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author | David Walter Seikel | 2014-01-13 19:47:58 +1000 |
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committer | David Walter Seikel | 2014-01-13 19:47:58 +1000 |
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tree | b16e389d7988700e21b4c9741044cefa536dcbae /libraries/irrlicht-1.8/source/Irrlicht/jpeglib/libjpeg.txt | |
parent | Libraries readme updated with change markers and more of the Irrlicht changes. (diff) | |
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Update Irrlicht to 1.8.1. Include actual change markers this time. lol
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1 | USING THE IJG JPEG LIBRARY | ||
2 | |||
3 | Copyright (C) 1994-2011, Thomas G. Lane, Guido Vollbeding. | ||
4 | This file is part of the Independent JPEG Group's software. | ||
5 | For conditions of distribution and use, see the accompanying README file. | ||
6 | |||
7 | |||
8 | This file describes how to use the IJG JPEG library within an application | ||
9 | program. Read it if you want to write a program that uses the library. | ||
10 | |||
11 | The file example.c provides heavily commented skeleton code for calling the | ||
12 | JPEG library. Also see jpeglib.h (the include file to be used by application | ||
13 | programs) for full details about data structures and function parameter lists. | ||
14 | The library source code, of course, is the ultimate reference. | ||
15 | |||
16 | Note that there have been *major* changes from the application interface | ||
17 | presented by IJG version 4 and earlier versions. The old design had several | ||
18 | inherent limitations, and it had accumulated a lot of cruft as we added | ||
19 | features while trying to minimize application-interface changes. We have | ||
20 | sacrificed backward compatibility in the version 5 rewrite, but we think the | ||
21 | improvements justify this. | ||
22 | |||
23 | |||
24 | TABLE OF CONTENTS | ||
25 | ----------------- | ||
26 | |||
27 | Overview: | ||
28 | Functions provided by the library | ||
29 | Outline of typical usage | ||
30 | Basic library usage: | ||
31 | Data formats | ||
32 | Compression details | ||
33 | Decompression details | ||
34 | Mechanics of usage: include files, linking, etc | ||
35 | Advanced features: | ||
36 | Compression parameter selection | ||
37 | Decompression parameter selection | ||
38 | Special color spaces | ||
39 | Error handling | ||
40 | Compressed data handling (source and destination managers) | ||
41 | I/O suspension | ||
42 | Progressive JPEG support | ||
43 | Buffered-image mode | ||
44 | Abbreviated datastreams and multiple images | ||
45 | Special markers | ||
46 | Raw (downsampled) image data | ||
47 | Really raw data: DCT coefficients | ||
48 | Progress monitoring | ||
49 | Memory management | ||
50 | Memory usage | ||
51 | Library compile-time options | ||
52 | Portability considerations | ||
53 | Notes for MS-DOS implementors | ||
54 | |||
55 | You should read at least the overview and basic usage sections before trying | ||
56 | to program with the library. The sections on advanced features can be read | ||
57 | if and when you need them. | ||
58 | |||
59 | |||
60 | OVERVIEW | ||
61 | ======== | ||
62 | |||
63 | Functions provided by the library | ||
64 | --------------------------------- | ||
65 | |||
66 | The IJG JPEG library provides C code to read and write JPEG-compressed image | ||
67 | files. The surrounding application program receives or supplies image data a | ||
68 | scanline at a time, using a straightforward uncompressed image format. All | ||
69 | details of color conversion and other preprocessing/postprocessing can be | ||
70 | handled by the library. | ||
71 | |||
72 | The library includes a substantial amount of code that is not covered by the | ||
73 | JPEG standard but is necessary for typical applications of JPEG. These | ||
74 | functions preprocess the image before JPEG compression or postprocess it after | ||
75 | decompression. They include colorspace conversion, downsampling/upsampling, | ||
76 | and color quantization. The application indirectly selects use of this code | ||
77 | by specifying the format in which it wishes to supply or receive image data. | ||
78 | For example, if colormapped output is requested, then the decompression | ||
79 | library automatically invokes color quantization. | ||
80 | |||
81 | A wide range of quality vs. speed tradeoffs are possible in JPEG processing, | ||
82 | and even more so in decompression postprocessing. The decompression library | ||
83 | provides multiple implementations that cover most of the useful tradeoffs, | ||
84 | ranging from very-high-quality down to fast-preview operation. On the | ||
85 | compression side we have generally not provided low-quality choices, since | ||
86 | compression is normally less time-critical. It should be understood that the | ||
87 | low-quality modes may not meet the JPEG standard's accuracy requirements; | ||
88 | nonetheless, they are useful for viewers. | ||
89 | |||
90 | A word about functions *not* provided by the library. We handle a subset of | ||
91 | the ISO JPEG standard; most baseline, extended-sequential, and progressive | ||
92 | JPEG processes are supported. (Our subset includes all features now in common | ||
93 | use.) Unsupported ISO options include: | ||
94 | * Hierarchical storage | ||
95 | * Lossless JPEG | ||
96 | * DNL marker | ||
97 | * Nonintegral subsampling ratios | ||
98 | We support both 8- and 12-bit data precision, but this is a compile-time | ||
99 | choice rather than a run-time choice; hence it is difficult to use both | ||
100 | precisions in a single application. | ||
101 | |||
102 | By itself, the library handles only interchange JPEG datastreams --- in | ||
103 | particular the widely used JFIF file format. The library can be used by | ||
104 | surrounding code to process interchange or abbreviated JPEG datastreams that | ||
105 | are embedded in more complex file formats. (For example, this library is | ||
106 | used by the free LIBTIFF library to support JPEG compression in TIFF.) | ||
107 | |||
108 | |||
109 | Outline of typical usage | ||
110 | ------------------------ | ||
111 | |||
112 | The rough outline of a JPEG compression operation is: | ||
113 | |||
114 | Allocate and initialize a JPEG compression object | ||
115 | Specify the destination for the compressed data (eg, a file) | ||
116 | Set parameters for compression, including image size & colorspace | ||
117 | jpeg_start_compress(...); | ||
118 | while (scan lines remain to be written) | ||
119 | jpeg_write_scanlines(...); | ||
120 | jpeg_finish_compress(...); | ||
121 | Release the JPEG compression object | ||
122 | |||
123 | A JPEG compression object holds parameters and working state for the JPEG | ||
124 | library. We make creation/destruction of the object separate from starting | ||
125 | or finishing compression of an image; the same object can be re-used for a | ||
126 | series of image compression operations. This makes it easy to re-use the | ||
127 | same parameter settings for a sequence of images. Re-use of a JPEG object | ||
128 | also has important implications for processing abbreviated JPEG datastreams, | ||
129 | as discussed later. | ||
130 | |||
131 | The image data to be compressed is supplied to jpeg_write_scanlines() from | ||
132 | in-memory buffers. If the application is doing file-to-file compression, | ||
133 | reading image data from the source file is the application's responsibility. | ||
134 | The library emits compressed data by calling a "data destination manager", | ||
135 | which typically will write the data into a file; but the application can | ||
136 | provide its own destination manager to do something else. | ||
137 | |||
138 | Similarly, the rough outline of a JPEG decompression operation is: | ||
139 | |||
140 | Allocate and initialize a JPEG decompression object | ||
141 | Specify the source of the compressed data (eg, a file) | ||
142 | Call jpeg_read_header() to obtain image info | ||
143 | Set parameters for decompression | ||
144 | jpeg_start_decompress(...); | ||
145 | while (scan lines remain to be read) | ||
146 | jpeg_read_scanlines(...); | ||
147 | jpeg_finish_decompress(...); | ||
148 | Release the JPEG decompression object | ||
149 | |||
150 | This is comparable to the compression outline except that reading the | ||
151 | datastream header is a separate step. This is helpful because information | ||
152 | about the image's size, colorspace, etc is available when the application | ||
153 | selects decompression parameters. For example, the application can choose an | ||
154 | output scaling ratio that will fit the image into the available screen size. | ||
155 | |||
156 | The decompression library obtains compressed data by calling a data source | ||
157 | manager, which typically will read the data from a file; but other behaviors | ||
158 | can be obtained with a custom source manager. Decompressed data is delivered | ||
159 | into in-memory buffers passed to jpeg_read_scanlines(). | ||
160 | |||
161 | It is possible to abort an incomplete compression or decompression operation | ||
162 | by calling jpeg_abort(); or, if you do not need to retain the JPEG object, | ||
163 | simply release it by calling jpeg_destroy(). | ||
164 | |||
165 | JPEG compression and decompression objects are two separate struct types. | ||
166 | However, they share some common fields, and certain routines such as | ||
167 | jpeg_destroy() can work on either type of object. | ||
168 | |||
169 | The JPEG library has no static variables: all state is in the compression | ||
170 | or decompression object. Therefore it is possible to process multiple | ||
171 | compression and decompression operations concurrently, using multiple JPEG | ||
172 | objects. | ||
173 | |||
174 | Both compression and decompression can be done in an incremental memory-to- | ||
175 | memory fashion, if suitable source/destination managers are used. See the | ||
176 | section on "I/O suspension" for more details. | ||
177 | |||
178 | |||
179 | BASIC LIBRARY USAGE | ||
180 | =================== | ||
181 | |||
182 | Data formats | ||
183 | ------------ | ||
184 | |||
185 | Before diving into procedural details, it is helpful to understand the | ||
186 | image data format that the JPEG library expects or returns. | ||
187 | |||
188 | The standard input image format is a rectangular array of pixels, with each | ||
189 | pixel having the same number of "component" or "sample" values (color | ||
190 | channels). You must specify how many components there are and the colorspace | ||
191 | interpretation of the components. Most applications will use RGB data | ||
192 | (three components per pixel) or grayscale data (one component per pixel). | ||
193 | PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE. | ||
194 | A remarkable number of people manage to miss this, only to find that their | ||
195 | programs don't work with grayscale JPEG files. | ||
196 | |||
197 | There is no provision for colormapped input. JPEG files are always full-color | ||
198 | or full grayscale (or sometimes another colorspace such as CMYK). You can | ||
199 | feed in a colormapped image by expanding it to full-color format. However | ||
200 | JPEG often doesn't work very well with source data that has been colormapped, | ||
201 | because of dithering noise. This is discussed in more detail in the JPEG FAQ | ||
202 | and the other references mentioned in the README file. | ||
203 | |||
204 | Pixels are stored by scanlines, with each scanline running from left to | ||
205 | right. The component values for each pixel are adjacent in the row; for | ||
206 | example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an | ||
207 | array of data type JSAMPLE --- which is typically "unsigned char", unless | ||
208 | you've changed jmorecfg.h. (You can also change the RGB pixel layout, say | ||
209 | to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in | ||
210 | that file before doing so.) | ||
211 | |||
212 | A 2-D array of pixels is formed by making a list of pointers to the starts of | ||
213 | scanlines; so the scanlines need not be physically adjacent in memory. Even | ||
214 | if you process just one scanline at a time, you must make a one-element | ||
215 | pointer array to conform to this structure. Pointers to JSAMPLE rows are of | ||
216 | type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY. | ||
217 | |||
218 | The library accepts or supplies one or more complete scanlines per call. | ||
219 | It is not possible to process part of a row at a time. Scanlines are always | ||
220 | processed top-to-bottom. You can process an entire image in one call if you | ||
221 | have it all in memory, but usually it's simplest to process one scanline at | ||
222 | a time. | ||
223 | |||
224 | For best results, source data values should have the precision specified by | ||
225 | BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress | ||
226 | data that's only 6 bits/channel, you should left-justify each value in a | ||
227 | byte before passing it to the compressor. If you need to compress data | ||
228 | that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12. | ||
229 | (See "Library compile-time options", later.) | ||
230 | |||
231 | |||
232 | The data format returned by the decompressor is the same in all details, | ||
233 | except that colormapped output is supported. (Again, a JPEG file is never | ||
234 | colormapped. But you can ask the decompressor to perform on-the-fly color | ||
235 | quantization to deliver colormapped output.) If you request colormapped | ||
236 | output then the returned data array contains a single JSAMPLE per pixel; | ||
237 | its value is an index into a color map. The color map is represented as | ||
238 | a 2-D JSAMPARRAY in which each row holds the values of one color component, | ||
239 | that is, colormap[i][j] is the value of the i'th color component for pixel | ||
240 | value (map index) j. Note that since the colormap indexes are stored in | ||
241 | JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE | ||
242 | (ie, at most 256 colors for an 8-bit JPEG library). | ||
243 | |||
244 | |||
245 | Compression details | ||
246 | ------------------- | ||
247 | |||
248 | Here we revisit the JPEG compression outline given in the overview. | ||
249 | |||
250 | 1. Allocate and initialize a JPEG compression object. | ||
251 | |||
252 | A JPEG compression object is a "struct jpeg_compress_struct". (It also has | ||
253 | a bunch of subsidiary structures which are allocated via malloc(), but the | ||
254 | application doesn't control those directly.) This struct can be just a local | ||
255 | variable in the calling routine, if a single routine is going to execute the | ||
256 | whole JPEG compression sequence. Otherwise it can be static or allocated | ||
257 | from malloc(). | ||
258 | |||
259 | You will also need a structure representing a JPEG error handler. The part | ||
260 | of this that the library cares about is a "struct jpeg_error_mgr". If you | ||
261 | are providing your own error handler, you'll typically want to embed the | ||
262 | jpeg_error_mgr struct in a larger structure; this is discussed later under | ||
263 | "Error handling". For now we'll assume you are just using the default error | ||
264 | handler. The default error handler will print JPEG error/warning messages | ||
265 | on stderr, and it will call exit() if a fatal error occurs. | ||
266 | |||
267 | You must initialize the error handler structure, store a pointer to it into | ||
268 | the JPEG object's "err" field, and then call jpeg_create_compress() to | ||
269 | initialize the rest of the JPEG object. | ||
270 | |||
271 | Typical code for this step, if you are using the default error handler, is | ||
272 | |||
273 | struct jpeg_compress_struct cinfo; | ||
274 | struct jpeg_error_mgr jerr; | ||
275 | ... | ||
276 | cinfo.err = jpeg_std_error(&jerr); | ||
277 | jpeg_create_compress(&cinfo); | ||
278 | |||
279 | jpeg_create_compress allocates a small amount of memory, so it could fail | ||
280 | if you are out of memory. In that case it will exit via the error handler; | ||
281 | that's why the error handler must be initialized first. | ||
282 | |||
283 | |||
284 | 2. Specify the destination for the compressed data (eg, a file). | ||
285 | |||
286 | As previously mentioned, the JPEG library delivers compressed data to a | ||
287 | "data destination" module. The library includes one data destination | ||
288 | module which knows how to write to a stdio stream. You can use your own | ||
289 | destination module if you want to do something else, as discussed later. | ||
290 | |||
291 | If you use the standard destination module, you must open the target stdio | ||
292 | stream beforehand. Typical code for this step looks like: | ||
293 | |||
294 | FILE * outfile; | ||
295 | ... | ||
296 | if ((outfile = fopen(filename, "wb")) == NULL) { | ||
297 | fprintf(stderr, "can't open %s\n", filename); | ||
298 | exit(1); | ||
299 | } | ||
300 | jpeg_stdio_dest(&cinfo, outfile); | ||
301 | |||
302 | where the last line invokes the standard destination module. | ||
303 | |||
304 | WARNING: it is critical that the binary compressed data be delivered to the | ||
305 | output file unchanged. On non-Unix systems the stdio library may perform | ||
306 | newline translation or otherwise corrupt binary data. To suppress this | ||
307 | behavior, you may need to use a "b" option to fopen (as shown above), or use | ||
308 | setmode() or another routine to put the stdio stream in binary mode. See | ||
309 | cjpeg.c and djpeg.c for code that has been found to work on many systems. | ||
310 | |||
311 | You can select the data destination after setting other parameters (step 3), | ||
312 | if that's more convenient. You may not change the destination between | ||
313 | calling jpeg_start_compress() and jpeg_finish_compress(). | ||
314 | |||
315 | |||
316 | 3. Set parameters for compression, including image size & colorspace. | ||
317 | |||
318 | You must supply information about the source image by setting the following | ||
319 | fields in the JPEG object (cinfo structure): | ||
320 | |||
321 | image_width Width of image, in pixels | ||
322 | image_height Height of image, in pixels | ||
323 | input_components Number of color channels (samples per pixel) | ||
324 | in_color_space Color space of source image | ||
325 | |||
326 | The image dimensions are, hopefully, obvious. JPEG supports image dimensions | ||
327 | of 1 to 64K pixels in either direction. The input color space is typically | ||
328 | RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special | ||
329 | color spaces", later, for more info.) The in_color_space field must be | ||
330 | assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or | ||
331 | JCS_GRAYSCALE. | ||
332 | |||
333 | JPEG has a large number of compression parameters that determine how the | ||
334 | image is encoded. Most applications don't need or want to know about all | ||
335 | these parameters. You can set all the parameters to reasonable defaults by | ||
336 | calling jpeg_set_defaults(); then, if there are particular values you want | ||
337 | to change, you can do so after that. The "Compression parameter selection" | ||
338 | section tells about all the parameters. | ||
339 | |||
340 | You must set in_color_space correctly before calling jpeg_set_defaults(), | ||
341 | because the defaults depend on the source image colorspace. However the | ||
342 | other three source image parameters need not be valid until you call | ||
343 | jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more | ||
344 | than once, if that happens to be convenient. | ||
345 | |||
346 | Typical code for a 24-bit RGB source image is | ||
347 | |||
348 | cinfo.image_width = Width; /* image width and height, in pixels */ | ||
349 | cinfo.image_height = Height; | ||
350 | cinfo.input_components = 3; /* # of color components per pixel */ | ||
351 | cinfo.in_color_space = JCS_RGB; /* colorspace of input image */ | ||
352 | |||
353 | jpeg_set_defaults(&cinfo); | ||
354 | /* Make optional parameter settings here */ | ||
355 | |||
356 | |||
357 | 4. jpeg_start_compress(...); | ||
358 | |||
359 | After you have established the data destination and set all the necessary | ||
360 | source image info and other parameters, call jpeg_start_compress() to begin | ||
361 | a compression cycle. This will initialize internal state, allocate working | ||
362 | storage, and emit the first few bytes of the JPEG datastream header. | ||
363 | |||
364 | Typical code: | ||
365 | |||
366 | jpeg_start_compress(&cinfo, TRUE); | ||
367 | |||
368 | The "TRUE" parameter ensures that a complete JPEG interchange datastream | ||
369 | will be written. This is appropriate in most cases. If you think you might | ||
370 | want to use an abbreviated datastream, read the section on abbreviated | ||
371 | datastreams, below. | ||
372 | |||
373 | Once you have called jpeg_start_compress(), you may not alter any JPEG | ||
374 | parameters or other fields of the JPEG object until you have completed | ||
375 | the compression cycle. | ||
376 | |||
377 | |||
378 | 5. while (scan lines remain to be written) | ||
379 | jpeg_write_scanlines(...); | ||
380 | |||
381 | Now write all the required image data by calling jpeg_write_scanlines() | ||
382 | one or more times. You can pass one or more scanlines in each call, up | ||
383 | to the total image height. In most applications it is convenient to pass | ||
384 | just one or a few scanlines at a time. The expected format for the passed | ||
385 | data is discussed under "Data formats", above. | ||
386 | |||
387 | Image data should be written in top-to-bottom scanline order. The JPEG spec | ||
388 | contains some weasel wording about how top and bottom are application-defined | ||
389 | terms (a curious interpretation of the English language...) but if you want | ||
390 | your files to be compatible with everyone else's, you WILL use top-to-bottom | ||
391 | order. If the source data must be read in bottom-to-top order, you can use | ||
392 | the JPEG library's virtual array mechanism to invert the data efficiently. | ||
393 | Examples of this can be found in the sample application cjpeg. | ||
394 | |||
395 | The library maintains a count of the number of scanlines written so far | ||
396 | in the next_scanline field of the JPEG object. Usually you can just use | ||
397 | this variable as the loop counter, so that the loop test looks like | ||
398 | "while (cinfo.next_scanline < cinfo.image_height)". | ||
399 | |||
400 | Code for this step depends heavily on the way that you store the source data. | ||
401 | example.c shows the following code for the case of a full-size 2-D source | ||
402 | array containing 3-byte RGB pixels: | ||
403 | |||
404 | JSAMPROW row_pointer[1]; /* pointer to a single row */ | ||
405 | int row_stride; /* physical row width in buffer */ | ||
406 | |||
407 | row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */ | ||
408 | |||
409 | while (cinfo.next_scanline < cinfo.image_height) { | ||
410 | row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride]; | ||
411 | jpeg_write_scanlines(&cinfo, row_pointer, 1); | ||
412 | } | ||
413 | |||
414 | jpeg_write_scanlines() returns the number of scanlines actually written. | ||
415 | This will normally be equal to the number passed in, so you can usually | ||
416 | ignore the return value. It is different in just two cases: | ||
417 | * If you try to write more scanlines than the declared image height, | ||
418 | the additional scanlines are ignored. | ||
419 | * If you use a suspending data destination manager, output buffer overrun | ||
420 | will cause the compressor to return before accepting all the passed lines. | ||
421 | This feature is discussed under "I/O suspension", below. The normal | ||
422 | stdio destination manager will NOT cause this to happen. | ||
423 | In any case, the return value is the same as the change in the value of | ||
424 | next_scanline. | ||
425 | |||
426 | |||
427 | 6. jpeg_finish_compress(...); | ||
428 | |||
429 | After all the image data has been written, call jpeg_finish_compress() to | ||
430 | complete the compression cycle. This step is ESSENTIAL to ensure that the | ||
431 | last bufferload of data is written to the data destination. | ||
432 | jpeg_finish_compress() also releases working memory associated with the JPEG | ||
433 | object. | ||
434 | |||
435 | Typical code: | ||
436 | |||
437 | jpeg_finish_compress(&cinfo); | ||
438 | |||
439 | If using the stdio destination manager, don't forget to close the output | ||
440 | stdio stream (if necessary) afterwards. | ||
441 | |||
442 | If you have requested a multi-pass operating mode, such as Huffman code | ||
443 | optimization, jpeg_finish_compress() will perform the additional passes using | ||
444 | data buffered by the first pass. In this case jpeg_finish_compress() may take | ||
445 | quite a while to complete. With the default compression parameters, this will | ||
446 | not happen. | ||
447 | |||
448 | It is an error to call jpeg_finish_compress() before writing the necessary | ||
449 | total number of scanlines. If you wish to abort compression, call | ||
450 | jpeg_abort() as discussed below. | ||
451 | |||
452 | After completing a compression cycle, you may dispose of the JPEG object | ||
453 | as discussed next, or you may use it to compress another image. In that case | ||
454 | return to step 2, 3, or 4 as appropriate. If you do not change the | ||
455 | destination manager, the new datastream will be written to the same target. | ||
456 | If you do not change any JPEG parameters, the new datastream will be written | ||
457 | with the same parameters as before. Note that you can change the input image | ||
458 | dimensions freely between cycles, but if you change the input colorspace, you | ||
459 | should call jpeg_set_defaults() to adjust for the new colorspace; and then | ||
460 | you'll need to repeat all of step 3. | ||
461 | |||
462 | |||
463 | 7. Release the JPEG compression object. | ||
464 | |||
465 | When you are done with a JPEG compression object, destroy it by calling | ||
466 | jpeg_destroy_compress(). This will free all subsidiary memory (regardless of | ||
467 | the previous state of the object). Or you can call jpeg_destroy(), which | ||
468 | works for either compression or decompression objects --- this may be more | ||
469 | convenient if you are sharing code between compression and decompression | ||
470 | cases. (Actually, these routines are equivalent except for the declared type | ||
471 | of the passed pointer. To avoid gripes from ANSI C compilers, jpeg_destroy() | ||
472 | should be passed a j_common_ptr.) | ||
473 | |||
474 | If you allocated the jpeg_compress_struct structure from malloc(), freeing | ||
475 | it is your responsibility --- jpeg_destroy() won't. Ditto for the error | ||
476 | handler structure. | ||
477 | |||
478 | Typical code: | ||
479 | |||
480 | jpeg_destroy_compress(&cinfo); | ||
481 | |||
482 | |||
483 | 8. Aborting. | ||
484 | |||
485 | If you decide to abort a compression cycle before finishing, you can clean up | ||
486 | in either of two ways: | ||
487 | |||
488 | * If you don't need the JPEG object any more, just call | ||
489 | jpeg_destroy_compress() or jpeg_destroy() to release memory. This is | ||
490 | legitimate at any point after calling jpeg_create_compress() --- in fact, | ||
491 | it's safe even if jpeg_create_compress() fails. | ||
492 | |||
493 | * If you want to re-use the JPEG object, call jpeg_abort_compress(), or call | ||
494 | jpeg_abort() which works on both compression and decompression objects. | ||
495 | This will return the object to an idle state, releasing any working memory. | ||
496 | jpeg_abort() is allowed at any time after successful object creation. | ||
497 | |||
498 | Note that cleaning up the data destination, if required, is your | ||
499 | responsibility; neither of these routines will call term_destination(). | ||
500 | (See "Compressed data handling", below, for more about that.) | ||
501 | |||
502 | jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG | ||
503 | object that has reported an error by calling error_exit (see "Error handling" | ||
504 | for more info). The internal state of such an object is likely to be out of | ||
505 | whack. Either of these two routines will return the object to a known state. | ||
506 | |||
507 | |||
508 | Decompression details | ||
509 | --------------------- | ||
510 | |||
511 | Here we revisit the JPEG decompression outline given in the overview. | ||
512 | |||
513 | 1. Allocate and initialize a JPEG decompression object. | ||
514 | |||
515 | This is just like initialization for compression, as discussed above, | ||
516 | except that the object is a "struct jpeg_decompress_struct" and you | ||
517 | call jpeg_create_decompress(). Error handling is exactly the same. | ||
518 | |||
519 | Typical code: | ||
520 | |||
521 | struct jpeg_decompress_struct cinfo; | ||
522 | struct jpeg_error_mgr jerr; | ||
523 | ... | ||
524 | cinfo.err = jpeg_std_error(&jerr); | ||
525 | jpeg_create_decompress(&cinfo); | ||
526 | |||
527 | (Both here and in the IJG code, we usually use variable name "cinfo" for | ||
528 | both compression and decompression objects.) | ||
529 | |||
530 | |||
531 | 2. Specify the source of the compressed data (eg, a file). | ||
532 | |||
533 | As previously mentioned, the JPEG library reads compressed data from a "data | ||
534 | source" module. The library includes one data source module which knows how | ||
535 | to read from a stdio stream. You can use your own source module if you want | ||
536 | to do something else, as discussed later. | ||
537 | |||
538 | If you use the standard source module, you must open the source stdio stream | ||
539 | beforehand. Typical code for this step looks like: | ||
540 | |||
541 | FILE * infile; | ||
542 | ... | ||
543 | if ((infile = fopen(filename, "rb")) == NULL) { | ||
544 | fprintf(stderr, "can't open %s\n", filename); | ||
545 | exit(1); | ||
546 | } | ||
547 | jpeg_stdio_src(&cinfo, infile); | ||
548 | |||
549 | where the last line invokes the standard source module. | ||
550 | |||
551 | WARNING: it is critical that the binary compressed data be read unchanged. | ||
552 | On non-Unix systems the stdio library may perform newline translation or | ||
553 | otherwise corrupt binary data. To suppress this behavior, you may need to use | ||
554 | a "b" option to fopen (as shown above), or use setmode() or another routine to | ||
555 | put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that | ||
556 | has been found to work on many systems. | ||
557 | |||
558 | You may not change the data source between calling jpeg_read_header() and | ||
559 | jpeg_finish_decompress(). If you wish to read a series of JPEG images from | ||
560 | a single source file, you should repeat the jpeg_read_header() to | ||
561 | jpeg_finish_decompress() sequence without reinitializing either the JPEG | ||
562 | object or the data source module; this prevents buffered input data from | ||
563 | being discarded. | ||
564 | |||
565 | |||
566 | 3. Call jpeg_read_header() to obtain image info. | ||
567 | |||
568 | Typical code for this step is just | ||
569 | |||
570 | jpeg_read_header(&cinfo, TRUE); | ||
571 | |||
572 | This will read the source datastream header markers, up to the beginning | ||
573 | of the compressed data proper. On return, the image dimensions and other | ||
574 | info have been stored in the JPEG object. The application may wish to | ||
575 | consult this information before selecting decompression parameters. | ||
576 | |||
577 | More complex code is necessary if | ||
578 | * A suspending data source is used --- in that case jpeg_read_header() | ||
579 | may return before it has read all the header data. See "I/O suspension", | ||
580 | below. The normal stdio source manager will NOT cause this to happen. | ||
581 | * Abbreviated JPEG files are to be processed --- see the section on | ||
582 | abbreviated datastreams. Standard applications that deal only in | ||
583 | interchange JPEG files need not be concerned with this case either. | ||
584 | |||
585 | It is permissible to stop at this point if you just wanted to find out the | ||
586 | image dimensions and other header info for a JPEG file. In that case, | ||
587 | call jpeg_destroy() when you are done with the JPEG object, or call | ||
588 | jpeg_abort() to return it to an idle state before selecting a new data | ||
589 | source and reading another header. | ||
590 | |||
591 | |||
592 | 4. Set parameters for decompression. | ||
593 | |||
594 | jpeg_read_header() sets appropriate default decompression parameters based on | ||
595 | the properties of the image (in particular, its colorspace). However, you | ||
596 | may well want to alter these defaults before beginning the decompression. | ||
597 | For example, the default is to produce full color output from a color file. | ||
598 | If you want colormapped output you must ask for it. Other options allow the | ||
599 | returned image to be scaled and allow various speed/quality tradeoffs to be | ||
600 | selected. "Decompression parameter selection", below, gives details. | ||
601 | |||
602 | If the defaults are appropriate, nothing need be done at this step. | ||
603 | |||
604 | Note that all default values are set by each call to jpeg_read_header(). | ||
605 | If you reuse a decompression object, you cannot expect your parameter | ||
606 | settings to be preserved across cycles, as you can for compression. | ||
607 | You must set desired parameter values each time. | ||
608 | |||
609 | |||
610 | 5. jpeg_start_decompress(...); | ||
611 | |||
612 | Once the parameter values are satisfactory, call jpeg_start_decompress() to | ||
613 | begin decompression. This will initialize internal state, allocate working | ||
614 | memory, and prepare for returning data. | ||
615 | |||
616 | Typical code is just | ||
617 | |||
618 | jpeg_start_decompress(&cinfo); | ||
619 | |||
620 | If you have requested a multi-pass operating mode, such as 2-pass color | ||
621 | quantization, jpeg_start_decompress() will do everything needed before data | ||
622 | output can begin. In this case jpeg_start_decompress() may take quite a while | ||
623 | to complete. With a single-scan (non progressive) JPEG file and default | ||
624 | decompression parameters, this will not happen; jpeg_start_decompress() will | ||
625 | return quickly. | ||
626 | |||
627 | After this call, the final output image dimensions, including any requested | ||
628 | scaling, are available in the JPEG object; so is the selected colormap, if | ||
629 | colormapped output has been requested. Useful fields include | ||
630 | |||
631 | output_width image width and height, as scaled | ||
632 | output_height | ||
633 | out_color_components # of color components in out_color_space | ||
634 | output_components # of color components returned per pixel | ||
635 | colormap the selected colormap, if any | ||
636 | actual_number_of_colors number of entries in colormap | ||
637 | |||
638 | output_components is 1 (a colormap index) when quantizing colors; otherwise it | ||
639 | equals out_color_components. It is the number of JSAMPLE values that will be | ||
640 | emitted per pixel in the output arrays. | ||
641 | |||
642 | Typically you will need to allocate data buffers to hold the incoming image. | ||
643 | You will need output_width * output_components JSAMPLEs per scanline in your | ||
644 | output buffer, and a total of output_height scanlines will be returned. | ||
645 | |||
646 | Note: if you are using the JPEG library's internal memory manager to allocate | ||
647 | data buffers (as djpeg does), then the manager's protocol requires that you | ||
648 | request large buffers *before* calling jpeg_start_decompress(). This is a | ||
649 | little tricky since the output_XXX fields are not normally valid then. You | ||
650 | can make them valid by calling jpeg_calc_output_dimensions() after setting the | ||
651 | relevant parameters (scaling, output color space, and quantization flag). | ||
652 | |||
653 | |||
654 | 6. while (scan lines remain to be read) | ||
655 | jpeg_read_scanlines(...); | ||
656 | |||
657 | Now you can read the decompressed image data by calling jpeg_read_scanlines() | ||
658 | one or more times. At each call, you pass in the maximum number of scanlines | ||
659 | to be read (ie, the height of your working buffer); jpeg_read_scanlines() | ||
660 | will return up to that many lines. The return value is the number of lines | ||
661 | actually read. The format of the returned data is discussed under "Data | ||
662 | formats", above. Don't forget that grayscale and color JPEGs will return | ||
663 | different data formats! | ||
664 | |||
665 | Image data is returned in top-to-bottom scanline order. If you must write | ||
666 | out the image in bottom-to-top order, you can use the JPEG library's virtual | ||
667 | array mechanism to invert the data efficiently. Examples of this can be | ||
668 | found in the sample application djpeg. | ||
669 | |||
670 | The library maintains a count of the number of scanlines returned so far | ||
671 | in the output_scanline field of the JPEG object. Usually you can just use | ||
672 | this variable as the loop counter, so that the loop test looks like | ||
673 | "while (cinfo.output_scanline < cinfo.output_height)". (Note that the test | ||
674 | should NOT be against image_height, unless you never use scaling. The | ||
675 | image_height field is the height of the original unscaled image.) | ||
676 | The return value always equals the change in the value of output_scanline. | ||
677 | |||
678 | If you don't use a suspending data source, it is safe to assume that | ||
679 | jpeg_read_scanlines() reads at least one scanline per call, until the | ||
680 | bottom of the image has been reached. | ||
681 | |||
682 | If you use a buffer larger than one scanline, it is NOT safe to assume that | ||
683 | jpeg_read_scanlines() fills it. (The current implementation returns only a | ||
684 | few scanlines per call, no matter how large a buffer you pass.) So you must | ||
685 | always provide a loop that calls jpeg_read_scanlines() repeatedly until the | ||
686 | whole image has been read. | ||
687 | |||
688 | |||
689 | 7. jpeg_finish_decompress(...); | ||
690 | |||
691 | After all the image data has been read, call jpeg_finish_decompress() to | ||
692 | complete the decompression cycle. This causes working memory associated | ||
693 | with the JPEG object to be released. | ||
694 | |||
695 | Typical code: | ||
696 | |||
697 | jpeg_finish_decompress(&cinfo); | ||
698 | |||
699 | If using the stdio source manager, don't forget to close the source stdio | ||
700 | stream if necessary. | ||
701 | |||
702 | It is an error to call jpeg_finish_decompress() before reading the correct | ||
703 | total number of scanlines. If you wish to abort decompression, call | ||
704 | jpeg_abort() as discussed below. | ||
705 | |||
706 | After completing a decompression cycle, you may dispose of the JPEG object as | ||
707 | discussed next, or you may use it to decompress another image. In that case | ||
708 | return to step 2 or 3 as appropriate. If you do not change the source | ||
709 | manager, the next image will be read from the same source. | ||
710 | |||
711 | |||
712 | 8. Release the JPEG decompression object. | ||
713 | |||
714 | When you are done with a JPEG decompression object, destroy it by calling | ||
715 | jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of | ||
716 | destroying compression objects applies here too. | ||
717 | |||
718 | Typical code: | ||
719 | |||
720 | jpeg_destroy_decompress(&cinfo); | ||
721 | |||
722 | |||
723 | 9. Aborting. | ||
724 | |||
725 | You can abort a decompression cycle by calling jpeg_destroy_decompress() or | ||
726 | jpeg_destroy() if you don't need the JPEG object any more, or | ||
727 | jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object. | ||
728 | The previous discussion of aborting compression cycles applies here too. | ||
729 | |||
730 | |||
731 | Mechanics of usage: include files, linking, etc | ||
732 | ----------------------------------------------- | ||
733 | |||
734 | Applications using the JPEG library should include the header file jpeglib.h | ||
735 | to obtain declarations of data types and routines. Before including | ||
736 | jpeglib.h, include system headers that define at least the typedefs FILE and | ||
737 | size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on | ||
738 | older Unix systems, you may need <sys/types.h> to define size_t. | ||
739 | |||
740 | If the application needs to refer to individual JPEG library error codes, also | ||
741 | include jerror.h to define those symbols. | ||
742 | |||
743 | jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are | ||
744 | installing the JPEG header files in a system directory, you will want to | ||
745 | install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h. | ||
746 | |||
747 | The most convenient way to include the JPEG code into your executable program | ||
748 | is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix | ||
749 | machines) and reference it at your link step. If you use only half of the | ||
750 | library (only compression or only decompression), only that much code will be | ||
751 | included from the library, unless your linker is hopelessly brain-damaged. | ||
752 | The supplied makefiles build libjpeg.a automatically (see install.txt). | ||
753 | |||
754 | While you can build the JPEG library as a shared library if the whim strikes | ||
755 | you, we don't really recommend it. The trouble with shared libraries is that | ||
756 | at some point you'll probably try to substitute a new version of the library | ||
757 | without recompiling the calling applications. That generally doesn't work | ||
758 | because the parameter struct declarations usually change with each new | ||
759 | version. In other words, the library's API is *not* guaranteed binary | ||
760 | compatible across versions; we only try to ensure source-code compatibility. | ||
761 | (In hindsight, it might have been smarter to hide the parameter structs from | ||
762 | applications and introduce a ton of access functions instead. Too late now, | ||
763 | however.) | ||
764 | |||
765 | On some systems your application may need to set up a signal handler to ensure | ||
766 | that temporary files are deleted if the program is interrupted. This is most | ||
767 | critical if you are on MS-DOS and use the jmemdos.c memory manager back end; | ||
768 | it will try to grab extended memory for temp files, and that space will NOT be | ||
769 | freed automatically. See cjpeg.c or djpeg.c for an example signal handler. | ||
770 | |||
771 | It may be worth pointing out that the core JPEG library does not actually | ||
772 | require the stdio library: only the default source/destination managers and | ||
773 | error handler need it. You can use the library in a stdio-less environment | ||
774 | if you replace those modules and use jmemnobs.c (or another memory manager of | ||
775 | your own devising). More info about the minimum system library requirements | ||
776 | may be found in jinclude.h. | ||
777 | |||
778 | |||
779 | ADVANCED FEATURES | ||
780 | ================= | ||
781 | |||
782 | Compression parameter selection | ||
783 | ------------------------------- | ||
784 | |||
785 | This section describes all the optional parameters you can set for JPEG | ||
786 | compression, as well as the "helper" routines provided to assist in this | ||
787 | task. Proper setting of some parameters requires detailed understanding | ||
788 | of the JPEG standard; if you don't know what a parameter is for, it's best | ||
789 | not to mess with it! See REFERENCES in the README file for pointers to | ||
790 | more info about JPEG. | ||
791 | |||
792 | It's a good idea to call jpeg_set_defaults() first, even if you plan to set | ||
793 | all the parameters; that way your code is more likely to work with future JPEG | ||
794 | libraries that have additional parameters. For the same reason, we recommend | ||
795 | you use a helper routine where one is provided, in preference to twiddling | ||
796 | cinfo fields directly. | ||
797 | |||
798 | The helper routines are: | ||
799 | |||
800 | jpeg_set_defaults (j_compress_ptr cinfo) | ||
801 | This routine sets all JPEG parameters to reasonable defaults, using | ||
802 | only the input image's color space (field in_color_space, which must | ||
803 | already be set in cinfo). Many applications will only need to use | ||
804 | this routine and perhaps jpeg_set_quality(). | ||
805 | |||
806 | jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace) | ||
807 | Sets the JPEG file's colorspace (field jpeg_color_space) as specified, | ||
808 | and sets other color-space-dependent parameters appropriately. See | ||
809 | "Special color spaces", below, before using this. A large number of | ||
810 | parameters, including all per-component parameters, are set by this | ||
811 | routine; if you want to twiddle individual parameters you should call | ||
812 | jpeg_set_colorspace() before rather than after. | ||
813 | |||
814 | jpeg_default_colorspace (j_compress_ptr cinfo) | ||
815 | Selects an appropriate JPEG colorspace based on cinfo->in_color_space, | ||
816 | and calls jpeg_set_colorspace(). This is actually a subroutine of | ||
817 | jpeg_set_defaults(). It's broken out in case you want to change | ||
818 | just the colorspace-dependent JPEG parameters. | ||
819 | |||
820 | jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline) | ||
821 | Constructs JPEG quantization tables appropriate for the indicated | ||
822 | quality setting. The quality value is expressed on the 0..100 scale | ||
823 | recommended by IJG (cjpeg's "-quality" switch uses this routine). | ||
824 | Note that the exact mapping from quality values to tables may change | ||
825 | in future IJG releases as more is learned about DCT quantization. | ||
826 | If the force_baseline parameter is TRUE, then the quantization table | ||
827 | entries are constrained to the range 1..255 for full JPEG baseline | ||
828 | compatibility. In the current implementation, this only makes a | ||
829 | difference for quality settings below 25, and it effectively prevents | ||
830 | very small/low quality files from being generated. The IJG decoder | ||
831 | is capable of reading the non-baseline files generated at low quality | ||
832 | settings when force_baseline is FALSE, but other decoders may not be. | ||
833 | |||
834 | jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor, | ||
835 | boolean force_baseline) | ||
836 | Same as jpeg_set_quality() except that the generated tables are the | ||
837 | sample tables given in the JPEC spec section K.1, multiplied by the | ||
838 | specified scale factor (which is expressed as a percentage; thus | ||
839 | scale_factor = 100 reproduces the spec's tables). Note that larger | ||
840 | scale factors give lower quality. This entry point is useful for | ||
841 | conforming to the Adobe PostScript DCT conventions, but we do not | ||
842 | recommend linear scaling as a user-visible quality scale otherwise. | ||
843 | force_baseline again constrains the computed table entries to 1..255. | ||
844 | |||
845 | int jpeg_quality_scaling (int quality) | ||
846 | Converts a value on the IJG-recommended quality scale to a linear | ||
847 | scaling percentage. Note that this routine may change or go away | ||
848 | in future releases --- IJG may choose to adopt a scaling method that | ||
849 | can't be expressed as a simple scalar multiplier, in which case the | ||
850 | premise of this routine collapses. Caveat user. | ||
851 | |||
852 | jpeg_default_qtables (j_compress_ptr cinfo, boolean force_baseline) | ||
853 | Set default quantization tables with linear q_scale_factor[] values | ||
854 | (see below). | ||
855 | |||
856 | jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl, | ||
857 | const unsigned int *basic_table, | ||
858 | int scale_factor, boolean force_baseline) | ||
859 | Allows an arbitrary quantization table to be created. which_tbl | ||
860 | indicates which table slot to fill. basic_table points to an array | ||
861 | of 64 unsigned ints given in normal array order. These values are | ||
862 | multiplied by scale_factor/100 and then clamped to the range 1..65535 | ||
863 | (or to 1..255 if force_baseline is TRUE). | ||
864 | CAUTION: prior to library version 6a, jpeg_add_quant_table expected | ||
865 | the basic table to be given in JPEG zigzag order. If you need to | ||
866 | write code that works with either older or newer versions of this | ||
867 | routine, you must check the library version number. Something like | ||
868 | "#if JPEG_LIB_VERSION >= 61" is the right test. | ||
869 | |||
870 | jpeg_simple_progression (j_compress_ptr cinfo) | ||
871 | Generates a default scan script for writing a progressive-JPEG file. | ||
872 | This is the recommended method of creating a progressive file, | ||
873 | unless you want to make a custom scan sequence. You must ensure that | ||
874 | the JPEG color space is set correctly before calling this routine. | ||
875 | |||
876 | |||
877 | Compression parameters (cinfo fields) include: | ||
878 | |||
879 | int block_size | ||
880 | Set DCT block size. All N from 1 to 16 are possible. | ||
881 | Default is 8 (baseline format). | ||
882 | Larger values produce higher compression, | ||
883 | smaller values produce higher quality. | ||
884 | An exact DCT stage is possible with 1 or 2. | ||
885 | With the default quality of 75 and default Luminance qtable | ||
886 | the DCT+Quantization stage is lossless for value 1. | ||
887 | Note that values other than 8 require a SmartScale capable decoder, | ||
888 | introduced with IJG JPEG 8. Setting the block_size parameter for | ||
889 | compression works with version 8c and later. | ||
890 | |||
891 | J_DCT_METHOD dct_method | ||
892 | Selects the algorithm used for the DCT step. Choices are: | ||
893 | JDCT_ISLOW: slow but accurate integer algorithm | ||
894 | JDCT_IFAST: faster, less accurate integer method | ||
895 | JDCT_FLOAT: floating-point method | ||
896 | JDCT_DEFAULT: default method (normally JDCT_ISLOW) | ||
897 | JDCT_FASTEST: fastest method (normally JDCT_IFAST) | ||
898 | The FLOAT method is very slightly more accurate than the ISLOW method, | ||
899 | but may give different results on different machines due to varying | ||
900 | roundoff behavior. The integer methods should give the same results | ||
901 | on all machines. On machines with sufficiently fast FP hardware, the | ||
902 | floating-point method may also be the fastest. The IFAST method is | ||
903 | considerably less accurate than the other two; its use is not | ||
904 | recommended if high quality is a concern. JDCT_DEFAULT and | ||
905 | JDCT_FASTEST are macros configurable by each installation. | ||
906 | |||
907 | unsigned int scale_num, scale_denom | ||
908 | Scale the image by the fraction scale_num/scale_denom. Default is | ||
909 | 1/1, or no scaling. Currently, the supported scaling ratios are | ||
910 | M/N with all N from 1 to 16, where M is the destination DCT size, | ||
911 | which is 8 by default (see block_size parameter above). | ||
912 | (The library design allows for arbitrary scaling ratios but this | ||
913 | is not likely to be implemented any time soon.) | ||
914 | |||
915 | J_COLOR_SPACE jpeg_color_space | ||
916 | int num_components | ||
917 | The JPEG color space and corresponding number of components; see | ||
918 | "Special color spaces", below, for more info. We recommend using | ||
919 | jpeg_set_color_space() if you want to change these. | ||
920 | |||
921 | boolean optimize_coding | ||
922 | TRUE causes the compressor to compute optimal Huffman coding tables | ||
923 | for the image. This requires an extra pass over the data and | ||
924 | therefore costs a good deal of space and time. The default is | ||
925 | FALSE, which tells the compressor to use the supplied or default | ||
926 | Huffman tables. In most cases optimal tables save only a few percent | ||
927 | of file size compared to the default tables. Note that when this is | ||
928 | TRUE, you need not supply Huffman tables at all, and any you do | ||
929 | supply will be overwritten. | ||
930 | |||
931 | unsigned int restart_interval | ||
932 | int restart_in_rows | ||
933 | To emit restart markers in the JPEG file, set one of these nonzero. | ||
934 | Set restart_interval to specify the exact interval in MCU blocks. | ||
935 | Set restart_in_rows to specify the interval in MCU rows. (If | ||
936 | restart_in_rows is not 0, then restart_interval is set after the | ||
937 | image width in MCUs is computed.) Defaults are zero (no restarts). | ||
938 | One restart marker per MCU row is often a good choice. | ||
939 | NOTE: the overhead of restart markers is higher in grayscale JPEG | ||
940 | files than in color files, and MUCH higher in progressive JPEGs. | ||
941 | If you use restarts, you may want to use larger intervals in those | ||
942 | cases. | ||
943 | |||
944 | const jpeg_scan_info * scan_info | ||
945 | int num_scans | ||
946 | By default, scan_info is NULL; this causes the compressor to write a | ||
947 | single-scan sequential JPEG file. If not NULL, scan_info points to | ||
948 | an array of scan definition records of length num_scans. The | ||
949 | compressor will then write a JPEG file having one scan for each scan | ||
950 | definition record. This is used to generate noninterleaved or | ||
951 | progressive JPEG files. The library checks that the scan array | ||
952 | defines a valid JPEG scan sequence. (jpeg_simple_progression creates | ||
953 | a suitable scan definition array for progressive JPEG.) This is | ||
954 | discussed further under "Progressive JPEG support". | ||
955 | |||
956 | boolean do_fancy_downsampling | ||
957 | If TRUE, use direct DCT scaling with DCT size > 8 for downsampling | ||
958 | of chroma components. | ||
959 | If FALSE, use only DCT size <= 8 and simple separate downsampling. | ||
960 | Default is TRUE. | ||
961 | For better image stability in multiple generation compression cycles | ||
962 | it is preferable that this value matches the corresponding | ||
963 | do_fancy_upsampling value in decompression. | ||
964 | |||
965 | int smoothing_factor | ||
966 | If non-zero, the input image is smoothed; the value should be 1 for | ||
967 | minimal smoothing to 100 for maximum smoothing. Consult jcsample.c | ||
968 | for details of the smoothing algorithm. The default is zero. | ||
969 | |||
970 | boolean write_JFIF_header | ||
971 | If TRUE, a JFIF APP0 marker is emitted. jpeg_set_defaults() and | ||
972 | jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space | ||
973 | (ie, YCbCr or grayscale) is selected, otherwise FALSE. | ||
974 | |||
975 | UINT8 JFIF_major_version | ||
976 | UINT8 JFIF_minor_version | ||
977 | The version number to be written into the JFIF marker. | ||
978 | jpeg_set_defaults() initializes the version to 1.01 (major=minor=1). | ||
979 | You should set it to 1.02 (major=1, minor=2) if you plan to write | ||
980 | any JFIF 1.02 extension markers. | ||
981 | |||
982 | UINT8 density_unit | ||
983 | UINT16 X_density | ||
984 | UINT16 Y_density | ||
985 | The resolution information to be written into the JFIF marker; | ||
986 | not used otherwise. density_unit may be 0 for unknown, | ||
987 | 1 for dots/inch, or 2 for dots/cm. The default values are 0,1,1 | ||
988 | indicating square pixels of unknown size. | ||
989 | |||
990 | boolean write_Adobe_marker | ||
991 | If TRUE, an Adobe APP14 marker is emitted. jpeg_set_defaults() and | ||
992 | jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK, | ||
993 | or YCCK is selected, otherwise FALSE. It is generally a bad idea | ||
994 | to set both write_JFIF_header and write_Adobe_marker. In fact, | ||
995 | you probably shouldn't change the default settings at all --- the | ||
996 | default behavior ensures that the JPEG file's color space can be | ||
997 | recognized by the decoder. | ||
998 | |||
999 | JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS] | ||
1000 | Pointers to coefficient quantization tables, one per table slot, | ||
1001 | or NULL if no table is defined for a slot. Usually these should | ||
1002 | be set via one of the above helper routines; jpeg_add_quant_table() | ||
1003 | is general enough to define any quantization table. The other | ||
1004 | routines will set up table slot 0 for luminance quality and table | ||
1005 | slot 1 for chrominance. | ||
1006 | |||
1007 | int q_scale_factor[NUM_QUANT_TBLS] | ||
1008 | Linear quantization scaling factors (percentage, initialized 100) | ||
1009 | for use with jpeg_default_qtables(). | ||
1010 | See rdswitch.c and cjpeg.c for an example of usage. | ||
1011 | Note that the q_scale_factor[] fields are the "linear" scales, so you | ||
1012 | have to convert from user-defined ratings via jpeg_quality_scaling(). | ||
1013 | Here is an example code which corresponds to cjpeg -quality 90,70: | ||
1014 | |||
1015 | jpeg_set_defaults(cinfo); | ||
1016 | |||
1017 | /* Set luminance quality 90. */ | ||
1018 | cinfo->q_scale_factor[0] = jpeg_quality_scaling(90); | ||
1019 | /* Set chrominance quality 70. */ | ||
1020 | cinfo->q_scale_factor[1] = jpeg_quality_scaling(70); | ||
1021 | |||
1022 | jpeg_default_qtables(cinfo, force_baseline); | ||
1023 | |||
1024 | CAUTION: You must also set 1x1 subsampling for efficient separate | ||
1025 | color quality selection, since the default value used by library | ||
1026 | is 2x2: | ||
1027 | |||
1028 | cinfo->comp_info[0].v_samp_factor = 1; | ||
1029 | cinfo->comp_info[0].h_samp_factor = 1; | ||
1030 | |||
1031 | JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS] | ||
1032 | JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS] | ||
1033 | Pointers to Huffman coding tables, one per table slot, or NULL if | ||
1034 | no table is defined for a slot. Slots 0 and 1 are filled with the | ||
1035 | JPEG sample tables by jpeg_set_defaults(). If you need to allocate | ||
1036 | more table structures, jpeg_alloc_huff_table() may be used. | ||
1037 | Note that optimal Huffman tables can be computed for an image | ||
1038 | by setting optimize_coding, as discussed above; there's seldom | ||
1039 | any need to mess with providing your own Huffman tables. | ||
1040 | |||
1041 | |||
1042 | The actual dimensions of the JPEG image that will be written to the file are | ||
1043 | given by the following fields. These are computed from the input image | ||
1044 | dimensions and the compression parameters by jpeg_start_compress(). You can | ||
1045 | also call jpeg_calc_jpeg_dimensions() to obtain the values that will result | ||
1046 | from the current parameter settings. This can be useful if you are trying | ||
1047 | to pick a scaling ratio that will get close to a desired target size. | ||
1048 | |||
1049 | JDIMENSION jpeg_width Actual dimensions of output image. | ||
1050 | JDIMENSION jpeg_height | ||
1051 | |||
1052 | |||
1053 | Per-component parameters are stored in the struct cinfo.comp_info[i] for | ||
1054 | component number i. Note that components here refer to components of the | ||
1055 | JPEG color space, *not* the source image color space. A suitably large | ||
1056 | comp_info[] array is allocated by jpeg_set_defaults(); if you choose not | ||
1057 | to use that routine, it's up to you to allocate the array. | ||
1058 | |||
1059 | int component_id | ||
1060 | The one-byte identifier code to be recorded in the JPEG file for | ||
1061 | this component. For the standard color spaces, we recommend you | ||
1062 | leave the default values alone. | ||
1063 | |||
1064 | int h_samp_factor | ||
1065 | int v_samp_factor | ||
1066 | Horizontal and vertical sampling factors for the component; must | ||
1067 | be 1..4 according to the JPEG standard. Note that larger sampling | ||
1068 | factors indicate a higher-resolution component; many people find | ||
1069 | this behavior quite unintuitive. The default values are 2,2 for | ||
1070 | luminance components and 1,1 for chrominance components, except | ||
1071 | for grayscale where 1,1 is used. | ||
1072 | |||
1073 | int quant_tbl_no | ||
1074 | Quantization table number for component. The default value is | ||
1075 | 0 for luminance components and 1 for chrominance components. | ||
1076 | |||
1077 | int dc_tbl_no | ||
1078 | int ac_tbl_no | ||
1079 | DC and AC entropy coding table numbers. The default values are | ||
1080 | 0 for luminance components and 1 for chrominance components. | ||
1081 | |||
1082 | int component_index | ||
1083 | Must equal the component's index in comp_info[]. (Beginning in | ||
1084 | release v6, the compressor library will fill this in automatically; | ||
1085 | you don't have to.) | ||
1086 | |||
1087 | |||
1088 | Decompression parameter selection | ||
1089 | --------------------------------- | ||
1090 | |||
1091 | Decompression parameter selection is somewhat simpler than compression | ||
1092 | parameter selection, since all of the JPEG internal parameters are | ||
1093 | recorded in the source file and need not be supplied by the application. | ||
1094 | (Unless you are working with abbreviated files, in which case see | ||
1095 | "Abbreviated datastreams", below.) Decompression parameters control | ||
1096 | the postprocessing done on the image to deliver it in a format suitable | ||
1097 | for the application's use. Many of the parameters control speed/quality | ||
1098 | tradeoffs, in which faster decompression may be obtained at the price of | ||
1099 | a poorer-quality image. The defaults select the highest quality (slowest) | ||
1100 | processing. | ||
1101 | |||
1102 | The following fields in the JPEG object are set by jpeg_read_header() and | ||
1103 | may be useful to the application in choosing decompression parameters: | ||
1104 | |||
1105 | JDIMENSION image_width Width and height of image | ||
1106 | JDIMENSION image_height | ||
1107 | int num_components Number of color components | ||
1108 | J_COLOR_SPACE jpeg_color_space Colorspace of image | ||
1109 | boolean saw_JFIF_marker TRUE if a JFIF APP0 marker was seen | ||
1110 | UINT8 JFIF_major_version Version information from JFIF marker | ||
1111 | UINT8 JFIF_minor_version | ||
1112 | UINT8 density_unit Resolution data from JFIF marker | ||
1113 | UINT16 X_density | ||
1114 | UINT16 Y_density | ||
1115 | boolean saw_Adobe_marker TRUE if an Adobe APP14 marker was seen | ||
1116 | UINT8 Adobe_transform Color transform code from Adobe marker | ||
1117 | |||
1118 | The JPEG color space, unfortunately, is something of a guess since the JPEG | ||
1119 | standard proper does not provide a way to record it. In practice most files | ||
1120 | adhere to the JFIF or Adobe conventions, and the decoder will recognize these | ||
1121 | correctly. See "Special color spaces", below, for more info. | ||
1122 | |||
1123 | |||
1124 | The decompression parameters that determine the basic properties of the | ||
1125 | returned image are: | ||
1126 | |||
1127 | J_COLOR_SPACE out_color_space | ||
1128 | Output color space. jpeg_read_header() sets an appropriate default | ||
1129 | based on jpeg_color_space; typically it will be RGB or grayscale. | ||
1130 | The application can change this field to request output in a different | ||
1131 | colorspace. For example, set it to JCS_GRAYSCALE to get grayscale | ||
1132 | output from a color file. (This is useful for previewing: grayscale | ||
1133 | output is faster than full color since the color components need not | ||
1134 | be processed.) Note that not all possible color space transforms are | ||
1135 | currently implemented; you may need to extend jdcolor.c if you want an | ||
1136 | unusual conversion. | ||
1137 | |||
1138 | unsigned int scale_num, scale_denom | ||
1139 | Scale the image by the fraction scale_num/scale_denom. Currently, | ||
1140 | the supported scaling ratios are M/N with all M from 1 to 16, where | ||
1141 | N is the source DCT size, which is 8 for baseline JPEG. (The library | ||
1142 | design allows for arbitrary scaling ratios but this is not likely | ||
1143 | to be implemented any time soon.) The values are initialized by | ||
1144 | jpeg_read_header() with the source DCT size. For baseline JPEG | ||
1145 | this is 8/8. If you change only the scale_num value while leaving | ||
1146 | the other unchanged, then this specifies the DCT scaled size to be | ||
1147 | applied on the given input. For baseline JPEG this is equivalent | ||
1148 | to M/8 scaling, since the source DCT size for baseline JPEG is 8. | ||
1149 | Smaller scaling ratios permit significantly faster decoding since | ||
1150 | fewer pixels need be processed and a simpler IDCT method can be used. | ||
1151 | |||
1152 | boolean quantize_colors | ||
1153 | If set TRUE, colormapped output will be delivered. Default is FALSE, | ||
1154 | meaning that full-color output will be delivered. | ||
1155 | |||
1156 | The next three parameters are relevant only if quantize_colors is TRUE. | ||
1157 | |||
1158 | int desired_number_of_colors | ||
1159 | Maximum number of colors to use in generating a library-supplied color | ||
1160 | map (the actual number of colors is returned in a different field). | ||
1161 | Default 256. Ignored when the application supplies its own color map. | ||
1162 | |||
1163 | boolean two_pass_quantize | ||
1164 | If TRUE, an extra pass over the image is made to select a custom color | ||
1165 | map for the image. This usually looks a lot better than the one-size- | ||
1166 | fits-all colormap that is used otherwise. Default is TRUE. Ignored | ||
1167 | when the application supplies its own color map. | ||
1168 | |||
1169 | J_DITHER_MODE dither_mode | ||
1170 | Selects color dithering method. Supported values are: | ||
1171 | JDITHER_NONE no dithering: fast, very low quality | ||
1172 | JDITHER_ORDERED ordered dither: moderate speed and quality | ||
1173 | JDITHER_FS Floyd-Steinberg dither: slow, high quality | ||
1174 | Default is JDITHER_FS. (At present, ordered dither is implemented | ||
1175 | only in the single-pass, standard-colormap case. If you ask for | ||
1176 | ordered dither when two_pass_quantize is TRUE or when you supply | ||
1177 | an external color map, you'll get F-S dithering.) | ||
1178 | |||
1179 | When quantize_colors is TRUE, the target color map is described by the next | ||
1180 | two fields. colormap is set to NULL by jpeg_read_header(). The application | ||
1181 | can supply a color map by setting colormap non-NULL and setting | ||
1182 | actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress() | ||
1183 | selects a suitable color map and sets these two fields itself. | ||
1184 | [Implementation restriction: at present, an externally supplied colormap is | ||
1185 | only accepted for 3-component output color spaces.] | ||
1186 | |||
1187 | JSAMPARRAY colormap | ||
1188 | The color map, represented as a 2-D pixel array of out_color_components | ||
1189 | rows and actual_number_of_colors columns. Ignored if not quantizing. | ||
1190 | CAUTION: if the JPEG library creates its own colormap, the storage | ||
1191 | pointed to by this field is released by jpeg_finish_decompress(). | ||
1192 | Copy the colormap somewhere else first, if you want to save it. | ||
1193 | |||
1194 | int actual_number_of_colors | ||
1195 | The number of colors in the color map. | ||
1196 | |||
1197 | Additional decompression parameters that the application may set include: | ||
1198 | |||
1199 | J_DCT_METHOD dct_method | ||
1200 | Selects the algorithm used for the DCT step. Choices are the same | ||
1201 | as described above for compression. | ||
1202 | |||
1203 | boolean do_fancy_upsampling | ||
1204 | If TRUE, use direct DCT scaling with DCT size > 8 for upsampling | ||
1205 | of chroma components. | ||
1206 | If FALSE, use only DCT size <= 8 and simple separate upsampling. | ||
1207 | Default is TRUE. | ||
1208 | For better image stability in multiple generation compression cycles | ||
1209 | it is preferable that this value matches the corresponding | ||
1210 | do_fancy_downsampling value in compression. | ||
1211 | |||
1212 | boolean do_block_smoothing | ||
1213 | If TRUE, interblock smoothing is applied in early stages of decoding | ||
1214 | progressive JPEG files; if FALSE, not. Default is TRUE. Early | ||
1215 | progression stages look "fuzzy" with smoothing, "blocky" without. | ||
1216 | In any case, block smoothing ceases to be applied after the first few | ||
1217 | AC coefficients are known to full accuracy, so it is relevant only | ||
1218 | when using buffered-image mode for progressive images. | ||
1219 | |||
1220 | boolean enable_1pass_quant | ||
1221 | boolean enable_external_quant | ||
1222 | boolean enable_2pass_quant | ||
1223 | These are significant only in buffered-image mode, which is | ||
1224 | described in its own section below. | ||
1225 | |||
1226 | |||
1227 | The output image dimensions are given by the following fields. These are | ||
1228 | computed from the source image dimensions and the decompression parameters | ||
1229 | by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions() | ||
1230 | to obtain the values that will result from the current parameter settings. | ||
1231 | This can be useful if you are trying to pick a scaling ratio that will get | ||
1232 | close to a desired target size. It's also important if you are using the | ||
1233 | JPEG library's memory manager to allocate output buffer space, because you | ||
1234 | are supposed to request such buffers *before* jpeg_start_decompress(). | ||
1235 | |||
1236 | JDIMENSION output_width Actual dimensions of output image. | ||
1237 | JDIMENSION output_height | ||
1238 | int out_color_components Number of color components in out_color_space. | ||
1239 | int output_components Number of color components returned. | ||
1240 | int rec_outbuf_height Recommended height of scanline buffer. | ||
1241 | |||
1242 | When quantizing colors, output_components is 1, indicating a single color map | ||
1243 | index per pixel. Otherwise it equals out_color_components. The output arrays | ||
1244 | are required to be output_width * output_components JSAMPLEs wide. | ||
1245 | |||
1246 | rec_outbuf_height is the recommended minimum height (in scanlines) of the | ||
1247 | buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the | ||
1248 | library will still work, but time will be wasted due to unnecessary data | ||
1249 | copying. In high-quality modes, rec_outbuf_height is always 1, but some | ||
1250 | faster, lower-quality modes set it to larger values (typically 2 to 4). | ||
1251 | If you are going to ask for a high-speed processing mode, you may as well | ||
1252 | go to the trouble of honoring rec_outbuf_height so as to avoid data copying. | ||
1253 | (An output buffer larger than rec_outbuf_height lines is OK, but won't | ||
1254 | provide any material speed improvement over that height.) | ||
1255 | |||
1256 | |||
1257 | Special color spaces | ||
1258 | -------------------- | ||
1259 | |||
1260 | The JPEG standard itself is "color blind" and doesn't specify any particular | ||
1261 | color space. It is customary to convert color data to a luminance/chrominance | ||
1262 | color space before compressing, since this permits greater compression. The | ||
1263 | existing de-facto JPEG file format standards specify YCbCr or grayscale data | ||
1264 | (JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special | ||
1265 | applications such as multispectral images, other color spaces can be used, | ||
1266 | but it must be understood that such files will be unportable. | ||
1267 | |||
1268 | The JPEG library can handle the most common colorspace conversions (namely | ||
1269 | RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown | ||
1270 | color space, passing it through without conversion. If you deal extensively | ||
1271 | with an unusual color space, you can easily extend the library to understand | ||
1272 | additional color spaces and perform appropriate conversions. | ||
1273 | |||
1274 | For compression, the source data's color space is specified by field | ||
1275 | in_color_space. This is transformed to the JPEG file's color space given | ||
1276 | by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color | ||
1277 | space depending on in_color_space, but you can override this by calling | ||
1278 | jpeg_set_colorspace(). Of course you must select a supported transformation. | ||
1279 | jccolor.c currently supports the following transformations: | ||
1280 | RGB => YCbCr | ||
1281 | RGB => GRAYSCALE | ||
1282 | YCbCr => GRAYSCALE | ||
1283 | CMYK => YCCK | ||
1284 | plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB, | ||
1285 | YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN. | ||
1286 | |||
1287 | The de-facto file format standards (JFIF and Adobe) specify APPn markers that | ||
1288 | indicate the color space of the JPEG file. It is important to ensure that | ||
1289 | these are written correctly, or omitted if the JPEG file's color space is not | ||
1290 | one of the ones supported by the de-facto standards. jpeg_set_colorspace() | ||
1291 | will set the compression parameters to include or omit the APPn markers | ||
1292 | properly, so long as it is told the truth about the JPEG color space. | ||
1293 | For example, if you are writing some random 3-component color space without | ||
1294 | conversion, don't try to fake out the library by setting in_color_space and | ||
1295 | jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an | ||
1296 | APPn marker of your own devising to identify the colorspace --- see "Special | ||
1297 | markers", below. | ||
1298 | |||
1299 | When told that the color space is UNKNOWN, the library will default to using | ||
1300 | luminance-quality compression parameters for all color components. You may | ||
1301 | well want to change these parameters. See the source code for | ||
1302 | jpeg_set_colorspace(), in jcparam.c, for details. | ||
1303 | |||
1304 | For decompression, the JPEG file's color space is given in jpeg_color_space, | ||
1305 | and this is transformed to the output color space out_color_space. | ||
1306 | jpeg_read_header's setting of jpeg_color_space can be relied on if the file | ||
1307 | conforms to JFIF or Adobe conventions, but otherwise it is no better than a | ||
1308 | guess. If you know the JPEG file's color space for certain, you can override | ||
1309 | jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also | ||
1310 | selects a default output color space based on (its guess of) jpeg_color_space; | ||
1311 | set out_color_space to override this. Again, you must select a supported | ||
1312 | transformation. jdcolor.c currently supports | ||
1313 | YCbCr => RGB | ||
1314 | YCbCr => GRAYSCALE | ||
1315 | RGB => GRAYSCALE | ||
1316 | GRAYSCALE => RGB | ||
1317 | YCCK => CMYK | ||
1318 | as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an | ||
1319 | application can force grayscale JPEGs to look like color JPEGs if it only | ||
1320 | wants to handle one case.) | ||
1321 | |||
1322 | The two-pass color quantizer, jquant2.c, is specialized to handle RGB data | ||
1323 | (it weights distances appropriately for RGB colors). You'll need to modify | ||
1324 | the code if you want to use it for non-RGB output color spaces. Note that | ||
1325 | jquant2.c is used to map to an application-supplied colormap as well as for | ||
1326 | the normal two-pass colormap selection process. | ||
1327 | |||
1328 | CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG | ||
1329 | files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect. | ||
1330 | This is arguably a bug in Photoshop, but if you need to work with Photoshop | ||
1331 | CMYK files, you will have to deal with it in your application. We cannot | ||
1332 | "fix" this in the library by inverting the data during the CMYK<=>YCCK | ||
1333 | transform, because that would break other applications, notably Ghostscript. | ||
1334 | Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK | ||
1335 | data in the same inverted-YCCK representation used in bare JPEG files, but | ||
1336 | the surrounding PostScript code performs an inversion using the PS image | ||
1337 | operator. I am told that Photoshop 3.0 will write uninverted YCCK in | ||
1338 | EPS/JPEG files, and will omit the PS-level inversion. (But the data | ||
1339 | polarity used in bare JPEG files will not change in 3.0.) In either case, | ||
1340 | the JPEG library must not invert the data itself, or else Ghostscript would | ||
1341 | read these EPS files incorrectly. | ||
1342 | |||
1343 | |||
1344 | Error handling | ||
1345 | -------------- | ||
1346 | |||
1347 | When the default error handler is used, any error detected inside the JPEG | ||
1348 | routines will cause a message to be printed on stderr, followed by exit(). | ||
1349 | You can supply your own error handling routines to override this behavior | ||
1350 | and to control the treatment of nonfatal warnings and trace/debug messages. | ||
1351 | The file example.c illustrates the most common case, which is to have the | ||
1352 | application regain control after an error rather than exiting. | ||
1353 | |||
1354 | The JPEG library never writes any message directly; it always goes through | ||
1355 | the error handling routines. Three classes of messages are recognized: | ||
1356 | * Fatal errors: the library cannot continue. | ||
1357 | * Warnings: the library can continue, but the data is corrupt, and a | ||
1358 | damaged output image is likely to result. | ||
1359 | * Trace/informational messages. These come with a trace level indicating | ||
1360 | the importance of the message; you can control the verbosity of the | ||
1361 | program by adjusting the maximum trace level that will be displayed. | ||
1362 | |||
1363 | You may, if you wish, simply replace the entire JPEG error handling module | ||
1364 | (jerror.c) with your own code. However, you can avoid code duplication by | ||
1365 | only replacing some of the routines depending on the behavior you need. | ||
1366 | This is accomplished by calling jpeg_std_error() as usual, but then overriding | ||
1367 | some of the method pointers in the jpeg_error_mgr struct, as illustrated by | ||
1368 | example.c. | ||
1369 | |||
1370 | All of the error handling routines will receive a pointer to the JPEG object | ||
1371 | (a j_common_ptr which points to either a jpeg_compress_struct or a | ||
1372 | jpeg_decompress_struct; if you need to tell which, test the is_decompressor | ||
1373 | field). This struct includes a pointer to the error manager struct in its | ||
1374 | "err" field. Frequently, custom error handler routines will need to access | ||
1375 | additional data which is not known to the JPEG library or the standard error | ||
1376 | handler. The most convenient way to do this is to embed either the JPEG | ||
1377 | object or the jpeg_error_mgr struct in a larger structure that contains | ||
1378 | additional fields; then casting the passed pointer provides access to the | ||
1379 | additional fields. Again, see example.c for one way to do it. (Beginning | ||
1380 | with IJG version 6b, there is also a void pointer "client_data" in each | ||
1381 | JPEG object, which the application can also use to find related data. | ||
1382 | The library does not touch client_data at all.) | ||
1383 | |||
1384 | The individual methods that you might wish to override are: | ||
1385 | |||
1386 | error_exit (j_common_ptr cinfo) | ||
1387 | Receives control for a fatal error. Information sufficient to | ||
1388 | generate the error message has been stored in cinfo->err; call | ||
1389 | output_message to display it. Control must NOT return to the caller; | ||
1390 | generally this routine will exit() or longjmp() somewhere. | ||
1391 | Typically you would override this routine to get rid of the exit() | ||
1392 | default behavior. Note that if you continue processing, you should | ||
1393 | clean up the JPEG object with jpeg_abort() or jpeg_destroy(). | ||
1394 | |||
1395 | output_message (j_common_ptr cinfo) | ||
1396 | Actual output of any JPEG message. Override this to send messages | ||
1397 | somewhere other than stderr. Note that this method does not know | ||
1398 | how to generate a message, only where to send it. | ||
1399 | |||
1400 | format_message (j_common_ptr cinfo, char * buffer) | ||
1401 | Constructs a readable error message string based on the error info | ||
1402 | stored in cinfo->err. This method is called by output_message. Few | ||
1403 | applications should need to override this method. One possible | ||
1404 | reason for doing so is to implement dynamic switching of error message | ||
1405 | language. | ||
1406 | |||
1407 | emit_message (j_common_ptr cinfo, int msg_level) | ||
1408 | Decide whether or not to emit a warning or trace message; if so, | ||
1409 | calls output_message. The main reason for overriding this method | ||
1410 | would be to abort on warnings. msg_level is -1 for warnings, | ||
1411 | 0 and up for trace messages. | ||
1412 | |||
1413 | Only error_exit() and emit_message() are called from the rest of the JPEG | ||
1414 | library; the other two are internal to the error handler. | ||
1415 | |||
1416 | The actual message texts are stored in an array of strings which is pointed to | ||
1417 | by the field err->jpeg_message_table. The messages are numbered from 0 to | ||
1418 | err->last_jpeg_message, and it is these code numbers that are used in the | ||
1419 | JPEG library code. You could replace the message texts (for instance, with | ||
1420 | messages in French or German) by changing the message table pointer. See | ||
1421 | jerror.h for the default texts. CAUTION: this table will almost certainly | ||
1422 | change or grow from one library version to the next. | ||
1423 | |||
1424 | It may be useful for an application to add its own message texts that are | ||
1425 | handled by the same mechanism. The error handler supports a second "add-on" | ||
1426 | message table for this purpose. To define an addon table, set the pointer | ||
1427 | err->addon_message_table and the message numbers err->first_addon_message and | ||
1428 | err->last_addon_message. If you number the addon messages beginning at 1000 | ||
1429 | or so, you won't have to worry about conflicts with the library's built-in | ||
1430 | messages. See the sample applications cjpeg/djpeg for an example of using | ||
1431 | addon messages (the addon messages are defined in cderror.h). | ||
1432 | |||
1433 | Actual invocation of the error handler is done via macros defined in jerror.h: | ||
1434 | ERREXITn(...) for fatal errors | ||
1435 | WARNMSn(...) for corrupt-data warnings | ||
1436 | TRACEMSn(...) for trace and informational messages. | ||
1437 | These macros store the message code and any additional parameters into the | ||
1438 | error handler struct, then invoke the error_exit() or emit_message() method. | ||
1439 | The variants of each macro are for varying numbers of additional parameters. | ||
1440 | The additional parameters are inserted into the generated message using | ||
1441 | standard printf() format codes. | ||
1442 | |||
1443 | See jerror.h and jerror.c for further details. | ||
1444 | |||
1445 | |||
1446 | Compressed data handling (source and destination managers) | ||
1447 | ---------------------------------------------------------- | ||
1448 | |||
1449 | The JPEG compression library sends its compressed data to a "destination | ||
1450 | manager" module. The default destination manager just writes the data to a | ||
1451 | memory buffer or to a stdio stream, but you can provide your own manager to | ||
1452 | do something else. Similarly, the decompression library calls a "source | ||
1453 | manager" to obtain the compressed data; you can provide your own source | ||
1454 | manager if you want the data to come from somewhere other than a memory | ||
1455 | buffer or a stdio stream. | ||
1456 | |||
1457 | In both cases, compressed data is processed a bufferload at a time: the | ||
1458 | destination or source manager provides a work buffer, and the library invokes | ||
1459 | the manager only when the buffer is filled or emptied. (You could define a | ||
1460 | one-character buffer to force the manager to be invoked for each byte, but | ||
1461 | that would be rather inefficient.) The buffer's size and location are | ||
1462 | controlled by the manager, not by the library. For example, the memory | ||
1463 | source manager just makes the buffer pointer and length point to the original | ||
1464 | data in memory. In this case the buffer-reload procedure will be invoked | ||
1465 | only if the decompressor ran off the end of the datastream, which would | ||
1466 | indicate an erroneous datastream. | ||
1467 | |||
1468 | The work buffer is defined as an array of datatype JOCTET, which is generally | ||
1469 | "char" or "unsigned char". On a machine where char is not exactly 8 bits | ||
1470 | wide, you must define JOCTET as a wider data type and then modify the data | ||
1471 | source and destination modules to transcribe the work arrays into 8-bit units | ||
1472 | on external storage. | ||
1473 | |||
1474 | A data destination manager struct contains a pointer and count defining the | ||
1475 | next byte to write in the work buffer and the remaining free space: | ||
1476 | |||
1477 | JOCTET * next_output_byte; /* => next byte to write in buffer */ | ||
1478 | size_t free_in_buffer; /* # of byte spaces remaining in buffer */ | ||
1479 | |||
1480 | The library increments the pointer and decrements the count until the buffer | ||
1481 | is filled. The manager's empty_output_buffer method must reset the pointer | ||
1482 | and count. The manager is expected to remember the buffer's starting address | ||
1483 | and total size in private fields not visible to the library. | ||
1484 | |||
1485 | A data destination manager provides three methods: | ||
1486 | |||
1487 | init_destination (j_compress_ptr cinfo) | ||
1488 | Initialize destination. This is called by jpeg_start_compress() | ||
1489 | before any data is actually written. It must initialize | ||
1490 | next_output_byte and free_in_buffer. free_in_buffer must be | ||
1491 | initialized to a positive value. | ||
1492 | |||
1493 | empty_output_buffer (j_compress_ptr cinfo) | ||
1494 | This is called whenever the buffer has filled (free_in_buffer | ||
1495 | reaches zero). In typical applications, it should write out the | ||
1496 | *entire* buffer (use the saved start address and buffer length; | ||
1497 | ignore the current state of next_output_byte and free_in_buffer). | ||
1498 | Then reset the pointer & count to the start of the buffer, and | ||
1499 | return TRUE indicating that the buffer has been dumped. | ||
1500 | free_in_buffer must be set to a positive value when TRUE is | ||
1501 | returned. A FALSE return should only be used when I/O suspension is | ||
1502 | desired (this operating mode is discussed in the next section). | ||
1503 | |||
1504 | term_destination (j_compress_ptr cinfo) | ||
1505 | Terminate destination --- called by jpeg_finish_compress() after all | ||
1506 | data has been written. In most applications, this must flush any | ||
1507 | data remaining in the buffer. Use either next_output_byte or | ||
1508 | free_in_buffer to determine how much data is in the buffer. | ||
1509 | |||
1510 | term_destination() is NOT called by jpeg_abort() or jpeg_destroy(). If you | ||
1511 | want the destination manager to be cleaned up during an abort, you must do it | ||
1512 | yourself. | ||
1513 | |||
1514 | You will also need code to create a jpeg_destination_mgr struct, fill in its | ||
1515 | method pointers, and insert a pointer to the struct into the "dest" field of | ||
1516 | the JPEG compression object. This can be done in-line in your setup code if | ||
1517 | you like, but it's probably cleaner to provide a separate routine similar to | ||
1518 | the jpeg_stdio_dest() or jpeg_mem_dest() routines of the supplied destination | ||
1519 | managers. | ||
1520 | |||
1521 | Decompression source managers follow a parallel design, but with some | ||
1522 | additional frammishes. The source manager struct contains a pointer and count | ||
1523 | defining the next byte to read from the work buffer and the number of bytes | ||
1524 | remaining: | ||
1525 | |||
1526 | const JOCTET * next_input_byte; /* => next byte to read from buffer */ | ||
1527 | size_t bytes_in_buffer; /* # of bytes remaining in buffer */ | ||
1528 | |||
1529 | The library increments the pointer and decrements the count until the buffer | ||
1530 | is emptied. The manager's fill_input_buffer method must reset the pointer and | ||
1531 | count. In most applications, the manager must remember the buffer's starting | ||
1532 | address and total size in private fields not visible to the library. | ||
1533 | |||
1534 | A data source manager provides five methods: | ||
1535 | |||
1536 | init_source (j_decompress_ptr cinfo) | ||
1537 | Initialize source. This is called by jpeg_read_header() before any | ||
1538 | data is actually read. Unlike init_destination(), it may leave | ||
1539 | bytes_in_buffer set to 0 (in which case a fill_input_buffer() call | ||
1540 | will occur immediately). | ||
1541 | |||
1542 | fill_input_buffer (j_decompress_ptr cinfo) | ||
1543 | This is called whenever bytes_in_buffer has reached zero and more | ||
1544 | data is wanted. In typical applications, it should read fresh data | ||
1545 | into the buffer (ignoring the current state of next_input_byte and | ||
1546 | bytes_in_buffer), reset the pointer & count to the start of the | ||
1547 | buffer, and return TRUE indicating that the buffer has been reloaded. | ||
1548 | It is not necessary to fill the buffer entirely, only to obtain at | ||
1549 | least one more byte. bytes_in_buffer MUST be set to a positive value | ||
1550 | if TRUE is returned. A FALSE return should only be used when I/O | ||
1551 | suspension is desired (this mode is discussed in the next section). | ||
1552 | |||
1553 | skip_input_data (j_decompress_ptr cinfo, long num_bytes) | ||
1554 | Skip num_bytes worth of data. The buffer pointer and count should | ||
1555 | be advanced over num_bytes input bytes, refilling the buffer as | ||
1556 | needed. This is used to skip over a potentially large amount of | ||
1557 | uninteresting data (such as an APPn marker). In some applications | ||
1558 | it may be possible to optimize away the reading of the skipped data, | ||
1559 | but it's not clear that being smart is worth much trouble; large | ||
1560 | skips are uncommon. bytes_in_buffer may be zero on return. | ||
1561 | A zero or negative skip count should be treated as a no-op. | ||
1562 | |||
1563 | resync_to_restart (j_decompress_ptr cinfo, int desired) | ||
1564 | This routine is called only when the decompressor has failed to find | ||
1565 | a restart (RSTn) marker where one is expected. Its mission is to | ||
1566 | find a suitable point for resuming decompression. For most | ||
1567 | applications, we recommend that you just use the default resync | ||
1568 | procedure, jpeg_resync_to_restart(). However, if you are able to back | ||
1569 | up in the input data stream, or if you have a-priori knowledge about | ||
1570 | the likely location of restart markers, you may be able to do better. | ||
1571 | Read the read_restart_marker() and jpeg_resync_to_restart() routines | ||
1572 | in jdmarker.c if you think you'd like to implement your own resync | ||
1573 | procedure. | ||
1574 | |||
1575 | term_source (j_decompress_ptr cinfo) | ||
1576 | Terminate source --- called by jpeg_finish_decompress() after all | ||
1577 | data has been read. Often a no-op. | ||
1578 | |||
1579 | For both fill_input_buffer() and skip_input_data(), there is no such thing | ||
1580 | as an EOF return. If the end of the file has been reached, the routine has | ||
1581 | a choice of exiting via ERREXIT() or inserting fake data into the buffer. | ||
1582 | In most cases, generating a warning message and inserting a fake EOI marker | ||
1583 | is the best course of action --- this will allow the decompressor to output | ||
1584 | however much of the image is there. In pathological cases, the decompressor | ||
1585 | may swallow the EOI and again demand data ... just keep feeding it fake EOIs. | ||
1586 | jdatasrc.c illustrates the recommended error recovery behavior. | ||
1587 | |||
1588 | term_source() is NOT called by jpeg_abort() or jpeg_destroy(). If you want | ||
1589 | the source manager to be cleaned up during an abort, you must do it yourself. | ||
1590 | |||
1591 | You will also need code to create a jpeg_source_mgr struct, fill in its method | ||
1592 | pointers, and insert a pointer to the struct into the "src" field of the JPEG | ||
1593 | decompression object. This can be done in-line in your setup code if you | ||
1594 | like, but it's probably cleaner to provide a separate routine similar to the | ||
1595 | jpeg_stdio_src() or jpeg_mem_src() routines of the supplied source managers. | ||
1596 | |||
1597 | For more information, consult the memory and stdio source and destination | ||
1598 | managers in jdatasrc.c and jdatadst.c. | ||
1599 | |||
1600 | |||
1601 | I/O suspension | ||
1602 | -------------- | ||
1603 | |||
1604 | Some applications need to use the JPEG library as an incremental memory-to- | ||
1605 | memory filter: when the compressed data buffer is filled or emptied, they want | ||
1606 | control to return to the outer loop, rather than expecting that the buffer can | ||
1607 | be emptied or reloaded within the data source/destination manager subroutine. | ||
1608 | The library supports this need by providing an "I/O suspension" mode, which we | ||
1609 | describe in this section. | ||
1610 | |||
1611 | The I/O suspension mode is not a panacea: nothing is guaranteed about the | ||
1612 | maximum amount of time spent in any one call to the library, so it will not | ||
1613 | eliminate response-time problems in single-threaded applications. If you | ||
1614 | need guaranteed response time, we suggest you "bite the bullet" and implement | ||
1615 | a real multi-tasking capability. | ||
1616 | |||
1617 | To use I/O suspension, cooperation is needed between the calling application | ||
1618 | and the data source or destination manager; you will always need a custom | ||
1619 | source/destination manager. (Please read the previous section if you haven't | ||
1620 | already.) The basic idea is that the empty_output_buffer() or | ||
1621 | fill_input_buffer() routine is a no-op, merely returning FALSE to indicate | ||
1622 | that it has done nothing. Upon seeing this, the JPEG library suspends | ||
1623 | operation and returns to its caller. The surrounding application is | ||
1624 | responsible for emptying or refilling the work buffer before calling the | ||
1625 | JPEG library again. | ||
1626 | |||
1627 | Compression suspension: | ||
1628 | |||
1629 | For compression suspension, use an empty_output_buffer() routine that returns | ||
1630 | FALSE; typically it will not do anything else. This will cause the | ||
1631 | compressor to return to the caller of jpeg_write_scanlines(), with the return | ||
1632 | value indicating that not all the supplied scanlines have been accepted. | ||
1633 | The application must make more room in the output buffer, adjust the output | ||
1634 | buffer pointer/count appropriately, and then call jpeg_write_scanlines() | ||
1635 | again, pointing to the first unconsumed scanline. | ||
1636 | |||
1637 | When forced to suspend, the compressor will backtrack to a convenient stopping | ||
1638 | point (usually the start of the current MCU); it will regenerate some output | ||
1639 | data when restarted. Therefore, although empty_output_buffer() is only | ||
1640 | called when the buffer is filled, you should NOT write out the entire buffer | ||
1641 | after a suspension. Write only the data up to the current position of | ||
1642 | next_output_byte/free_in_buffer. The data beyond that point will be | ||
1643 | regenerated after resumption. | ||
1644 | |||
1645 | Because of the backtracking behavior, a good-size output buffer is essential | ||
1646 | for efficiency; you don't want the compressor to suspend often. (In fact, an | ||
1647 | overly small buffer could lead to infinite looping, if a single MCU required | ||
1648 | more data than would fit in the buffer.) We recommend a buffer of at least | ||
1649 | several Kbytes. You may want to insert explicit code to ensure that you don't | ||
1650 | call jpeg_write_scanlines() unless there is a reasonable amount of space in | ||
1651 | the output buffer; in other words, flush the buffer before trying to compress | ||
1652 | more data. | ||
1653 | |||
1654 | The compressor does not allow suspension while it is trying to write JPEG | ||
1655 | markers at the beginning and end of the file. This means that: | ||
1656 | * At the beginning of a compression operation, there must be enough free | ||
1657 | space in the output buffer to hold the header markers (typically 600 or | ||
1658 | so bytes). The recommended buffer size is bigger than this anyway, so | ||
1659 | this is not a problem as long as you start with an empty buffer. However, | ||
1660 | this restriction might catch you if you insert large special markers, such | ||
1661 | as a JFIF thumbnail image, without flushing the buffer afterwards. | ||
1662 | * When you call jpeg_finish_compress(), there must be enough space in the | ||
1663 | output buffer to emit any buffered data and the final EOI marker. In the | ||
1664 | current implementation, half a dozen bytes should suffice for this, but | ||
1665 | for safety's sake we recommend ensuring that at least 100 bytes are free | ||
1666 | before calling jpeg_finish_compress(). | ||
1667 | |||
1668 | A more significant restriction is that jpeg_finish_compress() cannot suspend. | ||
1669 | This means you cannot use suspension with multi-pass operating modes, namely | ||
1670 | Huffman code optimization and multiple-scan output. Those modes write the | ||
1671 | whole file during jpeg_finish_compress(), which will certainly result in | ||
1672 | buffer overrun. (Note that this restriction applies only to compression, | ||
1673 | not decompression. The decompressor supports input suspension in all of its | ||
1674 | operating modes.) | ||
1675 | |||
1676 | Decompression suspension: | ||
1677 | |||
1678 | For decompression suspension, use a fill_input_buffer() routine that simply | ||
1679 | returns FALSE (except perhaps during error recovery, as discussed below). | ||
1680 | This will cause the decompressor to return to its caller with an indication | ||
1681 | that suspension has occurred. This can happen at four places: | ||
1682 | * jpeg_read_header(): will return JPEG_SUSPENDED. | ||
1683 | * jpeg_start_decompress(): will return FALSE, rather than its usual TRUE. | ||
1684 | * jpeg_read_scanlines(): will return the number of scanlines already | ||
1685 | completed (possibly 0). | ||
1686 | * jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE. | ||
1687 | The surrounding application must recognize these cases, load more data into | ||
1688 | the input buffer, and repeat the call. In the case of jpeg_read_scanlines(), | ||
1689 | increment the passed pointers past any scanlines successfully read. | ||
1690 | |||
1691 | Just as with compression, the decompressor will typically backtrack to a | ||
1692 | convenient restart point before suspending. When fill_input_buffer() is | ||
1693 | called, next_input_byte/bytes_in_buffer point to the current restart point, | ||
1694 | which is where the decompressor will backtrack to if FALSE is returned. | ||
1695 | The data beyond that position must NOT be discarded if you suspend; it needs | ||
1696 | to be re-read upon resumption. In most implementations, you'll need to shift | ||
1697 | this data down to the start of your work buffer and then load more data after | ||
1698 | it. Again, this behavior means that a several-Kbyte work buffer is essential | ||
1699 | for decent performance; furthermore, you should load a reasonable amount of | ||
1700 | new data before resuming decompression. (If you loaded, say, only one new | ||
1701 | byte each time around, you could waste a LOT of cycles.) | ||
1702 | |||
1703 | The skip_input_data() source manager routine requires special care in a | ||
1704 | suspension scenario. This routine is NOT granted the ability to suspend the | ||
1705 | decompressor; it can decrement bytes_in_buffer to zero, but no more. If the | ||
1706 | requested skip distance exceeds the amount of data currently in the input | ||
1707 | buffer, then skip_input_data() must set bytes_in_buffer to zero and record the | ||
1708 | additional skip distance somewhere else. The decompressor will immediately | ||
1709 | call fill_input_buffer(), which should return FALSE, which will cause a | ||
1710 | suspension return. The surrounding application must then arrange to discard | ||
1711 | the recorded number of bytes before it resumes loading the input buffer. | ||
1712 | (Yes, this design is rather baroque, but it avoids complexity in the far more | ||
1713 | common case where a non-suspending source manager is used.) | ||
1714 | |||
1715 | If the input data has been exhausted, we recommend that you emit a warning | ||
1716 | and insert dummy EOI markers just as a non-suspending data source manager | ||
1717 | would do. This can be handled either in the surrounding application logic or | ||
1718 | within fill_input_buffer(); the latter is probably more efficient. If | ||
1719 | fill_input_buffer() knows that no more data is available, it can set the | ||
1720 | pointer/count to point to a dummy EOI marker and then return TRUE just as | ||
1721 | though it had read more data in a non-suspending situation. | ||
1722 | |||
1723 | The decompressor does not attempt to suspend within standard JPEG markers; | ||
1724 | instead it will backtrack to the start of the marker and reprocess the whole | ||
1725 | marker next time. Hence the input buffer must be large enough to hold the | ||
1726 | longest standard marker in the file. Standard JPEG markers should normally | ||
1727 | not exceed a few hundred bytes each (DHT tables are typically the longest). | ||
1728 | We recommend at least a 2K buffer for performance reasons, which is much | ||
1729 | larger than any correct marker is likely to be. For robustness against | ||
1730 | damaged marker length counts, you may wish to insert a test in your | ||
1731 | application for the case that the input buffer is completely full and yet | ||
1732 | the decoder has suspended without consuming any data --- otherwise, if this | ||
1733 | situation did occur, it would lead to an endless loop. (The library can't | ||
1734 | provide this test since it has no idea whether "the buffer is full", or | ||
1735 | even whether there is a fixed-size input buffer.) | ||
1736 | |||
1737 | The input buffer would need to be 64K to allow for arbitrary COM or APPn | ||
1738 | markers, but these are handled specially: they are either saved into allocated | ||
1739 | memory, or skipped over by calling skip_input_data(). In the former case, | ||
1740 | suspension is handled correctly, and in the latter case, the problem of | ||
1741 | buffer overrun is placed on skip_input_data's shoulders, as explained above. | ||
1742 | Note that if you provide your own marker handling routine for large markers, | ||
1743 | you should consider how to deal with buffer overflow. | ||
1744 | |||
1745 | Multiple-buffer management: | ||
1746 | |||
1747 | In some applications it is desirable to store the compressed data in a linked | ||
1748 | list of buffer areas, so as to avoid data copying. This can be handled by | ||
1749 | having empty_output_buffer() or fill_input_buffer() set the pointer and count | ||
1750 | to reference the next available buffer; FALSE is returned only if no more | ||
1751 | buffers are available. Although seemingly straightforward, there is a | ||
1752 | pitfall in this approach: the backtrack that occurs when FALSE is returned | ||
1753 | could back up into an earlier buffer. For example, when fill_input_buffer() | ||
1754 | is called, the current pointer & count indicate the backtrack restart point. | ||
1755 | Since fill_input_buffer() will set the pointer and count to refer to a new | ||
1756 | buffer, the restart position must be saved somewhere else. Suppose a second | ||
1757 | call to fill_input_buffer() occurs in the same library call, and no | ||
1758 | additional input data is available, so fill_input_buffer must return FALSE. | ||
1759 | If the JPEG library has not moved the pointer/count forward in the current | ||
1760 | buffer, then *the correct restart point is the saved position in the prior | ||
1761 | buffer*. Prior buffers may be discarded only after the library establishes | ||
1762 | a restart point within a later buffer. Similar remarks apply for output into | ||
1763 | a chain of buffers. | ||
1764 | |||
1765 | The library will never attempt to backtrack over a skip_input_data() call, | ||
1766 | so any skipped data can be permanently discarded. You still have to deal | ||
1767 | with the case of skipping not-yet-received data, however. | ||
1768 | |||
1769 | It's much simpler to use only a single buffer; when fill_input_buffer() is | ||
1770 | called, move any unconsumed data (beyond the current pointer/count) down to | ||
1771 | the beginning of this buffer and then load new data into the remaining buffer | ||
1772 | space. This approach requires a little more data copying but is far easier | ||
1773 | to get right. | ||
1774 | |||
1775 | |||
1776 | Progressive JPEG support | ||
1777 | ------------------------ | ||
1778 | |||
1779 | Progressive JPEG rearranges the stored data into a series of scans of | ||
1780 | increasing quality. In situations where a JPEG file is transmitted across a | ||
1781 | slow communications link, a decoder can generate a low-quality image very | ||
1782 | quickly from the first scan, then gradually improve the displayed quality as | ||
1783 | more scans are received. The final image after all scans are complete is | ||
1784 | identical to that of a regular (sequential) JPEG file of the same quality | ||
1785 | setting. Progressive JPEG files are often slightly smaller than equivalent | ||
1786 | sequential JPEG files, but the possibility of incremental display is the main | ||
1787 | reason for using progressive JPEG. | ||
1788 | |||
1789 | The IJG encoder library generates progressive JPEG files when given a | ||
1790 | suitable "scan script" defining how to divide the data into scans. | ||
1791 | Creation of progressive JPEG files is otherwise transparent to the encoder. | ||
1792 | Progressive JPEG files can also be read transparently by the decoder library. | ||
1793 | If the decoding application simply uses the library as defined above, it | ||
1794 | will receive a final decoded image without any indication that the file was | ||
1795 | progressive. Of course, this approach does not allow incremental display. | ||
1796 | To perform incremental display, an application needs to use the decoder | ||
1797 | library's "buffered-image" mode, in which it receives a decoded image | ||
1798 | multiple times. | ||
1799 | |||
1800 | Each displayed scan requires about as much work to decode as a full JPEG | ||
1801 | image of the same size, so the decoder must be fairly fast in relation to the | ||
1802 | data transmission rate in order to make incremental display useful. However, | ||
1803 | it is possible to skip displaying the image and simply add the incoming bits | ||
1804 | to the decoder's coefficient buffer. This is fast because only Huffman | ||
1805 | decoding need be done, not IDCT, upsampling, colorspace conversion, etc. | ||
1806 | The IJG decoder library allows the application to switch dynamically between | ||
1807 | displaying the image and simply absorbing the incoming bits. A properly | ||
1808 | coded application can automatically adapt the number of display passes to | ||
1809 | suit the time available as the image is received. Also, a final | ||
1810 | higher-quality display cycle can be performed from the buffered data after | ||
1811 | the end of the file is reached. | ||
1812 | |||
1813 | Progressive compression: | ||
1814 | |||
1815 | To create a progressive JPEG file (or a multiple-scan sequential JPEG file), | ||
1816 | set the scan_info cinfo field to point to an array of scan descriptors, and | ||
1817 | perform compression as usual. Instead of constructing your own scan list, | ||
1818 | you can call the jpeg_simple_progression() helper routine to create a | ||
1819 | recommended progression sequence; this method should be used by all | ||
1820 | applications that don't want to get involved in the nitty-gritty of | ||
1821 | progressive scan sequence design. (If you want to provide user control of | ||
1822 | scan sequences, you may wish to borrow the scan script reading code found | ||
1823 | in rdswitch.c, so that you can read scan script files just like cjpeg's.) | ||
1824 | When scan_info is not NULL, the compression library will store DCT'd data | ||
1825 | into a buffer array as jpeg_write_scanlines() is called, and will emit all | ||
1826 | the requested scans during jpeg_finish_compress(). This implies that | ||
1827 | multiple-scan output cannot be created with a suspending data destination | ||
1828 | manager, since jpeg_finish_compress() does not support suspension. We | ||
1829 | should also note that the compressor currently forces Huffman optimization | ||
1830 | mode when creating a progressive JPEG file, because the default Huffman | ||
1831 | tables are unsuitable for progressive files. | ||
1832 | |||
1833 | Progressive decompression: | ||
1834 | |||
1835 | When buffered-image mode is not used, the decoder library will read all of | ||
1836 | a multi-scan file during jpeg_start_decompress(), so that it can provide a | ||
1837 | final decoded image. (Here "multi-scan" means either progressive or | ||
1838 | multi-scan sequential.) This makes multi-scan files transparent to the | ||
1839 | decoding application. However, existing applications that used suspending | ||
1840 | input with version 5 of the IJG library will need to be modified to check | ||
1841 | for a suspension return from jpeg_start_decompress(). | ||
1842 | |||
1843 | To perform incremental display, an application must use the library's | ||
1844 | buffered-image mode. This is described in the next section. | ||
1845 | |||
1846 | |||
1847 | Buffered-image mode | ||
1848 | ------------------- | ||
1849 | |||
1850 | In buffered-image mode, the library stores the partially decoded image in a | ||
1851 | coefficient buffer, from which it can be read out as many times as desired. | ||
1852 | This mode is typically used for incremental display of progressive JPEG files, | ||
1853 | but it can be used with any JPEG file. Each scan of a progressive JPEG file | ||
1854 | adds more data (more detail) to the buffered image. The application can | ||
1855 | display in lockstep with the source file (one display pass per input scan), | ||
1856 | or it can allow input processing to outrun display processing. By making | ||
1857 | input and display processing run independently, it is possible for the | ||
1858 | application to adapt progressive display to a wide range of data transmission | ||
1859 | rates. | ||
1860 | |||
1861 | The basic control flow for buffered-image decoding is | ||
1862 | |||
1863 | jpeg_create_decompress() | ||
1864 | set data source | ||
1865 | jpeg_read_header() | ||
1866 | set overall decompression parameters | ||
1867 | cinfo.buffered_image = TRUE; /* select buffered-image mode */ | ||
1868 | jpeg_start_decompress() | ||
1869 | for (each output pass) { | ||
1870 | adjust output decompression parameters if required | ||
1871 | jpeg_start_output() /* start a new output pass */ | ||
1872 | for (all scanlines in image) { | ||
1873 | jpeg_read_scanlines() | ||
1874 | display scanlines | ||
1875 | } | ||
1876 | jpeg_finish_output() /* terminate output pass */ | ||
1877 | } | ||
1878 | jpeg_finish_decompress() | ||
1879 | jpeg_destroy_decompress() | ||
1880 | |||
1881 | This differs from ordinary unbuffered decoding in that there is an additional | ||
1882 | level of looping. The application can choose how many output passes to make | ||
1883 | and how to display each pass. | ||
1884 | |||
1885 | The simplest approach to displaying progressive images is to do one display | ||
1886 | pass for each scan appearing in the input file. In this case the outer loop | ||
1887 | condition is typically | ||
1888 | while (! jpeg_input_complete(&cinfo)) | ||
1889 | and the start-output call should read | ||
1890 | jpeg_start_output(&cinfo, cinfo.input_scan_number); | ||
1891 | The second parameter to jpeg_start_output() indicates which scan of the input | ||
1892 | file is to be displayed; the scans are numbered starting at 1 for this | ||
1893 | purpose. (You can use a loop counter starting at 1 if you like, but using | ||
1894 | the library's input scan counter is easier.) The library automatically reads | ||
1895 | data as necessary to complete each requested scan, and jpeg_finish_output() | ||
1896 | advances to the next scan or end-of-image marker (hence input_scan_number | ||
1897 | will be incremented by the time control arrives back at jpeg_start_output()). | ||
1898 | With this technique, data is read from the input file only as needed, and | ||
1899 | input and output processing run in lockstep. | ||
1900 | |||
1901 | After reading the final scan and reaching the end of the input file, the | ||
1902 | buffered image remains available; it can be read additional times by | ||
1903 | repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output() | ||
1904 | sequence. For example, a useful technique is to use fast one-pass color | ||
1905 | quantization for display passes made while the image is arriving, followed by | ||
1906 | a final display pass using two-pass quantization for highest quality. This | ||
1907 | is done by changing the library parameters before the final output pass. | ||
1908 | Changing parameters between passes is discussed in detail below. | ||
1909 | |||
1910 | In general the last scan of a progressive file cannot be recognized as such | ||
1911 | until after it is read, so a post-input display pass is the best approach if | ||
1912 | you want special processing in the final pass. | ||
1913 | |||
1914 | When done with the image, be sure to call jpeg_finish_decompress() to release | ||
1915 | the buffered image (or just use jpeg_destroy_decompress()). | ||
1916 | |||
1917 | If input data arrives faster than it can be displayed, the application can | ||
1918 | cause the library to decode input data in advance of what's needed to produce | ||
1919 | output. This is done by calling the routine jpeg_consume_input(). | ||
1920 | The return value is one of the following: | ||
1921 | JPEG_REACHED_SOS: reached an SOS marker (the start of a new scan) | ||
1922 | JPEG_REACHED_EOI: reached the EOI marker (end of image) | ||
1923 | JPEG_ROW_COMPLETED: completed reading one MCU row of compressed data | ||
1924 | JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan | ||
1925 | JPEG_SUSPENDED: suspended before completing any of the above | ||
1926 | (JPEG_SUSPENDED can occur only if a suspending data source is used.) This | ||
1927 | routine can be called at any time after initializing the JPEG object. It | ||
1928 | reads some additional data and returns when one of the indicated significant | ||
1929 | events occurs. (If called after the EOI marker is reached, it will | ||
1930 | immediately return JPEG_REACHED_EOI without attempting to read more data.) | ||
1931 | |||
1932 | The library's output processing will automatically call jpeg_consume_input() | ||
1933 | whenever the output processing overtakes the input; thus, simple lockstep | ||
1934 | display requires no direct calls to jpeg_consume_input(). But by adding | ||
1935 | calls to jpeg_consume_input(), you can absorb data in advance of what is | ||
1936 | being displayed. This has two benefits: | ||
1937 | * You can limit buildup of unprocessed data in your input buffer. | ||
1938 | * You can eliminate extra display passes by paying attention to the | ||
1939 | state of the library's input processing. | ||
1940 | |||
1941 | The first of these benefits only requires interspersing calls to | ||
1942 | jpeg_consume_input() with your display operations and any other processing | ||
1943 | you may be doing. To avoid wasting cycles due to backtracking, it's best to | ||
1944 | call jpeg_consume_input() only after a hundred or so new bytes have arrived. | ||
1945 | This is discussed further under "I/O suspension", above. (Note: the JPEG | ||
1946 | library currently is not thread-safe. You must not call jpeg_consume_input() | ||
1947 | from one thread of control if a different library routine is working on the | ||
1948 | same JPEG object in another thread.) | ||
1949 | |||
1950 | When input arrives fast enough that more than one new scan is available | ||
1951 | before you start a new output pass, you may as well skip the output pass | ||
1952 | corresponding to the completed scan. This occurs for free if you pass | ||
1953 | cinfo.input_scan_number as the target scan number to jpeg_start_output(). | ||
1954 | The input_scan_number field is simply the index of the scan currently being | ||
1955 | consumed by the input processor. You can ensure that this is up-to-date by | ||
1956 | emptying the input buffer just before calling jpeg_start_output(): call | ||
1957 | jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or | ||
1958 | JPEG_REACHED_EOI. | ||
1959 | |||
1960 | The target scan number passed to jpeg_start_output() is saved in the | ||
1961 | cinfo.output_scan_number field. The library's output processing calls | ||
1962 | jpeg_consume_input() whenever the current input scan number and row within | ||
1963 | that scan is less than or equal to the current output scan number and row. | ||
1964 | Thus, input processing can "get ahead" of the output processing but is not | ||
1965 | allowed to "fall behind". You can achieve several different effects by | ||
1966 | manipulating this interlock rule. For example, if you pass a target scan | ||
1967 | number greater than the current input scan number, the output processor will | ||
1968 | wait until that scan starts to arrive before producing any output. (To avoid | ||
1969 | an infinite loop, the target scan number is automatically reset to the last | ||
1970 | scan number when the end of image is reached. Thus, if you specify a large | ||
1971 | target scan number, the library will just absorb the entire input file and | ||
1972 | then perform an output pass. This is effectively the same as what | ||
1973 | jpeg_start_decompress() does when you don't select buffered-image mode.) | ||
1974 | When you pass a target scan number equal to the current input scan number, | ||
1975 | the image is displayed no faster than the current input scan arrives. The | ||
1976 | final possibility is to pass a target scan number less than the current input | ||
1977 | scan number; this disables the input/output interlock and causes the output | ||
1978 | processor to simply display whatever it finds in the image buffer, without | ||
1979 | waiting for input. (However, the library will not accept a target scan | ||
1980 | number less than one, so you can't avoid waiting for the first scan.) | ||
1981 | |||
1982 | When data is arriving faster than the output display processing can advance | ||
1983 | through the image, jpeg_consume_input() will store data into the buffered | ||
1984 | image beyond the point at which the output processing is reading data out | ||
1985 | again. If the input arrives fast enough, it may "wrap around" the buffer to | ||
1986 | the point where the input is more than one whole scan ahead of the output. | ||
1987 | If the output processing simply proceeds through its display pass without | ||
1988 | paying attention to the input, the effect seen on-screen is that the lower | ||
1989 | part of the image is one or more scans better in quality than the upper part. | ||
1990 | Then, when the next output scan is started, you have a choice of what target | ||
1991 | scan number to use. The recommended choice is to use the current input scan | ||
1992 | number at that time, which implies that you've skipped the output scans | ||
1993 | corresponding to the input scans that were completed while you processed the | ||
1994 | previous output scan. In this way, the decoder automatically adapts its | ||
1995 | speed to the arriving data, by skipping output scans as necessary to keep up | ||
1996 | with the arriving data. | ||
1997 | |||
1998 | When using this strategy, you'll want to be sure that you perform a final | ||
1999 | output pass after receiving all the data; otherwise your last display may not | ||
2000 | be full quality across the whole screen. So the right outer loop logic is | ||
2001 | something like this: | ||
2002 | do { | ||
2003 | absorb any waiting input by calling jpeg_consume_input() | ||
2004 | final_pass = jpeg_input_complete(&cinfo); | ||
2005 | adjust output decompression parameters if required | ||
2006 | jpeg_start_output(&cinfo, cinfo.input_scan_number); | ||
2007 | ... | ||
2008 | jpeg_finish_output() | ||
2009 | } while (! final_pass); | ||
2010 | rather than quitting as soon as jpeg_input_complete() returns TRUE. This | ||
2011 | arrangement makes it simple to use higher-quality decoding parameters | ||
2012 | for the final pass. But if you don't want to use special parameters for | ||
2013 | the final pass, the right loop logic is like this: | ||
2014 | for (;;) { | ||
2015 | absorb any waiting input by calling jpeg_consume_input() | ||
2016 | jpeg_start_output(&cinfo, cinfo.input_scan_number); | ||
2017 | ... | ||
2018 | jpeg_finish_output() | ||
2019 | if (jpeg_input_complete(&cinfo) && | ||
2020 | cinfo.input_scan_number == cinfo.output_scan_number) | ||
2021 | break; | ||
2022 | } | ||
2023 | In this case you don't need to know in advance whether an output pass is to | ||
2024 | be the last one, so it's not necessary to have reached EOF before starting | ||
2025 | the final output pass; rather, what you want to test is whether the output | ||
2026 | pass was performed in sync with the final input scan. This form of the loop | ||
2027 | will avoid an extra output pass whenever the decoder is able (or nearly able) | ||
2028 | to keep up with the incoming data. | ||
2029 | |||
2030 | When the data transmission speed is high, you might begin a display pass, | ||
2031 | then find that much or all of the file has arrived before you can complete | ||
2032 | the pass. (You can detect this by noting the JPEG_REACHED_EOI return code | ||
2033 | from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().) | ||
2034 | In this situation you may wish to abort the current display pass and start a | ||
2035 | new one using the newly arrived information. To do so, just call | ||
2036 | jpeg_finish_output() and then start a new pass with jpeg_start_output(). | ||
2037 | |||
2038 | A variant strategy is to abort and restart display if more than one complete | ||
2039 | scan arrives during an output pass; this can be detected by noting | ||
2040 | JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number. This | ||
2041 | idea should be employed with caution, however, since the display process | ||
2042 | might never get to the bottom of the image before being aborted, resulting | ||
2043 | in the lower part of the screen being several passes worse than the upper. | ||
2044 | In most cases it's probably best to abort an output pass only if the whole | ||
2045 | file has arrived and you want to begin the final output pass immediately. | ||
2046 | |||
2047 | When receiving data across a communication link, we recommend always using | ||
2048 | the current input scan number for the output target scan number; if a | ||
2049 | higher-quality final pass is to be done, it should be started (aborting any | ||
2050 | incomplete output pass) as soon as the end of file is received. However, | ||
2051 | many other strategies are possible. For example, the application can examine | ||
2052 | the parameters of the current input scan and decide whether to display it or | ||
2053 | not. If the scan contains only chroma data, one might choose not to use it | ||
2054 | as the target scan, expecting that the scan will be small and will arrive | ||
2055 | quickly. To skip to the next scan, call jpeg_consume_input() until it | ||
2056 | returns JPEG_REACHED_SOS or JPEG_REACHED_EOI. Or just use the next higher | ||
2057 | number as the target scan for jpeg_start_output(); but that method doesn't | ||
2058 | let you inspect the next scan's parameters before deciding to display it. | ||
2059 | |||
2060 | |||
2061 | In buffered-image mode, jpeg_start_decompress() never performs input and | ||
2062 | thus never suspends. An application that uses input suspension with | ||
2063 | buffered-image mode must be prepared for suspension returns from these | ||
2064 | routines: | ||
2065 | * jpeg_start_output() performs input only if you request 2-pass quantization | ||
2066 | and the target scan isn't fully read yet. (This is discussed below.) | ||
2067 | * jpeg_read_scanlines(), as always, returns the number of scanlines that it | ||
2068 | was able to produce before suspending. | ||
2069 | * jpeg_finish_output() will read any markers following the target scan, | ||
2070 | up to the end of the file or the SOS marker that begins another scan. | ||
2071 | (But it reads no input if jpeg_consume_input() has already reached the | ||
2072 | end of the file or a SOS marker beyond the target output scan.) | ||
2073 | * jpeg_finish_decompress() will read until the end of file, and thus can | ||
2074 | suspend if the end hasn't already been reached (as can be tested by | ||
2075 | calling jpeg_input_complete()). | ||
2076 | jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress() | ||
2077 | all return TRUE if they completed their tasks, FALSE if they had to suspend. | ||
2078 | In the event of a FALSE return, the application must load more input data | ||
2079 | and repeat the call. Applications that use non-suspending data sources need | ||
2080 | not check the return values of these three routines. | ||
2081 | |||
2082 | |||
2083 | It is possible to change decoding parameters between output passes in the | ||
2084 | buffered-image mode. The decoder library currently supports only very | ||
2085 | limited changes of parameters. ONLY THE FOLLOWING parameter changes are | ||
2086 | allowed after jpeg_start_decompress() is called: | ||
2087 | * dct_method can be changed before each call to jpeg_start_output(). | ||
2088 | For example, one could use a fast DCT method for early scans, changing | ||
2089 | to a higher quality method for the final scan. | ||
2090 | * dither_mode can be changed before each call to jpeg_start_output(); | ||
2091 | of course this has no impact if not using color quantization. Typically | ||
2092 | one would use ordered dither for initial passes, then switch to | ||
2093 | Floyd-Steinberg dither for the final pass. Caution: changing dither mode | ||
2094 | can cause more memory to be allocated by the library. Although the amount | ||
2095 | of memory involved is not large (a scanline or so), it may cause the | ||
2096 | initial max_memory_to_use specification to be exceeded, which in the worst | ||
2097 | case would result in an out-of-memory failure. | ||
2098 | * do_block_smoothing can be changed before each call to jpeg_start_output(). | ||
2099 | This setting is relevant only when decoding a progressive JPEG image. | ||
2100 | During the first DC-only scan, block smoothing provides a very "fuzzy" look | ||
2101 | instead of the very "blocky" look seen without it; which is better seems a | ||
2102 | matter of personal taste. But block smoothing is nearly always a win | ||
2103 | during later stages, especially when decoding a successive-approximation | ||
2104 | image: smoothing helps to hide the slight blockiness that otherwise shows | ||
2105 | up on smooth gradients until the lowest coefficient bits are sent. | ||
2106 | * Color quantization mode can be changed under the rules described below. | ||
2107 | You *cannot* change between full-color and quantized output (because that | ||
2108 | would alter the required I/O buffer sizes), but you can change which | ||
2109 | quantization method is used. | ||
2110 | |||
2111 | When generating color-quantized output, changing quantization method is a | ||
2112 | very useful way of switching between high-speed and high-quality display. | ||
2113 | The library allows you to change among its three quantization methods: | ||
2114 | 1. Single-pass quantization to a fixed color cube. | ||
2115 | Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL. | ||
2116 | 2. Single-pass quantization to an application-supplied colormap. | ||
2117 | Selected by setting cinfo.colormap to point to the colormap (the value of | ||
2118 | two_pass_quantize is ignored); also set cinfo.actual_number_of_colors. | ||
2119 | 3. Two-pass quantization to a colormap chosen specifically for the image. | ||
2120 | Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL. | ||
2121 | (This is the default setting selected by jpeg_read_header, but it is | ||
2122 | probably NOT what you want for the first pass of progressive display!) | ||
2123 | These methods offer successively better quality and lesser speed. However, | ||
2124 | only the first method is available for quantizing in non-RGB color spaces. | ||
2125 | |||
2126 | IMPORTANT: because the different quantizer methods have very different | ||
2127 | working-storage requirements, the library requires you to indicate which | ||
2128 | one(s) you intend to use before you call jpeg_start_decompress(). (If we did | ||
2129 | not require this, the max_memory_to_use setting would be a complete fiction.) | ||
2130 | You do this by setting one or more of these three cinfo fields to TRUE: | ||
2131 | enable_1pass_quant Fixed color cube colormap | ||
2132 | enable_external_quant Externally-supplied colormap | ||
2133 | enable_2pass_quant Two-pass custom colormap | ||
2134 | All three are initialized FALSE by jpeg_read_header(). But | ||
2135 | jpeg_start_decompress() automatically sets TRUE the one selected by the | ||
2136 | current two_pass_quantize and colormap settings, so you only need to set the | ||
2137 | enable flags for any other quantization methods you plan to change to later. | ||
2138 | |||
2139 | After setting the enable flags correctly at jpeg_start_decompress() time, you | ||
2140 | can change to any enabled quantization method by setting two_pass_quantize | ||
2141 | and colormap properly just before calling jpeg_start_output(). The following | ||
2142 | special rules apply: | ||
2143 | 1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass | ||
2144 | or 2-pass mode from a different mode, or when you want the 2-pass | ||
2145 | quantizer to be re-run to generate a new colormap. | ||
2146 | 2. To switch to an external colormap, or to change to a different external | ||
2147 | colormap than was used on the prior pass, you must call | ||
2148 | jpeg_new_colormap() after setting cinfo.colormap. | ||
2149 | NOTE: if you want to use the same colormap as was used in the prior pass, | ||
2150 | you should not do either of these things. This will save some nontrivial | ||
2151 | switchover costs. | ||
2152 | (These requirements exist because cinfo.colormap will always be non-NULL | ||
2153 | after completing a prior output pass, since both the 1-pass and 2-pass | ||
2154 | quantizers set it to point to their output colormaps. Thus you have to | ||
2155 | do one of these two things to notify the library that something has changed. | ||
2156 | Yup, it's a bit klugy, but it's necessary to do it this way for backwards | ||
2157 | compatibility.) | ||
2158 | |||
2159 | Note that in buffered-image mode, the library generates any requested colormap | ||
2160 | during jpeg_start_output(), not during jpeg_start_decompress(). | ||
2161 | |||
2162 | When using two-pass quantization, jpeg_start_output() makes a pass over the | ||
2163 | buffered image to determine the optimum color map; it therefore may take a | ||
2164 | significant amount of time, whereas ordinarily it does little work. The | ||
2165 | progress monitor hook is called during this pass, if defined. It is also | ||
2166 | important to realize that if the specified target scan number is greater than | ||
2167 | or equal to the current input scan number, jpeg_start_output() will attempt | ||
2168 | to consume input as it makes this pass. If you use a suspending data source, | ||
2169 | you need to check for a FALSE return from jpeg_start_output() under these | ||
2170 | conditions. The combination of 2-pass quantization and a not-yet-fully-read | ||
2171 | target scan is the only case in which jpeg_start_output() will consume input. | ||
2172 | |||
2173 | |||
2174 | Application authors who support buffered-image mode may be tempted to use it | ||
2175 | for all JPEG images, even single-scan ones. This will work, but it is | ||
2176 | inefficient: there is no need to create an image-sized coefficient buffer for | ||
2177 | single-scan images. Requesting buffered-image mode for such an image wastes | ||
2178 | memory. Worse, it can cost time on large images, since the buffered data has | ||
2179 | to be swapped out or written to a temporary file. If you are concerned about | ||
2180 | maximum performance on baseline JPEG files, you should use buffered-image | ||
2181 | mode only when the incoming file actually has multiple scans. This can be | ||
2182 | tested by calling jpeg_has_multiple_scans(), which will return a correct | ||
2183 | result at any time after jpeg_read_header() completes. | ||
2184 | |||
2185 | It is also worth noting that when you use jpeg_consume_input() to let input | ||
2186 | processing get ahead of output processing, the resulting pattern of access to | ||
2187 | the coefficient buffer is quite nonsequential. It's best to use the memory | ||
2188 | manager jmemnobs.c if you can (ie, if you have enough real or virtual main | ||
2189 | memory). If not, at least make sure that max_memory_to_use is set as high as | ||
2190 | possible. If the JPEG memory manager has to use a temporary file, you will | ||
2191 | probably see a lot of disk traffic and poor performance. (This could be | ||
2192 | improved with additional work on the memory manager, but we haven't gotten | ||
2193 | around to it yet.) | ||
2194 | |||
2195 | In some applications it may be convenient to use jpeg_consume_input() for all | ||
2196 | input processing, including reading the initial markers; that is, you may | ||
2197 | wish to call jpeg_consume_input() instead of jpeg_read_header() during | ||
2198 | startup. This works, but note that you must check for JPEG_REACHED_SOS and | ||
2199 | JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes. | ||
2200 | Once the first SOS marker has been reached, you must call | ||
2201 | jpeg_start_decompress() before jpeg_consume_input() will consume more input; | ||
2202 | it'll just keep returning JPEG_REACHED_SOS until you do. If you read a | ||
2203 | tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI | ||
2204 | without ever returning JPEG_REACHED_SOS; be sure to check for this case. | ||
2205 | If this happens, the decompressor will not read any more input until you call | ||
2206 | jpeg_abort() to reset it. It is OK to call jpeg_consume_input() even when not | ||
2207 | using buffered-image mode, but in that case it's basically a no-op after the | ||
2208 | initial markers have been read: it will just return JPEG_SUSPENDED. | ||
2209 | |||
2210 | |||
2211 | Abbreviated datastreams and multiple images | ||
2212 | ------------------------------------------- | ||
2213 | |||
2214 | A JPEG compression or decompression object can be reused to process multiple | ||
2215 | images. This saves a small amount of time per image by eliminating the | ||
2216 | "create" and "destroy" operations, but that isn't the real purpose of the | ||
2217 | feature. Rather, reuse of an object provides support for abbreviated JPEG | ||
2218 | datastreams. Object reuse can also simplify processing a series of images in | ||
2219 | a single input or output file. This section explains these features. | ||
2220 | |||
2221 | A JPEG file normally contains several hundred bytes worth of quantization | ||
2222 | and Huffman tables. In a situation where many images will be stored or | ||
2223 | transmitted with identical tables, this may represent an annoying overhead. | ||
2224 | The JPEG standard therefore permits tables to be omitted. The standard | ||
2225 | defines three classes of JPEG datastreams: | ||
2226 | * "Interchange" datastreams contain an image and all tables needed to decode | ||
2227 | the image. These are the usual kind of JPEG file. | ||
2228 | * "Abbreviated image" datastreams contain an image, but are missing some or | ||
2229 | all of the tables needed to decode that image. | ||
2230 | * "Abbreviated table specification" (henceforth "tables-only") datastreams | ||
2231 | contain only table specifications. | ||
2232 | To decode an abbreviated image, it is necessary to load the missing table(s) | ||
2233 | into the decoder beforehand. This can be accomplished by reading a separate | ||
2234 | tables-only file. A variant scheme uses a series of images in which the first | ||
2235 | image is an interchange (complete) datastream, while subsequent ones are | ||
2236 | abbreviated and rely on the tables loaded by the first image. It is assumed | ||
2237 | that once the decoder has read a table, it will remember that table until a | ||
2238 | new definition for the same table number is encountered. | ||
2239 | |||
2240 | It is the application designer's responsibility to figure out how to associate | ||
2241 | the correct tables with an abbreviated image. While abbreviated datastreams | ||
2242 | can be useful in a closed environment, their use is strongly discouraged in | ||
2243 | any situation where data exchange with other applications might be needed. | ||
2244 | Caveat designer. | ||
2245 | |||
2246 | The JPEG library provides support for reading and writing any combination of | ||
2247 | tables-only datastreams and abbreviated images. In both compression and | ||
2248 | decompression objects, a quantization or Huffman table will be retained for | ||
2249 | the lifetime of the object, unless it is overwritten by a new table definition. | ||
2250 | |||
2251 | |||
2252 | To create abbreviated image datastreams, it is only necessary to tell the | ||
2253 | compressor not to emit some or all of the tables it is using. Each | ||
2254 | quantization and Huffman table struct contains a boolean field "sent_table", | ||
2255 | which normally is initialized to FALSE. For each table used by the image, the | ||
2256 | header-writing process emits the table and sets sent_table = TRUE unless it is | ||
2257 | already TRUE. (In normal usage, this prevents outputting the same table | ||
2258 | definition multiple times, as would otherwise occur because the chroma | ||
2259 | components typically share tables.) Thus, setting this field to TRUE before | ||
2260 | calling jpeg_start_compress() will prevent the table from being written at | ||
2261 | all. | ||
2262 | |||
2263 | If you want to create a "pure" abbreviated image file containing no tables, | ||
2264 | just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the | ||
2265 | tables. If you want to emit some but not all tables, you'll need to set the | ||
2266 | individual sent_table fields directly. | ||
2267 | |||
2268 | To create an abbreviated image, you must also call jpeg_start_compress() | ||
2269 | with a second parameter of FALSE, not TRUE. Otherwise jpeg_start_compress() | ||
2270 | will force all the sent_table fields to FALSE. (This is a safety feature to | ||
2271 | prevent abbreviated images from being created accidentally.) | ||
2272 | |||
2273 | To create a tables-only file, perform the same parameter setup that you | ||
2274 | normally would, but instead of calling jpeg_start_compress() and so on, call | ||
2275 | jpeg_write_tables(&cinfo). This will write an abbreviated datastream | ||
2276 | containing only SOI, DQT and/or DHT markers, and EOI. All the quantization | ||
2277 | and Huffman tables that are currently defined in the compression object will | ||
2278 | be emitted unless their sent_tables flag is already TRUE, and then all the | ||
2279 | sent_tables flags will be set TRUE. | ||
2280 | |||
2281 | A sure-fire way to create matching tables-only and abbreviated image files | ||
2282 | is to proceed as follows: | ||
2283 | |||
2284 | create JPEG compression object | ||
2285 | set JPEG parameters | ||
2286 | set destination to tables-only file | ||
2287 | jpeg_write_tables(&cinfo); | ||
2288 | set destination to image file | ||
2289 | jpeg_start_compress(&cinfo, FALSE); | ||
2290 | write data... | ||
2291 | jpeg_finish_compress(&cinfo); | ||
2292 | |||
2293 | Since the JPEG parameters are not altered between writing the table file and | ||
2294 | the abbreviated image file, the same tables are sure to be used. Of course, | ||
2295 | you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence | ||
2296 | many times to produce many abbreviated image files matching the table file. | ||
2297 | |||
2298 | You cannot suppress output of the computed Huffman tables when Huffman | ||
2299 | optimization is selected. (If you could, there'd be no way to decode the | ||
2300 | image...) Generally, you don't want to set optimize_coding = TRUE when | ||
2301 | you are trying to produce abbreviated files. | ||
2302 | |||
2303 | In some cases you might want to compress an image using tables which are | ||
2304 | not stored in the application, but are defined in an interchange or | ||
2305 | tables-only file readable by the application. This can be done by setting up | ||
2306 | a JPEG decompression object to read the specification file, then copying the | ||
2307 | tables into your compression object. See jpeg_copy_critical_parameters() | ||
2308 | for an example of copying quantization tables. | ||
2309 | |||
2310 | |||
2311 | To read abbreviated image files, you simply need to load the proper tables | ||
2312 | into the decompression object before trying to read the abbreviated image. | ||
2313 | If the proper tables are stored in the application program, you can just | ||
2314 | allocate the table structs and fill in their contents directly. For example, | ||
2315 | to load a fixed quantization table into table slot "n": | ||
2316 | |||
2317 | if (cinfo.quant_tbl_ptrs[n] == NULL) | ||
2318 | cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo); | ||
2319 | quant_ptr = cinfo.quant_tbl_ptrs[n]; /* quant_ptr is JQUANT_TBL* */ | ||
2320 | for (i = 0; i < 64; i++) { | ||
2321 | /* Qtable[] is desired quantization table, in natural array order */ | ||
2322 | quant_ptr->quantval[i] = Qtable[i]; | ||
2323 | } | ||
2324 | |||
2325 | Code to load a fixed Huffman table is typically (for AC table "n"): | ||
2326 | |||
2327 | if (cinfo.ac_huff_tbl_ptrs[n] == NULL) | ||
2328 | cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo); | ||
2329 | huff_ptr = cinfo.ac_huff_tbl_ptrs[n]; /* huff_ptr is JHUFF_TBL* */ | ||
2330 | for (i = 1; i <= 16; i++) { | ||
2331 | /* counts[i] is number of Huffman codes of length i bits, i=1..16 */ | ||
2332 | huff_ptr->bits[i] = counts[i]; | ||
2333 | } | ||
2334 | for (i = 0; i < 256; i++) { | ||
2335 | /* symbols[] is the list of Huffman symbols, in code-length order */ | ||
2336 | huff_ptr->huffval[i] = symbols[i]; | ||
2337 | } | ||
2338 | |||
2339 | (Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a | ||
2340 | constant JQUANT_TBL object is not safe. If the incoming file happened to | ||
2341 | contain a quantization table definition, your master table would get | ||
2342 | overwritten! Instead allocate a working table copy and copy the master table | ||
2343 | into it, as illustrated above. Ditto for Huffman tables, of course.) | ||
2344 | |||
2345 | You might want to read the tables from a tables-only file, rather than | ||
2346 | hard-wiring them into your application. The jpeg_read_header() call is | ||
2347 | sufficient to read a tables-only file. You must pass a second parameter of | ||
2348 | FALSE to indicate that you do not require an image to be present. Thus, the | ||
2349 | typical scenario is | ||
2350 | |||
2351 | create JPEG decompression object | ||
2352 | set source to tables-only file | ||
2353 | jpeg_read_header(&cinfo, FALSE); | ||
2354 | set source to abbreviated image file | ||
2355 | jpeg_read_header(&cinfo, TRUE); | ||
2356 | set decompression parameters | ||
2357 | jpeg_start_decompress(&cinfo); | ||
2358 | read data... | ||
2359 | jpeg_finish_decompress(&cinfo); | ||
2360 | |||
2361 | In some cases, you may want to read a file without knowing whether it contains | ||
2362 | an image or just tables. In that case, pass FALSE and check the return value | ||
2363 | from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found, | ||
2364 | JPEG_HEADER_TABLES_ONLY if only tables were found. (A third return value, | ||
2365 | JPEG_SUSPENDED, is possible when using a suspending data source manager.) | ||
2366 | Note that jpeg_read_header() will not complain if you read an abbreviated | ||
2367 | image for which you haven't loaded the missing tables; the missing-table check | ||
2368 | occurs later, in jpeg_start_decompress(). | ||
2369 | |||
2370 | |||
2371 | It is possible to read a series of images from a single source file by | ||
2372 | repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence, | ||
2373 | without releasing/recreating the JPEG object or the data source module. | ||
2374 | (If you did reinitialize, any partial bufferload left in the data source | ||
2375 | buffer at the end of one image would be discarded, causing you to lose the | ||
2376 | start of the next image.) When you use this method, stored tables are | ||
2377 | automatically carried forward, so some of the images can be abbreviated images | ||
2378 | that depend on tables from earlier images. | ||
2379 | |||
2380 | If you intend to write a series of images into a single destination file, | ||
2381 | you might want to make a specialized data destination module that doesn't | ||
2382 | flush the output buffer at term_destination() time. This would speed things | ||
2383 | up by some trifling amount. Of course, you'd need to remember to flush the | ||
2384 | buffer after the last image. You can make the later images be abbreviated | ||
2385 | ones by passing FALSE to jpeg_start_compress(). | ||
2386 | |||
2387 | |||
2388 | Special markers | ||
2389 | --------------- | ||
2390 | |||
2391 | Some applications may need to insert or extract special data in the JPEG | ||
2392 | datastream. The JPEG standard provides marker types "COM" (comment) and | ||
2393 | "APP0" through "APP15" (application) to hold application-specific data. | ||
2394 | Unfortunately, the use of these markers is not specified by the standard. | ||
2395 | COM markers are fairly widely used to hold user-supplied text. The JFIF file | ||
2396 | format spec uses APP0 markers with specified initial strings to hold certain | ||
2397 | data. Adobe applications use APP14 markers beginning with the string "Adobe" | ||
2398 | for miscellaneous data. Other APPn markers are rarely seen, but might | ||
2399 | contain almost anything. | ||
2400 | |||
2401 | If you wish to store user-supplied text, we recommend you use COM markers | ||
2402 | and place readable 7-bit ASCII text in them. Newline conventions are not | ||
2403 | standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR | ||
2404 | (Mac style). A robust COM reader should be able to cope with random binary | ||
2405 | garbage, including nulls, since some applications generate COM markers | ||
2406 | containing non-ASCII junk. (But yours should not be one of them.) | ||
2407 | |||
2408 | For program-supplied data, use an APPn marker, and be sure to begin it with an | ||
2409 | identifying string so that you can tell whether the marker is actually yours. | ||
2410 | It's probably best to avoid using APP0 or APP14 for any private markers. | ||
2411 | (NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you | ||
2412 | not use APP8 markers for any private purposes, either.) | ||
2413 | |||
2414 | Keep in mind that at most 65533 bytes can be put into one marker, but you | ||
2415 | can have as many markers as you like. | ||
2416 | |||
2417 | By default, the IJG compression library will write a JFIF APP0 marker if the | ||
2418 | selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if | ||
2419 | the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but | ||
2420 | we don't recommend it. The decompression library will recognize JFIF and | ||
2421 | Adobe markers and will set the JPEG colorspace properly when one is found. | ||
2422 | |||
2423 | |||
2424 | You can write special markers immediately following the datastream header by | ||
2425 | calling jpeg_write_marker() after jpeg_start_compress() and before the first | ||
2426 | call to jpeg_write_scanlines(). When you do this, the markers appear after | ||
2427 | the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before | ||
2428 | all else. Specify the marker type parameter as "JPEG_COM" for COM or | ||
2429 | "JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write | ||
2430 | any marker type, but we don't recommend writing any other kinds of marker.) | ||
2431 | For example, to write a user comment string pointed to by comment_text: | ||
2432 | jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text)); | ||
2433 | |||
2434 | If it's not convenient to store all the marker data in memory at once, | ||
2435 | you can instead call jpeg_write_m_header() followed by multiple calls to | ||
2436 | jpeg_write_m_byte(). If you do it this way, it's your responsibility to | ||
2437 | call jpeg_write_m_byte() exactly the number of times given in the length | ||
2438 | parameter to jpeg_write_m_header(). (This method lets you empty the | ||
2439 | output buffer partway through a marker, which might be important when | ||
2440 | using a suspending data destination module. In any case, if you are using | ||
2441 | a suspending destination, you should flush its buffer after inserting | ||
2442 | any special markers. See "I/O suspension".) | ||
2443 | |||
2444 | Or, if you prefer to synthesize the marker byte sequence yourself, | ||
2445 | you can just cram it straight into the data destination module. | ||
2446 | |||
2447 | If you are writing JFIF 1.02 extension markers (thumbnail images), don't | ||
2448 | forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the | ||
2449 | correct JFIF version number in the JFIF header marker. The library's default | ||
2450 | is to write version 1.01, but that's wrong if you insert any 1.02 extension | ||
2451 | markers. (We could probably get away with just defaulting to 1.02, but there | ||
2452 | used to be broken decoders that would complain about unknown minor version | ||
2453 | numbers. To reduce compatibility risks it's safest not to write 1.02 unless | ||
2454 | you are actually using 1.02 extensions.) | ||
2455 | |||
2456 | |||
2457 | When reading, two methods of handling special markers are available: | ||
2458 | 1. You can ask the library to save the contents of COM and/or APPn markers | ||
2459 | into memory, and then examine them at your leisure afterwards. | ||
2460 | 2. You can supply your own routine to process COM and/or APPn markers | ||
2461 | on-the-fly as they are read. | ||
2462 | The first method is simpler to use, especially if you are using a suspending | ||
2463 | data source; writing a marker processor that copes with input suspension is | ||
2464 | not easy (consider what happens if the marker is longer than your available | ||
2465 | input buffer). However, the second method conserves memory since the marker | ||
2466 | data need not be kept around after it's been processed. | ||
2467 | |||
2468 | For either method, you'd normally set up marker handling after creating a | ||
2469 | decompression object and before calling jpeg_read_header(), because the | ||
2470 | markers of interest will typically be near the head of the file and so will | ||
2471 | be scanned by jpeg_read_header. Once you've established a marker handling | ||
2472 | method, it will be used for the life of that decompression object | ||
2473 | (potentially many datastreams), unless you change it. Marker handling is | ||
2474 | determined separately for COM markers and for each APPn marker code. | ||
2475 | |||
2476 | |||
2477 | To save the contents of special markers in memory, call | ||
2478 | jpeg_save_markers(cinfo, marker_code, length_limit) | ||
2479 | where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n. | ||
2480 | (To arrange to save all the special marker types, you need to call this | ||
2481 | routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer | ||
2482 | than length_limit data bytes, only length_limit bytes will be saved; this | ||
2483 | parameter allows you to avoid chewing up memory when you only need to see the | ||
2484 | first few bytes of a potentially large marker. If you want to save all the | ||
2485 | data, set length_limit to 0xFFFF; that is enough since marker lengths are only | ||
2486 | 16 bits. As a special case, setting length_limit to 0 prevents that marker | ||
2487 | type from being saved at all. (That is the default behavior, in fact.) | ||
2488 | |||
2489 | After jpeg_read_header() completes, you can examine the special markers by | ||
2490 | following the cinfo->marker_list pointer chain. All the special markers in | ||
2491 | the file appear in this list, in order of their occurrence in the file (but | ||
2492 | omitting any markers of types you didn't ask for). Both the original data | ||
2493 | length and the saved data length are recorded for each list entry; the latter | ||
2494 | will not exceed length_limit for the particular marker type. Note that these | ||
2495 | lengths exclude the marker length word, whereas the stored representation | ||
2496 | within the JPEG file includes it. (Hence the maximum data length is really | ||
2497 | only 65533.) | ||
2498 | |||
2499 | It is possible that additional special markers appear in the file beyond the | ||
2500 | SOS marker at which jpeg_read_header stops; if so, the marker list will be | ||
2501 | extended during reading of the rest of the file. This is not expected to be | ||
2502 | common, however. If you are short on memory you may want to reset the length | ||
2503 | limit to zero for all marker types after finishing jpeg_read_header, to | ||
2504 | ensure that the max_memory_to_use setting cannot be exceeded due to addition | ||
2505 | of later markers. | ||
2506 | |||
2507 | The marker list remains stored until you call jpeg_finish_decompress or | ||
2508 | jpeg_abort, at which point the memory is freed and the list is set to empty. | ||
2509 | (jpeg_destroy also releases the storage, of course.) | ||
2510 | |||
2511 | Note that the library is internally interested in APP0 and APP14 markers; | ||
2512 | if you try to set a small nonzero length limit on these types, the library | ||
2513 | will silently force the length up to the minimum it wants. (But you can set | ||
2514 | a zero length limit to prevent them from being saved at all.) Also, in a | ||
2515 | 16-bit environment, the maximum length limit may be constrained to less than | ||
2516 | 65533 by malloc() limitations. It is therefore best not to assume that the | ||
2517 | effective length limit is exactly what you set it to be. | ||
2518 | |||
2519 | |||
2520 | If you want to supply your own marker-reading routine, you do it by calling | ||
2521 | jpeg_set_marker_processor(). A marker processor routine must have the | ||
2522 | signature | ||
2523 | boolean jpeg_marker_parser_method (j_decompress_ptr cinfo) | ||
2524 | Although the marker code is not explicitly passed, the routine can find it | ||
2525 | in cinfo->unread_marker. At the time of call, the marker proper has been | ||
2526 | read from the data source module. The processor routine is responsible for | ||
2527 | reading the marker length word and the remaining parameter bytes, if any. | ||
2528 | Return TRUE to indicate success. (FALSE should be returned only if you are | ||
2529 | using a suspending data source and it tells you to suspend. See the standard | ||
2530 | marker processors in jdmarker.c for appropriate coding methods if you need to | ||
2531 | use a suspending data source.) | ||
2532 | |||
2533 | If you override the default APP0 or APP14 processors, it is up to you to | ||
2534 | recognize JFIF and Adobe markers if you want colorspace recognition to occur | ||
2535 | properly. We recommend copying and extending the default processors if you | ||
2536 | want to do that. (A better idea is to save these marker types for later | ||
2537 | examination by calling jpeg_save_markers(); that method doesn't interfere | ||
2538 | with the library's own processing of these markers.) | ||
2539 | |||
2540 | jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive | ||
2541 | --- if you call one it overrides any previous call to the other, for the | ||
2542 | particular marker type specified. | ||
2543 | |||
2544 | A simple example of an external COM processor can be found in djpeg.c. | ||
2545 | Also, see jpegtran.c for an example of using jpeg_save_markers. | ||
2546 | |||
2547 | |||
2548 | Raw (downsampled) image data | ||
2549 | ---------------------------- | ||
2550 | |||
2551 | Some applications need to supply already-downsampled image data to the JPEG | ||
2552 | compressor, or to receive raw downsampled data from the decompressor. The | ||
2553 | library supports this requirement by allowing the application to write or | ||
2554 | read raw data, bypassing the normal preprocessing or postprocessing steps. | ||
2555 | The interface is different from the standard one and is somewhat harder to | ||
2556 | use. If your interest is merely in bypassing color conversion, we recommend | ||
2557 | that you use the standard interface and simply set jpeg_color_space = | ||
2558 | in_color_space (or jpeg_color_space = out_color_space for decompression). | ||
2559 | The mechanism described in this section is necessary only to supply or | ||
2560 | receive downsampled image data, in which not all components have the same | ||
2561 | dimensions. | ||
2562 | |||
2563 | |||
2564 | To compress raw data, you must supply the data in the colorspace to be used | ||
2565 | in the JPEG file (please read the earlier section on Special color spaces) | ||
2566 | and downsampled to the sampling factors specified in the JPEG parameters. | ||
2567 | You must supply the data in the format used internally by the JPEG library, | ||
2568 | namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional | ||
2569 | arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one | ||
2570 | color component. This structure is necessary since the components are of | ||
2571 | different sizes. If the image dimensions are not a multiple of the MCU size, | ||
2572 | you must also pad the data correctly (usually, this is done by replicating | ||
2573 | the last column and/or row). The data must be padded to a multiple of a DCT | ||
2574 | block in each component: that is, each downsampled row must contain a | ||
2575 | multiple of 8 valid samples, and there must be a multiple of 8 sample rows | ||
2576 | for each component. (For applications such as conversion of digital TV | ||
2577 | images, the standard image size is usually a multiple of the DCT block size, | ||
2578 | so that no padding need actually be done.) | ||
2579 | |||
2580 | The procedure for compression of raw data is basically the same as normal | ||
2581 | compression, except that you call jpeg_write_raw_data() in place of | ||
2582 | jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do | ||
2583 | the following: | ||
2584 | * Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().) | ||
2585 | This notifies the library that you will be supplying raw data. | ||
2586 | Furthermore, set cinfo->do_fancy_downsampling to FALSE if you want to use | ||
2587 | real downsampled data. (It is set TRUE by jpeg_set_defaults().) | ||
2588 | * Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace() | ||
2589 | call is a good idea. Note that since color conversion is bypassed, | ||
2590 | in_color_space is ignored, except that jpeg_set_defaults() uses it to | ||
2591 | choose the default jpeg_color_space setting. | ||
2592 | * Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and | ||
2593 | cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the | ||
2594 | dimensions of the data you are supplying, it's wise to set them | ||
2595 | explicitly, rather than assuming the library's defaults are what you want. | ||
2596 | |||
2597 | To pass raw data to the library, call jpeg_write_raw_data() in place of | ||
2598 | jpeg_write_scanlines(). The two routines work similarly except that | ||
2599 | jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY. | ||
2600 | The scanlines count passed to and returned from jpeg_write_raw_data is | ||
2601 | measured in terms of the component with the largest v_samp_factor. | ||
2602 | |||
2603 | jpeg_write_raw_data() processes one MCU row per call, which is to say | ||
2604 | v_samp_factor*DCTSIZE sample rows of each component. The passed num_lines | ||
2605 | value must be at least max_v_samp_factor*DCTSIZE, and the return value will | ||
2606 | be exactly that amount (or possibly some multiple of that amount, in future | ||
2607 | library versions). This is true even on the last call at the bottom of the | ||
2608 | image; don't forget to pad your data as necessary. | ||
2609 | |||
2610 | The required dimensions of the supplied data can be computed for each | ||
2611 | component as | ||
2612 | cinfo->comp_info[i].width_in_blocks*DCTSIZE samples per row | ||
2613 | cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image | ||
2614 | after jpeg_start_compress() has initialized those fields. If the valid data | ||
2615 | is smaller than this, it must be padded appropriately. For some sampling | ||
2616 | factors and image sizes, additional dummy DCT blocks are inserted to make | ||
2617 | the image a multiple of the MCU dimensions. The library creates such dummy | ||
2618 | blocks itself; it does not read them from your supplied data. Therefore you | ||
2619 | need never pad by more than DCTSIZE samples. An example may help here. | ||
2620 | Assume 2h2v downsampling of YCbCr data, that is | ||
2621 | cinfo->comp_info[0].h_samp_factor = 2 for Y | ||
2622 | cinfo->comp_info[0].v_samp_factor = 2 | ||
2623 | cinfo->comp_info[1].h_samp_factor = 1 for Cb | ||
2624 | cinfo->comp_info[1].v_samp_factor = 1 | ||
2625 | cinfo->comp_info[2].h_samp_factor = 1 for Cr | ||
2626 | cinfo->comp_info[2].v_samp_factor = 1 | ||
2627 | and suppose that the nominal image dimensions (cinfo->image_width and | ||
2628 | cinfo->image_height) are 101x101 pixels. Then jpeg_start_compress() will | ||
2629 | compute downsampled_width = 101 and width_in_blocks = 13 for Y, | ||
2630 | downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same | ||
2631 | for the height fields). You must pad the Y data to at least 13*8 = 104 | ||
2632 | columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows. The | ||
2633 | MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16 | ||
2634 | scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual | ||
2635 | sample rows of Y and 8 each of Cb and Cr. A total of 7 MCU rows are needed, | ||
2636 | so you must pass a total of 7*16 = 112 "scanlines". The last DCT block row | ||
2637 | of Y data is dummy, so it doesn't matter what you pass for it in the data | ||
2638 | arrays, but the scanlines count must total up to 112 so that all of the Cb | ||
2639 | and Cr data gets passed. | ||
2640 | |||
2641 | Output suspension is supported with raw-data compression: if the data | ||
2642 | destination module suspends, jpeg_write_raw_data() will return 0. | ||
2643 | In this case the same data rows must be passed again on the next call. | ||
2644 | |||
2645 | |||
2646 | Decompression with raw data output implies bypassing all postprocessing. | ||
2647 | You must deal with the color space and sampling factors present in the | ||
2648 | incoming file. If your application only handles, say, 2h1v YCbCr data, | ||
2649 | you must check for and fail on other color spaces or other sampling factors. | ||
2650 | The library will not convert to a different color space for you. | ||
2651 | |||
2652 | To obtain raw data output, set cinfo->raw_data_out = TRUE before | ||
2653 | jpeg_start_decompress() (it is set FALSE by jpeg_read_header()). Be sure to | ||
2654 | verify that the color space and sampling factors are ones you can handle. | ||
2655 | Furthermore, set cinfo->do_fancy_upsampling = FALSE if you want to get real | ||
2656 | downsampled data (it is set TRUE by jpeg_read_header()). | ||
2657 | Then call jpeg_read_raw_data() in place of jpeg_read_scanlines(). The | ||
2658 | decompression process is otherwise the same as usual. | ||
2659 | |||
2660 | jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a | ||
2661 | buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is | ||
2662 | the same as for raw-data compression). The buffer you pass must be large | ||
2663 | enough to hold the actual data plus padding to DCT-block boundaries. As with | ||
2664 | compression, any entirely dummy DCT blocks are not processed so you need not | ||
2665 | allocate space for them, but the total scanline count includes them. The | ||
2666 | above example of computing buffer dimensions for raw-data compression is | ||
2667 | equally valid for decompression. | ||
2668 | |||
2669 | Input suspension is supported with raw-data decompression: if the data source | ||
2670 | module suspends, jpeg_read_raw_data() will return 0. You can also use | ||
2671 | buffered-image mode to read raw data in multiple passes. | ||
2672 | |||
2673 | |||
2674 | Really raw data: DCT coefficients | ||
2675 | --------------------------------- | ||
2676 | |||
2677 | It is possible to read or write the contents of a JPEG file as raw DCT | ||
2678 | coefficients. This facility is mainly intended for use in lossless | ||
2679 | transcoding between different JPEG file formats. Other possible applications | ||
2680 | include lossless cropping of a JPEG image, lossless reassembly of a | ||
2681 | multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc. | ||
2682 | |||
2683 | To read the contents of a JPEG file as DCT coefficients, open the file and do | ||
2684 | jpeg_read_header() as usual. But instead of calling jpeg_start_decompress() | ||
2685 | and jpeg_read_scanlines(), call jpeg_read_coefficients(). This will read the | ||
2686 | entire image into a set of virtual coefficient-block arrays, one array per | ||
2687 | component. The return value is a pointer to an array of virtual-array | ||
2688 | descriptors. Each virtual array can be accessed directly using the JPEG | ||
2689 | memory manager's access_virt_barray method (see Memory management, below, | ||
2690 | and also read structure.txt's discussion of virtual array handling). Or, | ||
2691 | for simple transcoding to a different JPEG file format, the array list can | ||
2692 | just be handed directly to jpeg_write_coefficients(). | ||
2693 | |||
2694 | Each block in the block arrays contains quantized coefficient values in | ||
2695 | normal array order (not JPEG zigzag order). The block arrays contain only | ||
2696 | DCT blocks containing real data; any entirely-dummy blocks added to fill out | ||
2697 | interleaved MCUs at the right or bottom edges of the image are discarded | ||
2698 | during reading and are not stored in the block arrays. (The size of each | ||
2699 | block array can be determined from the width_in_blocks and height_in_blocks | ||
2700 | fields of the component's comp_info entry.) This is also the data format | ||
2701 | expected by jpeg_write_coefficients(). | ||
2702 | |||
2703 | When you are done using the virtual arrays, call jpeg_finish_decompress() | ||
2704 | to release the array storage and return the decompression object to an idle | ||
2705 | state; or just call jpeg_destroy() if you don't need to reuse the object. | ||
2706 | |||
2707 | If you use a suspending data source, jpeg_read_coefficients() will return | ||
2708 | NULL if it is forced to suspend; a non-NULL return value indicates successful | ||
2709 | completion. You need not test for a NULL return value when using a | ||
2710 | non-suspending data source. | ||
2711 | |||
2712 | It is also possible to call jpeg_read_coefficients() to obtain access to the | ||
2713 | decoder's coefficient arrays during a normal decode cycle in buffered-image | ||
2714 | mode. This frammish might be useful for progressively displaying an incoming | ||
2715 | image and then re-encoding it without loss. To do this, decode in buffered- | ||
2716 | image mode as discussed previously, then call jpeg_read_coefficients() after | ||
2717 | the last jpeg_finish_output() call. The arrays will be available for your use | ||
2718 | until you call jpeg_finish_decompress(). | ||
2719 | |||
2720 | |||
2721 | To write the contents of a JPEG file as DCT coefficients, you must provide | ||
2722 | the DCT coefficients stored in virtual block arrays. You can either pass | ||
2723 | block arrays read from an input JPEG file by jpeg_read_coefficients(), or | ||
2724 | allocate virtual arrays from the JPEG compression object and fill them | ||
2725 | yourself. In either case, jpeg_write_coefficients() is substituted for | ||
2726 | jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is | ||
2727 | * Create compression object | ||
2728 | * Set all compression parameters as necessary | ||
2729 | * Request virtual arrays if needed | ||
2730 | * jpeg_write_coefficients() | ||
2731 | * jpeg_finish_compress() | ||
2732 | * Destroy or re-use compression object | ||
2733 | jpeg_write_coefficients() is passed a pointer to an array of virtual block | ||
2734 | array descriptors; the number of arrays is equal to cinfo.num_components. | ||
2735 | |||
2736 | The virtual arrays need only have been requested, not realized, before | ||
2737 | jpeg_write_coefficients() is called. A side-effect of | ||
2738 | jpeg_write_coefficients() is to realize any virtual arrays that have been | ||
2739 | requested from the compression object's memory manager. Thus, when obtaining | ||
2740 | the virtual arrays from the compression object, you should fill the arrays | ||
2741 | after calling jpeg_write_coefficients(). The data is actually written out | ||
2742 | when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes | ||
2743 | the file header. | ||
2744 | |||
2745 | When writing raw DCT coefficients, it is crucial that the JPEG quantization | ||
2746 | tables and sampling factors match the way the data was encoded, or the | ||
2747 | resulting file will be invalid. For transcoding from an existing JPEG file, | ||
2748 | we recommend using jpeg_copy_critical_parameters(). This routine initializes | ||
2749 | all the compression parameters to default values (like jpeg_set_defaults()), | ||
2750 | then copies the critical information from a source decompression object. | ||
2751 | The decompression object should have just been used to read the entire | ||
2752 | JPEG input file --- that is, it should be awaiting jpeg_finish_decompress(). | ||
2753 | |||
2754 | jpeg_write_coefficients() marks all tables stored in the compression object | ||
2755 | as needing to be written to the output file (thus, it acts like | ||
2756 | jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid | ||
2757 | emitting abbreviated JPEG files by accident. If you really want to emit an | ||
2758 | abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables' | ||
2759 | individual sent_table flags, between calling jpeg_write_coefficients() and | ||
2760 | jpeg_finish_compress(). | ||
2761 | |||
2762 | |||
2763 | Progress monitoring | ||
2764 | ------------------- | ||
2765 | |||
2766 | Some applications may need to regain control from the JPEG library every so | ||
2767 | often. The typical use of this feature is to produce a percent-done bar or | ||
2768 | other progress display. (For a simple example, see cjpeg.c or djpeg.c.) | ||
2769 | Although you do get control back frequently during the data-transferring pass | ||
2770 | (the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes | ||
2771 | will occur inside jpeg_finish_compress or jpeg_start_decompress; those | ||
2772 | routines may take a long time to execute, and you don't get control back | ||
2773 | until they are done. | ||
2774 | |||
2775 | You can define a progress-monitor routine which will be called periodically | ||
2776 | by the library. No guarantees are made about how often this call will occur, | ||
2777 | so we don't recommend you use it for mouse tracking or anything like that. | ||
2778 | At present, a call will occur once per MCU row, scanline, or sample row | ||
2779 | group, whichever unit is convenient for the current processing mode; so the | ||
2780 | wider the image, the longer the time between calls. During the data | ||
2781 | transferring pass, only one call occurs per call of jpeg_read_scanlines or | ||
2782 | jpeg_write_scanlines, so don't pass a large number of scanlines at once if | ||
2783 | you want fine resolution in the progress count. (If you really need to use | ||
2784 | the callback mechanism for time-critical tasks like mouse tracking, you could | ||
2785 | insert additional calls inside some of the library's inner loops.) | ||
2786 | |||
2787 | To establish a progress-monitor callback, create a struct jpeg_progress_mgr, | ||
2788 | fill in its progress_monitor field with a pointer to your callback routine, | ||
2789 | and set cinfo->progress to point to the struct. The callback will be called | ||
2790 | whenever cinfo->progress is non-NULL. (This pointer is set to NULL by | ||
2791 | jpeg_create_compress or jpeg_create_decompress; the library will not change | ||
2792 | it thereafter. So if you allocate dynamic storage for the progress struct, | ||
2793 | make sure it will live as long as the JPEG object does. Allocating from the | ||
2794 | JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.) You | ||
2795 | can use the same callback routine for both compression and decompression. | ||
2796 | |||
2797 | The jpeg_progress_mgr struct contains four fields which are set by the library: | ||
2798 | long pass_counter; /* work units completed in this pass */ | ||
2799 | long pass_limit; /* total number of work units in this pass */ | ||
2800 | int completed_passes; /* passes completed so far */ | ||
2801 | int total_passes; /* total number of passes expected */ | ||
2802 | During any one pass, pass_counter increases from 0 up to (not including) | ||
2803 | pass_limit; the step size is usually but not necessarily 1. The pass_limit | ||
2804 | value may change from one pass to another. The expected total number of | ||
2805 | passes is in total_passes, and the number of passes already completed is in | ||
2806 | completed_passes. Thus the fraction of work completed may be estimated as | ||
2807 | completed_passes + (pass_counter/pass_limit) | ||
2808 | -------------------------------------------- | ||
2809 | total_passes | ||
2810 | ignoring the fact that the passes may not be equal amounts of work. | ||
2811 | |||
2812 | When decompressing, pass_limit can even change within a pass, because it | ||
2813 | depends on the number of scans in the JPEG file, which isn't always known in | ||
2814 | advance. The computed fraction-of-work-done may jump suddenly (if the library | ||
2815 | discovers it has overestimated the number of scans) or even decrease (in the | ||
2816 | opposite case). It is not wise to put great faith in the work estimate. | ||
2817 | |||
2818 | When using the decompressor's buffered-image mode, the progress monitor work | ||
2819 | estimate is likely to be completely unhelpful, because the library has no way | ||
2820 | to know how many output passes will be demanded of it. Currently, the library | ||
2821 | sets total_passes based on the assumption that there will be one more output | ||
2822 | pass if the input file end hasn't yet been read (jpeg_input_complete() isn't | ||
2823 | TRUE), but no more output passes if the file end has been reached when the | ||
2824 | output pass is started. This means that total_passes will rise as additional | ||
2825 | output passes are requested. If you have a way of determining the input file | ||
2826 | size, estimating progress based on the fraction of the file that's been read | ||
2827 | will probably be more useful than using the library's value. | ||
2828 | |||
2829 | |||
2830 | Memory management | ||
2831 | ----------------- | ||
2832 | |||
2833 | This section covers some key facts about the JPEG library's built-in memory | ||
2834 | manager. For more info, please read structure.txt's section about the memory | ||
2835 | manager, and consult the source code if necessary. | ||
2836 | |||
2837 | All memory and temporary file allocation within the library is done via the | ||
2838 | memory manager. If necessary, you can replace the "back end" of the memory | ||
2839 | manager to control allocation yourself (for example, if you don't want the | ||
2840 | library to use malloc() and free() for some reason). | ||
2841 | |||
2842 | Some data is allocated "permanently" and will not be freed until the JPEG | ||
2843 | object is destroyed. Most data is allocated "per image" and is freed by | ||
2844 | jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort. You can call the | ||
2845 | memory manager yourself to allocate structures that will automatically be | ||
2846 | freed at these times. Typical code for this is | ||
2847 | ptr = (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, size); | ||
2848 | Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object. | ||
2849 | Use alloc_large instead of alloc_small for anything bigger than a few Kbytes. | ||
2850 | There are also alloc_sarray and alloc_barray routines that automatically | ||
2851 | build 2-D sample or block arrays. | ||
2852 | |||
2853 | The library's minimum space requirements to process an image depend on the | ||
2854 | image's width, but not on its height, because the library ordinarily works | ||
2855 | with "strip" buffers that are as wide as the image but just a few rows high. | ||
2856 | Some operating modes (eg, two-pass color quantization) require full-image | ||
2857 | buffers. Such buffers are treated as "virtual arrays": only the current strip | ||
2858 | need be in memory, and the rest can be swapped out to a temporary file. | ||
2859 | |||
2860 | If you use the simplest memory manager back end (jmemnobs.c), then no | ||
2861 | temporary files are used; virtual arrays are simply malloc()'d. Images bigger | ||
2862 | than memory can be processed only if your system supports virtual memory. | ||
2863 | The other memory manager back ends support temporary files of various flavors | ||
2864 | and thus work in machines without virtual memory. They may also be useful on | ||
2865 | Unix machines if you need to process images that exceed available swap space. | ||
2866 | |||
2867 | When using temporary files, the library will make the in-memory buffers for | ||
2868 | its virtual arrays just big enough to stay within a "maximum memory" setting. | ||
2869 | Your application can set this limit by setting cinfo->mem->max_memory_to_use | ||
2870 | after creating the JPEG object. (Of course, there is still a minimum size for | ||
2871 | the buffers, so the max-memory setting is effective only if it is bigger than | ||
2872 | the minimum space needed.) If you allocate any large structures yourself, you | ||
2873 | must allocate them before jpeg_start_compress() or jpeg_start_decompress() in | ||
2874 | order to have them counted against the max memory limit. Also keep in mind | ||
2875 | that space allocated with alloc_small() is ignored, on the assumption that | ||
2876 | it's too small to be worth worrying about; so a reasonable safety margin | ||
2877 | should be left when setting max_memory_to_use. | ||
2878 | |||
2879 | If you use the jmemname.c or jmemdos.c memory manager back end, it is | ||
2880 | important to clean up the JPEG object properly to ensure that the temporary | ||
2881 | files get deleted. (This is especially crucial with jmemdos.c, where the | ||
2882 | "temporary files" may be extended-memory segments; if they are not freed, | ||
2883 | DOS will require a reboot to recover the memory.) Thus, with these memory | ||
2884 | managers, it's a good idea to provide a signal handler that will trap any | ||
2885 | early exit from your program. The handler should call either jpeg_abort() | ||
2886 | or jpeg_destroy() for any active JPEG objects. A handler is not needed with | ||
2887 | jmemnobs.c, and shouldn't be necessary with jmemansi.c or jmemmac.c either, | ||
2888 | since the C library is supposed to take care of deleting files made with | ||
2889 | tmpfile(). | ||
2890 | |||
2891 | |||
2892 | Memory usage | ||
2893 | ------------ | ||
2894 | |||
2895 | Working memory requirements while performing compression or decompression | ||
2896 | depend on image dimensions, image characteristics (such as colorspace and | ||
2897 | JPEG process), and operating mode (application-selected options). | ||
2898 | |||
2899 | As of v6b, the decompressor requires: | ||
2900 | 1. About 24K in more-or-less-fixed-size data. This varies a bit depending | ||
2901 | on operating mode and image characteristics (particularly color vs. | ||
2902 | grayscale), but it doesn't depend on image dimensions. | ||
2903 | 2. Strip buffers (of size proportional to the image width) for IDCT and | ||
2904 | upsampling results. The worst case for commonly used sampling factors | ||
2905 | is about 34 bytes * width in pixels for a color image. A grayscale image | ||
2906 | only needs about 8 bytes per pixel column. | ||
2907 | 3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG | ||
2908 | file (including progressive JPEGs), or whenever you select buffered-image | ||
2909 | mode. This takes 2 bytes/coefficient. At typical 2x2 sampling, that's | ||
2910 | 3 bytes per pixel for a color image. Worst case (1x1 sampling) requires | ||
2911 | 6 bytes/pixel. For grayscale, figure 2 bytes/pixel. | ||
2912 | 4. To perform 2-pass color quantization, the decompressor also needs a | ||
2913 | 128K color lookup table and a full-image pixel buffer (3 bytes/pixel). | ||
2914 | This does not count any memory allocated by the application, such as a | ||
2915 | buffer to hold the final output image. | ||
2916 | |||
2917 | The above figures are valid for 8-bit JPEG data precision and a machine with | ||
2918 | 32-bit ints. For 12-bit JPEG data, double the size of the strip buffers and | ||
2919 | quantization pixel buffer. The "fixed-size" data will be somewhat smaller | ||
2920 | with 16-bit ints, larger with 64-bit ints. Also, CMYK or other unusual | ||
2921 | color spaces will require different amounts of space. | ||
2922 | |||
2923 | The full-image coefficient and pixel buffers, if needed at all, do not | ||
2924 | have to be fully RAM resident; you can have the library use temporary | ||
2925 | files instead when the total memory usage would exceed a limit you set. | ||
2926 | (But if your OS supports virtual memory, it's probably better to just use | ||
2927 | jmemnobs and let the OS do the swapping.) | ||
2928 | |||
2929 | The compressor's memory requirements are similar, except that it has no need | ||
2930 | for color quantization. Also, it needs a full-image DCT coefficient buffer | ||
2931 | if Huffman-table optimization is asked for, even if progressive mode is not | ||
2932 | requested. | ||
2933 | |||
2934 | If you need more detailed information about memory usage in a particular | ||
2935 | situation, you can enable the MEM_STATS code in jmemmgr.c. | ||
2936 | |||
2937 | |||
2938 | Library compile-time options | ||
2939 | ---------------------------- | ||
2940 | |||
2941 | A number of compile-time options are available by modifying jmorecfg.h. | ||
2942 | |||
2943 | The JPEG standard provides for both the baseline 8-bit DCT process and | ||
2944 | a 12-bit DCT process. The IJG code supports 12-bit JPEG if you define | ||
2945 | BITS_IN_JSAMPLE as 12 rather than 8. Note that this causes JSAMPLE to be | ||
2946 | larger than a char, so it affects the surrounding application's image data. | ||
2947 | The sample applications cjpeg and djpeg can support 12-bit mode only for PPM | ||
2948 | and GIF file formats; you must disable the other file formats to compile a | ||
2949 | 12-bit cjpeg or djpeg. (install.txt has more information about that.) | ||
2950 | At present, a 12-bit library can handle *only* 12-bit images, not both | ||
2951 | precisions. (If you need to include both 8- and 12-bit libraries in a single | ||
2952 | application, you could probably do it by defining NEED_SHORT_EXTERNAL_NAMES | ||
2953 | for just one of the copies. You'd have to access the 8-bit and 12-bit copies | ||
2954 | from separate application source files. This is untested ... if you try it, | ||
2955 | we'd like to hear whether it works!) | ||
2956 | |||
2957 | Note that a 12-bit library always compresses in Huffman optimization mode, | ||
2958 | in order to generate valid Huffman tables. This is necessary because our | ||
2959 | default Huffman tables only cover 8-bit data. If you need to output 12-bit | ||
2960 | files in one pass, you'll have to supply suitable default Huffman tables. | ||
2961 | You may also want to supply your own DCT quantization tables; the existing | ||
2962 | quality-scaling code has been developed for 8-bit use, and probably doesn't | ||
2963 | generate especially good tables for 12-bit. | ||
2964 | |||
2965 | The maximum number of components (color channels) in the image is determined | ||
2966 | by MAX_COMPONENTS. The JPEG standard allows up to 255 components, but we | ||
2967 | expect that few applications will need more than four or so. | ||
2968 | |||
2969 | On machines with unusual data type sizes, you may be able to improve | ||
2970 | performance or reduce memory space by tweaking the various typedefs in | ||
2971 | jmorecfg.h. In particular, on some RISC CPUs, access to arrays of "short"s | ||
2972 | is quite slow; consider trading memory for speed by making JCOEF, INT16, and | ||
2973 | UINT16 be "int" or "unsigned int". UINT8 is also a candidate to become int. | ||
2974 | You probably don't want to make JSAMPLE be int unless you have lots of memory | ||
2975 | to burn. | ||
2976 | |||
2977 | You can reduce the size of the library by compiling out various optional | ||
2978 | functions. To do this, undefine xxx_SUPPORTED symbols as necessary. | ||
2979 | |||
2980 | You can also save a few K by not having text error messages in the library; | ||
2981 | the standard error message table occupies about 5Kb. This is particularly | ||
2982 | reasonable for embedded applications where there's no good way to display | ||
2983 | a message anyway. To do this, remove the creation of the message table | ||
2984 | (jpeg_std_message_table[]) from jerror.c, and alter format_message to do | ||
2985 | something reasonable without it. You could output the numeric value of the | ||
2986 | message code number, for example. If you do this, you can also save a couple | ||
2987 | more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing; | ||
2988 | you don't need trace capability anyway, right? | ||
2989 | |||
2990 | |||
2991 | Portability considerations | ||
2992 | -------------------------- | ||
2993 | |||
2994 | The JPEG library has been written to be extremely portable; the sample | ||
2995 | applications cjpeg and djpeg are slightly less so. This section summarizes | ||
2996 | the design goals in this area. (If you encounter any bugs that cause the | ||
2997 | library to be less portable than is claimed here, we'd appreciate hearing | ||
2998 | about them.) | ||
2999 | |||
3000 | The code works fine on ANSI C, C++, and pre-ANSI C compilers, using any of | ||
3001 | the popular system include file setups, and some not-so-popular ones too. | ||
3002 | See install.txt for configuration procedures. | ||
3003 | |||
3004 | The code is not dependent on the exact sizes of the C data types. As | ||
3005 | distributed, we make the assumptions that | ||
3006 | char is at least 8 bits wide | ||
3007 | short is at least 16 bits wide | ||
3008 | int is at least 16 bits wide | ||
3009 | long is at least 32 bits wide | ||
3010 | (These are the minimum requirements of the ANSI C standard.) Wider types will | ||
3011 | work fine, although memory may be used inefficiently if char is much larger | ||
3012 | than 8 bits or short is much bigger than 16 bits. The code should work | ||
3013 | equally well with 16- or 32-bit ints. | ||
3014 | |||
3015 | In a system where these assumptions are not met, you may be able to make the | ||
3016 | code work by modifying the typedefs in jmorecfg.h. However, you will probably | ||
3017 | have difficulty if int is less than 16 bits wide, since references to plain | ||
3018 | int abound in the code. | ||
3019 | |||
3020 | char can be either signed or unsigned, although the code runs faster if an | ||
3021 | unsigned char type is available. If char is wider than 8 bits, you will need | ||
3022 | to redefine JOCTET and/or provide custom data source/destination managers so | ||
3023 | that JOCTET represents exactly 8 bits of data on external storage. | ||
3024 | |||
3025 | The JPEG library proper does not assume ASCII representation of characters. | ||
3026 | But some of the image file I/O modules in cjpeg/djpeg do have ASCII | ||
3027 | dependencies in file-header manipulation; so does cjpeg's select_file_type() | ||
3028 | routine. | ||
3029 | |||
3030 | The JPEG library does not rely heavily on the C library. In particular, C | ||
3031 | stdio is used only by the data source/destination modules and the error | ||
3032 | handler, all of which are application-replaceable. (cjpeg/djpeg are more | ||
3033 | heavily dependent on stdio.) malloc and free are called only from the memory | ||
3034 | manager "back end" module, so you can use a different memory allocator by | ||
3035 | replacing that one file. | ||
3036 | |||
3037 | The code generally assumes that C names must be unique in the first 15 | ||
3038 | characters. However, global function names can be made unique in the | ||
3039 | first 6 characters by defining NEED_SHORT_EXTERNAL_NAMES. | ||
3040 | |||
3041 | More info about porting the code may be gleaned by reading jconfig.txt, | ||
3042 | jmorecfg.h, and jinclude.h. | ||
3043 | |||
3044 | |||
3045 | Notes for MS-DOS implementors | ||
3046 | ----------------------------- | ||
3047 | |||
3048 | The IJG code is designed to work efficiently in 80x86 "small" or "medium" | ||
3049 | memory models (i.e., data pointers are 16 bits unless explicitly declared | ||
3050 | "far"; code pointers can be either size). You may be able to use small | ||
3051 | model to compile cjpeg or djpeg by itself, but you will probably have to use | ||
3052 | medium model for any larger application. This won't make much difference in | ||
3053 | performance. You *will* take a noticeable performance hit if you use a | ||
3054 | large-data memory model (perhaps 10%-25%), and you should avoid "huge" model | ||
3055 | if at all possible. | ||
3056 | |||
3057 | The JPEG library typically needs 2Kb-3Kb of stack space. It will also | ||
3058 | malloc about 20K-30K of near heap space while executing (and lots of far | ||
3059 | heap, but that doesn't count in this calculation). This figure will vary | ||
3060 | depending on selected operating mode, and to a lesser extent on image size. | ||
3061 | There is also about 5Kb-6Kb of constant data which will be allocated in the | ||
3062 | near data segment (about 4Kb of this is the error message table). | ||
3063 | Thus you have perhaps 20K available for other modules' static data and near | ||
3064 | heap space before you need to go to a larger memory model. The C library's | ||
3065 | static data will account for several K of this, but that still leaves a good | ||
3066 | deal for your needs. (If you are tight on space, you could reduce the sizes | ||
3067 | of the I/O buffers allocated by jdatasrc.c and jdatadst.c, say from 4K to | ||
3068 | 1K. Another possibility is to move the error message table to far memory; | ||
3069 | this should be doable with only localized hacking on jerror.c.) | ||
3070 | |||
3071 | About 2K of the near heap space is "permanent" memory that will not be | ||
3072 | released until you destroy the JPEG object. This is only an issue if you | ||
3073 | save a JPEG object between compression or decompression operations. | ||
3074 | |||
3075 | Far data space may also be a tight resource when you are dealing with large | ||
3076 | images. The most memory-intensive case is decompression with two-pass color | ||
3077 | quantization, or single-pass quantization to an externally supplied color | ||
3078 | map. This requires a 128Kb color lookup table plus strip buffers amounting | ||
3079 | to about 40 bytes per column for typical sampling ratios (eg, about 25600 | ||
3080 | bytes for a 640-pixel-wide image). You may not be able to process wide | ||
3081 | images if you have large data structures of your own. | ||
3082 | |||
3083 | Of course, all of these concerns vanish if you use a 32-bit flat-memory-model | ||
3084 | compiler, such as DJGPP or Watcom C. We highly recommend flat model if you | ||
3085 | can use it; the JPEG library is significantly faster in flat model. | ||