1 USAGE instructions for the Independent JPEG Group's JPEG software
2 =================================================================
4 This file describes usage of the JPEG conversion programs cjpeg and djpeg,
5 as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See
6 the other documentation files if you wish to use the JPEG library within
9 If you are on a Unix machine you may prefer to read the Unix-style manual
10 pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.
15 These programs implement JPEG image encoding, decoding, and transcoding.
16 JPEG (pronounced "jay-peg") is a standardized compression method for
17 full-color and gray-scale images.
22 We provide two programs, cjpeg to compress an image file into JPEG format,
23 and djpeg to decompress a JPEG file back into a conventional image format.
25 On Unix-like systems, you say:
26 cjpeg [switches] [imagefile] >jpegfile
28 djpeg [switches] [jpegfile] >imagefile
29 The programs read the specified input file, or standard input if none is
30 named. They always write to standard output (with trace/error messages to
31 standard error). These conventions are handy for piping images between
34 On most non-Unix systems, you say:
35 cjpeg [switches] imagefile jpegfile
37 djpeg [switches] jpegfile imagefile
38 i.e., both the input and output files are named on the command line. This
39 style is a little more foolproof, and it loses no functionality if you don't
40 have pipes. (You can get this style on Unix too, if you prefer, by defining
41 TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.)
44 cjpeg [switches] -outfile jpegfile imagefile
46 djpeg [switches] -outfile imagefile jpegfile
47 This syntax works on all systems, so it is useful for scripts.
49 The currently supported image file formats are: PPM (PBMPLUS color format),
50 PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Toolkit
51 format). (RLE is supported only if the URT library is available.)
52 cjpeg recognizes the input image format automatically, with the exception
53 of some Targa-format files. You have to tell djpeg which format to generate.
55 JPEG files are in the defacto standard JFIF file format. There are other,
56 less widely used JPEG-based file formats, but we don't support them.
58 All switch names may be abbreviated; for example, -grayscale may be written
59 -gray or -gr. Most of the "basic" switches can be abbreviated to as little as
60 one letter. Upper and lower case are equivalent (-BMP is the same as -bmp).
61 British spellings are also accepted (e.g., -greyscale), though for brevity
62 these are not mentioned below.
67 The basic command line switches for cjpeg are:
69 -quality N[,...] Scale quantization tables to adjust image quality.
70 Quality is 0 (worst) to 100 (best); default is 75.
71 (See below for more info.)
73 -grayscale Create monochrome JPEG file from color input.
74 Be sure to use this switch when compressing a grayscale
75 BMP file, because cjpeg isn't bright enough to notice
76 whether a BMP file uses only shades of gray. By
77 saying -grayscale, you'll get a smaller JPEG file that
78 takes less time to process.
80 -optimize Perform optimization of entropy encoding parameters.
81 Without this, default encoding parameters are used.
82 -optimize usually makes the JPEG file a little smaller,
83 but cjpeg runs somewhat slower and needs much more
84 memory. Image quality and speed of decompression are
85 unaffected by -optimize.
87 -progressive Create progressive JPEG file (see below).
89 -scale M/N Scale the output image by a factor M/N. Currently
90 supported scale factors are M/N with all N from 1 to
91 16, where M is the destination DCT size, which is 8 by
92 default (see -block N switch below).
94 -targa Input file is Targa format. Targa files that contain
95 an "identification" field will not be automatically
96 recognized by cjpeg; for such files you must specify
97 -targa to make cjpeg treat the input as Targa format.
98 For most Targa files, you won't need this switch.
100 The -quality switch lets you trade off compressed file size against quality of
101 the reconstructed image: the higher the quality setting, the larger the JPEG
102 file, and the closer the output image will be to the original input. Normally
103 you want to use the lowest quality setting (smallest file) that decompresses
104 into something visually indistinguishable from the original image. For this
105 purpose the quality setting should be between 50 and 95; the default of 75 is
106 often about right. If you see defects at -quality 75, then go up 5 or 10
107 counts at a time until you are happy with the output image. (The optimal
108 setting will vary from one image to another.)
110 -quality 100 will generate a quantization table of all 1's, minimizing loss
111 in the quantization step (but there is still information loss in subsampling,
112 as well as roundoff error). This setting is mainly of interest for
113 experimental purposes. Quality values above about 95 are NOT recommended for
114 normal use; the compressed file size goes up dramatically for hardly any gain
115 in output image quality.
117 In the other direction, quality values below 50 will produce very small files
118 of low image quality. Settings around 5 to 10 might be useful in preparing an
119 index of a large image library, for example. Try -quality 2 (or so) for some
120 amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
121 quantization tables, which are considered optional in the JPEG standard.
122 cjpeg emits a warning message when you give such a quality value, because some
123 other JPEG programs may be unable to decode the resulting file. Use -baseline
124 if you need to ensure compatibility at low quality values.)
126 The -quality option has been extended in IJG version 7 for support of separate
127 quality settings for luminance and chrominance (or in general, for every
128 provided quantization table slot). This feature is useful for high-quality
129 applications which cannot accept the damage of color data by coarse
130 subsampling settings. You can now easily reduce the color data amount more
131 smoothly with finer control without separate subsampling. The resulting file
132 is fully compliant with standard JPEG decoders.
133 Note that the -quality ratings refer to the quantization table slots, and that
134 the last value is replicated if there are more q-table slots than parameters.
135 The default q-table slots are 0 for luminance and 1 for chrominance with
136 default tables as given in the JPEG standard. This is compatible with the old
137 behaviour in case that only one parameter is given, which is then used for
138 both luminance and chrominance (slots 0 and 1). More or custom quantization
139 tables can be set with -qtables and assigned to components with -qslots
140 parameter (see the "wizard" switches below).
141 CAUTION: You must explicitly add -sample 1x1 for efficient separate color
142 quality selection, since the default value used by library is 2x2!
144 The -progressive switch creates a "progressive JPEG" file. In this type of
145 JPEG file, the data is stored in multiple scans of increasing quality. If the
146 file is being transmitted over a slow communications link, the decoder can use
147 the first scan to display a low-quality image very quickly, and can then
148 improve the display with each subsequent scan. The final image is exactly
149 equivalent to a standard JPEG file of the same quality setting, and the total
150 file size is about the same --- often a little smaller.
152 Switches for advanced users:
154 -block N Set DCT block size. All N from 1 to 16 are possible.
155 Default is 8 (baseline format).
156 Larger values produce higher compression,
157 smaller values produce higher quality
158 (exact DCT stage possible with 1 or 2; with the
159 default quality of 75 and default Luminance qtable
160 the DCT+Quantization stage is lossless for N=1).
161 CAUTION: An implementation of the JPEG SmartScale
162 extension is required for this feature. SmartScale
163 enabled JPEG is not yet widely implemented, so many
164 decoders will be unable to view a SmartScale extended
167 -dct int Use integer DCT method (default).
168 -dct fast Use fast integer DCT (less accurate).
169 -dct float Use floating-point DCT method.
170 The float method is very slightly more accurate than
171 the int method, but is much slower unless your machine
172 has very fast floating-point hardware. Also note that
173 results of the floating-point method may vary slightly
174 across machines, while the integer methods should give
175 the same results everywhere. The fast integer method
176 is much less accurate than the other two.
178 -nosmooth Don't use high-quality downsampling.
180 -restart N Emit a JPEG restart marker every N MCU rows, or every
181 N MCU blocks if "B" is attached to the number.
182 -restart 0 (the default) means no restart markers.
184 -smooth N Smooth the input image to eliminate dithering noise.
185 N, ranging from 1 to 100, indicates the strength of
186 smoothing. 0 (the default) means no smoothing.
188 -maxmemory N Set limit for amount of memory to use in processing
189 large images. Value is in thousands of bytes, or
190 millions of bytes if "M" is attached to the number.
191 For example, -max 4m selects 4000000 bytes. If more
192 space is needed, temporary files will be used.
194 -verbose Enable debug printout. More -v's give more printout.
195 or -debug Also, version information is printed at startup.
197 The -restart option inserts extra markers that allow a JPEG decoder to
198 resynchronize after a transmission error. Without restart markers, any damage
199 to a compressed file will usually ruin the image from the point of the error
200 to the end of the image; with restart markers, the damage is usually confined
201 to the portion of the image up to the next restart marker. Of course, the
202 restart markers occupy extra space. We recommend -restart 1 for images that
203 will be transmitted across unreliable networks such as Usenet.
205 The -smooth option filters the input to eliminate fine-scale noise. This is
206 often useful when converting dithered images to JPEG: a moderate smoothing
207 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
208 in a smaller JPEG file and a better-looking image. Too large a smoothing
209 factor will visibly blur the image, however.
211 Switches for wizards:
213 -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG
214 is not yet widely implemented, so many decoders will
215 be unable to view an arithmetic coded JPEG file at
218 -baseline Force baseline-compatible quantization tables to be
219 generated. This clamps quantization values to 8 bits
220 even at low quality settings. (This switch is poorly
221 named, since it does not ensure that the output is
222 actually baseline JPEG. For example, you can use
223 -baseline and -progressive together.)
225 -qtables file Use the quantization tables given in the specified
228 -qslots N[,...] Select which quantization table to use for each color
231 -sample HxV[,...] Set JPEG sampling factors for each color component.
233 -scans file Use the scan script given in the specified text file.
235 The "wizard" switches are intended for experimentation with JPEG. If you
236 don't know what you are doing, DON'T USE THEM. These switches are documented
237 further in the file wizard.txt.
242 The basic command line switches for djpeg are:
244 -colors N Reduce image to at most N colors. This reduces the
245 or -quantize N number of colors used in the output image, so that it
246 can be displayed on a colormapped display or stored in
247 a colormapped file format. For example, if you have
248 an 8-bit display, you'd need to reduce to 256 or fewer
249 colors. (-colors is the recommended name, -quantize
250 is provided only for backwards compatibility.)
252 -fast Select recommended processing options for fast, low
253 quality output. (The default options are chosen for
254 highest quality output.) Currently, this is equivalent
255 to "-dct fast -nosmooth -onepass -dither ordered".
257 -grayscale Force gray-scale output even if JPEG file is color.
258 Useful for viewing on monochrome displays; also,
259 djpeg runs noticeably faster in this mode.
261 -scale M/N Scale the output image by a factor M/N. Currently
262 supported scale factors are M/N with all M from 1 to
263 16, where N is the source DCT size, which is 8 for
264 baseline JPEG. If the /N part is omitted, then M
265 specifies the DCT scaled size to be applied on the
266 given input. For baseline JPEG this is equivalent to
267 M/8 scaling, since the source DCT size for baseline
268 JPEG is 8. Scaling is handy if the image is larger
269 than your screen; also, djpeg runs much faster when
270 scaling down the output.
272 -bmp Select BMP output format (Windows flavor). 8-bit
273 colormapped format is emitted if -colors or -grayscale
274 is specified, or if the JPEG file is gray-scale;
275 otherwise, 24-bit full-color format is emitted.
277 -gif Select GIF output format. Since GIF does not support
278 more than 256 colors, -colors 256 is assumed (unless
279 you specify a smaller number of colors). If you
280 specify -fast, the default number of colors is 216.
282 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit
283 colormapped format is emitted if -colors or -grayscale
284 is specified, or if the JPEG file is gray-scale;
285 otherwise, 24-bit full-color format is emitted.
287 -pnm Select PBMPLUS (PPM/PGM) output format (this is the
288 default format). PGM is emitted if the JPEG file is
289 gray-scale or if -grayscale is specified; otherwise
292 -rle Select RLE output format. (Requires URT library.)
294 -targa Select Targa output format. Gray-scale format is
295 emitted if the JPEG file is gray-scale or if
296 -grayscale is specified; otherwise, colormapped format
297 is emitted if -colors is specified; otherwise, 24-bit
298 full-color format is emitted.
300 Switches for advanced users:
302 -dct int Use integer DCT method (default).
303 -dct fast Use fast integer DCT (less accurate).
304 -dct float Use floating-point DCT method.
305 The float method is very slightly more accurate than
306 the int method, but is much slower unless your machine
307 has very fast floating-point hardware. Also note that
308 results of the floating-point method may vary slightly
309 across machines, while the integer methods should give
310 the same results everywhere. The fast integer method
311 is much less accurate than the other two.
313 -dither fs Use Floyd-Steinberg dithering in color quantization.
314 -dither ordered Use ordered dithering in color quantization.
315 -dither none Do not use dithering in color quantization.
316 By default, Floyd-Steinberg dithering is applied when
317 quantizing colors; this is slow but usually produces
318 the best results. Ordered dither is a compromise
319 between speed and quality; no dithering is fast but
320 usually looks awful. Note that these switches have
321 no effect unless color quantization is being done.
322 Ordered dither is only available in -onepass mode.
324 -map FILE Quantize to the colors used in the specified image
325 file. This is useful for producing multiple files
326 with identical color maps, or for forcing a predefined
327 set of colors to be used. The FILE must be a GIF
328 or PPM file. This option overrides -colors and
331 -nosmooth Don't use high-quality upsampling.
333 -onepass Use one-pass instead of two-pass color quantization.
334 The one-pass method is faster and needs less memory,
335 but it produces a lower-quality image. -onepass is
336 ignored unless you also say -colors N. Also,
337 the one-pass method is always used for gray-scale
338 output (the two-pass method is no improvement then).
340 -maxmemory N Set limit for amount of memory to use in processing
341 large images. Value is in thousands of bytes, or
342 millions of bytes if "M" is attached to the number.
343 For example, -max 4m selects 4000000 bytes. If more
344 space is needed, temporary files will be used.
346 -verbose Enable debug printout. More -v's give more printout.
347 or -debug Also, version information is printed at startup.
352 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
353 compressing full-color (24-bit) images. In particular, don't try to convert
354 cartoons, line drawings, and other images that have only a few distinct
355 colors. GIF works great on these, JPEG does not. If you want to convert a
356 GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
357 to get a satisfactory conversion. -smooth 10 or so is often helpful.
359 Avoid running an image through a series of JPEG compression/decompression
360 cycles. Image quality loss will accumulate; after ten or so cycles the image
361 may be noticeably worse than it was after one cycle. It's best to use a
362 lossless format while manipulating an image, then convert to JPEG format when
363 you are ready to file the image away.
365 The -optimize option to cjpeg is worth using when you are making a "final"
366 version for posting or archiving. It's also a win when you are using low
367 quality settings to make very small JPEG files; the percentage improvement
368 is often a lot more than it is on larger files. (At present, -optimize
369 mode is always selected when generating progressive JPEG files.)
371 GIF input files are no longer supported, to avoid the Unisys LZW patent (now expired).
372 (Conversion of GIF files to JPEG is usually a bad idea anyway.)
377 To get a quick preview of an image, use the -grayscale and/or -scale switches.
378 "-grayscale -scale 1/8" is the fastest case.
380 Several options are available that trade off image quality to gain speed.
381 "-fast" turns on the recommended settings.
383 "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
384 When producing a color-quantized image, "-onepass -dither ordered" is fast but
385 much lower quality than the default behavior. "-dither none" may give
386 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
388 If you are fortunate enough to have very fast floating point hardware,
389 "-dct float" may be even faster than "-dct fast". But on most machines
390 "-dct float" is slower than "-dct int"; in this case it is not worth using,
391 because its theoretical accuracy advantage is too small to be significant
394 Two-pass color quantization requires a good deal of memory; on MS-DOS machines
395 it may run out of memory even with -maxmemory 0. In that case you can still
396 decompress, with some loss of image quality, by specifying -onepass for
397 one-pass quantization.
399 To avoid the Unisys LZW patent (now expired), djpeg produces uncompressed GIF files. These
400 are larger than they should be, but are readable by standard GIF decoders.
403 HINTS FOR BOTH PROGRAMS
405 If more space is needed than will fit in the available main memory (as
406 determined by -maxmemory), temporary files will be used. (MS-DOS versions
407 will try to get extended or expanded memory first.) The temporary files are
408 often rather large: in typical cases they occupy three bytes per pixel, for
409 example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
410 free disk space, leave out -progressive and -optimize (for cjpeg) or specify
411 -onepass (for djpeg).
413 On MS-DOS, the temporary files are created in the directory named by the TMP
414 or TEMP environment variable, or in the current directory if neither of those
415 exist. Amiga implementations put the temp files in the directory named by
416 JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
419 The default memory usage limit (-maxmemory) is set when the software is
420 compiled. If you get an "insufficient memory" error, try specifying a smaller
421 -maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
422 may want to recompile with a smaller default value if this happens often.
424 On machines that have "environment" variables, you can define the environment
425 variable JPEGMEM to set the default memory limit. The value is specified as
426 described for the -maxmemory switch. JPEGMEM overrides the default value
427 specified when the program was compiled, and itself is overridden by an
428 explicit -maxmemory switch.
430 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
431 use. (Extended or expanded memory is also used if available.) Most
432 DOS-specific versions of this software do their own memory space estimation
433 and do not need you to specify -maxmemory.
438 jpegtran performs various useful transformations of JPEG files.
439 It can translate the coded representation from one variant of JPEG to another,
440 for example from baseline JPEG to progressive JPEG or vice versa. It can also
441 perform some rearrangements of the image data, for example turning an image
442 from landscape to portrait format by rotation.
444 jpegtran works by rearranging the compressed data (DCT coefficients), without
445 ever fully decoding the image. Therefore, its transformations are lossless:
446 there is no image degradation at all, which would not be true if you used
447 djpeg followed by cjpeg to accomplish the same conversion. But by the same
448 token, jpegtran cannot perform lossy operations such as changing the image
451 jpegtran uses a command line syntax similar to cjpeg or djpeg.
452 On Unix-like systems, you say:
453 jpegtran [switches] [inputfile] >outputfile
454 On most non-Unix systems, you say:
455 jpegtran [switches] inputfile outputfile
456 where both the input and output files are JPEG files.
458 To specify the coded JPEG representation used in the output file,
459 jpegtran accepts a subset of the switches recognized by cjpeg:
460 -optimize Perform optimization of entropy encoding parameters.
461 -progressive Create progressive JPEG file.
462 -restart N Emit a JPEG restart marker every N MCU rows, or every
463 N MCU blocks if "B" is attached to the number.
464 -arithmetic Use arithmetic coding.
465 -scans file Use the scan script given in the specified text file.
466 See the previous discussion of cjpeg for more details about these switches.
467 If you specify none of these switches, you get a plain baseline-JPEG output
468 file. The quality setting and so forth are determined by the input file.
470 The image can be losslessly transformed by giving one of these switches:
471 -flip horizontal Mirror image horizontally (left-right).
472 -flip vertical Mirror image vertically (top-bottom).
473 -rotate 90 Rotate image 90 degrees clockwise.
474 -rotate 180 Rotate image 180 degrees.
475 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw).
476 -transpose Transpose image (across UL-to-LR axis).
477 -transverse Transverse transpose (across UR-to-LL axis).
479 The transpose transformation has no restrictions regarding image dimensions.
480 The other transformations operate rather oddly if the image dimensions are not
481 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
482 transform complete blocks of DCT coefficient data in the desired way.
484 jpegtran's default behavior when transforming an odd-size image is designed
485 to preserve exact reversibility and mathematical consistency of the
486 transformation set. As stated, transpose is able to flip the entire image
487 area. Horizontal mirroring leaves any partial iMCU column at the right edge
488 untouched, but is able to flip all rows of the image. Similarly, vertical
489 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
490 able to flip all columns. The other transforms can be built up as sequences
491 of transpose and flip operations; for consistency, their actions on edge
492 pixels are defined to be the same as the end result of the corresponding
493 transpose-and-flip sequence.
495 For practical use, you may prefer to discard any untransformable edge pixels
496 rather than having a strange-looking strip along the right and/or bottom edges
497 of a transformed image. To do this, add the -trim switch:
498 -trim Drop non-transformable edge blocks.
499 Obviously, a transformation with -trim is not reversible, so strictly speaking
500 jpegtran with this switch is not lossless. Also, the expected mathematical
501 equivalences between the transformations no longer hold. For example,
502 "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
503 "-rot 180 -trim" trims both edges.
505 If you are only interested in perfect transformation, add the -perfect switch:
506 -perfect Fails with an error if the transformation is not
508 For example you may want to do
509 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
510 to do a perfect rotation if available or an approximated one if not.
512 We also offer a lossless-crop option, which discards data outside a given
513 image region but losslessly preserves what is inside. Like the rotate and
514 flip transforms, lossless crop is restricted by the current JPEG format: the
515 upper left corner of the selected region must fall on an iMCU boundary. If
516 this does not hold for the given crop parameters, we silently move the upper
517 left corner up and/or left to make it so, simultaneously increasing the region
518 dimensions to keep the lower right crop corner unchanged. (Thus, the output
519 image covers at least the requested region, but may cover more.)
521 The image can be losslessly cropped by giving the switch:
522 -crop WxH+X+Y Crop to a rectangular subarea of width W, height H
523 starting at point X,Y.
525 Other not-strictly-lossless transformation switches are:
527 -grayscale Force grayscale output.
528 This option discards the chrominance channels if the input image is YCbCr
529 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
530 luminance channel is preserved exactly, so this is a better method of reducing
531 to grayscale than decompression, conversion, and recompression. This switch
532 is particularly handy for fixing a monochrome picture that was mistakenly
533 encoded as a color JPEG. (In such a case, the space savings from getting rid
534 of the near-empty chroma channels won't be large; but the decoding time for
535 a grayscale JPEG is substantially less than that for a color JPEG.)
537 -scale M/N Scale the output image by a factor M/N.
538 Currently supported scale factors are M/N with all M from 1 to 16, where N is
539 the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
540 then M specifies the DCT scaled size to be applied on the given input. For
541 baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
542 for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale
543 extension is required for this feature. SmartScale enabled JPEG is not yet
544 widely implemented, so many decoders will be unable to view a SmartScale
545 extended JPEG file at all.
547 jpegtran also recognizes these switches that control what to do with "extra"
548 markers, such as comment blocks:
549 -copy none Copy no extra markers from source file. This setting
550 suppresses all comments and other excess baggage
551 present in the source file.
552 -copy comments Copy only comment markers. This setting copies
553 comments from the source file, but discards
554 any other inessential (for image display) data.
555 -copy all Copy all extra markers. This setting preserves
556 miscellaneous markers found in the source file, such
557 as JFIF thumbnails, Exif data, and Photoshop settings.
558 In some files these extra markers can be sizable.
559 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
560 jpegtran always did the equivalent of -copy none.)
562 Additional switches recognized by jpegtran are:
567 These work the same as in cjpeg or djpeg.
570 THE COMMENT UTILITIES
572 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
573 Although the standard doesn't actually define what COM blocks are for, they
574 are widely used to hold user-supplied text strings. This lets you add
575 annotations, titles, index terms, etc to your JPEG files, and later retrieve
576 them as text. COM blocks do not interfere with the image stored in the JPEG
577 file. The maximum size of a COM block is 64K, but you can have as many of
578 them as you like in one JPEG file.
580 We provide two utility programs to display COM block contents and add COM
581 blocks to a JPEG file.
583 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
584 standard output. The command line syntax is
585 rdjpgcom [-raw] [-verbose] [inputfilename]
586 The switch "-raw" (or just "-r") causes rdjpgcom to also output non-printable
587 characters in comments, which are normally escaped for security reasons.
588 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
589 image dimensions. If you omit the input file name from the command line,
590 the JPEG file is read from standard input. (This may not work on some
591 operating systems, if binary data can't be read from stdin.)
593 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
594 Ordinarily, the COM block is added after any existing COM blocks, but you
595 can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
596 file; it does not modify the input file. DO NOT try to overwrite the input
597 file by directing wrjpgcom's output back into it; on most systems this will
598 just destroy your file.
600 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
602 wrjpgcom [switches] [inputfilename]
603 The output file is written to standard output. The input file comes from
604 the named file, or from standard input if no input file is named.
606 On most non-Unix systems, the syntax is
607 wrjpgcom [switches] inputfilename outputfilename
608 where both input and output file names must be given explicitly.
610 wrjpgcom understands three switches:
611 -replace Delete any existing COM blocks from the file.
612 -comment "Comment text" Supply new COM text on command line.
613 -cfile name Read text for new COM block from named file.
614 (Switch names can be abbreviated.) If you have only one line of comment text
615 to add, you can provide it on the command line with -comment. The comment
616 text must be surrounded with quotes so that it is treated as a single
617 argument. Longer comments can be read from a text file.
619 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
620 text from standard input. (In this case an input image file name MUST be
621 supplied, so that the source JPEG file comes from somewhere else.) You can
622 enter multiple lines, up to 64KB worth. Type an end-of-file indicator
623 (usually control-D or control-Z) to terminate the comment text entry.
625 wrjpgcom will not add a COM block if the provided comment string is empty.
626 Therefore -replace -comment "" can be used to delete all COM blocks from a
629 These utility programs do not depend on the IJG JPEG library. In
630 particular, the source code for rdjpgcom is intended as an illustration of
631 the minimum amount of code required to parse a JPEG file header correctly.
projects / framework / multimedia / libjpeg8.git / blob