Real-time collaboration for Jupyter Notebooks, Linux Terminals, LaTeX, VS Code, R IDE, and more,
all in one place.

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

435