1 NOTE: This file was modified by The libjpeg-turbo Project to include only
2 information relevant to libjpeg-turbo and to wordsmith certain sections.
4 USAGE instructions for the Independent JPEG Group's JPEG software
5 =================================================================
7 This file describes usage of the JPEG conversion programs cjpeg and djpeg,
8 as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See
9 the other documentation files if you wish to use the JPEG library within
12 If you are on a Unix machine you may prefer to read the Unix-style manual
13 pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.
18 These programs implement JPEG image encoding, decoding, and transcoding.
19 JPEG (pronounced "jay-peg") is a standardized compression method for
20 full-color and grayscale images.
25 We provide two programs, cjpeg to compress an image file into JPEG format,
26 and djpeg to decompress a JPEG file back into a conventional image format.
28 On Unix-like systems, you say:
29 cjpeg [switches] [imagefile] >jpegfile
31 djpeg [switches] [jpegfile] >imagefile
32 The programs read the specified input file, or standard input if none is
33 named. They always write to standard output (with trace/error messages to
34 standard error). These conventions are handy for piping images between
37 On most non-Unix systems, you say:
38 cjpeg [switches] imagefile jpegfile
40 djpeg [switches] jpegfile imagefile
41 i.e., both the input and output files are named on the command line. This
42 style is a little more foolproof, and it loses no functionality if you don't
43 have pipes. (You can get this style on Unix too, if you prefer, by defining
44 TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.)
47 cjpeg [switches] -outfile jpegfile imagefile
49 djpeg [switches] -outfile imagefile jpegfile
50 This syntax works on all systems, so it is useful for scripts.
52 The currently supported image file formats are: PPM (PBMPLUS color format),
53 PGM (PBMPLUS grayscale format), BMP, Targa, and RLE (Utah Raster Toolkit
54 format). (RLE is supported only if the URT library is available, which it
55 isn't on most non-Unix systems.) cjpeg recognizes the input image format
56 automatically, with the exception of some Targa files. You have to tell djpeg
57 which format to generate.
59 JPEG files are in the defacto standard JFIF file format. There are other,
60 less widely used JPEG-based file formats, but we don't support them.
62 All switch names may be abbreviated; for example, -grayscale may be written
63 -gray or -gr. Most of the "basic" switches can be abbreviated to as little as
64 one letter. Upper and lower case are equivalent (-BMP is the same as -bmp).
65 British spellings are also accepted (e.g., -greyscale), though for brevity
66 these are not mentioned below.
71 The basic command line switches for cjpeg are:
73 -quality N[,...] Scale quantization tables to adjust image quality.
74 Quality is 0 (worst) to 100 (best); default is 75.
75 (See below for more info.)
77 -grayscale Create monochrome JPEG file from color input.
78 Be sure to use this switch when compressing a grayscale
79 BMP file, because cjpeg isn't bright enough to notice
80 whether a BMP file uses only shades of gray. By
81 saying -grayscale, you'll get a smaller JPEG file that
82 takes less time to process.
84 -rgb Create RGB JPEG file.
85 Using this switch suppresses the conversion from RGB
86 colorspace input to the default YCbCr JPEG colorspace.
88 -optimize Perform optimization of entropy encoding parameters.
89 Without this, default encoding parameters are used.
90 -optimize usually makes the JPEG file a little smaller,
91 but cjpeg runs somewhat slower and needs much more
92 memory. Image quality and speed of decompression are
93 unaffected by -optimize.
95 -progressive Create progressive JPEG file (see below).
97 -targa Input file is Targa format. Targa files that contain
98 an "identification" field will not be automatically
99 recognized by cjpeg; for such files you must specify
100 -targa to make cjpeg treat the input as Targa format.
101 For most Targa files, you won't need this switch.
103 The -quality switch lets you trade off compressed file size against quality of
104 the reconstructed image: the higher the quality setting, the larger the JPEG
105 file, and the closer the output image will be to the original input. Normally
106 you want to use the lowest quality setting (smallest file) that decompresses
107 into something visually indistinguishable from the original image. For this
108 purpose the quality setting should generally be between 50 and 95 (the default
109 is 75) for photographic images. If you see defects at -quality 75, then go up
110 5 or 10 counts at a time until you are happy with the output image. (The
111 optimal setting will vary from one image to another.)
113 -quality 100 will generate a quantization table of all 1's, minimizing loss
114 in the quantization step (but there is still information loss in subsampling,
115 as well as roundoff error.) For most images, specifying a quality value above
116 about 95 will increase the size of the compressed file dramatically, and while
117 the quality gain from these higher quality values is measurable (using metrics
118 such as PSNR or SSIM), it is rarely perceivable by human vision.
120 In the other direction, quality values below 50 will produce very small files
121 of low image quality. Settings around 5 to 10 might be useful in preparing an
122 index of a large image library, for example. Try -quality 2 (or so) for some
123 amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
124 quantization tables, which are considered optional in the JPEG standard.
125 cjpeg emits a warning message when you give such a quality value, because some
126 other JPEG programs may be unable to decode the resulting file. Use -baseline
127 if you need to ensure compatibility at low quality values.)
129 The -quality option has been extended in this version of cjpeg to support
130 separate quality settings for luminance and chrominance (or, in general,
131 separate settings for every quantization table slot.) The principle is the
132 same as chrominance subsampling: since the human eye is more sensitive to
133 spatial changes in brightness than spatial changes in color, the chrominance
134 components can be quantized more than the luminance components without
135 incurring any visible image quality loss. However, unlike subsampling, this
136 feature reduces data in the frequency domain instead of the spatial domain,
137 which allows for more fine-grained control. This option is useful in
138 quality-sensitive applications, for which the artifacts generated by
139 subsampling may be unacceptable.
141 The -quality option accepts a comma-separated list of parameters, which
142 respectively refer to the quality levels that should be assigned to the
143 quantization table slots. If there are more q-table slots than parameters,
144 then the last parameter is replicated. Thus, if only one quality parameter is
145 given, this is used for both luminance and chrominance (slots 0 and 1,
146 respectively), preserving the legacy behavior of cjpeg v6b and prior. More (or
147 customized) quantization tables can be set with the -qtables option and
148 assigned to components with the -qslots option (see the "wizard" switches
151 JPEG files generated with separate luminance and chrominance quality are
152 fully compliant with standard JPEG decoders.
154 CAUTION: For this setting to be useful, be sure to pass an argument of
155 -sample 1x1 to cjpeg to disable chrominance subsampling. Otherwise, the
156 default subsampling level (2x2, AKA "4:2:0") will be used.
158 The -progressive switch creates a "progressive JPEG" file. In this type of
159 JPEG file, the data is stored in multiple scans of increasing quality. If the
160 file is being transmitted over a slow communications link, the decoder can use
161 the first scan to display a low-quality image very quickly, and can then
162 improve the display with each subsequent scan. The final image is exactly
163 equivalent to a standard JPEG file of the same quality setting, and the total
164 file size is about the same --- often a little smaller.
166 Switches for advanced users:
168 -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG
169 is not yet widely implemented, so many decoders will
170 be unable to view an arithmetic coded JPEG file at
173 -dct int Use integer DCT method (default).
174 -dct fast Use fast integer DCT (less accurate).
175 In libjpeg-turbo, the fast method is generally about
176 5-15% faster than the int method when using the
177 x86/x86-64 SIMD extensions (results may vary with other
178 SIMD implementations, or when using libjpeg-turbo
179 without SIMD extensions.) For quality levels of 90 and
180 below, there should be little or no perceptible
181 difference between the two algorithms. For quality
182 levels above 90, however, the difference between
183 the fast and the int methods becomes more pronounced.
184 With quality=97, for instance, the fast method incurs
185 generally about a 1-3 dB loss (in PSNR) relative to
186 the int method, but this can be larger for some images.
187 Do not use the fast method with quality levels above
188 97. The algorithm often degenerates at quality=98 and
189 above and can actually produce a more lossy image than
190 if lower quality levels had been used. Also, in
191 libjpeg-turbo, the fast method is not fully accerated
192 for quality levels above 97, so it will be slower than
194 -dct float Use floating-point DCT method.
195 The float method is mainly a legacy feature. It does
196 not produce significantly more accurate results than
197 the int method, and it is much slower. The float
198 method may also give different results on different
199 machines due to varying roundoff behavior, whereas the
200 integer methods should give the same results on all
203 -restart N Emit a JPEG restart marker every N MCU rows, or every
204 N MCU blocks if "B" is attached to the number.
205 -restart 0 (the default) means no restart markers.
207 -smooth N Smooth the input image to eliminate dithering noise.
208 N, ranging from 1 to 100, indicates the strength of
209 smoothing. 0 (the default) means no smoothing.
211 -maxmemory N Set limit for amount of memory to use in processing
212 large images. Value is in thousands of bytes, or
213 millions of bytes if "M" is attached to the number.
214 For example, -max 4m selects 4000000 bytes. If more
215 space is needed, an error will occur.
217 -verbose Enable debug printout. More -v's give more printout.
218 or -debug Also, version information is printed at startup.
220 The -restart option inserts extra markers that allow a JPEG decoder to
221 resynchronize after a transmission error. Without restart markers, any damage
222 to a compressed file will usually ruin the image from the point of the error
223 to the end of the image; with restart markers, the damage is usually confined
224 to the portion of the image up to the next restart marker. Of course, the
225 restart markers occupy extra space. We recommend -restart 1 for images that
226 will be transmitted across unreliable networks such as Usenet.
228 The -smooth option filters the input to eliminate fine-scale noise. This is
229 often useful when converting dithered images to JPEG: a moderate smoothing
230 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
231 in a smaller JPEG file and a better-looking image. Too large a smoothing
232 factor will visibly blur the image, however.
234 Switches for wizards:
236 -baseline Force baseline-compatible quantization tables to be
237 generated. This clamps quantization values to 8 bits
238 even at low quality settings. (This switch is poorly
239 named, since it does not ensure that the output is
240 actually baseline JPEG. For example, you can use
241 -baseline and -progressive together.)
243 -qtables file Use the quantization tables given in the specified
246 -qslots N[,...] Select which quantization table to use for each color
249 -sample HxV[,...] Set JPEG sampling factors for each color component.
251 -scans file Use the scan script given in the specified text file.
253 The "wizard" switches are intended for experimentation with JPEG. If you
254 don't know what you are doing, DON'T USE THEM. These switches are documented
255 further in the file wizard.txt.
260 The basic command line switches for djpeg are:
262 -colors N Reduce image to at most N colors. This reduces the
263 or -quantize N number of colors used in the output image, so that it
264 can be displayed on a colormapped display or stored in
265 a colormapped file format. For example, if you have
266 an 8-bit display, you'd need to reduce to 256 or fewer
267 colors. (-colors is the recommended name, -quantize
268 is provided only for backwards compatibility.)
270 -fast Select recommended processing options for fast, low
271 quality output. (The default options are chosen for
272 highest quality output.) Currently, this is equivalent
273 to "-dct fast -nosmooth -onepass -dither ordered".
275 -grayscale Force grayscale output even if JPEG file is color.
276 Useful for viewing on monochrome displays; also,
277 djpeg runs noticeably faster in this mode.
279 -rgb Force RGB output even if JPEG file is grayscale.
281 -scale M/N Scale the output image by a factor M/N. Currently
282 the scale factor must be M/8, where M is an integer
283 between 1 and 16 inclusive, or any reduced fraction
284 thereof (such as 1/2, 3/4, etc. Scaling is handy if
285 the image is larger than your screen; also, djpeg runs
286 much faster when scaling down the output.
288 -bmp Select BMP output format (Windows flavor). 8-bit
289 colormapped format is emitted if -colors or -grayscale
290 is specified, or if the JPEG file is grayscale;
291 otherwise, 24-bit full-color format is emitted.
293 -gif Select GIF output format. Since GIF does not support
294 more than 256 colors, -colors 256 is assumed (unless
295 you specify a smaller number of colors). If you
296 specify -fast, the default number of colors is 216.
298 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit
299 colormapped format is emitted if -colors or -grayscale
300 is specified, or if the JPEG file is grayscale;
301 otherwise, 24-bit full-color format is emitted.
303 -pnm Select PBMPLUS (PPM/PGM) output format (this is the
304 default format). PGM is emitted if the JPEG file is
305 grayscale or if -grayscale is specified; otherwise
308 -rle Select RLE output format. (Requires URT library.)
310 -targa Select Targa output format. Grayscale format is
311 emitted if the JPEG file is grayscale or if
312 -grayscale is specified; otherwise, colormapped format
313 is emitted if -colors is specified; otherwise, 24-bit
314 full-color format is emitted.
316 Switches for advanced users:
318 -dct int Use integer DCT method (default).
319 -dct fast Use fast integer DCT (less accurate).
320 In libjpeg-turbo, the fast method is generally about
321 5-15% faster than the int method when using the
322 x86/x86-64 SIMD extensions (results may vary with other
323 SIMD implementations, or when using libjpeg-turbo
324 without SIMD extensions.) If the JPEG image was
325 compressed using a quality level of 85 or below, then
326 there should be little or no perceptible difference
327 between the two algorithms. When decompressing images
328 that were compressed using quality levels above 85,
329 however, the difference between the fast and int
330 methods becomes more pronounced. With images
331 compressed using quality=97, for instance, the fast
332 method incurs generally about a 4-6 dB loss (in PSNR)
333 relative to the int method, but this can be larger for
334 some images. If you can avoid it, do not use the fast
335 method when decompressing images that were compressed
336 using quality levels above 97. The algorithm often
337 degenerates for such images and can actually produce
338 a more lossy output image than if the JPEG image had
339 been compressed using lower quality levels.
340 -dct float Use floating-point DCT method.
341 The float method is mainly a legacy feature. It does
342 Â not produce significantly more accurate results than
343 the int method, and it is much slower. The float
344 method may also give different results on different
345 machines due to varying roundoff behavior, whereas the
346 integer methods should give the same results on all
349 -dither fs Use Floyd-Steinberg dithering in color quantization.
350 -dither ordered Use ordered dithering in color quantization.
351 -dither none Do not use dithering in color quantization.
352 By default, Floyd-Steinberg dithering is applied when
353 quantizing colors; this is slow but usually produces
354 the best results. Ordered dither is a compromise
355 between speed and quality; no dithering is fast but
356 usually looks awful. Note that these switches have
357 no effect unless color quantization is being done.
358 Ordered dither is only available in -onepass mode.
360 -map FILE Quantize to the colors used in the specified image
361 file. This is useful for producing multiple files
362 with identical color maps, or for forcing a predefined
363 set of colors to be used. The FILE must be a GIF
364 or PPM file. This option overrides -colors and
367 -nosmooth Use a faster, lower-quality upsampling routine.
369 -onepass Use one-pass instead of two-pass color quantization.
370 The one-pass method is faster and needs less memory,
371 but it produces a lower-quality image. -onepass is
372 ignored unless you also say -colors N. Also,
373 the one-pass method is always used for grayscale
374 output (the two-pass method is no improvement then).
376 -maxmemory N Set limit for amount of memory to use in processing
377 large images. Value is in thousands of bytes, or
378 millions of bytes if "M" is attached to the number.
379 For example, -max 4m selects 4000000 bytes. If more
380 space is needed, an error will occur.
382 -verbose Enable debug printout. More -v's give more printout.
383 or -debug Also, version information is printed at startup.
388 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
389 compressing full-color (24-bit) images. In particular, don't try to convert
390 cartoons, line drawings, and other images that have only a few distinct
391 colors. GIF works great on these, JPEG does not. If you want to convert a
392 GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
393 to get a satisfactory conversion. -smooth 10 or so is often helpful.
395 Avoid running an image through a series of JPEG compression/decompression
396 cycles. Image quality loss will accumulate; after ten or so cycles the image
397 may be noticeably worse than it was after one cycle. It's best to use a
398 lossless format while manipulating an image, then convert to JPEG format when
399 you are ready to file the image away.
401 The -optimize option to cjpeg is worth using when you are making a "final"
402 version for posting or archiving. It's also a win when you are using low
403 quality settings to make very small JPEG files; the percentage improvement
404 is often a lot more than it is on larger files. (At present, -optimize
405 mode is always selected when generating progressive JPEG files.)
407 Support for GIF input files was removed in cjpeg v6b due to concerns over
408 the Unisys LZW patent. Although this patent expired in 2006, cjpeg still
409 lacks GIF support, for these historical reasons. (Conversion of GIF files to
410 JPEG is usually a bad idea anyway.)
415 To get a quick preview of an image, use the -grayscale and/or -scale switches.
416 "-grayscale -scale 1/8" is the fastest case.
418 Several options are available that trade off image quality to gain speed.
419 "-fast" turns on the recommended settings.
421 "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
422 When producing a color-quantized image, "-onepass -dither ordered" is fast but
423 much lower quality than the default behavior. "-dither none" may give
424 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
426 To avoid the Unisys LZW patent (now expired), djpeg produces uncompressed GIF
427 files. These are larger than they should be, but are readable by standard GIF
431 HINTS FOR BOTH PROGRAMS
433 If the memory needed by cjpeg or djpeg exceeds the limit specified by
434 -maxmemory, an error will occur. You can leave out -progressive and -optimize
435 (for cjpeg) or specify -onepass (for djpeg) to reduce memory usage.
437 On machines that have "environment" variables, you can define the environment
438 variable JPEGMEM to set the default memory limit. The value is specified as
439 described for the -maxmemory switch. JPEGMEM overrides the default value
440 specified when the program was compiled, and itself is overridden by an
441 explicit -maxmemory switch.
446 jpegtran performs various useful transformations of JPEG files.
447 It can translate the coded representation from one variant of JPEG to another,
448 for example from baseline JPEG to progressive JPEG or vice versa. It can also
449 perform some rearrangements of the image data, for example turning an image
450 from landscape to portrait format by rotation. For EXIF files and JPEG files
451 containing Exif data, you may prefer to use exiftran instead.
453 jpegtran works by rearranging the compressed data (DCT coefficients), without
454 ever fully decoding the image. Therefore, its transformations are lossless:
455 there is no image degradation at all, which would not be true if you used
456 djpeg followed by cjpeg to accomplish the same conversion. But by the same
457 token, jpegtran cannot perform lossy operations such as changing the image
458 quality. However, while the image data is losslessly transformed, metadata
459 can be removed. See the -copy option for specifics.
461 jpegtran uses a command line syntax similar to cjpeg or djpeg.
462 On Unix-like systems, you say:
463 jpegtran [switches] [inputfile] >outputfile
464 On most non-Unix systems, you say:
465 jpegtran [switches] inputfile outputfile
466 where both the input and output files are JPEG files.
468 To specify the coded JPEG representation used in the output file,
469 jpegtran accepts a subset of the switches recognized by cjpeg:
470 -optimize Perform optimization of entropy encoding parameters.
471 -progressive Create progressive JPEG file.
472 -arithmetic Use arithmetic coding.
473 -restart N Emit a JPEG restart marker every N MCU rows, or every
474 N MCU blocks if "B" is attached to the number.
475 -scans file Use the scan script given in the specified text file.
476 See the previous discussion of cjpeg for more details about these switches.
477 If you specify none of these switches, you get a plain baseline-JPEG output
478 file. The quality setting and so forth are determined by the input file.
480 The image can be losslessly transformed by giving one of these switches:
481 -flip horizontal Mirror image horizontally (left-right).
482 -flip vertical Mirror image vertically (top-bottom).
483 -rotate 90 Rotate image 90 degrees clockwise.
484 -rotate 180 Rotate image 180 degrees.
485 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw).
486 -transpose Transpose image (across UL-to-LR axis).
487 -transverse Transverse transpose (across UR-to-LL axis).
489 The transpose transformation has no restrictions regarding image dimensions.
490 The other transformations operate rather oddly if the image dimensions are not
491 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
492 transform complete blocks of DCT coefficient data in the desired way.
494 jpegtran's default behavior when transforming an odd-size image is designed
495 to preserve exact reversibility and mathematical consistency of the
496 transformation set. As stated, transpose is able to flip the entire image
497 area. Horizontal mirroring leaves any partial iMCU column at the right edge
498 untouched, but is able to flip all rows of the image. Similarly, vertical
499 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
500 able to flip all columns. The other transforms can be built up as sequences
501 of transpose and flip operations; for consistency, their actions on edge
502 pixels are defined to be the same as the end result of the corresponding
503 transpose-and-flip sequence.
505 For practical use, you may prefer to discard any untransformable edge pixels
506 rather than having a strange-looking strip along the right and/or bottom edges
507 of a transformed image. To do this, add the -trim switch:
508 -trim Drop non-transformable edge blocks.
509 Obviously, a transformation with -trim is not reversible, so strictly speaking
510 jpegtran with this switch is not lossless. Also, the expected mathematical
511 equivalences between the transformations no longer hold. For example,
512 "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
513 "-rot 180 -trim" trims both edges.
515 If you are only interested in perfect transformations, add the -perfect switch:
516 -perfect Fail with an error if the transformation is not
518 For example, you may want to do
519 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
520 to do a perfect rotation, if available, or an approximated one if not.
522 This version of jpegtran also offers a lossless crop option, which discards
523 data outside of a given image region but losslessly preserves what is inside.
524 Like the rotate and flip transforms, lossless crop is restricted by the current
525 JPEG format; the upper left corner of the selected region must fall on an iMCU
526 boundary. If it doesn't, then it is silently moved up and/or left to the
527 nearest iMCU boundary (the lower right corner is unchanged.) Thus, the output
528 image covers at least the requested region, but it may cover more. The
529 adjustment of the region dimensions may be optionally disabled by attaching an
530 'f' character ("force") to the width or height number.
532 The image can be losslessly cropped by giving the switch:
533 -crop WxH+X+Y Crop to a rectangular region of width W and height H,
534 starting at point X,Y.
536 Other not-strictly-lossless transformation switches are:
538 -grayscale Force grayscale output.
539 This option discards the chrominance channels if the input image is YCbCr
540 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
541 luminance channel is preserved exactly, so this is a better method of reducing
542 to grayscale than decompression, conversion, and recompression. This switch
543 is particularly handy for fixing a monochrome picture that was mistakenly
544 encoded as a color JPEG. (In such a case, the space savings from getting rid
545 of the near-empty chroma channels won't be large; but the decoding time for
546 a grayscale JPEG is substantially less than that for a color JPEG.)
548 jpegtran also recognizes these switches that control what to do with "extra"
549 markers, such as comment blocks:
550 -copy none Copy no extra markers from source file. This setting
551 suppresses all comments and other metadata in the
553 -copy comments Copy only comment markers. This setting copies
554 comments from the source file but discards any other
556 -copy all Copy all extra markers. This setting preserves
557 miscellaneous markers found in the source file, such
558 as JFIF thumbnails, Exif data, and Photoshop settings.
559 In some files, these extra markers can be sizable.
560 Note that this option will copy thumbnails as-is;
561 they will not be transformed.
562 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
563 jpegtran always did the equivalent of -copy none.)
565 Additional switches recognized by jpegtran are:
570 These work the same as in cjpeg or djpeg.
573 THE COMMENT UTILITIES
575 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
576 Although the standard doesn't actually define what COM blocks are for, they
577 are widely used to hold user-supplied text strings. This lets you add
578 annotations, titles, index terms, etc to your JPEG files, and later retrieve
579 them as text. COM blocks do not interfere with the image stored in the JPEG
580 file. The maximum size of a COM block is 64K, but you can have as many of
581 them as you like in one JPEG file.
583 We provide two utility programs to display COM block contents and add COM
584 blocks to a JPEG file.
586 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
587 standard output. The command line syntax is
588 rdjpgcom [-raw] [-verbose] [inputfilename]
589 The switch "-raw" (or just "-r") causes rdjpgcom to output non-printable
590 characters in JPEG comments. These characters are normally escaped for
592 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
593 image dimensions. If you omit the input file name from the command line,
594 the JPEG file is read from standard input. (This may not work on some
595 operating systems, if binary data can't be read from stdin.)
597 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
598 Ordinarily, the COM block is added after any existing COM blocks, but you
599 can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
600 file; it does not modify the input file. DO NOT try to overwrite the input
601 file by directing wrjpgcom's output back into it; on most systems this will
602 just destroy your file.
604 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
606 wrjpgcom [switches] [inputfilename]
607 The output file is written to standard output. The input file comes from
608 the named file, or from standard input if no input file is named.
610 On most non-Unix systems, the syntax is
611 wrjpgcom [switches] inputfilename outputfilename
612 where both input and output file names must be given explicitly.
614 wrjpgcom understands three switches:
615 -replace Delete any existing COM blocks from the file.
616 -comment "Comment text" Supply new COM text on command line.
617 -cfile name Read text for new COM block from named file.
618 (Switch names can be abbreviated.) If you have only one line of comment text
619 to add, you can provide it on the command line with -comment. The comment
620 text must be surrounded with quotes so that it is treated as a single
621 argument. Longer comments can be read from a text file.
623 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
624 text from standard input. (In this case an input image file name MUST be
625 supplied, so that the source JPEG file comes from somewhere else.) You can
626 enter multiple lines, up to 64KB worth. Type an end-of-file indicator
627 (usually control-D or control-Z) to terminate the comment text entry.
629 wrjpgcom will not add a COM block if the provided comment string is empty.
630 Therefore -replace -comment "" can be used to delete all COM blocks from a
633 These utility programs do not depend on the IJG JPEG library. In
634 particular, the source code for rdjpgcom is intended as an illustration of
635 the minimum amount of code required to parse a JPEG file header correctly.
projects / platform / upstream / libjpeg-turbo.git / blob