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)
write_JPEG_file(char * filename,int quality)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)
my_error_exit(j_common_ptr cinfo)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)
read_JPEG_file(char * filename)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