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 8/N with all N from 1 to
93 -targa Input file is Targa format. Targa files that contain
94 an "identification" field will not be automatically
95 recognized by cjpeg; for such files you must specify
96 -targa to make cjpeg treat the input as Targa format.
97 For most Targa files, you won't need this switch.
99 The -quality switch lets you trade off compressed file size against quality of
100 the reconstructed image: the higher the quality setting, the larger the JPEG
101 file, and the closer the output image will be to the original input. Normally
102 you want to use the lowest quality setting (smallest file) that decompresses
103 into something visually indistinguishable from the original image. For this
104 purpose the quality setting should be between 50 and 95; the default of 75 is
105 often about right. If you see defects at -quality 75, then go up 5 or 10
106 counts at a time until you are happy with the output image. (The optimal
107 setting will vary from one image to another.)
109 -quality 100 will generate a quantization table of all 1's, minimizing loss
110 in the quantization step (but there is still information loss in subsampling,
111 as well as roundoff error). This setting is mainly of interest for
112 experimental purposes. Quality values above about 95 are NOT recommended for
113 normal use; the compressed file size goes up dramatically for hardly any gain
114 in output image quality.
116 In the other direction, quality values below 50 will produce very small files
117 of low image quality. Settings around 5 to 10 might be useful in preparing an
118 index of a large image library, for example. Try -quality 2 (or so) for some
119 amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
120 quantization tables, which are considered optional in the JPEG standard.
121 cjpeg emits a warning message when you give such a quality value, because some
122 other JPEG programs may be unable to decode the resulting file. Use -baseline
123 if you need to ensure compatibility at low quality values.)
125 The -quality option has been extended in IJG version 7 for support of separate
126 quality settings for luminance and chrominance (or in general, for every
127 provided quantization table slot). This feature is useful for high-quality
128 applications which cannot accept the damage of color data by coarse
129 subsampling settings. You can now easily reduce the color data amount more
130 smoothly with finer control without separate subsampling. The resulting file
131 is fully compliant with standard JPEG decoders.
132 Note that the -quality ratings refer to the quantization table slots, and that
133 the last value is replicated if there are more q-table slots than parameters.
134 The default q-table slots are 0 for luminance and 1 for chrominance with
135 default tables as given in the JPEG standard. This is compatible with the old
136 behaviour in case that only one parameter is given, which is then used for
137 both luminance and chrominance (slots 0 and 1). More or custom quantization
138 tables can be set with -qtables and assigned to components with -qslots
139 parameter (see the "wizard" switches below).
140 CAUTION: You must explicitly add -sample 1x1 for efficient separate color
141 quality selection, since the default value used by library is 2x2!
143 The -progressive switch creates a "progressive JPEG" file. In this type of
144 JPEG file, the data is stored in multiple scans of increasing quality. If the
145 file is being transmitted over a slow communications link, the decoder can use
146 the first scan to display a low-quality image very quickly, and can then
147 improve the display with each subsequent scan. The final image is exactly
148 equivalent to a standard JPEG file of the same quality setting, and the total
149 file size is about the same --- often a little smaller.
151 Switches for advanced users:
153 -dct int Use integer DCT method (default).
154 -dct fast Use fast integer DCT (less accurate).
155 -dct float Use floating-point DCT method.
156 The float method is very slightly more accurate than
157 the int method, but is much slower unless your machine
158 has very fast floating-point hardware. Also note that
159 results of the floating-point method may vary slightly
160 across machines, while the integer methods should give
161 the same results everywhere. The fast integer method
162 is much less accurate than the other two.
164 -nosmooth Don't use high-quality downsampling.
166 -restart N Emit a JPEG restart marker every N MCU rows, or every
167 N MCU blocks if "B" is attached to the number.
168 -restart 0 (the default) means no restart markers.
170 -smooth N Smooth the input image to eliminate dithering noise.
171 N, ranging from 1 to 100, indicates the strength of
172 smoothing. 0 (the default) means no smoothing.
174 -maxmemory N Set limit for amount of memory to use in processing
175 large images. Value is in thousands of bytes, or
176 millions of bytes if "M" is attached to the number.
177 For example, -max 4m selects 4000000 bytes. If more
178 space is needed, temporary files will be used.
180 -verbose Enable debug printout. More -v's give more printout.
181 or -debug Also, version information is printed at startup.
183 The -restart option inserts extra markers that allow a JPEG decoder to
184 resynchronize after a transmission error. Without restart markers, any damage
185 to a compressed file will usually ruin the image from the point of the error
186 to the end of the image; with restart markers, the damage is usually confined
187 to the portion of the image up to the next restart marker. Of course, the
188 restart markers occupy extra space. We recommend -restart 1 for images that
189 will be transmitted across unreliable networks such as Usenet.
191 The -smooth option filters the input to eliminate fine-scale noise. This is
192 often useful when converting dithered images to JPEG: a moderate smoothing
193 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
194 in a smaller JPEG file and a better-looking image. Too large a smoothing
195 factor will visibly blur the image, however.
197 Switches for wizards:
199 -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG
200 is not yet widely implemented, so many decoders will
201 be unable to view an arithmetic coded JPEG file at
204 -baseline Force baseline-compatible quantization tables to be
205 generated. This clamps quantization values to 8 bits
206 even at low quality settings. (This switch is poorly
207 named, since it does not ensure that the output is
208 actually baseline JPEG. For example, you can use
209 -baseline and -progressive together.)
211 -qtables file Use the quantization tables given in the specified
214 -qslots N[,...] Select which quantization table to use for each color
217 -sample HxV[,...] Set JPEG sampling factors for each color component.
219 -scans file Use the scan script given in the specified text file.
221 The "wizard" switches are intended for experimentation with JPEG. If you
222 don't know what you are doing, DON'T USE THEM. These switches are documented
223 further in the file wizard.txt.
228 The basic command line switches for djpeg are:
230 -colors N Reduce image to at most N colors. This reduces the
231 or -quantize N number of colors used in the output image, so that it
232 can be displayed on a colormapped display or stored in
233 a colormapped file format. For example, if you have
234 an 8-bit display, you'd need to reduce to 256 or fewer
235 colors. (-colors is the recommended name, -quantize
236 is provided only for backwards compatibility.)
238 -fast Select recommended processing options for fast, low
239 quality output. (The default options are chosen for
240 highest quality output.) Currently, this is equivalent
241 to "-dct fast -nosmooth -onepass -dither ordered".
243 -grayscale Force gray-scale output even if JPEG file is color.
244 Useful for viewing on monochrome displays; also,
245 djpeg runs noticeably faster in this mode.
247 -scale M/N Scale the output image by a factor M/N. Currently
248 supported scale factors are M/N with all M from 1 to
249 16, where N is the source DCT size, which is 8 for
250 baseline JPEG. If the /N part is omitted, then M
251 specifies the DCT scaled size to be applied on the
252 given input. For baseline JPEG this is equivalent to
253 M/8 scaling, since the source DCT size for baseline
254 JPEG is 8. Scaling is handy if the image is larger
255 than your screen; also, djpeg runs much faster when
256 scaling down the output.
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 Don't use high-quality upsampling.
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 GIF input files are no longer supported, to avoid the Unisys LZW patent.
358 (Conversion of GIF files to JPEG is usually a bad idea anyway.)
363 To get a quick preview of an image, use the -grayscale and/or -scale switches.
364 "-grayscale -scale 1/8" is the fastest case.
366 Several options are available that trade off image quality to gain speed.
367 "-fast" turns on the recommended settings.
369 "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
370 When producing a color-quantized image, "-onepass -dither ordered" is fast but
371 much lower quality than the default behavior. "-dither none" may give
372 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
374 If you are fortunate enough to have very fast floating point hardware,
375 "-dct float" may be even faster than "-dct fast". But on most machines
376 "-dct float" is slower than "-dct int"; in this case it is not worth using,
377 because its theoretical accuracy advantage is too small to be significant
380 Two-pass color quantization requires a good deal of memory; on MS-DOS machines
381 it may run out of memory even with -maxmemory 0. In that case you can still
382 decompress, with some loss of image quality, by specifying -onepass for
383 one-pass quantization.
385 To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These
386 are larger than they should be, but are readable by standard GIF decoders.
389 HINTS FOR BOTH PROGRAMS
391 If more space is needed than will fit in the available main memory (as
392 determined by -maxmemory), temporary files will be used. (MS-DOS versions
393 will try to get extended or expanded memory first.) The temporary files are
394 often rather large: in typical cases they occupy three bytes per pixel, for
395 example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
396 free disk space, leave out -progressive and -optimize (for cjpeg) or specify
397 -onepass (for djpeg).
399 On MS-DOS, the temporary files are created in the directory named by the TMP
400 or TEMP environment variable, or in the current directory if neither of those
401 exist. Amiga implementations put the temp files in the directory named by
402 JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
405 The default memory usage limit (-maxmemory) is set when the software is
406 compiled. If you get an "insufficient memory" error, try specifying a smaller
407 -maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
408 may want to recompile with a smaller default value if this happens often.
410 On machines that have "environment" variables, you can define the environment
411 variable JPEGMEM to set the default memory limit. The value is specified as
412 described for the -maxmemory switch. JPEGMEM overrides the default value
413 specified when the program was compiled, and itself is overridden by an
414 explicit -maxmemory switch.
416 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
417 use. (Extended or expanded memory is also used if available.) Most
418 DOS-specific versions of this software do their own memory space estimation
419 and do not need you to specify -maxmemory.
424 jpegtran performs various useful transformations of JPEG files.
425 It can translate the coded representation from one variant of JPEG to another,
426 for example from baseline JPEG to progressive JPEG or vice versa. It can also
427 perform some rearrangements of the image data, for example turning an image
428 from landscape to portrait format by rotation.
430 jpegtran works by rearranging the compressed data (DCT coefficients), without
431 ever fully decoding the image. Therefore, its transformations are lossless:
432 there is no image degradation at all, which would not be true if you used
433 djpeg followed by cjpeg to accomplish the same conversion. But by the same
434 token, jpegtran cannot perform lossy operations such as changing the image
437 jpegtran uses a command line syntax similar to cjpeg or djpeg.
438 On Unix-like systems, you say:
439 jpegtran [switches] [inputfile] >outputfile
440 On most non-Unix systems, you say:
441 jpegtran [switches] inputfile outputfile
442 where both the input and output files are JPEG files.
444 To specify the coded JPEG representation used in the output file,
445 jpegtran accepts a subset of the switches recognized by cjpeg:
446 -optimize Perform optimization of entropy encoding parameters.
447 -progressive Create progressive JPEG file.
448 -restart N Emit a JPEG restart marker every N MCU rows, or every
449 N MCU blocks if "B" is attached to the number.
450 -arithmetic Use arithmetic coding.
451 -scans file Use the scan script given in the specified text file.
452 See the previous discussion of cjpeg for more details about these switches.
453 If you specify none of these switches, you get a plain baseline-JPEG output
454 file. The quality setting and so forth are determined by the input file.
456 The image can be losslessly transformed by giving one of these switches:
457 -flip horizontal Mirror image horizontally (left-right).
458 -flip vertical Mirror image vertically (top-bottom).
459 -rotate 90 Rotate image 90 degrees clockwise.
460 -rotate 180 Rotate image 180 degrees.
461 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw).
462 -transpose Transpose image (across UL-to-LR axis).
463 -transverse Transverse transpose (across UR-to-LL axis).
465 The transpose transformation has no restrictions regarding image dimensions.
466 The other transformations operate rather oddly if the image dimensions are not
467 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
468 transform complete blocks of DCT coefficient data in the desired way.
470 jpegtran's default behavior when transforming an odd-size image is designed
471 to preserve exact reversibility and mathematical consistency of the
472 transformation set. As stated, transpose is able to flip the entire image
473 area. Horizontal mirroring leaves any partial iMCU column at the right edge
474 untouched, but is able to flip all rows of the image. Similarly, vertical
475 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
476 able to flip all columns. The other transforms can be built up as sequences
477 of transpose and flip operations; for consistency, their actions on edge
478 pixels are defined to be the same as the end result of the corresponding
479 transpose-and-flip sequence.
481 For practical use, you may prefer to discard any untransformable edge pixels
482 rather than having a strange-looking strip along the right and/or bottom edges
483 of a transformed image. To do this, add the -trim switch:
484 -trim Drop non-transformable edge blocks.
485 Obviously, a transformation with -trim is not reversible, so strictly speaking
486 jpegtran with this switch is not lossless. Also, the expected mathematical
487 equivalences between the transformations no longer hold. For example,
488 "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
489 "-rot 180 -trim" trims both edges.
491 If you are only interested in perfect transformation, add the -perfect switch:
492 -perfect Fails with an error if the transformation is not
494 For example you may want to do
495 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
496 to do a perfect rotation if available or an approximated one if not.
498 We also offer a lossless-crop option, which discards data outside a given
499 image region but losslessly preserves what is inside. Like the rotate and
500 flip transforms, lossless crop is restricted by the current JPEG format: the
501 upper left corner of the selected region must fall on an iMCU boundary. If
502 this does not hold for the given crop parameters, we silently move the upper
503 left corner up and/or left to make it so, simultaneously increasing the region
504 dimensions to keep the lower right crop corner unchanged. (Thus, the output
505 image covers at least the requested region, but may cover more.)
507 The image can be losslessly cropped by giving the switch:
508 -crop WxH+X+Y Crop to a rectangular subarea of width W, 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 -scale M/N Scale the output image by a factor M/N.
524 Currently supported scale factors are M/N with all M from 1 to 16, where N is
525 the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
526 then M specifies the DCT scaled size to be applied on the given input. For
527 baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
528 for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale
529 extension is required for this feature. SmartScale enabled JPEG is not yet
530 widely implemented, so many decoders will be unable to view a SmartScale
531 extended JPEG file at all.
533 jpegtran also recognizes these switches that control what to do with "extra"
534 markers, such as comment blocks:
535 -copy none Copy no extra markers from source file. This setting
536 suppresses all comments and other excess baggage
537 present in the source file.
538 -copy comments Copy only comment markers. This setting copies
539 comments from the source file, but discards
540 any other inessential (for image display) data.
541 -copy all Copy all extra markers. This setting preserves
542 miscellaneous markers found in the source file, such
543 as JFIF thumbnails, Exif data, and Photoshop settings.
544 In some files these extra markers can be sizable.
545 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
546 jpegtran always did the equivalent of -copy none.)
548 Additional switches recognized by jpegtran are:
553 These work the same as in cjpeg or djpeg.
556 THE COMMENT UTILITIES
558 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
559 Although the standard doesn't actually define what COM blocks are for, they
560 are widely used to hold user-supplied text strings. This lets you add
561 annotations, titles, index terms, etc to your JPEG files, and later retrieve
562 them as text. COM blocks do not interfere with the image stored in the JPEG
563 file. The maximum size of a COM block is 64K, but you can have as many of
564 them as you like in one JPEG file.
566 We provide two utility programs to display COM block contents and add COM
567 blocks to a JPEG file.
569 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
570 standard output. The command line syntax is
571 rdjpgcom [-raw] [-verbose] [inputfilename]
572 The switch "-raw" (or just "-r") causes rdjpgcom to also output non-printable
573 characters in comments, which are normally escaped for security reasons.
574 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
575 image dimensions. If you omit the input file name from the command line,
576 the JPEG file is read from standard input. (This may not work on some
577 operating systems, if binary data can't be read from stdin.)
579 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
580 Ordinarily, the COM block is added after any existing COM blocks, but you
581 can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
582 file; it does not modify the input file. DO NOT try to overwrite the input
583 file by directing wrjpgcom's output back into it; on most systems this will
584 just destroy your file.
586 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
588 wrjpgcom [switches] [inputfilename]
589 The output file is written to standard output. The input file comes from
590 the named file, or from standard input if no input file is named.
592 On most non-Unix systems, the syntax is
593 wrjpgcom [switches] inputfilename outputfilename
594 where both input and output file names must be given explicitly.
596 wrjpgcom understands three switches:
597 -replace Delete any existing COM blocks from the file.
598 -comment "Comment text" Supply new COM text on command line.
599 -cfile name Read text for new COM block from named file.
600 (Switch names can be abbreviated.) If you have only one line of comment text
601 to add, you can provide it on the command line with -comment. The comment
602 text must be surrounded with quotes so that it is treated as a single
603 argument. Longer comments can be read from a text file.
605 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
606 text from standard input. (In this case an input image file name MUST be
607 supplied, so that the source JPEG file comes from somewhere else.) You can
608 enter multiple lines, up to 64KB worth. Type an end-of-file indicator
609 (usually control-D or control-Z) to terminate the comment text entry.
611 wrjpgcom will not add a COM block if the provided comment string is empty.
612 Therefore -replace -comment "" can be used to delete all COM blocks from a
615 These utility programs do not depend on the IJG JPEG library. In
616 particular, the source code for rdjpgcom is intended as an illustration of
617 the minimum amount of code required to parse a JPEG file header correctly.
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