1 /* 2 * example.c 3 * 4 * This file illustrates how to use the IJG code as a subroutine library 5 * to read or write JPEG image files. You should look at this code in 6 * conjunction with the documentation file libjpeg.txt. 7 * 8 * This code will not do anything useful as-is, but it may be helpful as a 9 * skeleton for constructing routines that call the JPEG library. 10 * 11 * We present these routines in the same coding style used in the JPEG code 12 * (ANSI function definitions, etc); but you are of course free to code your 13 * routines in a different style if you prefer. 14 */ 15 16 #include <stdio.h> 17 18 /* 19 * Include file for users of JPEG library. 20 * You will need to have included system headers that define at least 21 * the typedefs FILE and size_t before you can include jpeglib.h. 22 * (stdio.h is sufficient on ANSI-conforming systems.) 23 * You may also wish to include "jerror.h". 24 */ 25 26 #include "jpeglib.h" 27 28 /* 29 * <setjmp.h> is used for the optional error recovery mechanism shown in 30 * the second part of the example. 31 */ 32 33 #include <setjmp.h> 34 35 36 37 /******************** JPEG COMPRESSION SAMPLE INTERFACE *******************/ 38 39 /* This half of the example shows how to feed data into the JPEG compressor. 40 * We present a minimal version that does not worry about refinements such 41 * as error recovery (the JPEG code will just exit() if it gets an error). 42 */ 43 44 45 /* 46 * IMAGE DATA FORMATS: 47 * 48 * The standard input image format is a rectangular array of pixels, with 49 * each pixel having the same number of "component" values (color channels). 50 * Each pixel row is an array of JSAMPLEs (which typically are unsigned chars). 51 * If you are working with color data, then the color values for each pixel 52 * must be adjacent in the row; for example, R,G,B,R,G,B,R,G,B,... for 24-bit 53 * RGB color. 54 * 55 * For this example, we'll assume that this data structure matches the way 56 * our application has stored the image in memory, so we can just pass a 57 * pointer to our image buffer. In particular, let's say that the image is 58 * RGB color and is described by: 59 */ 60 61 extern JSAMPLE *image_buffer; /* Points to large array of R,G,B-order data */ 62 extern int image_height; /* Number of rows in image */ 63 extern int image_width; /* Number of columns in image */ 64 65 66 /* 67 * Sample routine for JPEG compression. We assume that the target file name 68 * and a compression quality factor are passed in. 69 */ 70 71 GLOBAL(void) 72 write_JPEG_file (char *filename, int quality) 73 { 74 /* This struct contains the JPEG compression parameters and pointers to 75 * working space (which is allocated as needed by the JPEG library). 76 * It is possible to have several such structures, representing multiple 77 * compression/decompression processes, in existence at once. We refer 78 * to any one struct (and its associated working data) as a "JPEG object". 79 */ 80 struct jpeg_compress_struct cinfo; 81 /* This struct represents a JPEG error handler. It is declared separately 82 * because applications often want to supply a specialized error handler 83 * (see the second half of this file for an example). But here we just 84 * take the easy way out and use the standard error handler, which will 85 * print a message on stderr and call exit() if compression fails. 86 * Note that this struct must live as long as the main JPEG parameter 87 * struct, to avoid dangling-pointer problems. 88 */ 89 struct jpeg_error_mgr jerr; 90 /* More stuff */ 91 FILE *outfile; /* target file */ 92 JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */ 93 int row_stride; /* physical row width in image buffer */ 94 95 /* Step 1: allocate and initialize JPEG compression object */ 96 97 /* We have to set up the error handler first, in case the initialization 98 * step fails. (Unlikely, but it could happen if you are out of memory.) 99 * This routine fills in the contents of struct jerr, and returns jerr's 100 * address which we place into the link field in cinfo. 101 */ 102 cinfo.err = jpeg_std_error(&jerr); 103 /* Now we can initialize the JPEG compression object. */ 104 jpeg_create_compress(&cinfo); 105 106 /* Step 2: specify data destination (eg, a file) */ 107 /* Note: steps 2 and 3 can be done in either order. */ 108 109 /* Here we use the library-supplied code to send compressed data to a 110 * stdio stream. You can also write your own code to do something else. 111 * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that 112 * requires it in order to write binary files. 113 */ 114 if ((outfile = fopen(filename, "wb")) == NULL) { 115 fprintf(stderr, "can't open %s\n", filename); 116 exit(1); 117 } 118 jpeg_stdio_dest(&cinfo, outfile); 119 120 /* Step 3: set parameters for compression */ 121 122 /* First we supply a description of the input image. 123 * Four fields of the cinfo struct must be filled in: 124 */ 125 cinfo.image_width = image_width; /* image width and height, in pixels */ 126 cinfo.image_height = image_height; 127 cinfo.input_components = 3; /* # of color components per pixel */ 128 cinfo.in_color_space = JCS_RGB; /* colorspace of input image */ 129 /* Now use the library's routine to set default compression parameters. 130 * (You must set at least cinfo.in_color_space before calling this, 131 * since the defaults depend on the source color space.) 132 */ 133 jpeg_set_defaults(&cinfo); 134 /* Now you can set any non-default parameters you wish to. 135 * Here we just illustrate the use of quality (quantization table) scaling: 136 */ 137 jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */); 138 139 /* Step 4: Start compressor */ 140 141 /* TRUE ensures that we will write a complete interchange-JPEG file. 142 * Pass TRUE unless you are very sure of what you're doing. 143 */ 144 jpeg_start_compress(&cinfo, TRUE); 145 146 /* Step 5: while (scan lines remain to be written) */ 147 /* jpeg_write_scanlines(...); */ 148 149 /* Here we use the library's state variable cinfo.next_scanline as the 150 * loop counter, so that we don't have to keep track ourselves. 151 * To keep things simple, we pass one scanline per call; you can pass 152 * more if you wish, though. 153 */ 154 row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */ 155 156 while (cinfo.next_scanline < cinfo.image_height) { 157 /* jpeg_write_scanlines expects an array of pointers to scanlines. 158 * Here the array is only one element long, but you could pass 159 * more than one scanline at a time if that's more convenient. 160 */ 161 row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride]; 162 (void) jpeg_write_scanlines(&cinfo, row_pointer, 1); 163 } 164 165 /* Step 6: Finish compression */ 166 167 jpeg_finish_compress(&cinfo); 168 /* After finish_compress, we can close the output file. */ 169 fclose(outfile); 170 171 /* Step 7: release JPEG compression object */ 172 173 /* This is an important step since it will release a good deal of memory. */ 174 jpeg_destroy_compress(&cinfo); 175 176 /* And we're done! */ 177 } 178 179 180 /* 181 * SOME FINE POINTS: 182 * 183 * In the above loop, we ignored the return value of jpeg_write_scanlines, 184 * which is the number of scanlines actually written. We could get away 185 * with this because we were only relying on the value of cinfo.next_scanline, 186 * which will be incremented correctly. If you maintain additional loop 187 * variables then you should be careful to increment them properly. 188 * Actually, for output to a stdio stream you needn't worry, because 189 * then jpeg_write_scanlines will write all the lines passed (or else exit 190 * with a fatal error). Partial writes can only occur if you use a data 191 * destination module that can demand suspension of the compressor. 192 * (If you don't know what that's for, you don't need it.) 193 * 194 * If the compressor requires full-image buffers (for entropy-coding 195 * optimization or a multi-scan JPEG file), it will create temporary 196 * files for anything that doesn't fit within the maximum-memory setting. 197 * (Note that temp files are NOT needed if you use the default parameters.) 198 * On some systems you may need to set up a signal handler to ensure that 199 * temporary files are deleted if the program is interrupted. See libjpeg.txt. 200 * 201 * Scanlines MUST be supplied in top-to-bottom order if you want your JPEG 202 * files to be compatible with everyone else's. If you cannot readily read 203 * your data in that order, you'll need an intermediate array to hold the 204 * image. See rdtarga.c or rdbmp.c for examples of handling bottom-to-top 205 * source data using the JPEG code's internal virtual-array mechanisms. 206 */ 207 208 209 210 /******************** JPEG DECOMPRESSION SAMPLE INTERFACE *******************/ 211 212 /* This half of the example shows how to read data from the JPEG decompressor. 213 * It's a bit more refined than the above, in that we show: 214 * (a) how to modify the JPEG library's standard error-reporting behavior; 215 * (b) how to allocate workspace using the library's memory manager. 216 * 217 * Just to make this example a little different from the first one, we'll 218 * assume that we do not intend to put the whole image into an in-memory 219 * buffer, but to send it line-by-line someplace else. We need a one- 220 * scanline-high JSAMPLE array as a work buffer, and we will let the JPEG 221 * memory manager allocate it for us. This approach is actually quite useful 222 * because we don't need to remember to deallocate the buffer separately: it 223 * will go away automatically when the JPEG object is cleaned up. 224 */ 225 226 227 /* 228 * ERROR HANDLING: 229 * 230 * The JPEG library's standard error handler (jerror.c) is divided into 231 * several "methods" which you can override individually. This lets you 232 * adjust the behavior without duplicating a lot of code, which you might 233 * have to update with each future release. 234 * 235 * Our example here shows how to override the "error_exit" method so that 236 * control is returned to the library's caller when a fatal error occurs, 237 * rather than calling exit() as the standard error_exit method does. 238 * 239 * We use C's setjmp/longjmp facility to return control. This means that the 240 * routine which calls the JPEG library must first execute a setjmp() call to 241 * establish the return point. We want the replacement error_exit to do a 242 * longjmp(). But we need to make the setjmp buffer accessible to the 243 * error_exit routine. To do this, we make a private extension of the 244 * standard JPEG error handler object. (If we were using C++, we'd say we 245 * were making a subclass of the regular error handler.) 246 * 247 * Here's the extended error handler struct: 248 */ 249 250 struct my_error_mgr { 251 struct jpeg_error_mgr pub; /* "public" fields */ 252 253 jmp_buf setjmp_buffer; /* for return to caller */ 254 }; 255 256 typedef struct my_error_mgr *my_error_ptr; 257 258 /* 259 * Here's the routine that will replace the standard error_exit method: 260 */ 261 262 METHODDEF(void) 263 my_error_exit (j_common_ptr cinfo) 264 { 265 /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */ 266 my_error_ptr myerr = (my_error_ptr) cinfo->err; 267 268 /* Always display the message. */ 269 /* We could postpone this until after returning, if we chose. */ 270 (*cinfo->err->output_message) (cinfo); 271 272 /* Return control to the setjmp point */ 273 longjmp(myerr->setjmp_buffer, 1); 274 } 275 276 277 /* 278 * Sample routine for JPEG decompression. We assume that the source file name 279 * is passed in. We want to return 1 on success, 0 on error. 280 */ 281 282 283 GLOBAL(int) 284 read_JPEG_file (char *filename) 285 { 286 /* This struct contains the JPEG decompression parameters and pointers to 287 * working space (which is allocated as needed by the JPEG library). 288 */ 289 struct jpeg_decompress_struct cinfo; 290 /* We use our private extension JPEG error handler. 291 * Note that this struct must live as long as the main JPEG parameter 292 * struct, to avoid dangling-pointer problems. 293 */ 294 struct my_error_mgr jerr; 295 /* More stuff */ 296 FILE *infile; /* source file */ 297 JSAMPARRAY buffer; /* Output row buffer */ 298 int row_stride; /* physical row width in output buffer */ 299 300 /* In this example we want to open the input file before doing anything else, 301 * so that the setjmp() error recovery below can assume the file is open. 302 * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that 303 * requires it in order to read binary files. 304 */ 305 306 if ((infile = fopen(filename, "rb")) == NULL) { 307 fprintf(stderr, "can't open %s\n", filename); 308 return 0; 309 } 310 311 /* Step 1: allocate and initialize JPEG decompression object */ 312 313 /* We set up the normal JPEG error routines, then override error_exit. */ 314 cinfo.err = jpeg_std_error(&jerr.pub); 315 jerr.pub.error_exit = my_error_exit; 316 /* Establish the setjmp return context for my_error_exit to use. */ 317 if (setjmp(jerr.setjmp_buffer)) { 318 /* If we get here, the JPEG code has signaled an error. 319 * We need to clean up the JPEG object, close the input file, and return. 320 */ 321 jpeg_destroy_decompress(&cinfo); 322 fclose(infile); 323 return 0; 324 } 325 /* Now we can initialize the JPEG decompression object. */ 326 jpeg_create_decompress(&cinfo); 327 328 /* Step 2: specify data source (eg, a file) */ 329 330 jpeg_stdio_src(&cinfo, infile); 331 332 /* Step 3: read file parameters with jpeg_read_header() */ 333 334 (void) jpeg_read_header(&cinfo, TRUE); 335 /* We can ignore the return value from jpeg_read_header since 336 * (a) suspension is not possible with the stdio data source, and 337 * (b) we passed TRUE to reject a tables-only JPEG file as an error. 338 * See libjpeg.txt for more info. 339 */ 340 341 /* Step 4: set parameters for decompression */ 342 343 /* In this example, we don't need to change any of the defaults set by 344 * jpeg_read_header(), so we do nothing here. 345 */ 346 347 /* Step 5: Start decompressor */ 348 349 (void) jpeg_start_decompress(&cinfo); 350 /* We can ignore the return value since suspension is not possible 351 * with the stdio data source. 352 */ 353 354 /* We may need to do some setup of our own at this point before reading 355 * the data. After jpeg_start_decompress() we have the correct scaled 356 * output image dimensions available, as well as the output colormap 357 * if we asked for color quantization. 358 * In this example, we need to make an output work buffer of the right size. 359 */ 360 /* JSAMPLEs per row in output buffer */ 361 row_stride = cinfo.output_width * cinfo.output_components; 362 /* Make a one-row-high sample array that will go away when done with image */ 363 buffer = (*cinfo.mem->alloc_sarray) 364 ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1); 365 366 /* Step 6: while (scan lines remain to be read) */ 367 /* jpeg_read_scanlines(...); */ 368 369 /* Here we use the library's state variable cinfo.output_scanline as the 370 * loop counter, so that we don't have to keep track ourselves. 371 */ 372 while (cinfo.output_scanline < cinfo.output_height) { 373 /* jpeg_read_scanlines expects an array of pointers to scanlines. 374 * Here the array is only one element long, but you could ask for 375 * more than one scanline at a time if that's more convenient. 376 */ 377 (void) jpeg_read_scanlines(&cinfo, buffer, 1); 378 /* Assume put_scanline_someplace wants a pointer and sample count. */ 379 put_scanline_someplace(buffer[0], row_stride); 380 } 381 382 /* Step 7: Finish decompression */ 383 384 (void) jpeg_finish_decompress(&cinfo); 385 /* We can ignore the return value since suspension is not possible 386 * with the stdio data source. 387 */ 388 389 /* Step 8: Release JPEG decompression object */ 390 391 /* This is an important step since it will release a good deal of memory. */ 392 jpeg_destroy_decompress(&cinfo); 393 394 /* After finish_decompress, we can close the input file. 395 * Here we postpone it until after no more JPEG errors are possible, 396 * so as to simplify the setjmp error logic above. (Actually, I don't 397 * think that jpeg_destroy can do an error exit, but why assume anything...) 398 */ 399 fclose(infile); 400 401 /* At this point you may want to check to see whether any corrupt-data 402 * warnings occurred (test whether jerr.pub.num_warnings is nonzero). 403 */ 404 405 /* And we're done! */ 406 return 1; 407 } 408 409 410 /* 411 * SOME FINE POINTS: 412 * 413 * In the above code, we ignored the return value of jpeg_read_scanlines, 414 * which is the number of scanlines actually read. We could get away with 415 * this because we asked for only one line at a time and we weren't using 416 * a suspending data source. See libjpeg.txt for more info. 417 * 418 * We cheated a bit by calling alloc_sarray() after jpeg_start_decompress(); 419 * we should have done it beforehand to ensure that the space would be 420 * counted against the JPEG max_memory setting. In some systems the above 421 * code would risk an out-of-memory error. However, in general we don't 422 * know the output image dimensions before jpeg_start_decompress(), unless we 423 * call jpeg_calc_output_dimensions(). See libjpeg.txt for more about this. 424 * 425 * Scanlines are returned in the same order as they appear in the JPEG file, 426 * which is standardly top-to-bottom. If you must emit data bottom-to-top, 427 * you can use one of the virtual arrays provided by the JPEG memory manager 428 * to invert the data. See wrbmp.c for an example. 429 * 430 * As with compression, some operating modes may require temporary files. 431 * On some systems you may need to set up a signal handler to ensure that 432 * temporary files are deleted if the program is interrupted. See libjpeg.txt. 433 */ 434
