source: trunk/libjpeg/example.c @ 283

Last change on this file since 283 was 283, checked in by rbri, 12 years ago

JPEG plugin: libjpeg updated to version 7

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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.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
61extern JSAMPLE * image_buffer;  /* Points to large array of R,G,B-order data */
62extern int image_height;        /* Number of rows in image */
63extern 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
71GLOBAL(void)
72write_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
250struct my_error_mgr {
251  struct jpeg_error_mgr pub;    /* "public" fields */
252
253  jmp_buf setjmp_buffer;        /* for return to caller */
254};
255
256typedef struct my_error_mgr * my_error_ptr;
257
258/*
259 * Here's the routine that will replace the standard error_exit method:
260 */
261
262METHODDEF(void)
263my_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
283GLOBAL(int)
284read_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 */
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