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 -targa Input file is Targa format. Targa files that contain
90 an "identification" field will not be automatically
91 recognized by cjpeg; for such files you must specify
92 -targa to make cjpeg treat the input as Targa format.
93 For most Targa files, you won't need this switch.
95 The -quality switch lets you trade off compressed file size against quality of
96 the reconstructed image: the higher the quality setting, the larger the JPEG
97 file, and the closer the output image will be to the original input. Normally
98 you want to use the lowest quality setting (smallest file) that decompresses
99 into something visually indistinguishable from the original image. For this
100 purpose the quality setting should be between 50 and 95; the default of 75 is
101 often about right. If you see defects at -quality 75, then go up 5 or 10
102 counts at a time until you are happy with the output image. (The optimal
103 setting will vary from one image to another.)
105 -quality 100 will generate a quantization table of all 1's, minimizing loss
106 in the quantization step (but there is still information loss in subsampling,
107 as well as roundoff error). This setting is mainly of interest for
108 experimental purposes. Quality values above about 95 are NOT recommended for
109 normal use; the compressed file size goes up dramatically for hardly any gain
110 in output image quality.
112 In the other direction, quality values below 50 will produce very small files
113 of low image quality. Settings around 5 to 10 might be useful in preparing an
114 index of a large image library, for example. Try -quality 2 (or so) for some
115 amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
116 quantization tables, which are considered optional in the JPEG standard.
117 cjpeg emits a warning message when you give such a quality value, because some
118 other JPEG programs may be unable to decode the resulting file. Use -baseline
119 if you need to ensure compatibility at low quality values.)
121 The -quality option has been extended in this version of cjpeg to support
122 separate quality settings for luminance and chrominance (or, in general,
123 separate settings for every quantization table slot.) The principle is the
124 same as chrominance subsampling: since the human eye is more sensitive to
125 spatial changes in brightness than spatial changes in color, the chrominance
126 components can be quantized more than the luminance components without
127 incurring any visible image quality loss. However, unlike subsampling, this
128 feature reduces data in the frequency domain instead of the spatial domain,
129 which allows for more fine-grained control. This option is useful in
130 quality-sensitive applications, for which the artifacts generated by
131 subsampling may be unacceptable.
133 The -quality option accepts a comma-separated list of parameters, which
134 respectively refer to the quality levels that should be assigned to the
135 quantization table slots. If there are more q-table slots than parameters,
136 then the last parameter is replicated. Thus, if only one quality parameter is
137 given, this is used for both luminance and chrominance (slots 0 and 1,
138 respectively), preserving the legacy behavior of cjpeg v6b and prior. More (or
139 customized) quantization tables can be set with the -qtables option and
140 assigned to components with the -qslots option (see the "wizard" switches
143 JPEG files generated with separate luminance and chrominance quality are
144 fully compliant with standard JPEG decoders.
146 CAUTION: For this setting to be useful, be sure to pass an argument of
147 -sample 1x1 to cjpeg to disable chrominance subsampling. Otherwise, the
148 default subsampling level (2x2, AKA "4:2:0") will be used.
150 The -progressive switch creates a "progressive JPEG" file. In this type of
151 JPEG file, the data is stored in multiple scans of increasing quality. If the
152 file is being transmitted over a slow communications link, the decoder can use
153 the first scan to display a low-quality image very quickly, and can then
154 improve the display with each subsequent scan. The final image is exactly
155 equivalent to a standard JPEG file of the same quality setting, and the total
156 file size is about the same --- often a little smaller.
158 Switches for advanced users:
160 -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG
161 is not yet widely implemented, so many decoders will
162 be unable to view an arithmetic coded JPEG file at
165 -dct int Use integer DCT method (default).
166 -dct fast Use fast integer DCT (less accurate).
167 -dct float Use floating-point DCT method.
168 The float method is very slightly more accurate than
169 the int method, but is much slower unless your machine
170 has very fast floating-point hardware. Also note that
171 results of the floating-point method may vary slightly
172 across machines, while the integer methods should give
173 the same results everywhere. The fast integer method
174 is much less accurate than the other two.
176 -restart N Emit a JPEG restart marker every N MCU rows, or every
177 N MCU blocks if "B" is attached to the number.
178 -restart 0 (the default) means no restart markers.
180 -smooth N Smooth the input image to eliminate dithering noise.
181 N, ranging from 1 to 100, indicates the strength of
182 smoothing. 0 (the default) means no smoothing.
184 -maxmemory N Set limit for amount of memory to use in processing
185 large images. Value is in thousands of bytes, or
186 millions of bytes if "M" is attached to the number.
187 For example, -max 4m selects 4000000 bytes. If more
188 space is needed, temporary files will be used.
190 -verbose Enable debug printout. More -v's give more printout.
191 or -debug Also, version information is printed at startup.
193 The -restart option inserts extra markers that allow a JPEG decoder to
194 resynchronize after a transmission error. Without restart markers, any damage
195 to a compressed file will usually ruin the image from the point of the error
196 to the end of the image; with restart markers, the damage is usually confined
197 to the portion of the image up to the next restart marker. Of course, the
198 restart markers occupy extra space. We recommend -restart 1 for images that
199 will be transmitted across unreliable networks such as Usenet.
201 The -smooth option filters the input to eliminate fine-scale noise. This is
202 often useful when converting dithered images to JPEG: a moderate smoothing
203 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
204 in a smaller JPEG file and a better-looking image. Too large a smoothing
205 factor will visibly blur the image, however.
207 Switches for wizards:
209 -baseline Force baseline-compatible quantization tables to be
210 generated. This clamps quantization values to 8 bits
211 even at low quality settings. (This switch is poorly
212 named, since it does not ensure that the output is
213 actually baseline JPEG. For example, you can use
214 -baseline and -progressive together.)
216 -qtables file Use the quantization tables given in the specified
219 -qslots N[,...] Select which quantization table to use for each color
222 -sample HxV[,...] Set JPEG sampling factors for each color component.
224 -scans file Use the scan script given in the specified text file.
226 The "wizard" switches are intended for experimentation with JPEG. If you
227 don't know what you are doing, DON'T USE THEM. These switches are documented
228 further in the file wizard.txt.
233 The basic command line switches for djpeg are:
235 -colors N Reduce image to at most N colors. This reduces the
236 or -quantize N number of colors used in the output image, so that it
237 can be displayed on a colormapped display or stored in
238 a colormapped file format. For example, if you have
239 an 8-bit display, you'd need to reduce to 256 or fewer
240 colors. (-colors is the recommended name, -quantize
241 is provided only for backwards compatibility.)
243 -fast Select recommended processing options for fast, low
244 quality output. (The default options are chosen for
245 highest quality output.) Currently, this is equivalent
246 to "-dct fast -nosmooth -onepass -dither ordered".
248 -grayscale Force gray-scale output even if JPEG file is color.
249 Useful for viewing on monochrome displays; also,
250 djpeg runs noticeably faster in this mode.
252 -scale M/N Scale the output image by a factor M/N. Currently
253 the scale factor must be 1/1, 1/2, 1/4, or 1/8.
254 Scaling is handy if the image is larger than your
255 screen; also, djpeg runs much faster when scaling
258 -bmp Select BMP output format (Windows flavor). 8-bit
259 colormapped format is emitted if -colors or -grayscale
260 is specified, or if the JPEG file is gray-scale;
261 otherwise, 24-bit full-color format is emitted.
263 -gif Select GIF output format. Since GIF does not support
264 more than 256 colors, -colors 256 is assumed (unless
265 you specify a smaller number of colors). If you
266 specify -fast, the default number of colors is 216.
268 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit
269 colormapped format is emitted if -colors or -grayscale
270 is specified, or if the JPEG file is gray-scale;
271 otherwise, 24-bit full-color format is emitted.
273 -pnm Select PBMPLUS (PPM/PGM) output format (this is the
274 default format). PGM is emitted if the JPEG file is
275 gray-scale or if -grayscale is specified; otherwise
278 -rle Select RLE output format. (Requires URT library.)
280 -targa Select Targa output format. Gray-scale format is
281 emitted if the JPEG file is gray-scale or if
282 -grayscale is specified; otherwise, colormapped format
283 is emitted if -colors is specified; otherwise, 24-bit
284 full-color format is emitted.
286 Switches for advanced users:
288 -dct int Use integer DCT method (default).
289 -dct fast Use fast integer DCT (less accurate).
290 -dct float Use floating-point DCT method.
291 The float method is very slightly more accurate than
292 the int method, but is much slower unless your machine
293 has very fast floating-point hardware. Also note that
294 results of the floating-point method may vary slightly
295 across machines, while the integer methods should give
296 the same results everywhere. The fast integer method
297 is much less accurate than the other two.
299 -dither fs Use Floyd-Steinberg dithering in color quantization.
300 -dither ordered Use ordered dithering in color quantization.
301 -dither none Do not use dithering in color quantization.
302 By default, Floyd-Steinberg dithering is applied when
303 quantizing colors; this is slow but usually produces
304 the best results. Ordered dither is a compromise
305 between speed and quality; no dithering is fast but
306 usually looks awful. Note that these switches have
307 no effect unless color quantization is being done.
308 Ordered dither is only available in -onepass mode.
310 -map FILE Quantize to the colors used in the specified image
311 file. This is useful for producing multiple files
312 with identical color maps, or for forcing a predefined
313 set of colors to be used. The FILE must be a GIF
314 or PPM file. This option overrides -colors and
317 -nosmooth Use a faster, lower-quality upsampling routine.
319 -onepass Use one-pass instead of two-pass color quantization.
320 The one-pass method is faster and needs less memory,
321 but it produces a lower-quality image. -onepass is
322 ignored unless you also say -colors N. Also,
323 the one-pass method is always used for gray-scale
324 output (the two-pass method is no improvement then).
326 -maxmemory N Set limit for amount of memory to use in processing
327 large images. Value is in thousands of bytes, or
328 millions of bytes if "M" is attached to the number.
329 For example, -max 4m selects 4000000 bytes. If more
330 space is needed, temporary files will be used.
332 -verbose Enable debug printout. More -v's give more printout.
333 or -debug Also, version information is printed at startup.
338 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
339 compressing full-color (24-bit) images. In particular, don't try to convert
340 cartoons, line drawings, and other images that have only a few distinct
341 colors. GIF works great on these, JPEG does not. If you want to convert a
342 GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
343 to get a satisfactory conversion. -smooth 10 or so is often helpful.
345 Avoid running an image through a series of JPEG compression/decompression
346 cycles. Image quality loss will accumulate; after ten or so cycles the image
347 may be noticeably worse than it was after one cycle. It's best to use a
348 lossless format while manipulating an image, then convert to JPEG format when
349 you are ready to file the image away.
351 The -optimize option to cjpeg is worth using when you are making a "final"
352 version for posting or archiving. It's also a win when you are using low
353 quality settings to make very small JPEG files; the percentage improvement
354 is often a lot more than it is on larger files. (At present, -optimize
355 mode is always selected when generating progressive JPEG files.)
357 Support for GIF input files was removed in cjpeg v6b due to concerns over
358 the Unisys LZW patent. Although this patent expired in 2006, cjpeg still
359 lacks GIF support, for these historical reasons. (Conversion of GIF files to
360 JPEG is usually a bad idea anyway.)
365 To get a quick preview of an image, use the -grayscale and/or -scale switches.
366 "-grayscale -scale 1/8" is the fastest case.
368 Several options are available that trade off image quality to gain speed.
369 "-fast" turns on the recommended settings.
371 "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
372 When producing a color-quantized image, "-onepass -dither ordered" is fast but
373 much lower quality than the default behavior. "-dither none" may give
374 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
376 If you are fortunate enough to have very fast floating point hardware,
377 "-dct float" may be even faster than "-dct fast". But on most machines
378 "-dct float" is slower than "-dct int"; in this case it is not worth using,
379 because its theoretical accuracy advantage is too small to be significant
382 Two-pass color quantization requires a good deal of memory; on MS-DOS machines
383 it may run out of memory even with -maxmemory 0. In that case you can still
384 decompress, with some loss of image quality, by specifying -onepass for
385 one-pass quantization.
387 To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These
388 are larger than they should be, but are readable by standard GIF decoders.
391 HINTS FOR BOTH PROGRAMS
393 If more space is needed than will fit in the available main memory (as
394 determined by -maxmemory), temporary files will be used. (MS-DOS versions
395 will try to get extended or expanded memory first.) The temporary files are
396 often rather large: in typical cases they occupy three bytes per pixel, for
397 example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
398 free disk space, leave out -progressive and -optimize (for cjpeg) or specify
399 -onepass (for djpeg).
401 On MS-DOS, the temporary files are created in the directory named by the TMP
402 or TEMP environment variable, or in the current directory if neither of those
403 exist. Amiga implementations put the temp files in the directory named by
404 JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
407 The default memory usage limit (-maxmemory) is set when the software is
408 compiled. If you get an "insufficient memory" error, try specifying a smaller
409 -maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
410 may want to recompile with a smaller default value if this happens often.
412 On machines that have "environment" variables, you can define the environment
413 variable JPEGMEM to set the default memory limit. The value is specified as
414 described for the -maxmemory switch. JPEGMEM overrides the default value
415 specified when the program was compiled, and itself is overridden by an
416 explicit -maxmemory switch.
418 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
419 use. (Extended or expanded memory is also used if available.) Most
420 DOS-specific versions of this software do their own memory space estimation
421 and do not need you to specify -maxmemory.
426 jpegtran performs various useful transformations of JPEG files.
427 It can translate the coded representation from one variant of JPEG to another,
428 for example from baseline JPEG to progressive JPEG or vice versa. It can also
429 perform some rearrangements of the image data, for example turning an image
430 from landscape to portrait format by rotation.
432 jpegtran works by rearranging the compressed data (DCT coefficients), without
433 ever fully decoding the image. Therefore, its transformations are lossless:
434 there is no image degradation at all, which would not be true if you used
435 djpeg followed by cjpeg to accomplish the same conversion. But by the same
436 token, jpegtran cannot perform lossy operations such as changing the image
439 jpegtran uses a command line syntax similar to cjpeg or djpeg.
440 On Unix-like systems, you say:
441 jpegtran [switches] [inputfile] >outputfile
442 On most non-Unix systems, you say:
443 jpegtran [switches] inputfile outputfile
444 where both the input and output files are JPEG files.
446 To specify the coded JPEG representation used in the output file,
447 jpegtran accepts a subset of the switches recognized by cjpeg:
448 -optimize Perform optimization of entropy encoding parameters.
449 -progressive Create progressive JPEG file.
450 -arithmetic Use arithmetic coding.
451 -restart N Emit a JPEG restart marker every N MCU rows, or every
452 N MCU blocks if "B" is attached to the number.
453 -scans file Use the scan script given in the specified text file.
454 See the previous discussion of cjpeg for more details about these switches.
455 If you specify none of these switches, you get a plain baseline-JPEG output
456 file. The quality setting and so forth are determined by the input file.
458 The image can be losslessly transformed by giving one of these switches:
459 -flip horizontal Mirror image horizontally (left-right).
460 -flip vertical Mirror image vertically (top-bottom).
461 -rotate 90 Rotate image 90 degrees clockwise.
462 -rotate 180 Rotate image 180 degrees.
463 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw).
464 -transpose Transpose image (across UL-to-LR axis).
465 -transverse Transverse transpose (across UR-to-LL axis).
467 The transpose transformation has no restrictions regarding image dimensions.
468 The other transformations operate rather oddly if the image dimensions are not
469 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
470 transform complete blocks of DCT coefficient data in the desired way.
472 jpegtran's default behavior when transforming an odd-size image is designed
473 to preserve exact reversibility and mathematical consistency of the
474 transformation set. As stated, transpose is able to flip the entire image
475 area. Horizontal mirroring leaves any partial iMCU column at the right edge
476 untouched, but is able to flip all rows of the image. Similarly, vertical
477 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
478 able to flip all columns. The other transforms can be built up as sequences
479 of transpose and flip operations; for consistency, their actions on edge
480 pixels are defined to be the same as the end result of the corresponding
481 transpose-and-flip sequence.
483 For practical use, you may prefer to discard any untransformable edge pixels
484 rather than having a strange-looking strip along the right and/or bottom edges
485 of a transformed image. To do this, add the -trim switch:
486 -trim Drop non-transformable edge blocks.
487 Obviously, a transformation with -trim is not reversible, so strictly speaking
488 jpegtran with this switch is not lossless. Also, the expected mathematical
489 equivalences between the transformations no longer hold. For example,
490 "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
491 "-rot 180 -trim" trims both edges.
493 If you are only interested in perfect transformations, add the -perfect switch:
494 -perfect Fail with an error if the transformation is not
496 For example, you may want to do
497 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
498 to do a perfect rotation, if available, or an approximated one if not.
500 This version of jpegtran also offers a lossless crop option, which discards
501 data outside of a given image region but losslessly preserves what is inside.
502 Like the rotate and flip transforms, lossless crop is restricted by the current
503 JPEG format; the upper left corner of the selected region must fall on an iMCU
504 boundary. If it doesn't, then it is silently moved up and/or left to the
505 nearest iMCU boundary (the lower right corner is unchanged.)
507 The image can be losslessly cropped by giving the switch:
508 -crop WxH+X+Y Crop to a rectangular region of width W and height H,
509 starting at point X,Y.
511 Other not-strictly-lossless transformation switches are:
513 -grayscale Force grayscale output.
514 This option discards the chrominance channels if the input image is YCbCr
515 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
516 luminance channel is preserved exactly, so this is a better method of reducing
517 to grayscale than decompression, conversion, and recompression. This switch
518 is particularly handy for fixing a monochrome picture that was mistakenly
519 encoded as a color JPEG. (In such a case, the space savings from getting rid
520 of the near-empty chroma channels won't be large; but the decoding time for
521 a grayscale JPEG is substantially less than that for a color JPEG.)
523 jpegtran also recognizes these switches that control what to do with "extra"
524 markers, such as comment blocks:
525 -copy none Copy no extra markers from source file. This setting
526 suppresses all comments and other excess baggage
527 present in the source file.
528 -copy comments Copy only comment markers. This setting copies
529 comments from the source file but discards
530 any other data that is inessential for image display.
531 -copy all Copy all extra markers. This setting preserves
532 miscellaneous markers found in the source file, such
533 as JFIF thumbnails, Exif data, and Photoshop settings.
534 In some files, these extra markers can be sizable.
535 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
536 jpegtran always did the equivalent of -copy none.)
538 Additional switches recognized by jpegtran are:
543 These work the same as in cjpeg or djpeg.
546 THE COMMENT UTILITIES
548 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
549 Although the standard doesn't actually define what COM blocks are for, they
550 are widely used to hold user-supplied text strings. This lets you add
551 annotations, titles, index terms, etc to your JPEG files, and later retrieve
552 them as text. COM blocks do not interfere with the image stored in the JPEG
553 file. The maximum size of a COM block is 64K, but you can have as many of
554 them as you like in one JPEG file.
556 We provide two utility programs to display COM block contents and add COM
557 blocks to a JPEG file.
559 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
560 standard output. The command line syntax is
561 rdjpgcom [-raw] [-verbose] [inputfilename]
562 The switch "-raw" (or just "-r") causes rdjpgcom to output non-printable
563 characters in JPEG comments. These characters are normally escaped for
565 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
566 image dimensions. If you omit the input file name from the command line,
567 the JPEG file is read from standard input. (This may not work on some
568 operating systems, if binary data can't be read from stdin.)
570 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
571 Ordinarily, the COM block is added after any existing COM blocks, but you
572 can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
573 file; it does not modify the input file. DO NOT try to overwrite the input
574 file by directing wrjpgcom's output back into it; on most systems this will
575 just destroy your file.
577 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
579 wrjpgcom [switches] [inputfilename]
580 The output file is written to standard output. The input file comes from
581 the named file, or from standard input if no input file is named.
583 On most non-Unix systems, the syntax is
584 wrjpgcom [switches] inputfilename outputfilename
585 where both input and output file names must be given explicitly.
587 wrjpgcom understands three switches:
588 -replace Delete any existing COM blocks from the file.
589 -comment "Comment text" Supply new COM text on command line.
590 -cfile name Read text for new COM block from named file.
591 (Switch names can be abbreviated.) If you have only one line of comment text
592 to add, you can provide it on the command line with -comment. The comment
593 text must be surrounded with quotes so that it is treated as a single
594 argument. Longer comments can be read from a text file.
596 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
597 text from standard input. (In this case an input image file name MUST be
598 supplied, so that the source JPEG file comes from somewhere else.) You can
599 enter multiple lines, up to 64KB worth. Type an end-of-file indicator
600 (usually control-D or control-Z) to terminate the comment text entry.
602 wrjpgcom will not add a COM block if the provided comment string is empty.
603 Therefore -replace -comment "" can be used to delete all COM blocks from a
606 These utility programs do not depend on the IJG JPEG library. In
607 particular, the source code for rdjpgcom is intended as an illustration of
608 the minimum amount of code required to parse a JPEG file header correctly.
projects / external / libjpeg-turbo.git / blob