jpeg/example.c

   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  */