don't call png_read_end(), then your file pointer will be 2379 left pointing to the first chunk after the last IDAT, which is probably 2380 not what you want if you expect to read something beyond the end of 2381 the PNG datastream. 2382 2383 When you are done, you can free all memory allocated by libpng like this: 2384 2385 png_destroy_read_struct(&png_ptr, &info_ptr, 2386 &end_info); 2387 2388 or, if you didn't create an end_info structure, 2389 2390 png_destroy_read_struct(&png_ptr, &info_ptr, 2391 (png_infopp)NULL); 2392 2393 It is also possible to individually free the info_ptr members that 2394 point to libpng-allocated storage with the following function: 2395 2396 png_free_data(png_ptr, info_ptr, mask, seq) 2397 2398 mask - identifies data to be freed, a mask 2399 containing the bitwise OR of one or 2400 more of 2401 PNG_FREE_PLTE, PNG_FREE_TRNS, 2402 PNG_FREE_HIST, PNG_FREE_ICCP, 2403 PNG_FREE_PCAL, PNG_FREE_ROWS, 2404 PNG_FREE_SCAL, PNG_FREE_SPLT, 2405 PNG_FREE_TEXT, PNG_FREE_UNKN, 2406 or simply PNG_FREE_ALL 2407 2408 seq - sequence number of item to be freed 2409 (-1 for all items) 2410 2411 This function may be safely called when the relevant storage has 2412 already been freed, or has not yet been allocated, or was allocated 2413 by the user and not by libpng, and will in those cases do nothing. 2414 The "seq" parameter is ignored if only one item of the selected data 2415 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items 2416 are allowed for the data type identified in the mask, such as text or 2417 sPLT, only the n'th item in the structure is freed, where n is "seq". 2418 2419 The default behavior is only to free data that was allocated internally 2420 by libpng. This can be changed, so that libpng will not free the data, 2421 or so that it will free data that was allocated by the user with png_malloc() 2422 or png_calloc() and passed in via a png_set_*() function, with 2423 2424 png_data_freer(png_ptr, info_ptr, freer, mask) 2425 2426 freer - one of 2427 PNG_DESTROY_WILL_FREE_DATA 2428 PNG_SET_WILL_FREE_DATA 2429 PNG_USER_WILL_FREE_DATA 2430 2431 mask - which data elements are affected 2432 same choices as in png_free_data() 2433 2434 This function only affects data that has already been allocated. 2435 You can call this function after reading the PNG data but before calling 2436 any png_set_*() functions, to control whether the user or the png_set_*() 2437 function is responsible for freeing any existing data that might be present, 2438 and again after the png_set_*() functions to control whether the user 2439 or png_destroy_*() is supposed to free the data. When the user assumes 2440 responsibility for libpng-allocated data, the application must use 2441 png_free() to free it, and when the user transfers responsibility to libpng 2442 for data that the user has allocated, the user must have used png_malloc() 2443 or png_calloc() to allocate it. 2444 2445 If you allocated your row_pointers in a single block, as suggested above in 2446 the description of the high level read interface, you must not transfer 2447 responsibility for freeing it to the png_set_rows or png_read_destroy function, 2448 because they would also try to free the individual row_pointers[i]. 2449 2450 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword 2451 separately, do not transfer responsibility for freeing text_ptr to libpng, 2452 because when libpng fills a png_text structure it combines these members with 2453 the key member, and png_free_data() will free only text_ptr.key. Similarly, 2454 if you transfer responsibility for free'ing text_ptr from libpng to your 2455 application, your application must not separately free those members. 2456 2457 The png_free_data() function will turn off the "valid" flag for anything 2458 it frees. If you need to turn the flag off for a chunk that was freed by 2459 your application instead of by libpng, you can use 2460 2461 png_set_invalid(png_ptr, info_ptr, mask); 2462 2463 mask - identifies the chunks to be made invalid, 2464 containing the bitwise OR of one or 2465 more of 2466 PNG_INFO_gAMA, PNG_INFO_sBIT, 2467 PNG_INFO_cHRM, PNG_INFO_PLTE, 2468 PNG_INFO_tRNS, PNG_INFO_bKGD, 2469 PNG_INFO_hIST, PNG_INFO_pHYs, 2470 PNG_INFO_oFFs, PNG_INFO_tIME, 2471 PNG_INFO_pCAL, PNG_INFO_sRGB, 2472 PNG_INFO_iCCP, PNG_INFO_sPLT, 2473 PNG_INFO_sCAL, PNG_INFO_IDAT 2474 2475 For a more compact example of reading a PNG image, see the file example.c. 2476 2477 Reading PNG files progressively 2478 2479 The progressive reader is slightly different from the non-progressive 2480 reader. Instead of calling png_read_info(), png_read_rows(), and 2481 png_read_end(), you make one call to png_process_data(), which calls 2482 callbacks when it has the info, a row, or the end of the image. You 2483 set up these callbacks with png_set_progressive_read_fn(). You don't 2484 have to worry about the input/output functions of libpng, as you are 2485 giving the library the data directly in png_process_data(). I will 2486 assume that you have read the section on reading PNG files above, 2487 so I will only highlight the differences (although I will show 2488 all of the code). 2489 2490 png_structp png_ptr; 2491 png_infop info_ptr; 2492 2493 /* An example code fragment of how you would 2494 initialize the progressive reader in your 2495 application. */ 2496 int 2497 initialize_png_reader() 2498 { 2499 png_ptr = png_create_read_struct 2500 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 2501 user_error_fn, user_warning_fn); 2502 2503 if (!png_ptr) 2504 return (ERROR); 2505 2506 info_ptr = png_create_info_struct(png_ptr); 2507 2508 if (!info_ptr) 2509 { 2510 png_destroy_read_struct(&png_ptr, 2511 (png_infopp)NULL, (png_infopp)NULL); 2512 return (ERROR); 2513 } 2514 2515 if (setjmp(png_jmpbuf(png_ptr))) 2516 { 2517 png_destroy_read_struct(&png_ptr, &info_ptr, 2518 (png_infopp)NULL); 2519 return (ERROR); 2520 } 2521 2522 /* This one's new. You can provide functions 2523 to be called when the header info is valid, 2524 when each row is completed, and when the image 2525 is finished. If you aren't using all functions, 2526 you can specify NULL parameters. Even when all 2527 three functions are NULL, you need to call 2528 png_set_progressive_read_fn(). You can use 2529 any struct as the user_ptr (cast to a void pointer 2530 for the function call), and retrieve the pointer 2531 from inside the callbacks using the function 2532 2533 png_get_progressive_ptr(png_ptr); 2534 2535 which will return a void pointer, which you have 2536 to cast appropriately. 2537 */ 2538 png_set_progressive_read_fn(png_ptr, (void *)user_ptr, 2539 info_callback, row_callback, end_callback); 2540 2541 return 0; 2542 } 2543 2544 /* A code fragment that you call as you receive blocks 2545 of data */ 2546 int 2547 process_data(png_bytep buffer, png_uint_32 length) 2548 { 2549 if (setjmp(png_jmpbuf(png_ptr))) 2550 { 2551 png_destroy_read_struct(&png_ptr, &info_ptr, 2552 (png_infopp)NULL); 2553 return (ERROR); 2554 } 2555 2556 /* This one's new also. Simply give it a chunk 2557 of data from the file stream (in order, of 2558 course). On machines with segmented memory 2559 models machines, don't give it any more than 2560 64K. The library seems to run fine with sizes 2561 of 4K. Although you can give it much less if 2562 necessary (I assume you can give it chunks of 2563 1 byte, I haven't tried less than 256 bytes 2564 yet). When this function returns, you may 2565 want to display any rows that were generated 2566 in the row callback if you don't already do 2567 so there. 2568 */ 2569 png_process_data(png_ptr, info_ptr, buffer, length); 2570 2571 /* At this point you can call png_process_data_skip if 2572 you want to handle data the library will skip yourself; 2573 it simply returns the number of bytes to skip (and stops 2574 libpng skipping that number of bytes on the next 2575 png_process_data call). 2576 return 0; 2577 } 2578 2579 /* This function is called (as set by 2580 png_set_progressive_read_fn() above) when enough data 2581 has been supplied so all of the header has been 2582 read. 2583 */ 2584 void 2585 info_callback(png_structp png_ptr, png_infop info) 2586 { 2587 /* Do any setup here, including setting any of 2588 the transformations mentioned in the Reading 2589 PNG files section. For now, you _must_ call 2590 either png_start_read_image() or 2591 png_read_update_info() after all the 2592 transformations are set (even if you don't set 2593 any). You may start getting rows before 2594 png_process_data() returns, so this is your 2595 last chance to prepare for that. 2596 2597 This is where you turn on interlace handling, 2598 assuming you don't want to do it yourself. 2599 2600 If you need to you can stop the processing of 2601 your original input data at this point by calling 2602 png_process_data_pause. This returns the number 2603 of unprocessed bytes from the last png_process_data 2604 call - it is up to you to ensure that the next call 2605 sees these bytes again. If you don't want to bother 2606 with this you can get libpng to cache the unread 2607 bytes by setting the 'save' parameter (see png.h) but 2608 then libpng will have to copy the data internally. 2609 */ 2610 } 2611 2612 /* This function is called when each row of image 2613 data is complete */ 2614 void 2615 row_callback(png_structp png_ptr, png_bytep new_row, 2616 png_uint_32 row_num, int pass) 2617 { 2618 /* If the image is interlaced, and you turned 2619 on the interlace handler, this function will 2620 be called for every row in every pass. Some 2621 of these rows will not be changed from the 2622 previous pass. When the row is not changed, 2623 the new_row variable will be NULL. The rows 2624 and passes are called in order, so you don't 2625 really need the row_num and pass, but I'm 2626 supplying them because it may make your life 2627 easier. 2628 2629 If you did not turn on interlace handling then 2630 the callback is called for each row of each 2631 sub-image when the image is interlaced. In this 2632 case 'row_num' is the row in the sub-image, not 2633 the row in the output image as it is in all other 2634 cases. 2635 2636 For the non-NULL rows of interlaced images when 2637 you have switched on libpng interlace handling, 2638 you must call png_progressive_combine_row() 2639 passing in the row and the old row. You can 2640 call this function for NULL rows (it will just 2641 return) and for non-interlaced images (it just 2642 does the memcpy for you) if it will make the 2643 code easier. Thus, you can just do this for 2644 all cases if you switch on interlace handling; 2645 */ 2646 2647 png_progressive_combine_row(png_ptr, old_row, 2648 new_row); 2649 2650 /* where old_row is what was displayed 2651 previously for the row. Note that the first 2652 pass (pass == 0, really) will completely cover 2653 the old row, so the rows do not have to be 2654 initialized. After the first pass (and only 2655 for interlaced images), you will have to pass 2656 the current row, and the function will combine 2657 the old row and the new row. 2658 2659 You can also call png_process_data_pause in this 2660 callback - see above. 2661 */ 2662 } 2663 2664 void 2665 end_callback(png_structp png_ptr, png_infop info) 2666 { 2667 /* This function is called after the whole image 2668 has been read, including any chunks after the 2669 image (up to and including the IEND). You 2670 will usually have the same info chunk as you 2671 had in the header, although some data may have 2672 been added to the comments and time fields. 2673 2674 Most people won't do much here, perhaps setting 2675 a flag that marks the image as finished. 2676 */ 2677 } 2678 2679 2680 2681 IV. Writing 2682 2683 Much of this is very similar to reading. However, everything of 2684 importance is repeated here, so you won't have to constantly look 2685 back up in the reading section to understand writing. 2686 2687 Setup 2688 2689 You will want to do the I/O initialization before you get into libpng, 2690 so if it doesn't work, you don't have anything to undo. If you are not 2691 using the standard I/O functions, you will need to replace them with 2692 custom writing functions. See the discussion under Customizing libpng. 2693 2694 FILE *fp = fopen(file_name, "wb"); 2695 2696 if (!fp) 2697 return (ERROR); 2698 2699 Next, png_struct and png_info need to be allocated and initialized. 2700 As these can be both relatively large, you may not want to store these 2701 on the stack, unless you have stack space to spare. Of course, you 2702 will want to check if they return NULL. If you are also reading, 2703 you won't want to name your read structure and your write structure 2704 both "png_ptr"; you can call them anything you like, such as 2705 "read_ptr" and "write_ptr". Look at pngtest.c, for example. 2706 2707 png_structp png_ptr = png_create_write_struct 2708 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 2709 user_error_fn, user_warning_fn); 2710 2711 if (!png_ptr) 2712 return (ERROR); 2713 2714 png_infop info_ptr = png_create_info_struct(png_ptr); 2715 if (!info_ptr) 2716 { 2717 png_destroy_write_struct(&png_ptr, 2718 (png_infopp)NULL); 2719 return (ERROR); 2720 } 2721 2722 If you want to use your own memory allocation routines, 2723 define PNG_USER_MEM_SUPPORTED and use 2724 png_create_write_struct_2() instead of png_create_write_struct(): 2725 2726 png_structp png_ptr = png_create_write_struct_2 2727 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 2728 user_error_fn, user_warning_fn, (png_voidp) 2729 user_mem_ptr, user_malloc_fn, user_free_fn); 2730 2731 After you have these structures, you will need to set up the 2732 error handling. When libpng encounters an error, it expects to 2733 longjmp() back to your routine. Therefore, you will need to call 2734 setjmp() and pass the png_jmpbuf(png_ptr). If you 2735 write the file from different routines, you will need to update 2736 the png_jmpbuf(png_ptr) every time you enter a new routine that will 2737 call a png_*() function. See your documentation of setjmp/longjmp 2738 for your compiler for more information on setjmp/longjmp. See 2739 the discussion on libpng error handling in the Customizing Libpng 2740 section below for more information on the libpng error handling. 2741 2742 if (setjmp(png_jmpbuf(png_ptr))) 2743 { 2744 png_destroy_write_struct(&png_ptr, &info_ptr); 2745 fclose(fp); 2746 return (ERROR); 2747 } 2748 ... 2749 return; 2750 2751 If you would rather avoid the complexity of setjmp/longjmp issues, 2752 you can compile libpng with PNG_NO_SETJMP, in which case 2753 errors will result in a call to PNG_ABORT() which defaults to abort(). 2754 2755 You can #define PNG_ABORT() to a function that does something 2756 more useful than abort(), as long as your function does not 2757 return. 2758 2759 Checking for invalid palette index on write was added at libpng 2760 1.5.10. If a pixel contains an invalid (out-of-range) index libpng issues 2761 a benign error. This is enabled by default because this condition is an 2762 error according to the PNG specification, Clause 11.3.2, but the error can 2763 be ignored in each png_ptr with 2764 2765 png_set_check_for_invalid_index(png_ptr, 0); 2766 2767 If the error is ignored, or if png_benign_error() treats it as a warning, 2768 any invalid pixels are written as-is by the encoder, resulting in an 2769 invalid PNG datastream as output. In this case the application is 2770 responsible for ensuring that the pixel indexes are in range when it writes 2771 a PLTE chunk with fewer entries than the bit depth would allow. 2772 2773 Now you need to set up the output code. The default for libpng is to 2774 use the C function fwrite(). If you use this, you will need to pass a 2775 valid FILE * in the function png_init_io(). Be sure that the file is 2776 opened in binary mode. Again, if you wish to handle writing data in 2777 another way, see the discussion on libpng I/O handling in the Customizing 2778 Libpng section below. 2779 2780 png_init_io(png_ptr, fp); 2781 2782 If you are embedding your PNG into a datastream such as MNG, and don't 2783 want libpng to write the 8-byte signature, or if you have already 2784 written the signature in your application, use 2785 2786 png_set_sig_bytes(png_ptr, 8); 2787 2788 to inform libpng that it should not write a signature. 2789 2790 Write callbacks 2791 2792 At this point, you can set up a callback function that will be 2793 called after each row has been written, which you can use to control 2794 a progress meter or the like. It's demonstrated in pngtest.c. 2795 You must supply a function 2796 2797 void write_row_callback(png_structp png_ptr, png_uint_32 row, 2798 int pass); 2799 { 2800 /* put your code here */ 2801 } 2802 2803 (You can give it another name that you like instead of "write_row_callback") 2804 2805 To inform libpng about your function, use 2806 2807 png_set_write_status_fn(png_ptr, write_row_callback); 2808 2809 When this function is called the row has already been completely processed and 2810 it has also been written out. The 'row' and 'pass' refer to the next row to be 2811 handled. For the 2812 non-interlaced case the row that was just handled is simply one less than the 2813 passed in row number, and pass will always be 0. For the interlaced case the 2814 same applies unless the row value is 0, in which case the row just handled was 2815 the last one from one of the preceding passes. Because interlacing may skip a 2816 pass you cannot be sure that the preceding pass is just 'pass-1', if you really 2817 need to know what the last pass is record (row,pass) from the callback and use 2818 the last recorded value each time. 2819 2820 As with the user transform you can find the output row using the 2821 PNG_ROW_FROM_PASS_ROW macro. 2822 2823 You now have the option of modifying how the compression library will 2824 run. The following functions are mainly for testing, but may be useful 2825 in some cases, like if you need to write PNG files extremely fast and 2826 are willing to give up some compression, or if you want to get the 2827 maximum possible compression at the expense of slower writing. If you 2828 have no special needs in this area, let the library do what it wants by 2829 not calling this function at all, as it has been tuned to deliver a good 2830 speed/compression ratio. The second parameter to png_set_filter() is 2831 the filter method, for which the only valid values are 0 (as of the 2832 July 1999 PNG specification, version 1.2) or 64 (if you are writing 2833 a PNG datastream that is to be embedded in a MNG datastream). The third 2834 parameter is a flag that indicates which filter type(s) are to be tested 2835 for each scanline. See the PNG specification for details on the specific 2836 filter types. 2837 2838 2839 /* turn on or off filtering, and/or choose 2840 specific filters. You can use either a single 2841 PNG_FILTER_VALUE_NAME or the bitwise OR of one 2842 or more PNG_FILTER_NAME masks. 2843 */ 2844 png_set_filter(png_ptr, 0, 2845 PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE | 2846 PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB | 2847 PNG_FILTER_UP | PNG_FILTER_VALUE_UP | 2848 PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG | 2849 PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH| 2850 PNG_ALL_FILTERS | PNG_FAST_FILTERS); 2851 2852 If an application wants to start and stop using particular filters during 2853 compression, it should start out with all of the filters (to ensure that 2854 the previous row of pixels will be stored in case it's needed later), 2855 and then add and remove them after the start of compression. 2856 2857 If you are writing a PNG datastream that is to be embedded in a MNG 2858 datastream, the second parameter can be either 0 or 64. 2859 2860 The png_set_compression_*() functions interface to the zlib compression 2861 library, and should mostly be ignored unless you really know what you are 2862 doing. The only generally useful call is png_set_compression_level() 2863 which changes how much time zlib spends on trying to compress the image 2864 data. See the Compression Library (zlib.h and algorithm.txt, distributed 2865 with zlib) for details on the compression levels. 2866 2867 #include zlib.h 2868 2869 /* Set the zlib compression level */ 2870 png_set_compression_level(png_ptr, 2871 Z_BEST_COMPRESSION); 2872 2873 /* Set other zlib parameters for compressing IDAT */ 2874 png_set_compression_mem_level(png_ptr, 8); 2875 png_set_compression_strategy(png_ptr, 2876 Z_DEFAULT_STRATEGY); 2877 png_set_compression_window_bits(png_ptr, 15); 2878 png_set_compression_method(png_ptr, 8); 2879 png_set_compression_buffer_size(png_ptr, 8192) 2880 2881 /* Set zlib parameters for text compression 2882 * If you don't call these, the parameters 2883 * fall back on those defined for IDAT chunks 2884 */ 2885 png_set_text_compression_mem_level(png_ptr, 8); 2886 png_set_text_compression_strategy(png_ptr, 2887 Z_DEFAULT_STRATEGY); 2888 png_set_text_compression_window_bits(png_ptr, 15); 2889 png_set_text_compression_method(png_ptr, 8); 2890 2891 Setting the contents of info for output 2892 2893 You now need to fill in the png_info structure with all the data you 2894 wish to write before the actual image. Note that the only thing you 2895 are allowed to write after the image is the text chunks and the time 2896 chunk (as of PNG Specification 1.2, anyway). See png_write_end() and 2897 the latest PNG specification for more information on that. If you 2898 wish to write them before the image, fill them in now, and flag that 2899 data as being valid. If you want to wait until after the data, don't 2900 fill them until png_write_end(). For all the fields in png_info and 2901 their data types, see png.h. For explanations of what the fields 2902 contain, see the PNG specification. 2903 2904 Some of the more important parts of the png_info are: 2905 2906 png_set_IHDR(png_ptr, info_ptr, width, height, 2907 bit_depth, color_type, interlace_type, 2908 compression_type, filter_method) 2909 2910 width - holds the width of the image 2911 in pixels (up to 2^31). 2912 2913 height - holds the height of the image 2914 in pixels (up to 2^31). 2915 2916 bit_depth - holds the bit depth of one of the 2917 image channels. 2918 (valid values are 1, 2, 4, 8, 16 2919 and depend also on the 2920 color_type. See also significant 2921 bits (sBIT) below). 2922 2923 color_type - describes which color/alpha 2924 channels are present. 2925 PNG_COLOR_TYPE_GRAY 2926 (bit depths 1, 2, 4, 8, 16) 2927 PNG_COLOR_TYPE_GRAY_ALPHA 2928 (bit depths 8, 16) 2929 PNG_COLOR_TYPE_PALETTE 2930 (bit depths 1, 2, 4, 8) 2931 PNG_COLOR_TYPE_RGB 2932 (bit_depths 8, 16) 2933 PNG_COLOR_TYPE_RGB_ALPHA 2934 (bit_depths 8, 16) 2935 2936 PNG_COLOR_MASK_PALETTE 2937 PNG_COLOR_MASK_COLOR 2938 PNG_COLOR_MASK_ALPHA 2939 2940 interlace_type - PNG_INTERLACE_NONE or 2941 PNG_INTERLACE_ADAM7 2942 2943 compression_type - (must be 2944 PNG_COMPRESSION_TYPE_DEFAULT) 2945 2946 filter_method - (must be PNG_FILTER_TYPE_DEFAULT 2947 or, if you are writing a PNG to 2948 be embedded in a MNG datastream, 2949 can also be 2950 PNG_INTRAPIXEL_DIFFERENCING) 2951 2952 If you call png_set_IHDR(), the call must appear before any of the 2953 other png_set_*() functions, because they might require access to some of 2954 the IHDR settings. The remaining png_set_*() functions can be called 2955 in any order. 2956 2957 If you wish, you can reset the compression_type, interlace_type, or 2958 filter_method later by calling png_set_IHDR() again; if you do this, the 2959 width, height, bit_depth, and color_type must be the same in each call. 2960 2961 png_set_PLTE(png_ptr, info_ptr, palette, 2962 num_palette); 2963 2964 palette - the palette for the file 2965 (array of png_color) 2966 num_palette - number of entries in the palette 2967 2968 2969 png_set_gAMA(png_ptr, info_ptr, file_gamma); 2970 png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma); 2971 2972 file_gamma - the gamma at which the image was 2973 created (PNG_INFO_gAMA) 2974 2975 int_file_gamma - 100,000 times the gamma at which 2976 the image was created 2977 2978 png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y, 2979 green_x, green_y, blue_x, blue_y) 2980 png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X, 2981 green_Y, green_Z, blue_X, blue_Y, blue_Z) 2982 png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y, 2983 int_red_x, int_red_y, int_green_x, int_green_y, 2984 int_blue_x, int_blue_y) 2985 png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y, 2986 int_red_Z, int_green_X, int_green_Y, int_green_Z, 2987 int_blue_X, int_blue_Y, int_blue_Z) 2988 2989 {white,red,green,blue}_{x,y} 2990 A color space encoding specified using the chromaticities 2991 of the end points and the white point. 2992 2993 {red,green,blue}_{X,Y,Z} 2994 A color space encoding specified using the encoding end 2995 points - the CIE tristimulus specification of the intended 2996 color of the red, green and blue channels in the PNG RGB 2997 data. The white point is simply the sum of the three end 2998 points. 2999 3000 png_set_sRGB(png_ptr, info_ptr, srgb_intent); 3001 3002 srgb_intent - the rendering intent 3003 (PNG_INFO_sRGB) The presence of 3004 the sRGB chunk means that the pixel 3005 data is in the sRGB color space. 3006 This chunk also implies specific 3007 values of gAMA and cHRM. Rendering 3008 intent is the CSS-1 property that 3009 has been defined by the International 3010 Color Consortium 3011 (http://www.color.org). 3012 It can be one of 3013 PNG_sRGB_INTENT_SATURATION, 3014 PNG_sRGB_INTENT_PERCEPTUAL, 3015 PNG_sRGB_INTENT_ABSOLUTE, or 3016 PNG_sRGB_INTENT_RELATIVE. 3017 3018 3019 png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, 3020 srgb_intent); 3021 3022 srgb_intent - the rendering intent 3023 (PNG_INFO_sRGB) The presence of the 3024 sRGB chunk means that the pixel 3025 data is in the sRGB color space. 3026 This function also causes gAMA and 3027 cHRM chunks with the specific values 3028 that are consistent with sRGB to be 3029 written. 3030 3031 png_set_iCCP(png_ptr, info_ptr, name, compression_type, 3032 profile, proflen); 3033 3034 name - The profile name. 3035 3036 compression_type - The compression type; always 3037 PNG_COMPRESSION_TYPE_BASE for PNG 1.0. 3038 You may give NULL to this argument to 3039 ignore it. 3040 3041 profile - International Color Consortium color 3042 profile data. May contain NULs. 3043 3044 proflen - length of profile data in bytes. 3045 3046 png_set_sBIT(png_ptr, info_ptr, sig_bit); 3047 3048 sig_bit - the number of significant bits for 3049 (PNG_INFO_sBIT) each of the gray, red, 3050 green, and blue channels, whichever are 3051 appropriate for the given color type 3052 (png_color_16) 3053 3054 png_set_tRNS(png_ptr, info_ptr, trans_alpha, 3055 num_trans, trans_color); 3056 3057 trans_alpha - array of alpha (transparency) 3058 entries for palette (PNG_INFO_tRNS) 3059 3060 num_trans - number of transparent entries 3061 (PNG_INFO_tRNS) 3062 3063 trans_color - graylevel or color sample values 3064 (in order red, green, blue) of the 3065 single transparent color for 3066 non-paletted images (PNG_INFO_tRNS) 3067 3068 png_set_hIST(png_ptr, info_ptr, hist); 3069 3070 hist - histogram of palette (array of 3071 png_uint_16) (PNG_INFO_hIST) 3072 3073 png_set_tIME(png_ptr, info_ptr, mod_time); 3074 3075 mod_time - time image was last modified 3076 (PNG_VALID_tIME) 3077 3078 png_set_bKGD(png_ptr, info_ptr, background); 3079 3080 background - background color (of type 3081 png_color_16p) (PNG_VALID_bKGD) 3082 3083 png_set_text(png_ptr, info_ptr, text_ptr, num_text); 3084 3085 text_ptr - array of png_text holding image 3086 comments 3087 3088 text_ptr[i].compression - type of compression used 3089 on "text" PNG_TEXT_COMPRESSION_NONE 3090 PNG_TEXT_COMPRESSION_zTXt 3091 PNG_ITXT_COMPRESSION_NONE 3092 PNG_ITXT_COMPRESSION_zTXt 3093 text_ptr[i].key - keyword for comment. Must contain 3094 1-79 characters. 3095 text_ptr[i].text - text comments for current 3096 keyword. Can be NULL or empty. 3097 text_ptr[i].text_length - length of text string, 3098 after decompression, 0 for iTXt 3099 text_ptr[i].itxt_length - length of itxt string, 3100 after decompression, 0 for tEXt/zTXt 3101 text_ptr[i].lang - language of comment (NULL or 3102 empty for unknown). 3103 text_ptr[i].translated_keyword - keyword in UTF-8 (NULL 3104 or empty for unknown). 3105 3106 Note that the itxt_length, lang, and lang_key 3107 members of the text_ptr structure only exist when the 3108 library is built with iTXt chunk support. Prior to 3109 libpng-1.4.0 the library was built by default without 3110 iTXt support. Also note that when iTXt is supported, 3111 they contain NULL pointers when the "compression" 3112 field contains PNG_TEXT_COMPRESSION_NONE or 3113 PNG_TEXT_COMPRESSION_zTXt. 3114 3115 num_text - number of comments 3116 3117 png_set_sPLT(png_ptr, info_ptr, &palette_ptr, 3118 num_spalettes); 3119 3120 palette_ptr - array of png_sPLT_struct structures 3121 to be added to the list of palettes 3122 in the info structure. 3123 num_spalettes - number of palette structures to be 3124 added. 3125 3126 png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, 3127 unit_type); 3128 3129 offset_x - positive offset from the left 3130 edge of the screen 3131 3132 offset_y - positive offset from the top 3133 edge of the screen 3134 3135 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER 3136 3137 png_set_pHYs(png_ptr, info_ptr, res_x, res_y, 3138 unit_type); 3139 3140 res_x - pixels/unit physical resolution 3141 in x direction 3142 3143 res_y - pixels/unit physical resolution 3144 in y direction 3145 3146 unit_type - PNG_RESOLUTION_UNKNOWN, 3147 PNG_RESOLUTION_METER 3148 3149 png_set_sCAL(png_ptr, info_ptr, unit, width, height) 3150 3151 unit - physical scale units (an integer) 3152 3153 width - width of a pixel in physical scale units 3154 3155 height - height of a pixel in physical scale units 3156 (width and height are doubles) 3157 3158 png_set_sCAL_s(png_ptr, info_ptr, unit, width, height) 3159 3160 unit - physical scale units (an integer) 3161 3162 width - width of a pixel in physical scale units 3163 expressed as a string 3164 3165 height - height of a pixel in physical scale units 3166 (width and height are strings like "2.54") 3167 3168 png_set_unknown_chunks(png_ptr, info_ptr, &unknowns, 3169 num_unknowns) 3170 3171 unknowns - array of png_unknown_chunk 3172 structures holding unknown chunks 3173 unknowns[i].name - name of unknown chunk 3174 unknowns[i].data - data of unknown chunk 3175 unknowns[i].size - size of unknown chunk's data 3176 unknowns[i].location - position to write chunk in file 3177 0: do not write chunk 3178 PNG_HAVE_IHDR: before PLTE 3179 PNG_HAVE_PLTE: before IDAT 3180 PNG_AFTER_IDAT: after IDAT 3181 3182 The "location" member is set automatically according to 3183 what part of the output file has already been written. 3184 You can change its value after calling png_set_unknown_chunks() 3185 as demonstrated in pngtest.c. Within each of the "locations", 3186 the chunks are sequenced according to their position in the 3187 structure (that is, the value of "i", which is the order in which 3188 the chunk was either read from the input file or defined with 3189 png_set_unknown_chunks). 3190 3191 A quick word about text and num_text. text is an array of png_text 3192 structures. num_text is the number of valid structures in the array. 3193 Each png_text structure holds a language code, a keyword, a text value, 3194 and a compression type. 3195 3196 The compression types have the same valid numbers as the compression 3197 types of the image data. Currently, the only valid number is zero. 3198 However, you can store text either compressed or uncompressed, unlike 3199 images, which always have to be compressed. So if you don't want the 3200 text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE. 3201 Because tEXt and zTXt chunks don't have a language field, if you 3202 specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt 3203 any language code or translated keyword will not be written out. 3204 3205 Until text gets around a few hundred bytes, it is not worth compressing it. 3206 After the text has been written out to the file, the compression type 3207 is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, 3208 so that it isn't written out again at the end (in case you are calling 3209 png_write_end() with the same struct). 3210 3211 The keywords that are given in the PNG Specification are: 3212 3213 Title Short (one line) title or 3214 caption for image 3215 3216 Author Name of image's creator 3217 3218 Description Description of image (possibly long) 3219 3220 Copyright Copyright notice 3221 3222 Creation Time Time of original image creation 3223 (usually RFC 1123 format, see below) 3224 3225 Software Software used to create the image 3226 3227 Disclaimer Legal disclaimer 3228 3229 Warning Warning of nature of content 3230 3231 Source Device used to create the image 3232 3233 Comment Miscellaneous comment; conversion 3234 from other image format 3235 3236 The keyword-text pairs work like this. Keywords should be short 3237 simple descriptions of what the comment is about. Some typical 3238 keywords are found in the PNG specification, as is some recommendations 3239 on keywords. You can repeat keywords in a file. You can even write 3240 some text before the image and some after. For example, you may want 3241 to put a description of the image before the image, but leave the 3242 disclaimer until after, so viewers working over modem connections 3243 don't have to wait for the disclaimer to go over the modem before 3244 they start seeing the image. Finally, keywords should be full 3245 words, not abbreviations. Keywords and text are in the ISO 8859-1 3246 (Latin-1) character set (a superset of regular ASCII) and can not 3247 contain NUL characters, and should not contain control or other 3248 unprintable characters. To make the comments widely readable, stick 3249 with basic ASCII, and avoid machine specific character set extensions 3250 like the IBM-PC character set. The keyword must be present, but 3251 you can leave off the text string on non-compressed pairs. 3252 Compressed pairs must have a text string, as only the text string 3253 is compressed anyway, so the compression would be meaningless. 3254 3255 PNG supports modification time via the png_time structure. Two 3256 conversion routines are provided, png_convert_from_time_t() for 3257 time_t and png_convert_from_struct_tm() for struct tm. The 3258 time_t routine uses gmtime(). You don't have to use either of 3259 these, but if you wish to fill in the png_time structure directly, 3260 you should provide the time in universal time (GMT) if possible 3261 instead of your local time. Note that the year number is the full 3262 year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and 3263 that months start with 1. 3264 3265 If you want to store the time of the original image creation, you should 3266 use a plain tEXt chunk with the "Creation Time" keyword. This is 3267 necessary because the "creation time" of a PNG image is somewhat vague, 3268 depending on whether you mean the PNG file, the time the image was 3269 created in a non-PNG format, a still photo from which the image was 3270 scanned, or possibly the subject matter itself. In order to facilitate 3271 machine-readable dates, it is recommended that the "Creation Time" 3272 tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), 3273 although this isn't a requirement. Unlike the tIME chunk, the 3274 "Creation Time" tEXt chunk is not expected to be automatically changed 3275 by the software. To facilitate the use of RFC 1123 dates, a function 3276 png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to 3277 convert from PNG time to an RFC 1123 format string. The caller must provide 3278 a writeable buffer of at least 29 bytes. 3279 3280 Writing unknown chunks 3281 3282 You can use the png_set_unknown_chunks function to queue up private chunks 3283 for writing. You give it a chunk name, location, raw data, and a size. You 3284 also must use png_set_keep_unknown_chunks() to ensure that libpng will 3285 handle them. That's all there is to it. The chunks will be written by the 3286 next following png_write_info_before_PLTE, png_write_info, or png_write_end 3287 function, depending upon the specified location. Any chunks previously 3288 read into the info structure's unknown-chunk list will also be written out 3289 in a sequence that satisfies the PNG specification's ordering rules. 3290 3291 Here is an example of writing two private chunks, prVt and miNE: 3292 3293 #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED 3294 /* Set unknown chunk data */ 3295 png_unknown_chunk unk_chunk[2]; 3296 strcpy((char *) unk_chunk[0].name, "prVt"; 3297 unk_chunk[0].data = (unsigned char *) "PRIVATE DATA"; 3298 unk_chunk[0].size = strlen(unk_chunk[0].data)+1; 3299 unk_chunk[0].location = PNG_HAVE_IHDR; 3300 strcpy((char *) unk_chunk[1].name, "miNE"; 3301 unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA"; 3302 unk_chunk[1].size = strlen(unk_chunk[0].data)+1; 3303 unk_chunk[1].location = PNG_AFTER_IDAT; 3304 png_set_unknown_chunks(write_ptr, write_info_ptr, 3305 unk_chunk, 2); 3306 /* Needed because miNE is not safe-to-copy */ 3307 png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS, 3308 (png_bytep) "miNE", 1); 3309 # if PNG_LIBPNG_VER < 10600 3310 /* Deal with unknown chunk location bug in 1.5.x and earlier */ 3311 png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR); 3312 png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT); 3313 # endif 3314 # if PNG_LIBPNG_VER < 10500 3315 /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0, 3316 * one before IDAT and another after IDAT, so don't use it; only use 3317 * PNG_HAVE_IHDR location. This call resets the location previously 3318 * set by assignment and png_set_unknown_chunk_location() for chunk 1. 3319 */ 3320 png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR); 3321 # endif 3322 #endif 3323 3324 The high-level write interface 3325 3326 At this point there are two ways to proceed; through the high-level 3327 write interface, or through a sequence of low-level write operations. 3328 You can use the high-level interface if your image data is present 3329 in the info structure. All defined output 3330 transformations are permitted, enabled by the following masks. 3331 3332 PNG_TRANSFORM_IDENTITY No transformation 3333 PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples 3334 PNG_TRANSFORM_PACKSWAP Change order of packed 3335 pixels to LSB first 3336 PNG_TRANSFORM_INVERT_MONO Invert monochrome images 3337 PNG_TRANSFORM_SHIFT Normalize pixels to the 3338 sBIT depth 3339 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA 3340 to BGRA 3341 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA 3342 to AG 3343 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity 3344 to transparency 3345 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples 3346 PNG_TRANSFORM_STRIP_FILLER Strip out filler 3347 bytes (deprecated). 3348 PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading 3349 filler bytes 3350 PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing 3351 filler bytes 3352 3353 If you have valid image data in the info structure (you can use 3354 png_set_rows() to put image data in the info structure), simply do this: 3355 3356 png_write_png(png_ptr, info_ptr, png_transforms, NULL) 3357 3358 where png_transforms is an integer containing the bitwise OR of some set of 3359 transformation flags. This call is equivalent to png_write_info(), 3360 followed the set of transformations indicated by the transform mask, 3361 then png_write_image(), and finally png_write_end(). 3362 3363 (The final parameter of this call is not yet used. Someday it might point 3364 to transformation parameters required by some future output transform.) 3365 3366 You must use png_transforms and not call any png_set_transform() functions 3367 when you use png_write_png(). 3368 3369 The low-level write interface 3370 3371 If you are going the low-level route instead, you are now ready to 3372 write all the file information up to the actual image data. You do 3373 this with a call to png_write_info(). 3374 3375 png_write_info(png_ptr, info_ptr); 3376 3377 Note that there is one transformation you may need to do before 3378 png_write_info(). In PNG files, the alpha channel in an image is the 3379 level of opacity. If your data is supplied as a level of transparency, 3380 you can invert the alpha channel before you write it, so that 0 is 3381 fully transparent and 255 (in 8-bit or paletted images) or 65535 3382 (in 16-bit images) is fully opaque, with 3383 3384 png_set_invert_alpha(png_ptr); 3385 3386 This must appear before png_write_info() instead of later with the 3387 other transformations because in the case of paletted images the tRNS 3388 chunk data has to be inverted before the tRNS chunk is written. If 3389 your image is not a paletted image, the tRNS data (which in such cases 3390 represents a single color to be rendered as transparent) won't need to 3391 be changed, and you can safely do this transformation after your 3392 png_write_info() call. 3393 3394 If you need to write a private chunk that you want to appear before 3395 the PLTE chunk when PLTE is present, you can write the PNG info in 3396 two steps, and insert code to write your own chunk between them: 3397 3398 png_write_info_before_PLTE(png_ptr, info_ptr); 3399 png_set_unknown_chunks(png_ptr, info_ptr, ...); 3400 png_write_info(png_ptr, info_ptr); 3401 3402 After you've written the file information, you can set up the library 3403 to handle any special transformations of the image data. The various 3404 ways to transform the data will be described in the order that they 3405 should occur. This is important, as some of these change the color 3406 type and/or bit depth of the data, and some others only work on 3407 certain color types and bit depths. Even though each transformation 3408 checks to see if it has data that it can do something with, you should 3409 make sure to only enable a transformation if it will be valid for the 3410 data. For example, don't swap red and blue on grayscale data. 3411 3412 PNG files store RGB pixels packed into 3 or 6 bytes. This code tells 3413 the library to strip input data that has 4 or 8 bytes per pixel down 3414 to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2 3415 bytes per pixel). 3416 3417 png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); 3418 3419 where the 0 is unused, and the location is either PNG_FILLER_BEFORE or 3420 PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel 3421 is stored XRGB or RGBX. 3422 3423 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as 3424 they can, resulting in, for example, 8 pixels per byte for 1 bit files. 3425 If the data is supplied at 1 pixel per byte, use this code, which will 3426 correctly pack the pixels into a single byte: 3427 3428 png_set_packing(png_ptr); 3429 3430 PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your 3431 data is of another bit depth, you can write an sBIT chunk into the 3432 file so that decoders can recover the original data if desired. 3433 3434 /* Set the true bit depth of the image data */ 3435 if (color_type & PNG_COLOR_MASK_COLOR) 3436 { 3437 sig_bit.red = true_bit_depth; 3438 sig_bit.green = true_bit_depth; 3439 sig_bit.blue = true_bit_depth; 3440 } 3441 3442 else 3443 { 3444 sig_bit.gray = true_bit_depth; 3445 } 3446 3447 if (color_type & PNG_COLOR_MASK_ALPHA) 3448 { 3449 sig_bit.alpha = true_bit_depth; 3450 } 3451 3452 png_set_sBIT(png_ptr, info_ptr, &sig_bit); 3453 3454 If the data is stored in the row buffer in a bit depth other than 3455 one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), 3456 this will scale the values to appear to be the correct bit depth as 3457 is required by PNG. 3458 3459 png_set_shift(png_ptr, &sig_bit); 3460 3461 PNG files store 16-bit pixels in network byte order (big-endian, 3462 ie. most significant bits first). This code would be used if they are 3463 supplied the other way (little-endian, i.e. least significant bits 3464 first, the way PCs store them): 3465 3466 if (bit_depth > 8) 3467 png_set_swap(png_ptr); 3468 3469 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you 3470 need to change the order the pixels are packed into bytes, you can use: 3471 3472 if (bit_depth < 8) 3473 png_set_packswap(png_ptr); 3474 3475 PNG files store 3 color pixels in red, green, blue order. This code 3476 would be used if they are supplied as blue, green, red: 3477 3478 png_set_bgr(png_ptr); 3479 3480 PNG files describe monochrome as black being zero and white being 3481 one. This code would be used if the pixels are supplied with this reversed 3482 (black being one and white being zero): 3483 3484 png_set_invert_mono(png_ptr); 3485 3486 Finally, you can write your own transformation function if none of 3487 the existing ones meets your needs. This is done by setting a callback 3488 with 3489 3490 png_set_write_user_transform_fn(png_ptr, 3491 write_transform_fn); 3492 3493 You must supply the function 3494 3495 void write_transform_fn(png_structp png_ptr, png_row_infop 3496 row_info, png_bytep data) 3497 3498 See pngtest.c for a working example. Your function will be called 3499 before any of the other transformations are processed. If supported 3500 libpng also supplies an information routine that may be called from 3501 your callback: 3502 3503 png_get_current_row_number(png_ptr); 3504 png_get_current_pass_number(png_ptr); 3505 3506 This returns the current row passed to the transform. With interlaced 3507 images the value returned is the row in the input sub-image image. Use 3508 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to 3509 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). 3510 3511 The discussion of interlace handling above contains more information on how to 3512 use these values. 3513 3514 You can also set up a pointer to a user structure for use by your 3515 callback function. 3516 3517 png_set_user_transform_info(png_ptr, user_ptr, 0, 0); 3518 3519 The user_channels and user_depth parameters of this function are ignored 3520 when writing; you can set them to zero as shown. 3521 3522 You can retrieve the pointer via the function png_get_user_transform_ptr(). 3523 For example: 3524 3525 voidp write_user_transform_ptr = 3526 png_get_user_transform_ptr(png_ptr); 3527 3528 It is possible to have libpng flush any pending output, either manually, 3529 or automatically after a certain number of lines have been written. To 3530 flush the output stream a single time call: 3531 3532 png_write_flush(png_ptr); 3533 3534 and to have libpng flush the output stream periodically after a certain 3535 number of scanlines have been written, call: 3536 3537 png_set_flush(png_ptr, nrows); 3538 3539 Note that the distance between rows is from the last time png_write_flush() 3540 was called, or the first row of the image if it has never been called. 3541 So if you write 50 lines, and then png_set_flush 25, it will flush the 3542 output on the next scanline, and every 25 lines thereafter, unless 3543 png_write_flush() is called before 25 more lines have been written. 3544 If nrows is too small (less than about 10 lines for a 640 pixel wide 3545 RGB image) the image compression may decrease noticeably (although this 3546 may be acceptable for real-time applications). Infrequent flushing will 3547 only degrade the compression performance by a few percent over images 3548 that do not use flushing. 3549 3550 Writing the image data 3551 3552 That's it for the transformations. Now you can write the image data. 3553 The simplest way to do this is in one function call. If you have the 3554 whole image in memory, you can just call png_write_image() and libpng 3555 will write the image. You will need to pass in an array of pointers to 3556 each row. This function automatically handles interlacing, so you don't 3557 need to call png_set_interlace_handling() or call this function multiple 3558 times, or any of that other stuff necessary with png_write_rows(). 3559 3560 png_write_image(png_ptr, row_pointers); 3561 3562 where row_pointers is: 3563 3564 png_byte *row_pointers[height]; 3565 3566 You can point to void or char or whatever you use for pixels. 3567 3568 If you don't want to write the whole image at once, you can 3569 use png_write_rows() instead. If the file is not interlaced, 3570 this is simple: 3571 3572 png_write_rows(png_ptr, row_pointers, 3573 number_of_rows); 3574 3575 row_pointers is the same as in the png_write_image() call. 3576 3577 If you are just writing one row at a time, you can do this with 3578 a single row_pointer instead of an array of row_pointers: 3579 3580 png_bytep row_pointer = row; 3581 3582 png_write_row(png_ptr, row_pointer); 3583 3584 When the file is interlaced, things can get a good deal more complicated. 3585 The only currently (as of the PNG Specification version 1.2, dated July 3586 1999) defined interlacing scheme for PNG files is the "Adam7" interlace 3587 scheme, that breaks down an image into seven smaller images of varying 3588 size. libpng will build these images for you, or you can do them 3589 yourself. If you want to build them yourself, see the PNG specification 3590 for details of which pixels to write when. 3591 3592 If you don't want libpng to handle the interlacing details, just 3593 use png_set_interlace_handling() and call png_write_rows() the 3594 correct number of times to write all the sub-images 3595 (png_set_interlace_handling() returns the number of sub-images.) 3596 3597 If you want libpng to build the sub-images, call this before you start 3598 writing any rows: 3599 3600 number_of_passes = png_set_interlace_handling(png_ptr); 3601 3602 This will return the number of passes needed. Currently, this is seven, 3603 but may change if another interlace type is added. 3604 3605 Then write the complete image number_of_passes times. 3606 3607 png_write_rows(png_ptr, row_pointers, number_of_rows); 3608 3609 Think carefully before you write an interlaced image. Typically code that 3610 reads such images reads all the image data into memory, uncompressed, before 3611 doing any processing. Only code that can display an image on the fly can 3612 take advantage of the interlacing and even then the image has to be exactly 3613 the correct size for the output device, because scaling an image requires 3614 adjacent pixels and these are not available until all the passes have been 3615 read. 3616 3617 If you do write an interlaced image you will hardly ever need to handle 3618 the interlacing yourself. Call png_set_interlace_handling() and use the 3619 approach described above. 3620 3621 The only time it is conceivable that you will really need to write an 3622 interlaced image pass-by-pass is when you have read one pass by pass and 3623 made some pixel-by-pixel transformation to it, as described in the read 3624 code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros 3625 to determine the size of each sub-image in turn and simply write the rows 3626 you obtained from the read code. 3627 3628 Finishing a sequential write 3629 3630 After you are finished writing the image, you should finish writing 3631 the file. If you are interested in writing comments or time, you should 3632 pass an appropriately filled png_info pointer. If you are not interested, 3633 you can pass NULL. 3634 3635 png_write_end(png_ptr, info_ptr); 3636 3637 When you are done, you can free all memory used by libpng like this: 3638 3639 png_destroy_write_struct(&png_ptr, &info_ptr); 3640 3641 It is also possible to individually free the info_ptr members that 3642 point to libpng-allocated storage with the following function: 3643 3644 png_free_data(png_ptr, info_ptr, mask, seq) 3645 3646 mask - identifies data to be freed, a mask 3647 containing the bitwise OR of one or 3648 more of 3649 PNG_FREE_PLTE, PNG_FREE_TRNS, 3650 PNG_FREE_HIST, PNG_FREE_ICCP, 3651 PNG_FREE_PCAL, PNG_FREE_ROWS, 3652 PNG_FREE_SCAL, PNG_FREE_SPLT, 3653 PNG_FREE_TEXT, PNG_FREE_UNKN, 3654 or simply PNG_FREE_ALL 3655 3656 seq - sequence number of item to be freed 3657 (-1 for all items) 3658 3659 This function may be safely called when the relevant storage has 3660 already been freed, or has not yet been allocated, or was allocated 3661 by the user and not by libpng, and will in those cases do nothing. 3662 The "seq" parameter is ignored if only one item of the selected data 3663 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items 3664 are allowed for the data type identified in the mask, such as text or 3665 sPLT, only the n'th item in the structure is freed, where n is "seq". 3666 3667 If you allocated data such as a palette that you passed in to libpng 3668 with png_set_*, you must not free it until just before the call to 3669 png_destroy_write_struct(). 3670 3671 The default behavior is only to free data that was allocated internally 3672 by libpng. This can be changed, so that libpng will not free the data, 3673 or so that it will free data that was allocated by the user with png_malloc() 3674 or png_calloc() and passed in via a png_set_*() function, with 3675 3676 png_data_freer(png_ptr, info_ptr, freer, mask) 3677 3678 freer - one of 3679 PNG_DESTROY_WILL_FREE_DATA 3680 PNG_SET_WILL_FREE_DATA 3681 PNG_USER_WILL_FREE_DATA 3682 3683 mask - which data elements are affected 3684 same choices as in png_free_data() 3685 3686 For example, to transfer responsibility for some data from a read structure 3687 to a write structure, you could use 3688 3689 png_data_freer(read_ptr, read_info_ptr, 3690 PNG_USER_WILL_FREE_DATA, 3691 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) 3692 3693 png_data_freer(write_ptr, write_info_ptr, 3694 PNG_DESTROY_WILL_FREE_DATA, 3695 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) 3696 3697 thereby briefly reassigning responsibility for freeing to the user but 3698 immediately afterwards reassigning it once more to the write_destroy 3699 function. Having done this, it would then be safe to destroy the read 3700 structure and continue to use the PLTE, tRNS, and hIST data in the write 3701 structure. 3702 3703 This function only affects data that has already been allocated. 3704 You can call this function before calling after the png_set_*() functions 3705 to control whether the user or png_destroy_*() is supposed to free the data. 3706 When the user assumes responsibility for libpng-allocated data, the 3707 application must use 3708 png_free() to free it, and when the user transfers responsibility to libpng 3709 for data that the user has allocated, the user must have used png_malloc() 3710 or png_calloc() to allocate it. 3711 3712 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword 3713 separately, do not transfer responsibility for freeing text_ptr to libpng, 3714 because when libpng fills a png_text structure it combines these members with 3715 the key member, and png_free_data() will free only text_ptr.key. Similarly, 3716 if you transfer responsibility for free'ing text_ptr from libpng to your 3717 application, your application must not separately free those members. 3718 For a more compact example of writing a PNG image, see the file example.c. 3719 3720 V. Simplified API 3721 3722 The simplified API, which became available in libpng-1.6.0, hides the details 3723 of both libpng and the PNG file format itself. 3724 It allows PNG files to be read into a very limited number of 3725 in-memory bitmap formats or to be written from the same formats. If these 3726 formats do not accommodate your needs then you can, and should, use the more 3727 sophisticated APIs above - these support a wide variety of in-memory formats 3728 and a wide variety of sophisticated transformations to those formats as well 3729 as a wide variety of APIs to manipulate ancilliary information. 3730 3731 To read a PNG file using the simplified API: 3732 3733 1) Declare a 'png_image' structure (see below) on the stack, set the 3734 version field to PNG_IMAGE_VERSION and the 'opaque' pointer to NULL 3735 (this is REQUIRED, your program may crash if you don't do it.) 3736 3737 2) Call the appropriate png_image_begin_read... function. 3738 3739 3) Set the png_image 'format' member to the required sample format. 3740 3741 4) Allocate a buffer for the image and, if required, the color-map. 3742 3743 5) Call png_image_finish_read to read the image and, if required, the 3744 color-map into your buffers. 3745 3746 There are no restrictions on the format of the PNG input itself; all valid 3747 color types, bit depths, and interlace methods are acceptable, and the 3748 input image is transformed as necessary to the requested in-memory format 3749 during the png_image_finish_read() step. The only caveat is that if you 3750 request a color-mapped image from a PNG that is full-color or makes 3751 complex use of an alpha channel the transformation is extremely lossy and the 3752 result may look terrible. 3753 3754 To write a PNG file using the simplified API: 3755 3756 1) Declare a 'png_image' structure on the stack and memset() 3757 it to all zero. 3758 3759 2) Initialize the members of the structure that describe the 3760 image, setting the 'format' member to the format of the 3761 image samples. 3762 3763 3) Call the appropriate png_image_write... function with a 3764 pointer to the image and, if necessary, the color-map to write 3765 the PNG data. 3766 3767 png_image is a structure that describes the in-memory format of an image 3768 when it is being read or defines the in-memory format of an image that you 3769 need to write. The "png_image" structure contains the following members: 3770 3771 png_controlp opaque Initialize to NULL, free with png_image_free 3772 png_uint_32 version Set to PNG_IMAGE_VERSION 3773 png_uint_32 width Image width in pixels (columns) 3774 png_uint_32 height Image height in pixels (rows) 3775 png_uint_32 format Image format as defined below 3776 png_uint_32 flags A bit mask containing informational flags 3777 png_uint_32 colormap_entries; Number of entries in the color-map 3778 png_uint_32 warning_or_error; 3779 char message[64]; 3780 3781 In the event of an error or warning the "warning_or_error" 3782 field will be set to a non-zero value and the 'message' field will contain 3783 a '\0' terminated string with the libpng error or warning message. If both 3784 warnings and an error were encountered, only the error is recorded. If there 3785 are multiple warnings, only the first one is recorded. 3786 3787 The upper 30 bits of the "warning_or_error" value are reserved; the low two 3788 bits contain a two bit code such that a value more than 1 indicates a failure 3789 in the API just called: 3790 3791 0 - no warning or error 3792 1 - warning 3793 2 - error 3794 3 - error preceded by warning 3795 3796 The pixels (samples) of the image have one to four channels whose components 3797 have original values in the range 0 to 1.0: 3798 3799 1: A single gray or luminance channel (G). 3800 2: A gray/luminance channel and an alpha channel (GA). 3801 3: Three red, green, blue color channels (RGB). 3802 4: Three color channels and an alpha channel (RGBA). 3803 3804 The channels are encoded in one of two ways: 3805 3806 a) As a small integer, value 0..255, contained in a single byte. For the 3807 alpha channel the original value is simply value/255. For the color or 3808 luminance channels the value is encoded according to the sRGB specification 3809 and matches the 8-bit format expected by typical display devices. 3810 3811 The color/gray channels are not scaled (pre-multiplied) by the alpha 3812 channel and are suitable for passing to color management software. 3813 3814 b) As a value in the range 0..65535, contained in a 2-byte integer, in 3815 the native byte order of the platform on which the application is running. 3816 All channels can be converted to the original value by dividing by 65535; all 3817 channels are linear. Color channels use the RGB encoding (RGB end-points) of 3818 the sRGB specification. This encoding is identified by the 3819 PNG_FORMAT_FLAG_LINEAR flag below. 3820 3821 When the simplified API needs to convert between sRGB and linear colorspaces, 3822 the actual sRGB transfer curve defined in the sRGB specification (see the 3823 article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2 3824 approximation used elsewhere in libpng. 3825 3826 When an alpha channel is present it is expected to denote pixel coverage 3827 of the color or luminance channels and is returned as an associated alpha 3828 channel: the color/gray channels are scaled (pre-multiplied) by the alpha 3829 value. 3830 3831 The samples are either contained directly in the image data, between 1 and 8 3832 bytes per pixel according to the encoding, or are held in a color-map indexed 3833 by bytes in the image data. In the case of a color-map the color-map entries 3834 are individual samples, encoded as above, and the image data has one byte per 3835 pixel to select the relevant sample from the color-map. 3836 3837 PNG_FORMAT_* 3838 3839 The #defines to be used in png_image::format. Each #define identifies a 3840 particular layout of channel data and, if present, alpha values. There are 3841 separate defines for each of the two component encodings. 3842 3843 A format is built up using single bit flag values. All combinations are 3844 valid. Formats can be built up from the flag values or you can use one of 3845 the predefined values below. When testing formats always use the FORMAT_FLAG 3846 macros to test for individual features - future versions of the library may 3847 add new flags. 3848 3849 When reading or writing color-mapped images the format should be set to the 3850 format of the entries in the color-map then png_image_{read,write}_colormap 3851 called to read or write the color-map and set the format correctly for the 3852 image data. Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly! 3853 3854 NOTE: libpng can be built with particular features disabled. If you see 3855 compiler errors because the definition of one of the following flags has been 3856 compiled out it is because libpng does not have the required support. It is 3857 possible, however, for the libpng configuration to enable the format on just 3858 read or just write; in that case you may see an error at run time. 3859 You can guard against this by checking for the definition of the 3860 appropriate "_SUPPORTED" macro, one of: 3861 3862 PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED 3863 3864 PNG_FORMAT_FLAG_ALPHA format with an alpha channel 3865 PNG_FORMAT_FLAG_COLOR color format: otherwise grayscale 3866 PNG_FORMAT_FLAG_LINEAR 2-byte channels else 1-byte 3867 PNG_FORMAT_FLAG_COLORMAP image data is color-mapped 3868 PNG_FORMAT_FLAG_BGR BGR colors, else order is RGB 3869 PNG_FORMAT_FLAG_AFIRST alpha channel comes first 3870 3871 Supported formats are as follows. Future versions of libpng may support more 3872 formats; for compatibility with older versions simply check if the format 3873 macro is defined using #ifdef. These defines describe the in-memory layout 3874 of the components of the pixels of the image. 3875 3876 First the single byte (sRGB) formats: 3877 3878 PNG_FORMAT_GRAY 3879 PNG_FORMAT_GA 3880 PNG_FORMAT_AG 3881 PNG_FORMAT_RGB 3882 PNG_FORMAT_BGR 3883 PNG_FORMAT_RGBA 3884 PNG_FORMAT_ARGB 3885 PNG_FORMAT_BGRA 3886 PNG_FORMAT_ABGR 3887 3888 Then the linear 2-byte formats. When naming these "Y" is used to 3889 indicate a luminance (gray) channel. The component order within the pixel 3890 is always the same - there is no provision for swapping the order of the 3891 components in the linear format. The components are 16-bit integers in 3892 the native byte order for your platform, and there is no provision for 3893 swapping the bytes to a different endian condition. 3894 3895 PNG_FORMAT_LINEAR_Y 3896 PNG_FORMAT_LINEAR_Y_ALPHA 3897 PNG_FORMAT_LINEAR_RGB 3898 PNG_FORMAT_LINEAR_RGB_ALPHA 3899 3900 With color-mapped formats the image data is one byte for each pixel. The byte 3901 is an index into the color-map which is formatted as above. To obtain a 3902 color-mapped format it is sufficient just to add the PNG_FOMAT_FLAG_COLORMAP 3903 to one of the above definitions, or you can use one of the definitions below. 3904 3905 PNG_FORMAT_RGB_COLORMAP 3906 PNG_FORMAT_BGR_COLORMAP 3907 PNG_FORMAT_RGBA_COLORMAP 3908 PNG_FORMAT_ARGB_COLORMAP 3909 PNG_FORMAT_BGRA_COLORMAP 3910 PNG_FORMAT_ABGR_COLORMAP 3911 3912 PNG_IMAGE macros 3913 3914 These are convenience macros to derive information from a png_image 3915 structure. The PNG_IMAGE_SAMPLE_ macros return values appropriate to the 3916 actual image sample values - either the entries in the color-map or the 3917 pixels in the image. The PNG_IMAGE_PIXEL_ macros return corresponding values 3918 for the pixels and will always return 1 for color-mapped formats. The 3919 remaining macros return information about the rows in the image and the 3920 complete image. 3921 3922 NOTE: All the macros that take a png_image::format parameter are compile time 3923 constants if the format parameter is, itself, a constant. Therefore these 3924 macros can be used in array declarations and case labels where required. 3925 Similarly the macros are also pre-processor constants (sizeof is not used) so 3926 they can be used in #if tests. 3927 3928 PNG_IMAGE_SAMPLE_CHANNELS(fmt) 3929 Returns the total number of channels in a given format: 1..4 3930 3931 PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt) 3932 Returns the size in bytes of a single component of a pixel or color-map 3933 entry (as appropriate) in the image: 1 or 2. 3934 3935 PNG_IMAGE_SAMPLE_SIZE(fmt) 3936 This is the size of the sample data for one sample. If the image is 3937 color-mapped it is the size of one color-map entry (and image pixels are 3938 one byte in size), otherwise it is the size of one image pixel. 3939 3940 PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt) 3941 The maximum size of the color-map required by the format expressed in a 3942 count of components. This can be used to compile-time allocate a 3943 color-map: 3944 3945 png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)]; 3946 3947 png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)]; 3948 3949 Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the 3950 information from one of the png_image_begin_read_ APIs and dynamically 3951 allocate the required memory. 3952 3953 PNG_IMAGE_COLORMAP_SIZE(fmt) 3954 The size of the color-map required by the format; this is the size of the 3955 color-map buffer passed to the png_image_{read,write}_colormap APIs. It is 3956 a fixed number determined by the format so can easily be allocated on the 3957 stack if necessary. 3958 3959 Corresponding information about the pixels 3960 3961 PNG_IMAGE_PIXEL_CHANNELS(fmt) 3962 The number of separate channels (components) in a pixel; 1 for a 3963 color-mapped image. 3964 3965 PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)\ 3966 The size, in bytes, of each component in a pixel; 1 for a color-mapped 3967 image. 3968 3969 PNG_IMAGE_PIXEL_SIZE(fmt) 3970 The size, in bytes, of a complete pixel; 1 for a color-mapped image. 3971 3972 Information about the whole row, or whole image 3973 3974 PNG_IMAGE_ROW_STRIDE(image) 3975 Returns the total number of components in a single row of the image; this 3976 is the minimum 'row stride', the minimum count of components between each 3977 row. For a color-mapped image this is the minimum number of bytes in a 3978 row. 3979 3980 If you need the stride measured in bytes, row_stride_bytes is 3981 PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt) 3982 plus any padding bytes that your application might need, for example 3983 to start the next row on a 4-byte boundary. 3984 3985 PNG_IMAGE_BUFFER_SIZE(image, row_stride) 3986 Return the size, in bytes, of an image buffer given a png_image and a row 3987 stride - the number of components to leave space for in each row. 3988 3989 PNG_IMAGE_SIZE(image) 3990 Return the size, in bytes, of the image in memory given just a png_image; 3991 the row stride is the minimum stride required for the image. 3992 3993 PNG_IMAGE_COLORMAP_SIZE(image) 3994 Return the size, in bytes, of the color-map of this image. If the image 3995 format is not a color-map format this will return a size sufficient for 3996 256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if 3997 you don't want to allocate a color-map in this case. 3998 3999 PNG_IMAGE_FLAG_* 4000 4001 Flags containing additional information about the image are held in 4002 the 'flags' field of png_image. 4003 4004 PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01 4005 This indicates the the RGB values of the in-memory bitmap do not 4006 correspond to the red, green and blue end-points defined by sRGB. 4007 4008 PNG_IMAGE_FLAG_FAST == 0x02 4009 On write emphasise speed over compression; the resultant PNG file will be 4010 larger but will be produced significantly faster, particular for large 4011 images. Do not use this option for images which will be distributed, only 4012 used it when producing intermediate files that will be read back in 4013 repeatedly. For a typical 24-bit image the option will double the read 4014 speed at the cost of increasing the image size by 25%, however for many 4015 more compressible images the PNG file can be 10 times larger with only a 4016 slight speed gain. 4017 4018 PNG_IMAGE_FLAG_16BIT_sRGB == 0x04 4019 On read if the image is a 16-bit per component image and there is no gAMA 4020 or sRGB chunk assume that the components are sRGB encoded. Notice that 4021 images output by the simplified API always have gamma information; setting 4022 this flag only affects the interpretation of 16-bit images from an 4023 external source. It is recommended that the application expose this flag 4024 to the user; the user can normally easily recognize the difference between 4025 linear and sRGB encoding. This flag has no effect on write - the data 4026 passed to the write APIs must have the correct encoding (as defined 4027 above.) 4028 4029 If the flag is not set (the default) input 16-bit per component data is 4030 assumed to be linear. 4031 4032 NOTE: the flag can only be set after the png_image_begin_read_ call, 4033 because that call initializes the 'flags' field. 4034 4035 READ APIs 4036 4037 The png_image passed to the read APIs must have been initialized by setting 4038 the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.) 4039 4040 int png_image_begin_read_from_file( png_imagep image, 4041 const char *file_name) 4042 4043 The named file is opened for read and the image header 4044 is filled in from the PNG header in the file. 4045 4046 int png_image_begin_read_from_stdio (png_imagep image, 4047 FILE* file) 4048 4049 The PNG header is read from the stdio FILE object. 4050 4051 int png_image_begin_read_from_memory(png_imagep image, 4052 png_const_voidp memory, png_size_t size) 4053 4054 The PNG header is read from the given memory buffer. 4055 4056 int png_image_finish_read(png_imagep image, 4057 png_colorp background, void *buffer, 4058 png_int_32 row_stride, void *colormap)); 4059 4060 Finish reading the image into the supplied buffer and 4061 clean up the png_image structure. 4062 4063 row_stride is the step, in png_byte or png_uint_16 units 4064 as appropriate, between adjacent rows. A positive stride 4065 indicates that the top-most row is first in the buffer - 4066 the normal top-down arrangement. A negative stride 4067 indicates that the bottom-most row is first in the buffer. 4068 4069 background need only be supplied if an alpha channel must 4070 be removed from a png_byte format and the removal is to be 4071 done by compositing on a solid color; otherwise it may be 4072 NULL and any composition will be done directly onto the 4073 buffer. The value is an sRGB color to use for the 4074 background, for grayscale output the green channel is used. 4075 4076 For linear output removing the alpha channel is always done 4077 by compositing on black. 4078 4079 void png_image_free(png_imagep image) 4080 4081 Free any data allocated by libpng in image->opaque, 4082 setting the pointer to NULL. May be called at any time 4083 after the structure is initialized. 4084 4085 When the simplified API needs to convert between sRGB and linear colorspaces, 4086 the actual sRGB transfer curve defined in the sRGB specification (see the 4087 article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2 4088 approximation used elsewhere in libpng. 4089 4090 WRITE APIS 4091 4092 For write you must initialize a png_image structure to describe the image to 4093 be written: 4094 4095 version: must be set to PNG_IMAGE_VERSION 4096 opaque: must be initialized to NULL 4097 width: image width in pixels 4098 height: image height in rows 4099 format: the format of the data you wish to write 4100 flags: set to 0 unless one of the defined flags applies; set 4101 PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images 4102 where the RGB values do not correspond to the colors in sRGB. 4103 colormap_entries: set to the number of entries in the color-map (0 to 256) 4104 4105 int png_image_write_to_file, (png_imagep image, 4106 const char *file, int convert_to_8bit, const void *buffer, 4107 png_int_32 row_stride, const void *colormap)); 4108 4109 Write the image to the named file. 4110 4111 int png_image_write_to_memory (png_imagep image, void *memory, 4112 png_alloc_size_t * PNG_RESTRICT memory_bytes, 4113 int convert_to_8_bit, const void *buffer, ptrdiff_t row_stride, 4114 const void *colormap)); 4115 4116 Write the image to memory. 4117 4118 int png_image_write_to_stdio(png_imagep image, FILE *file, 4119 int convert_to_8_bit, const void *buffer, 4120 png_int_32 row_stride, const void *colormap) 4121 4122 Write the image to the given (FILE*). 4123 4124 With all write APIs if image is in one of the linear formats with 4125 (png_uint_16) data then setting convert_to_8_bit will cause the output to be 4126 a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise 4127 a 16-bit linear encoded PNG file is written. 4128 4129 With all APIs row_stride is handled as in the read APIs - it is the spacing 4130 from one row to the next in component sized units (float) and if negative 4131 indicates a bottom-up row layout in the buffer. If you pass zero, libpng will 4132 calculate the row_stride for you from the width and number of channels. 4133 4134 Note that the write API does not support interlacing, sub-8-bit pixels, 4135 indexed (paletted) images, or most ancillary chunks. 4136 4137 VI. Modifying/Customizing libpng 4138 4139 There are two issues here. The first is changing how libpng does 4140 standard things like memory allocation, input/output, and error handling. 4141 The second deals with more complicated things like adding new chunks, 4142 adding new transformations, and generally changing how libpng works. 4143 Both of those are compile-time issues; that is, they are generally 4144 determined at the time the code is written, and there is rarely a need 4145 to provide the user with a means of changing them. 4146 4147 Memory allocation, input/output, and error handling 4148 4149 All of the memory allocation, input/output, and error handling in libpng 4150 goes through callbacks that are user-settable. The default routines are 4151 in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change 4152 these functions, call the appropriate png_set_*_fn() function. 4153 4154 Memory allocation is done through the functions png_malloc(), png_calloc(), 4155 and png_free(). The png_malloc() and png_free() functions currently just 4156 call the standard C functions and png_calloc() calls png_malloc() and then 4157 clears the newly allocated memory to zero; note that png_calloc(png_ptr, size) 4158 is not the same as the calloc(number, size) function provided by stdlib.h. 4159 There is limited support for certain systems with segmented memory 4160 architectures and the types of pointers declared by png.h match this; you 4161 will have to use appropriate pointers in your application. If you prefer 4162 to use a different method of allocating and freeing data, you can use 4163 png_create_read_struct_2() or png_create_write_struct_2() to register your 4164 own functions as described above. These functions also provide a void 4165 pointer that can be retrieved via 4166 4167 mem_ptr=png_get_mem_ptr(png_ptr); 4168 4169 Your replacement memory functions must have prototypes as follows: 4170 4171 png_voidp malloc_fn(png_structp png_ptr, 4172 png_alloc_size_t size); 4173 4174 void free_fn(png_structp png_ptr, png_voidp ptr); 4175 4176 Your malloc_fn() must return NULL in case of failure. The png_malloc() 4177 function will normally call png_error() if it receives a NULL from the 4178 system memory allocator or from your replacement malloc_fn(). 4179 4180 Your free_fn() will never be called with a NULL ptr, since libpng's 4181 png_free() checks for NULL before calling free_fn(). 4182 4183 Input/Output in libpng is done through png_read() and png_write(), 4184 which currently just call fread() and fwrite(). The FILE * is stored in 4185 png_struct and is initialized via png_init_io(). If you wish to change 4186 the method of I/O, the library supplies callbacks that you can set 4187 through the function png_set_read_fn() and png_set_write_fn() at run 4188 time, instead of calling the png_init_io() function. These functions 4189 also provide a void pointer that can be retrieved via the function 4190 png_get_io_ptr(). For example: 4191 4192 png_set_read_fn(png_structp read_ptr, 4193 voidp read_io_ptr, png_rw_ptr read_data_fn) 4194 4195 png_set_write_fn(png_structp write_ptr, 4196 voidp write_io_ptr, png_rw_ptr write_data_fn, 4197 png_flush_ptr output_flush_fn); 4198 4199 voidp read_io_ptr = png_get_io_ptr(read_ptr); 4200 voidp write_io_ptr = png_get_io_ptr(write_ptr); 4201 4202 The replacement I/O functions must have prototypes as follows: 4203 4204 void user_read_data(png_structp png_ptr, 4205 png_bytep data, png_size_t length); 4206 4207 void user_write_data(png_structp png_ptr, 4208 png_bytep data, png_size_t length); 4209 4210 void user_flush_data(png_structp png_ptr); 4211 4212 The user_read_data() function is responsible for detecting and 4213 handling end-of-data errors. 4214 4215 Supplying NULL for the read, write, or flush functions sets them back 4216 to using the default C stream functions, which expect the io_ptr to 4217 point to a standard *FILE structure. It is probably a mistake 4218 to use NULL for one of write_data_fn and output_flush_fn but not both 4219 of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined. 4220 It is an error to read from a write stream, and vice versa. 4221 4222 Error handling in libpng is done through png_error() and png_warning(). 4223 Errors handled through png_error() are fatal, meaning that png_error() 4224 should never return to its caller. Currently, this is handled via 4225 setjmp() and longjmp() (unless you have compiled libpng with 4226 PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()), 4227 but you could change this to do things like exit() if you should wish, 4228 as long as your function does not return. 4229 4230 On non-fatal errors, png_warning() is called 4231 to print a warning message, and then control returns to the calling code. 4232 By default png_error() and png_warning() print a message on stderr via 4233 fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined 4234 (because you don't want the messages) or PNG_NO_STDIO defined (because 4235 fprintf() isn't available). If you wish to change the behavior of the error 4236 functions, you will need to set up your own message callbacks. These 4237 functions are normally supplied at the time that the png_struct is created. 4238 It is also possible to redirect errors and warnings to your own replacement 4239 functions after png_create_*_struct() has been called by calling: 4240 4241 png_set_error_fn(png_structp png_ptr, 4242 png_voidp error_ptr, png_error_ptr error_fn, 4243 png_error_ptr warning_fn); 4244 4245 png_voidp error_ptr = png_get_error_ptr(png_ptr); 4246 4247 If NULL is supplied for either error_fn or warning_fn, then the libpng 4248 default function will be used, calling fprintf() and/or longjmp() if a 4249 problem is encountered. The replacement error functions should have 4250 parameters as follows: 4251 4252 void user_error_fn(png_structp png_ptr, 4253 png_const_charp error_msg); 4254 4255 void user_warning_fn(png_structp png_ptr, 4256 png_const_charp warning_msg); 4257 4258 The motivation behind using setjmp() and longjmp() is the C++ throw and 4259 catch exception handling methods. This makes the code much easier to write, 4260 as there is no need to check every return code of every function call. 4261 However, there are some uncertainties about the status of local variables 4262 after a longjmp, so the user may want to be careful about doing anything 4263 after setjmp returns non-zero besides returning itself. Consult your 4264 compiler documentation for more details. For an alternative approach, you 4265 may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net), 4266 which is illustrated in pngvalid.c and in contrib/visupng. 4267 4268 Beginning in libpng-1.4.0, the png_set_benign_errors() API became available. 4269 You can use this to handle certain errors (normally handled as errors) 4270 as warnings. 4271 4272 png_set_benign_errors (png_ptr, int allowed); 4273 4274 allowed: 0: treat png_benign_error() as an error. 4275 1: treat png_benign_error() as a warning. 4276 4277 As of libpng-1.6.0, the default condition is to treat benign errors as 4278 warnings while reading and as errors while writing. 4279 4280 Custom chunks 4281 4282 If you need to read or write custom chunks, you may need to get deeper 4283 into the libpng code. The library now has mechanisms for storing 4284 and writing chunks of unknown type; you can even declare callbacks 4285 for custom chunks. However, this may not be good enough if the 4286 library code itself needs to know about interactions between your 4287 chunk and existing `intrinsic' chunks. 4288 4289 If you need to write a new intrinsic chunk, first read the PNG 4290 specification. Acquire a first level of understanding of how it works. 4291 Pay particular attention to the sections that describe chunk names, 4292 and look at how other chunks were designed, so you can do things 4293 similarly. Second, check out the sections of libpng that read and 4294 write chunks. Try to find a chunk that is similar to yours and use 4295 it as a template. More details can be found in the comments inside 4296 the code. It is best to handle private or unknown chunks in a generic method, 4297 via callback functions, instead of by modifying libpng functions. This 4298 is illustrated in pngtest.c, which uses a callback function to handle a 4299 private "vpAg" chunk and the new "sTER" chunk, which are both unknown to 4300 libpng. 4301 4302 If you wish to write your own transformation for the data, look through 4303 the part of the code that does the transformations, and check out some of 4304 the simpler ones to get an idea of how they work. Try to find a similar 4305 transformation to the one you want to add and copy off of it. More details 4306 can be found in the comments inside the code itself. 4307 4308 Configuring for gui/windowing platforms: 4309 4310 You will need to write new error and warning functions that use the GUI 4311 interface, as described previously, and set them to be the error and 4312 warning functions at the time that png_create_*_struct() is called, 4313 in order to have them available during the structure initialization. 4314 They can be changed later via png_set_error_fn(). On some compilers, 4315 you may also have to change the memory allocators (png_malloc, etc.). 4316 4317 Configuring zlib: 4318 4319 There are special functions to configure the compression. Perhaps the 4320 most useful one changes the compression level, which currently uses 4321 input compression values in the range 0 - 9. The library normally 4322 uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests 4323 have shown that for a large majority of images, compression values in 4324 the range 3-6 compress nearly as well as higher levels, and do so much 4325 faster. For online applications it may be desirable to have maximum speed 4326 (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also 4327 specify no compression (Z_NO_COMPRESSION = 0), but this would create 4328 files larger than just storing the raw bitmap. You can specify the 4329 compression level by calling: 4330 4331 #include zlib.h 4332 png_set_compression_level(png_ptr, level); 4333 4334 Another useful one is to reduce the memory level used by the library. 4335 The memory level defaults to 8, but it can be lowered if you are 4336 short on memory (running DOS, for example, where you only have 640K). 4337 Note that the memory level does have an effect on compression; among 4338 other things, lower levels will result in sections of incompressible 4339 data being emitted in smaller stored blocks, with a correspondingly 4340 larger relative overhead of up to 15% in the worst case. 4341 4342 #include zlib.h 4343 png_set_compression_mem_level(png_ptr, level); 4344 4345 The other functions are for configuring zlib. They are not recommended 4346 for normal use and may result in writing an invalid PNG file. See 4347 zlib.h for more information on what these mean. 4348 4349 #include zlib.h 4350 png_set_compression_strategy(png_ptr, 4351 strategy); 4352 4353 png_set_compression_window_bits(png_ptr, 4354 window_bits); 4355 4356 png_set_compression_method(png_ptr, method); 4357 4358 This controls the size of the IDAT chunks (default 8192): 4359 4360 png_set_compression_buffer_size(png_ptr, size); 4361 4362 As of libpng version 1.5.4, additional APIs became 4363 available to set these separately for non-IDAT 4364 compressed chunks such as zTXt, iTXt, and iCCP: 4365 4366 #include zlib.h 4367 #if PNG_LIBPNG_VER >= 10504 4368 png_set_text_compression_level(png_ptr, level); 4369 4370 png_set_text_compression_mem_level(png_ptr, level); 4371 4372 png_set_text_compression_strategy(png_ptr, 4373 strategy); 4374 4375 png_set_text_compression_window_bits(png_ptr, 4376 window_bits); 4377 4378 png_set_text_compression_method(png_ptr, method); 4379 #endif 4380 4381 Controlling row filtering 4382 4383 If you want to control whether libpng uses filtering or not, which 4384 filters are used, and how it goes about picking row filters, you 4385 can call one of these functions. The selection and configuration 4386 of row filters can have a significant impact on the size and 4387 encoding speed and a somewhat lesser impact on the decoding speed 4388 of an image. Filtering is enabled by default for RGB and grayscale 4389 images (with and without alpha), but not for paletted images nor 4390 for any images with bit depths less than 8 bits/pixel. 4391 4392 The 'method' parameter sets the main filtering method, which is 4393 currently only '0' in the PNG 1.2 specification. The 'filters' 4394 parameter sets which filter(s), if any, should be used for each 4395 scanline. Possible values are PNG_ALL_FILTERS, PNG_NO_FILTERS, 4396 or PNG_FAST_FILTERS to turn filtering on and off, or to turn on 4397 just the fast-decoding subset of filters, respectively. 4398 4399 Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, 4400 PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise 4401 ORed together with '|' to specify one or more filters to use. 4402 These filters are described in more detail in the PNG specification. 4403 If you intend to change the filter type during the course of writing 4404 the image, you should start with flags set for all of the filters 4405 you intend to use so that libpng can initialize its internal 4406 structures appropriately for all of the filter types. (Note that this 4407 means the first row must always be adaptively filtered, because libpng 4408 currently does not allocate the filter buffers until png_write_row() 4409 is called for the first time.) 4410 4411 filters = PNG_NO_FILTERS; 4412 filters = PNG_ALL_FILTERS; 4413 filters = PNG_FAST_FILTERS; 4414 4415 or 4416 4417 filters = PNG_FILTER_NONE | PNG_FILTER_SUB | 4418 PNG_FILTER_UP | PNG_FILTER_AVG | 4419 PNG_FILTER_PAETH; 4420 4421 png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, 4422 filters); 4423 4424 The second parameter can also be 4425 PNG_INTRAPIXEL_DIFFERENCING if you are 4426 writing a PNG to be embedded in a MNG 4427 datastream. This parameter must be the 4428 same as the value of filter_method used 4429 in png_set_IHDR(). 4430 4431 Requesting debug printout 4432 4433 The macro definition PNG_DEBUG can be used to request debugging 4434 printout. Set it to an integer value in the range 0 to 3. Higher 4435 numbers result in increasing amounts of debugging information. The 4436 information is printed to the "stderr" file, unless another file 4437 name is specified in the PNG_DEBUG_FILE macro definition. 4438 4439 When PNG_DEBUG > 0, the following functions (macros) become available: 4440 4441 png_debug(level, message) 4442 png_debug1(level, message, p1) 4443 png_debug2(level, message, p1, p2) 4444 4445 in which "level" is compared to PNG_DEBUG to decide whether to print 4446 the message, "message" is the formatted string to be printed, 4447 and p1 and p2 are parameters that are to be embedded in the string 4448 according to printf-style formatting directives. For example, 4449 4450 png_debug1(2, "foo=%d", foo); 4451 4452 is expanded to 4453 4454 if (PNG_DEBUG > 2) 4455 fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo); 4456 4457 When PNG_DEBUG is defined but is zero, the macros aren't defined, but you 4458 can still use PNG_DEBUG to control your own debugging: 4459 4460 #ifdef PNG_DEBUG 4461 fprintf(stderr, ... 4462 #endif 4463 4464 When PNG_DEBUG = 1, the macros are defined, but only png_debug statements 4465 having level = 0 will be printed. There aren't any such statements in 4466 this version of libpng, but if you insert some they will be printed. 4467 4468 VII. MNG support 4469 4470 The MNG specification (available at http://www.libpng.org/pub/mng) allows 4471 certain extensions to PNG for PNG images that are embedded in MNG datastreams. 4472 Libpng can support some of these extensions. To enable them, use the 4473 png_permit_mng_features() function: 4474 4475 feature_set = png_permit_mng_features(png_ptr, mask) 4476 4477 mask is a png_uint_32 containing the bitwise OR of the 4478 features you want to enable. These include 4479 PNG_FLAG_MNG_EMPTY_PLTE 4480 PNG_FLAG_MNG_FILTER_64 4481 PNG_ALL_MNG_FEATURES 4482 4483 feature_set is a png_uint_32 that is the bitwise AND of 4484 your mask with the set of MNG features that is 4485 supported by the version of libpng that you are using. 4486 4487 It is an error to use this function when reading or writing a standalone 4488 PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped 4489 in a MNG datastream. As a minimum, it must have the MNG 8-byte signature 4490 and the MHDR and MEND chunks. Libpng does not provide support for these 4491 or any other MNG chunks; your application must provide its own support for 4492 them. You may wish to consider using libmng (available at 4493 http://www.libmng.com) instead. 4494 4495 VIII. Changes to Libpng from version 0.88 4496 4497 It should be noted that versions of libpng later than 0.96 are not 4498 distributed by the original libpng author, Guy Schalnat, nor by 4499 Andreas Dilger, who had taken over from Guy during 1996 and 1997, and 4500 distributed versions 0.89 through 0.96, but rather by another member 4501 of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are 4502 still alive and well, but they have moved on to other things. 4503 4504 The old libpng functions png_read_init(), png_write_init(), 4505 png_info_init(), png_read_destroy(), and png_write_destroy() have been 4506 moved to PNG_INTERNAL in version 0.95 to discourage their use. These 4507 functions will be removed from libpng version 1.4.0. 4508 4509 The preferred method of creating and initializing the libpng structures is 4510 via the png_create_read_struct(), png_create_write_struct(), and 4511 png_create_info_struct() because they isolate the size of the structures 4512 from the application, allow version error checking, and also allow the 4513 use of custom error handling routines during the initialization, which 4514 the old functions do not. The functions png_read_destroy() and 4515 png_write_destroy() do not actually free the memory that libpng 4516 allocated for these structs, but just reset the data structures, so they 4517 can be used instead of png_destroy_read_struct() and 4518 png_destroy_write_struct() if you feel there is too much system overhead 4519 allocating and freeing the png_struct for each image read. 4520 4521 Setting the error callbacks via png_set_message_fn() before 4522 png_read_init() as was suggested in libpng-0.88 is no longer supported 4523 because this caused applications that do not use custom error functions 4524 to fail if the png_ptr was not initialized to zero. It is still possible 4525 to set the error callbacks AFTER png_read_init(), or to change them with 4526 png_set_error_fn(), which is essentially the same function, but with a new 4527 name to force compilation errors with applications that try to use the old 4528 method. 4529 4530 Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6; 4531 however, iTXt support was not enabled by default. 4532 4533 Starting with version 1.0.7, you can find out which version of the library 4534 you are using at run-time: 4535 4536 png_uint_32 libpng_vn = png_access_version_number(); 4537 4538 The number libpng_vn is constructed from the major version, minor 4539 version with leading zero, and release number with leading zero, 4540 (e.g., libpng_vn for version 1.0.7 is 10007). 4541 4542 Note that this function does not take a png_ptr, so you can call it 4543 before you've created one. 4544 4545 You can also check which version of png.h you used when compiling your 4546 application: 4547 4548 png_uint_32 application_vn = PNG_LIBPNG_VER; 4549 4550 IX. Changes to Libpng from version 1.0.x to 1.2.x 4551 4552 Support for user memory management was enabled by default. To 4553 accomplish this, the functions png_create_read_struct_2(), 4554 png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(), 4555 png_malloc_default(), and png_free_default() were added. 4556 4557 Support for the iTXt chunk has been enabled by default as of 4558 version 1.2.41. 4559 4560 Support for certain MNG features was enabled. 4561 4562 Support for numbered error messages was added. However, we never got 4563 around to actually numbering the error messages. The function 4564 png_set_strip_error_numbers() was added (Note: the prototype for this 4565 function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE 4566 builds of libpng-1.2.15. It was restored in libpng-1.2.36). 4567 4568 The png_malloc_warn() function was added at libpng-1.2.3. This issues 4569 a png_warning and returns NULL instead of aborting when it fails to 4570 acquire the requested memory allocation. 4571 4572 Support for setting user limits on image width and height was enabled 4573 by default. The functions png_set_user_limits(), png_get_user_width_max(), 4574 and png_get_user_height_max() were added at libpng-1.2.6. 4575 4576 The png_set_add_alpha() function was added at libpng-1.2.7. 4577 4578 The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9. 4579 Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the 4580 tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is 4581 deprecated. 4582 4583 A number of macro definitions in support of runtime selection of 4584 assembler code features (especially Intel MMX code support) were 4585 added at libpng-1.2.0: 4586 4587 PNG_ASM_FLAG_MMX_SUPPORT_COMPILED 4588 PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU 4589 PNG_ASM_FLAG_MMX_READ_COMBINE_ROW 4590 PNG_ASM_FLAG_MMX_READ_INTERLACE 4591 PNG_ASM_FLAG_MMX_READ_FILTER_SUB 4592 PNG_ASM_FLAG_MMX_READ_FILTER_UP 4593 PNG_ASM_FLAG_MMX_READ_FILTER_AVG 4594 PNG_ASM_FLAG_MMX_READ_FILTER_PAETH 4595 PNG_ASM_FLAGS_INITIALIZED 4596 PNG_MMX_READ_FLAGS 4597 PNG_MMX_FLAGS 4598 PNG_MMX_WRITE_FLAGS 4599 PNG_MMX_FLAGS 4600 4601 We added the following functions in support of runtime 4602 selection of assembler code features: 4603 4604 png_get_mmx_flagmask() 4605 png_set_mmx_thresholds() 4606 png_get_asm_flags() 4607 png_get_mmx_bitdepth_threshold() 4608 png_get_mmx_rowbytes_threshold() 4609 png_set_asm_flags() 4610 4611 We replaced all of these functions with simple stubs in libpng-1.2.20, 4612 when the Intel assembler code was removed due to a licensing issue. 4613 4614 These macros are deprecated: 4615 4616 PNG_READ_TRANSFORMS_NOT_SUPPORTED 4617 PNG_PROGRESSIVE_READ_NOT_SUPPORTED 4618 PNG_NO_SEQUENTIAL_READ_SUPPORTED 4619 PNG_WRITE_TRANSFORMS_NOT_SUPPORTED 4620 PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED 4621 PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED 4622 4623 They have been replaced, respectively, by: 4624 4625 PNG_NO_READ_TRANSFORMS 4626 PNG_NO_PROGRESSIVE_READ 4627 PNG_NO_SEQUENTIAL_READ 4628 PNG_NO_WRITE_TRANSFORMS 4629 PNG_NO_READ_ANCILLARY_CHUNKS 4630 PNG_NO_WRITE_ANCILLARY_CHUNKS 4631 4632 PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been 4633 deprecated since libpng-1.0.16 and libpng-1.2.6. 4634 4635 The function 4636 png_check_sig(sig, num) 4637 was replaced with 4638 !png_sig_cmp(sig, 0, num) 4639 It has been deprecated since libpng-0.90. 4640 4641 The function 4642 png_set_gray_1_2_4_to_8() 4643 which also expands tRNS to alpha was replaced with 4644 png_set_expand_gray_1_2_4_to_8() 4645 which does not. It has been deprecated since libpng-1.0.18 and 1.2.9. 4646 4647 X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x 4648 4649 Private libpng prototypes and macro definitions were moved from 4650 png.h and pngconf.h into a new pngpriv.h header file. 4651 4652 Functions png_set_benign_errors(), png_benign_error(), and 4653 png_chunk_benign_error() were added. 4654 4655 Support for setting the maximum amount of memory that the application 4656 will allocate for reading chunks was added, as a security measure. 4657 The functions png_set_chunk_cache_max() and png_get_chunk_cache_max() 4658 were added to the library. 4659 4660 We implemented support for I/O states by adding png_ptr member io_state 4661 and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c 4662 4663 We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level 4664 input transforms. 4665 4666 Checking for and reporting of errors in the IHDR chunk is more thorough. 4667 4668 Support for global arrays was removed, to improve thread safety. 4669 4670 Some obsolete/deprecated macros and functions have been removed. 4671 4672 Typecasted NULL definitions such as 4673 #define png_voidp_NULL (png_voidp)NULL 4674 were eliminated. If you used these in your application, just use 4675 NULL instead. 4676 4677 The png_struct and info_struct members "trans" and "trans_values" were 4678 changed to "trans_alpha" and "trans_color", respectively. 4679 4680 The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles 4681 were removed. 4682 4683 The PNG_1_0_X and PNG_1_2_X macros were eliminated. 4684 4685 The PNG_LEGACY_SUPPORTED macro was eliminated. 4686 4687 Many WIN32_WCE #ifdefs were removed. 4688 4689 The functions png_read_init(info_ptr), png_write_init(info_ptr), 4690 png_info_init(info_ptr), png_read_destroy(), and png_write_destroy() 4691 have been removed. They have been deprecated since libpng-0.95. 4692 4693 The png_permit_empty_plte() was removed. It has been deprecated 4694 since libpng-1.0.9. Use png_permit_mng_features() instead. 4695 4696 We removed the obsolete stub functions png_get_mmx_flagmask(), 4697 png_set_mmx_thresholds(), png_get_asm_flags(), 4698 png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(), 4699 png_set_asm_flags(), and png_mmx_supported() 4700 4701 We removed the obsolete png_check_sig(), png_memcpy_check(), and 4702 png_memset_check() functions. Instead use !png_sig_cmp(), memcpy(), 4703 and memset(), respectively. 4704 4705 The function png_set_gray_1_2_4_to_8() was removed. It has been 4706 deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with 4707 png_set_expand_gray_1_2_4_to_8() because the former function also 4708 expanded any tRNS chunk to an alpha channel. 4709 4710 Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32 4711 were added and are used by default instead of the corresponding 4712 functions. Unfortunately, 4713 from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the 4714 function) incorrectly returned a value of type png_uint_32. 4715 4716 We changed the prototype for png_malloc() from 4717 png_malloc(png_structp png_ptr, png_uint_32 size) 4718 to 4719 png_malloc(png_structp png_ptr, png_alloc_size_t size) 4720 4721 This also applies to the prototype for the user replacement malloc_fn(). 4722 4723 The png_calloc() function was added and is used in place of 4724 of "png_malloc(); memset();" except in the case in png_read_png() 4725 where the array consists of pointers; in this case a "for" loop is used 4726 after the png_malloc() to set the pointers to NULL, to give robust. 4727 behavior in case the application runs out of memory part-way through 4728 the process. 4729 4730 We changed the prototypes of png_get_compression_buffer_size() and 4731 png_set_compression_buffer_size() to work with png_size_t instead of 4732 png_uint_32. 4733 4734 Support for numbered error messages was removed by default, since we 4735 never got around to actually numbering the error messages. The function 4736 png_set_strip_error_numbers() was removed from the library by default. 4737 4738 The png_zalloc() and png_zfree() functions are no longer exported. 4739 The png_zalloc() function no longer zeroes out the memory that it 4740 allocates. Applications that called png_zalloc(png_ptr, number, size) 4741 can call png_calloc(png_ptr, number*size) instead, and can call 4742 png_free() instead of png_zfree(). 4743 4744 Support for dithering was disabled by default in libpng-1.4.0, because 4745 it has not been well tested and doesn't actually "dither". 4746 The code was not 4747 removed, however, and could be enabled by building libpng with 4748 PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support 4749 was re-enabled, but the function was renamed png_set_quantize() to 4750 reflect more accurately what it actually does. At the same time, 4751 the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to 4752 PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED 4753 was renamed to PNG_READ_QUANTIZE_SUPPORTED. 4754 4755 We removed the trailing '.' from the warning and error messages. 4756 4757 XI. Changes to Libpng from version 1.4.x to 1.5.x 4758 4759 From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the 4760 function) incorrectly returned a value of type png_uint_32. 4761 The incorrect macro was removed from libpng-1.4.5. 4762 4763 Checking for invalid palette index on write was added at libpng 4764 1.5.10. If a pixel contains an invalid (out-of-range) index libpng issues 4765 a benign error. This is enabled by default because this condition is an 4766 error according to the PNG specification, Clause 11.3.2, but the error can 4767 be ignored in each png_ptr with 4768 4769 png_set_check_for_invalid_index(png_ptr, allowed); 4770 4771 allowed - one of 4772 0: disable benign error (accept the 4773 invalid data without warning). 4774 1: enable benign error (treat the 4775 invalid data as an error or a 4776 warning). 4777 4778 If the error is ignored, or if png_benign_error() treats it as a warning, 4779 any invalid pixels are decoded as opaque black by the decoder and written 4780 as-is by the encoder. 4781 4782 Retrieving the maximum palette index found was added at libpng-1.5.15. 4783 This statement must appear after png_read_png() or png_read_image() while 4784 reading, and after png_write_png() or png_write_image() while writing. 4785 4786 int max_palette = png_get_palette_max(png_ptr, info_ptr); 4787 4788 This will return the maximum palette index found in the image, or "-1" if 4789 the palette was not checked, or "0" if no palette was found. Note that this 4790 does not account for any palette index used by ancillary chunks such as the 4791 bKGD chunk; you must check those separately to determine the maximum 4792 palette index actually used. 4793 4794 There are no substantial API changes between the non-deprecated parts of 4795 the 1.4.5 API and the 1.5.0 API; however, the ability to directly access 4796 members of the main libpng control structures, png_struct and png_info, 4797 deprecated in earlier versions of libpng, has been completely removed from 4798 libpng 1.5, and new private "pngstruct.h", "pnginfo.h", and "pngdebug.h" 4799 header files were created. 4800 4801 We no longer include zlib.h in png.h. The include statement has been moved 4802 to pngstruct.h, where it is not accessible by applications. Applications that 4803 need access to information in zlib.h will need to add the '#include "zlib.h"' 4804 directive. It does not matter whether this is placed prior to or after 4805 the '"#include png.h"' directive. 4806 4807 The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer used 4808 and were removed. 4809 4810 We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp() 4811 macros into a private header file (pngpriv.h) that is not accessible to 4812 applications. 4813 4814 In png_get_iCCP, the type of "profile" was changed from png_charpp 4815 to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep. 4816 4817 There are changes of form in png.h, including new and changed macros to 4818 declare parts of the API. Some API functions with arguments that are 4819 pointers to data not modified within the function have been corrected to 4820 declare these arguments with PNG_CONST. 4821 4822 Much of the internal use of C macros to control the library build has also 4823 changed and some of this is visible in the exported header files, in 4824 particular the use of macros to control data and API elements visible 4825 during application compilation may require significant revision to 4826 application code. (It is extremely rare for an application to do this.) 4827 4828 Any program that compiled against libpng 1.4 and did not use deprecated 4829 features or access internal library structures should compile and work 4830 against libpng 1.5, except for the change in the prototype for 4831 png_get_iCCP() and png_set_iCCP() API functions mentioned above. 4832 4833 libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of 4834 interlaced images. The macros return the number of rows and columns in 4835 each pass and information that can be used to de-interlace and (if 4836 absolutely necessary) interlace an image. 4837 4838 libpng 1.5.0 adds an API png_longjmp(png_ptr, value). This API calls 4839 the application-provided png_longjmp_ptr on the internal, but application 4840 initialized, longjmp buffer. It is provided as a convenience to avoid 4841 the need to use the png_jmpbuf macro, which had the unnecessary side 4842 effect of resetting the internal png_longjmp_ptr value. 4843 4844 libpng 1.5.0 includes a complete fixed point API. By default this is 4845 present along with the corresponding floating point API. In general the 4846 fixed point API is faster and smaller than the floating point one because 4847 the PNG file format used fixed point, not floating point. This applies 4848 even if the library uses floating point in internal calculations. A new 4849 macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library 4850 uses floating point arithmetic (the default) or fixed point arithmetic 4851 internally for performance critical calculations such as gamma correction. 4852 In some cases, the gamma calculations may produce slightly different 4853 results. This has changed the results in png_rgb_to_gray and in alpha 4854 composition (png_set_background for example). This applies even if the 4855 original image was already linear (gamma == 1.0) and, therefore, it is 4856 not necessary to linearize the image. This is because libpng has *not* 4857 been changed to optimize that case correctly, yet. 4858 4859 Fixed point support for the sCAL chunk comes with an important caveat; 4860 the sCAL specification uses a decimal encoding of floating point values 4861 and the accuracy of PNG fixed point values is insufficient for 4862 representation of these values. Consequently a "string" API 4863 (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading 4864 arbitrary sCAL chunks in the absence of either the floating point API or 4865 internal floating point calculations. Starting with libpng-1.5.0, both 4866 of these functions are present when PNG_sCAL_SUPPORTED is defined. Prior 4867 to libpng-1.5.0, their presence also depended upon PNG_FIXED_POINT_SUPPORTED 4868 being defined and PNG_FLOATING_POINT_SUPPORTED not being defined. 4869 4870 Applications no longer need to include the optional distribution header 4871 file pngusr.h or define the corresponding macros during application 4872 build in order to see the correct variant of the libpng API. From 1.5.0 4873 application code can check for the corresponding _SUPPORTED macro: 4874 4875 #ifdef PNG_INCH_CONVERSIONS_SUPPORTED 4876 /* code that uses the inch conversion APIs. */ 4877 #endif 4878 4879 This macro will only be defined if the inch conversion functions have been 4880 compiled into libpng. The full set of macros, and whether or not support 4881 has been compiled in, are available in the header file pnglibconf.h. 4882 This header file is specific to the libpng build. Notice that prior to 4883 1.5.0 the _SUPPORTED macros would always have the default definition unless 4884 reset by pngusr.h or by explicit settings on the compiler command line. 4885 These settings may produce compiler warnings or errors in 1.5.0 because 4886 of macro redefinition. 4887 4888 Applications can now choose whether to use these macros or to call the 4889 corresponding function by defining PNG_USE_READ_MACROS or 4890 PNG_NO_USE_READ_MACROS before including png.h. Notice that this is 4891 only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to 1.5.0 4892 will lead to a link failure. 4893 4894 Prior to libpng-1.5.4, the zlib compressor used the same set of parameters 4895 when compressing the IDAT data and textual data such as zTXt and iCCP. 4896 In libpng-1.5.4 we reinitialized the zlib stream for each type of data. 4897 We added five png_set_text_*() functions for setting the parameters to 4898 use with textual data. 4899 4900 Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED 4901 option was off by default, and slightly inaccurate scaling occurred. 4902 This option can no longer be turned off, and the choice of accurate 4903 or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8() 4904 API for accurate scaling or the old png_set_strip_16_to_8() API for simple 4905 chopping. In libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED 4906 macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8 4907 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two 4908 png_set_*_16_to_8() functions separately. 4909 4910 Prior to libpng-1.5.4, the png_set_user_limits() function could only be 4911 used to reduce the width and height limits from the value of 4912 PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said 4913 that it could be used to override them. Now this function will reduce or 4914 increase the limits. 4915 4916 Starting in libpng-1.5.10, the user limits can be set en masse with the 4917 configuration option PNG_SAFE_LIMITS_SUPPORTED. If this option is enabled, 4918 a set of "safe" limits is applied in pngpriv.h. These can be overridden by 4919 application calls to png_set_user_limits(), png_set_user_chunk_cache_max(), 4920 and/or png_set_user_malloc_max() that increase or decrease the limits. Also, 4921 in libpng-1.5.10 the default width and height limits were increased 4922 from 1,000,000 to 0x7fffffff (i.e., made unlimited). Therefore, the 4923 limits are now 4924 default safe 4925 png_user_width_max 0x7fffffff 1,000,000 4926 png_user_height_max 0x7fffffff 1,000,000 4927 png_user_chunk_cache_max 0 (unlimited) 128 4928 png_user_chunk_malloc_max 0 (unlimited) 8,000,000 4929 4930 The png_set_option() function (and the "options" member of the png struct) was 4931 added to libpng-1.5.15, with option PNG_ARM_NEON. 4932 4933 The library now supports a complete fixed point implementation and can 4934 thus be used on systems that have no floating point support or very 4935 limited or slow support. Previously gamma correction, an essential part 4936 of complete PNG support, required reasonably fast floating point. 4937 4938 As part of this the choice of internal implementation has been made 4939 independent of the choice of fixed versus floating point APIs and all the 4940 missing fixed point APIs have been implemented. 4941 4942 The exact mechanism used to control attributes of API functions has 4943 changed, as described in the INSTALL file. 4944 4945 A new test program, pngvalid, is provided in addition to pngtest. 4946 pngvalid validates the arithmetic accuracy of the gamma correction 4947 calculations and includes a number of validations of the file format. 4948 A subset of the full range of tests is run when "make check" is done 4949 (in the 'configure' build.) pngvalid also allows total allocated memory 4950 usage to be evaluated and performs additional memory overwrite validation. 4951 4952 Many changes to individual feature macros have been made. The following 4953 are the changes most likely to be noticed by library builders who 4954 configure libpng: 4955 4956 1) All feature macros now have consistent naming: 4957 4958 #define PNG_NO_feature turns the feature off 4959 #define PNG_feature_SUPPORTED turns the feature on 4960 4961 pnglibconf.h contains one line for each feature macro which is either: 4962 4963 #define PNG_feature_SUPPORTED 4964 4965 if the feature is supported or: 4966 4967 /*#undef PNG_feature_SUPPORTED*/ 4968 4969 if it is not. Library code consistently checks for the 'SUPPORTED' macro. 4970 It does not, and libpng applications should not, check for the 'NO' macro 4971 which will not normally be defined even if the feature is not supported. 4972 The 'NO' macros are only used internally for setting or not setting the 4973 corresponding 'SUPPORTED' macros. 4974 4975 Compatibility with the old names is provided as follows: 4976 4977 PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED 4978 4979 And the following definitions disable the corresponding feature: 4980 4981 PNG_SETJMP_NOT_SUPPORTED disables SETJMP 4982 PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS 4983 PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV 4984 PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS 4985 PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS 4986 PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS 4987 4988 Library builders should remove use of the above, inconsistent, names. 4989 4990 2) Warning and error message formatting was previously conditional on 4991 the STDIO feature. The library has been changed to use the 4992 CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled 4993 the library no longer uses the printf(3) functions, even though the 4994 default read/write implementations use (FILE) style stdio.h functions. 4995 4996 3) Three feature macros now control the fixed/floating point decisions: 4997 4998 PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs 4999 5000 PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in 5001 practice these are normally required internally anyway (because the PNG 5002 file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT 5003 merely stops the function from being exported. 5004 5005 PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating 5006 point implementation or the fixed point one. Typically the fixed point 5007 implementation is larger and slower than the floating point implementation 5008 on a system that supports floating point; however, it may be faster on a 5009 system which lacks floating point hardware and therefore uses a software 5010 emulation. 5011 5012 4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED. This allows the 5013 functions to read and write ints to be disabled independently of 5014 PNG_USE_READ_MACROS, which allows libpng to be built with the functions 5015 even though the default is to use the macros - this allows applications 5016 to choose at app buildtime whether or not to use macros (previously 5017 impossible because the functions weren't in the default build.) 5018 5019 XII. Changes to Libpng from version 1.5.x to 1.6.x 5020 5021 A "simplified API" has been added (see documentation in png.h and a simple 5022 example in contrib/examples/pngtopng.c). The new publicly visible API 5023 includes the following: 5024 5025 macros: 5026 PNG_FORMAT_* 5027 PNG_IMAGE_* 5028 structures: 5029 png_control 5030 png_image 5031 read functions 5032 png_image_begin_read_from_file() 5033 png_image_begin_read_from_stdio() 5034 png_image_begin_read_from_memory() 5035 png_image_finish_read() 5036 png_image_free() 5037 write functions 5038 png_image_write_to_file() 5039 png_image_write_to_memory() 5040 png_image_write_to_stdio() 5041 5042 Starting with libpng-1.6.0, you can configure libpng to prefix all exported 5043 symbols, using the PNG_PREFIX macro. 5044 5045 We no longer include string.h in png.h. The include statement has been moved 5046 to pngpriv.h, where it is not accessible by applications. Applications that 5047 need access to information in string.h must add an '#include <string.h>' 5048 directive. It does not matter whether this is placed prior to or after 5049 the '#include "png.h"' directive. 5050 5051 The following API are now DEPRECATED: 5052 png_info_init_3() 5053 png_convert_to_rfc1123() which has been replaced 5054 with png_convert_to_rfc1123_buffer() 5055 png_malloc_default() 5056 png_free_default() 5057 png_reset_zstream() 5058 5059 The following have been removed: 5060 png_get_io_chunk_name(), which has been replaced 5061 with png_get_io_chunk_type(). The new 5062 function returns a 32-bit integer instead of 5063 a string. 5064 The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and 5065 png_memset() macros are no longer used in the libpng sources and 5066 have been removed. These had already been made invisible to applications 5067 (i.e., defined in the private pngpriv.h header file) since libpng-1.5.0. 5068 5069 The signatures of many exported functions were changed, such that 5070 png_structp became png_structrp or png_const_structrp 5071 png_infop became png_inforp or png_const_inforp 5072 where "rp" indicates a "restricted pointer". 5073 5074 Dropped support for 16-bit platforms. The support for FAR/far types has 5075 been eliminated and the definition of png_alloc_size_t is now controlled 5076 by a flag so that 'small size_t' systems can select it if necessary. 5077 5078 Error detection in some chunks has improved; in particular the iCCP chunk 5079 reader now does pretty complete validation of the basic format. Some bad 5080 profiles that were previously accepted are now accepted with a warning or 5081 rejected, depending upon the png_set_benign_errors() setting, in particular 5082 the very old broken Microsoft/HP 3144-byte sRGB profile. Starting with 5083 libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by 5084 means of 5085 5086 #if defined(PNG_SKIP_sRGB_CHECK_PROFILE) && \ 5087 defined(PNG_SET_OPTION_SUPPORTED) 5088 png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE, 5089 PNG_OPTION_ON); 5090 #endif 5091 5092 It's not a good idea to do this if you are using the "simplified API", 5093 which needs to be able to recognize sRGB profiles conveyed via the iCCP 5094 chunk. 5095 5096 The PNG spec requirement that only grayscale profiles may appear in images 5097 with color type 0 or 4 and that even if the image only contains gray pixels, 5098 only RGB profiles may appear in images with color type 2, 3, or 6, is now 5099 enforced. The sRGB chunk is allowed to appear in images with any color type 5100 and is interpreted by libpng to convey a one-tracer-curve gray profile or a 5101 three-tracer-curve RGB profile as appropriate. 5102 5103 Libpng 1.5.x erroneously used /MD for Debug DLL builds; if you used the debug 5104 builds in your app and you changed your app to use /MD you will need to 5105 change it back to /MDd for libpng 1.6.x. 5106 5107 Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained 5108 an empty language field or an empty translated keyword. Both of these 5109 are allowed by the PNG specification, so these warnings are no longer issued. 5110 5111 The library now issues an error if the application attempts to set a 5112 transform after it calls png_read_update_info() or if it attempts to call 5113 both png_read_update_info() and png_start_read_image() or to call either 5114 of them more than once. 5115 5116 The default condition for benign_errors is now to treat benign errors as 5117 warnings while reading and as errors while writing. 5118 5119 The library now issues a warning if both background processing and RGB to 5120 gray are used when gamma correction happens. As with previous versions of 5121 the library the results are numerically very incorrect in this case. 5122 5123 There are some minor arithmetic changes in some transforms such as 5124 png_set_background(), that might be detected by certain regression tests. 5125 5126 Unknown chunk handling has been improved internally, without any API change. 5127 This adds more correct option control of the unknown handling, corrects 5128 a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes 5129 it possible to skip IDAT chunks in the sequential reader. 5130 5131 The machine-generated configure files are no longer included in branches 5132 libpng16 and later of the GIT repository. They continue to be included 5133 in the tarball releases, however. 5134 5135 Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT 5136 stream to set the size of the sliding window for reading instead of using the 5137 default 32-kbyte sliding window size. It was discovered that there are 5138 hundreds of PNG files in the wild that have incorrect CMF bytes that caused 5139 zlib to issue the "invalid distance too far back" error and reject the file. 5140 Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions, 5141 provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes 5142 and using a 32-kbyte sliding window), by using 5143 5144 png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW, 5145 PNG_OPTION_ON); 5146 5147 and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while 5148 optimizing the CMF bytes in its IDAT chunk correctly. 5149 5150 Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong 5151 length, which resulted in PNG files that cannot be read beyond the bad iTXt 5152 chunk. This error was fixed in libpng-1.6.3, and a tool (called 5153 contrib/tools/png-fix-itxt) has been added to the libpng distribution. 5154 5155 Starting with libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated 5156 and safe limits are used by default (users who need larger limits 5157 can still override them at compile time or run time, as described above). 5158 5159 The new limits are 5160 default spec limit 5161 png_user_width_max 1,000,000 2,147,483,647 5162 png_user_height_max 1,000,000 2,147,483,647 5163 png_user_chunk_cache_max 128 unlimited 5164 png_user_chunk_malloc_max 8,000,000 unlimited 5165 5166 Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which allows 5167 library builders to control compilation for an installed system (a release build). 5168 It can be set for testing debug or beta builds to ensure that they will compile 5169 when the build type is switched to RC or STABLE. In essence this overrides the 5170 PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable. 5171 5172 Starting with libpng-1.6.19, attempting to set an over-length PLTE chunk 5173 is an error. Previously this requirement of the PNG specification was not 5174 enforced, and the palette was always limited to 256 entries. An over-length 5175 PLTE chunk found in an input PNG is silently truncated. 5176 5177 XIII. Detecting libpng 5178 5179 The png_get_io_ptr() function has been present since libpng-0.88, has never 5180 changed, and is unaffected by conditional compilation macros. It is the 5181 best choice for use in configure scripts for detecting the presence of any 5182 libpng version since 0.88. In an autoconf "configure.in" you could use 5183 5184 AC_CHECK_LIB(png, png_get_io_ptr, ... 5185 5186 XV. Source code repository 5187 5188 Since about February 2009, version 1.2.34, libpng has been under "git" source 5189 control. The git repository was built from old libpng-x.y.z.tar.gz files 5190 going back to version 0.70. You can access the git repository (read only) 5191 at 5192 5193 git://git.code.sf.net/p/libpng/code 5194 5195 or you can browse it with a web browser by selecting the "code" button at 5196 5197 https://sourceforge.net/projects/libpng 5198 5199 Patches can be sent to glennrp at users.sourceforge.net or to 5200 png-mng-implement at lists.sourceforge.net or you can upload them to 5201 the libpng bug tracker at 5202 5203 http://libpng.sourceforge.net 5204 5205 We also accept patches built from the tar or zip distributions, and 5206 simple verbal discriptions of bug fixes, reported either to the 5207 SourceForge bug tracker, to the png-mng-implement at lists.sf.net 5208 mailing list, or directly to glennrp. 5209 5210 XV. Coding style 5211 5212 Our coding style is similar to the "Allman" style 5213 (See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly 5214 braces on separate lines: 5215 5216 if (condition) 5217 { 5218 action; 5219 } 5220 5221 else if (another condition) 5222 { 5223 another action; 5224 } 5225 5226 The braces can be omitted from simple one-line actions: 5227 5228 if (condition) 5229 return (0); 5230 5231 We use 3-space indentation, except for continued statements which 5232 are usually indented the same as the first line of the statement 5233 plus four more spaces. 5234 5235 For macro definitions we use 2-space indentation, always leaving the "#" 5236 in the first column. 5237 5238 #ifndef PNG_NO_FEATURE 5239 # ifndef PNG_FEATURE_SUPPORTED 5240 # define PNG_FEATURE_SUPPORTED 5241 # endif 5242 #endif 5243 5244 Comments appear with the leading "/*" at the same indentation as 5245 the statement that follows the comment: 5246 5247 /* Single-line comment */ 5248 statement; 5249 5250 /* This is a multiple-line 5251 * comment. 5252 */ 5253 statement; 5254 5255 Very short comments can be placed after the end of the statement 5256 to which they pertain: 5257 5258 statement; /* comment */ 5259 5260 We don't use C++ style ("//") comments. We have, however, 5261 used them in the past in some now-abandoned MMX assembler 5262 code. 5263 5264 Functions and their curly braces are not indented, and 5265 exported functions are marked with PNGAPI: 5266 5267 /* This is a public function that is visible to 5268 * application programmers. It does thus-and-so. 5269 */ 5270 void PNGAPI 5271 png_exported_function(png_ptr, png_info, foo) 5272 { 5273 body; 5274 } 5275 5276 The return type and decorations are placed on a separate line 5277 ahead of the function name, as illustrated above. 5278 5279 The prototypes for all exported functions appear in png.h, 5280 above the comment that says 5281 5282 /* Maintainer: Put new public prototypes here ... */ 5283 5284 We mark all non-exported functions with "/* PRIVATE */"": 5285 5286 void /* PRIVATE */ 5287 png_non_exported_function(png_ptr, png_info, foo) 5288 { 5289 body; 5290 } 5291 5292 The prototypes for non-exported functions (except for those in 5293 pngtest) appear in pngpriv.h above the comment that says 5294 5295 /* Maintainer: Put new private prototypes here ^ */ 5296 5297 To avoid polluting the global namespace, the names of all exported 5298 functions and variables begin with "png_", and all publicly visible C 5299 preprocessor macros begin with "PNG". We request that applications that 5300 use libpng *not* begin any of their own symbols with either of these strings. 5301 5302 We put a space after the "sizeof" operator and we omit the 5303 optional parentheses around its argument when the argument 5304 is an expression, not a type name, and we always enclose the 5305 sizeof operator, with its argument, in parentheses: 5306 5307 (sizeof (png_uint_32)) 5308 (sizeof array) 5309 5310 Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as 5311 though it were a function. 5312 5313 Control keywords if, for, while, and switch are always followed by a space 5314 to distinguish them from function calls, which have no trailing space. 5315 5316 We put a space after each comma and after each semicolon 5317 in "for" statements, and we put spaces before and after each 5318 C binary operator and after "for" or "while", and before 5319 "?". We don't put a space between a typecast and the expression 5320 being cast, nor do we put one between a function name and the 5321 left parenthesis that follows it: 5322 5323 for (i = 2; i > 0; --i) 5324 y[i] = a(x) + (int)b; 5325 5326 We prefer #ifdef and #ifndef to #if defined() and #if !defined() 5327 when there is only one macro being tested. We always use parentheses 5328 with "defined". 5329 5330 We express integer constants that are used as bit masks in hex format, 5331 with an even number of lower-case hex digits, and to make them unsigned 5332 (e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater than 0x7fff 5333 (e.g., 0xffffUL). 5334 5335 We prefer to use underscores rather than camelCase in names, except 5336 for a few type names that we inherit from zlib.h. 5337 5338 We prefer "if (something != 0)" and "if (something == 0)" 5339 over "if (something)" and if "(!something)", respectively. 5340 5341 We do not use the TAB character for indentation in the C sources. 5342 5343 Lines do not exceed 80 characters. 5344 5345 Other rules can be inferred by inspecting the libpng source. 5346 5347 XVI. Y2K Compliance in libpng 5348 5349 Since the PNG Development group is an ad-hoc body, we can't make 5350 an official declaration. 5351 5352 This is your unofficial assurance that libpng from version 0.71 and 5353 upward through 1.6.25 are Y2K compliant. It is my belief that earlier 5354 versions were also Y2K compliant. 5355 5356 Libpng only has two year fields. One is a 2-byte unsigned integer 5357 that will hold years up to 65535. The other, which is deprecated, 5358 holds the date in text format, and will hold years up to 9999. 5359 5360 The integer is 5361 "png_uint_16 year" in png_time_struct. 5362 5363 The string is 5364 "char time_buffer[29]" in png_struct. This is no longer used 5365 in libpng-1.6.x and will be removed from libpng-1.7.0. 5366 5367 There are seven time-related functions: 5368 5369 png_convert_to_rfc_1123_buffer() in png.c 5370 (formerly png_convert_to_rfc_1152() in error, and 5371 also formerly png_convert_to_rfc_1123()) 5372 png_convert_from_struct_tm() in pngwrite.c, called 5373 in pngwrite.c 5374 png_convert_from_time_t() in pngwrite.c 5375 png_get_tIME() in pngget.c 5376 png_handle_tIME() in pngrutil.c, called in pngread.c 5377 png_set_tIME() in pngset.c 5378 png_write_tIME() in pngwutil.c, called in pngwrite.c 5379 5380 All appear to handle dates properly in a Y2K environment. The 5381 png_convert_from_time_t() function calls gmtime() to convert from system 5382 clock time, which returns (year - 1900), which we properly convert to 5383 the full 4-digit year. There is a possibility that applications using 5384 libpng are not passing 4-digit years into the png_convert_to_rfc_1123() 5385 function, or that they are incorrectly passing only a 2-digit year 5386 instead of "year - 1900" into the png_convert_from_struct_tm() function, 5387 but this is not under our control. The libpng documentation has always 5388 stated that it works with 4-digit years, and the APIs have been 5389 documented as such. 5390 5391 The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned 5392 integer to hold the year, and can hold years as large as 65535. 5393 5394 zlib, upon which libpng depends, is also Y2K compliant. It contains 5395 no date-related code. 5396 5397 5398 Glenn Randers-Pehrson 5399 libpng maintainer 5400 PNG Development Group 5401