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      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.doc.
      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.doc.
    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.doc 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.doc 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.doc 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.doc.
    433  */
    434