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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
7 your own programs.)
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.
13 INTRODUCTION
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 grayscale images.
20 GENERAL USAGE
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
27 or
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
32 programs.
34 On most non-Unix systems, you say:
35 cjpeg [switches] imagefile jpegfile
36 or
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.)
43 You can also say:
44 cjpeg [switches] -outfile jpegfile imagefile
45 or
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 grayscale format), BMP, Targa, and RLE (Utah Raster Toolkit
51 format). (RLE is supported only if the URT library is available, which it
52 isn't on most non-Unix systems.) cjpeg recognizes the input image format
53 automatically, with the exception of some Targa-format files. You have to
54 tell djpeg which format to generate.
56 JPEG files are in the standard JFIF file format. There are other,
57 less widely used JPEG-based file formats, but we don't support them.
59 All switch names may be abbreviated; for example, -grayscale may be written
60 -gray or -gr. Most of the "basic" switches can be abbreviated to as little as
61 one letter. Upper and lower case are equivalent (-BMP is the same as -bmp).
62 British spellings are also accepted (e.g., -greyscale), though for brevity
63 these are not mentioned below.
66 CJPEG DETAILS
68 The basic command line switches for cjpeg are:
70 -quality N[,...] Scale quantization tables to adjust image quality.
71 Quality is 0 (worst) to 100 (best); default is 75.
72 (See below for more info.)
74 -grayscale Create monochrome JPEG file from color input.
75 Be sure to use this switch when compressing a grayscale
76 BMP file, because cjpeg isn't bright enough to notice
77 whether a BMP file uses only shades of gray. By
78 saying -grayscale, you'll get a smaller JPEG file that
79 takes less time to process.
81 -rgb Create RGB JPEG file.
82 Using this switch suppresses the conversion from RGB
83 colorspace input to the default YCbCr JPEG colorspace.
84 You can use this switch in combination with the
85 -block N switch (see below) for lossless JPEG coding.
86 See also the -rgb1 switch below.
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 -scale M/N Scale the output image by a factor M/N. Currently
98 supported scale factors are M/N with all N from 1 to
99 16, where M is the destination DCT size, which is 8 by
100 default (see -block N switch below).
102 -targa Input file is Targa format. Targa files that contain
103 an "identification" field will not be automatically
104 recognized by cjpeg; for such files you must specify
105 -targa to make cjpeg treat the input as Targa format.
106 For most Targa files, you won't need this switch.
108 The -quality switch lets you trade off compressed file size against quality of
109 the reconstructed image: the higher the quality setting, the larger the JPEG
110 file, and the closer the output image will be to the original input. Normally
111 you want to use the lowest quality setting (smallest file) that decompresses
112 into something visually indistinguishable from the original image. For this
113 purpose the quality setting should be between 50 and 95; the default of 75 is
114 often about right. If you see defects at -quality 75, then go up 5 or 10
115 counts at a time until you are happy with the output image. (The optimal
116 setting will vary from one image to another.)
118 -quality 100 will generate a quantization table of all 1's, minimizing loss
119 in the quantization step (but there is still information loss in subsampling,
120 as well as roundoff error). This setting is mainly of interest for
121 experimental purposes. Quality values above about 95 are NOT recommended for
122 normal use; the compressed file size goes up dramatically for hardly any gain
123 in output image quality.
125 In the other direction, quality values below 50 will produce very small files
126 of low image quality. Settings around 5 to 10 might be useful in preparing an
127 index of a large image library, for example. Try -quality 2 (or so) for some
128 amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
129 quantization tables, which are considered optional in the JPEG standard.
130 cjpeg emits a warning message when you give such a quality value, because some
131 other JPEG programs may be unable to decode the resulting file. Use -baseline
132 if you need to ensure compatibility at low quality values.)
134 The -quality option has been extended in IJG version 7 for support of separate
135 quality settings for luminance and chrominance (or in general, for every
136 provided quantization table slot). This feature is useful for high-quality
137 applications which cannot accept the damage of color data by coarse
138 subsampling settings. You can now easily reduce the color data amount more
139 smoothly with finer control without separate subsampling. The resulting file
140 is fully compliant with standard JPEG decoders.
141 Note that the -quality ratings refer to the quantization table slots, and that
142 the last value is replicated if there are more q-table slots than parameters.
143 The default q-table slots are 0 for luminance and 1 for chrominance with
144 default tables as given in the JPEG standard. This is compatible with the old
145 behaviour in case that only one parameter is given, which is then used for
146 both luminance and chrominance (slots 0 and 1). More or custom quantization
147 tables can be set with -qtables and assigned to components with -qslots
148 parameter (see the "wizard" switches below).
149 CAUTION: You must explicitly add -sample 1x1 for efficient separate color
150 quality selection, since the default value used by library is 2x2!
152 The -progressive switch creates a "progressive JPEG" file. In this type of
153 JPEG file, the data is stored in multiple scans of increasing quality. If the
154 file is being transmitted over a slow communications link, the decoder can use
155 the first scan to display a low-quality image very quickly, and can then
156 improve the display with each subsequent scan. The final image is exactly
157 equivalent to a standard JPEG file of the same quality setting, and the total
158 file size is about the same --- often a little smaller.
160 Switches for advanced users:
162 -arithmetic Use arithmetic coding.
163 CAUTION: arithmetic coded JPEG is not yet widely
164 implemented, so many decoders will be unable to
165 view an arithmetic coded JPEG file at all.
167 -block N Set DCT block size. All N from 1 to 16 are possible.
168 Default is 8 (baseline format).
169 Larger values produce higher compression,
170 smaller values produce higher quality
171 (exact DCT stage possible with 1 or 2; with the
172 default quality of 75 and default Luminance qtable
173 the DCT+Quantization stage is lossless for N=1).
174 CAUTION: An implementation of the JPEG SmartScale
175 extension is required for this feature. SmartScale
176 enabled JPEG is not yet widely implemented, so many
177 decoders will be unable to view a SmartScale extended
178 JPEG file at all.
180 -rgb1 Create RGB JPEG file with reversible color transform.
181 Works like the -rgb switch (see above) and inserts a
182 simple reversible color transform into the processing
183 which significantly improves the compression.
184 Use this switch in combination with the -block N
185 switch (see above) for lossless JPEG coding.
186 CAUTION: A decoder with inverse color transform
187 support is required for this feature. Reversible
188 color transform support is not yet widely implemented,
189 so many decoders will be unable to view a reversible
190 color transformed JPEG file at all.
192 -bgycc Create big gamut YCC JPEG file.
193 In this type of encoding the color difference
194 components are quantized further by a factor of 2
195 compared to the normal Cb/Cr values, thus creating
196 space to allow larger color values with higher
197 saturation than the normal gamut limits to be encoded.
198 In order to compensate for the loss of color fidelity
199 compared to a normal YCC encoded file, the color
200 quantization tables can be adjusted accordingly.
201 For example, cjpeg -bgycc -quality 80,90 will give
202 similar results as cjpeg -quality 80.
203 CAUTION: For correct decompression a decoder with big
204 gamut YCC support (JFIF version 2) is required.
205 An old decoder may or may not display a big gamut YCC
206 encoded JPEG file, depending on JFIF version check
207 and corresponding warning/error configuration.
208 In case of a granted decompression the old decoder
209 will display the image with half saturated colors.
211 -dct int Use integer DCT method (default).
212 -dct fast Use fast integer DCT (less accurate).
213 -dct float Use floating-point DCT method.
214 The float method is very slightly more accurate than
215 the int method, but is much slower unless your machine
216 has very fast floating-point hardware. Also note that
217 results of the floating-point method may vary slightly
218 across machines, while the integer methods should give
219 the same results everywhere. The fast integer method
220 is much less accurate than the other two.
222 -nosmooth Don't use high-quality downsampling.
224 -restart N Emit a JPEG restart marker every N MCU rows, or every
225 N MCU blocks if "B" is attached to the number.
226 -restart 0 (the default) means no restart markers.
228 -smooth N Smooth the input image to eliminate dithering noise.
229 N, ranging from 1 to 100, indicates the strength of
230 smoothing. 0 (the default) means no smoothing.
232 -maxmemory N Set limit for amount of memory to use in processing
233 large images. Value is in thousands of bytes, or
234 millions of bytes if "M" is attached to the number.
235 For example, -max 4m selects 4000000 bytes. If more
236 space is needed, temporary files will be used.
238 -verbose Enable debug printout. More -v's give more printout.
239 or -debug Also, version information is printed at startup.
241 The -restart option inserts extra markers that allow a JPEG decoder to
242 resynchronize after a transmission error. Without restart markers, any damage
243 to a compressed file will usually ruin the image from the point of the error
244 to the end of the image; with restart markers, the damage is usually confined
245 to the portion of the image up to the next restart marker. Of course, the
246 restart markers occupy extra space. We recommend -restart 1 for images that
247 will be transmitted across unreliable networks such as Usenet.
249 The -smooth option filters the input to eliminate fine-scale noise. This is
250 often useful when converting dithered images to JPEG: a moderate smoothing
251 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
252 in a smaller JPEG file and a better-looking image. Too large a smoothing
253 factor will visibly blur the image, however.
255 Switches for wizards:
257 -baseline Force baseline-compatible quantization tables to be
258 generated. This clamps quantization values to 8 bits
259 even at low quality settings. (This switch is poorly
260 named, since it does not ensure that the output is
261 actually baseline JPEG. For example, you can use
262 -baseline and -progressive together.)
264 -qtables file Use the quantization tables given in the specified
265 text file.
267 -qslots N[,...] Select which quantization table to use for each color
268 component.
270 -sample HxV[,...] Set JPEG sampling factors for each color component.
272 -scans file Use the scan script given in the specified text file.
274 The "wizard" switches are intended for experimentation with JPEG. If you
275 don't know what you are doing, DON'T USE THEM. These switches are documented
276 further in the file wizard.txt.
279 DJPEG DETAILS
281 The basic command line switches for djpeg are:
283 -colors N Reduce image to at most N colors. This reduces the
284 or -quantize N number of colors used in the output image, so that it
285 can be displayed on a colormapped display or stored in
286 a colormapped file format. For example, if you have
287 an 8-bit display, you'd need to reduce to 256 or fewer
288 colors. (-colors is the recommended name, -quantize
289 is provided only for backwards compatibility.)
291 -fast Select recommended processing options for fast, low
292 quality output. (The default options are chosen for
293 highest quality output.) Currently, this is equivalent
294 to "-dct fast -nosmooth -onepass -dither ordered".
296 -grayscale Force grayscale output even if JPEG file is color.
297 Useful for viewing on monochrome displays; also,
298 djpeg runs noticeably faster in this mode.
300 -rgb Force RGB output even if JPEG file is grayscale.
301 This is provided to support applications that don't
302 want to cope with grayscale as a separate case.
304 -scale M/N Scale the output image by a factor M/N. Currently
305 supported scale factors are M/N with all M from 1 to
306 16, where N is the source DCT size, which is 8 for
307 baseline JPEG. If the /N part is omitted, then M
308 specifies the DCT scaled size to be applied on the
309 given input. For baseline JPEG this is equivalent to
310 M/8 scaling, since the source DCT size for baseline
311 JPEG is 8. Scaling is handy if the image is larger
312 than your screen; also, djpeg runs much faster when
313 scaling down the output.
315 -bmp Select BMP output format (Windows flavor). 8-bit
316 colormapped format is emitted if -colors or -grayscale
317 is specified, or if the JPEG file is grayscale;
318 otherwise, 24-bit full-color format is emitted.
320 -gif Select GIF output format. Since GIF does not support
321 more than 256 colors, -colors 256 is assumed (unless
322 you specify a smaller number of colors). If you
323 specify -fast, the default number of colors is 216.
325 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit
326 colormapped format is emitted if -colors or -grayscale
327 is specified, or if the JPEG file is grayscale;
328 otherwise, 24-bit full-color format is emitted.
330 -pnm Select PBMPLUS (PPM/PGM) output format (this is the
331 default format). PGM is emitted if the JPEG file is
332 grayscale or if -grayscale is specified; otherwise
333 PPM is emitted.
335 -rle Select RLE output format. (Requires URT library.)
337 -targa Select Targa output format. Grayscale format is
338 emitted if the JPEG file is grayscale or if
339 -grayscale is specified; otherwise, colormapped format
340 is emitted if -colors is specified; otherwise, 24-bit
341 full-color format is emitted.
343 Switches for advanced users:
345 -dct int Use integer DCT method (default).
346 -dct fast Use fast integer DCT (less accurate).
347 -dct float Use floating-point DCT method.
348 The float method is very slightly more accurate than
349 the int method, but is much slower unless your machine
350 has very fast floating-point hardware. Also note that
351 results of the floating-point method may vary slightly
352 across machines, while the integer methods should give
353 the same results everywhere. The fast integer method
354 is much less accurate than the other two.
356 -dither fs Use Floyd-Steinberg dithering in color quantization.
357 -dither ordered Use ordered dithering in color quantization.
358 -dither none Do not use dithering in color quantization.
359 By default, Floyd-Steinberg dithering is applied when
360 quantizing colors; this is slow but usually produces
361 the best results. Ordered dither is a compromise
362 between speed and quality; no dithering is fast but
363 usually looks awful. Note that these switches have
364 no effect unless color quantization is being done.
365 Ordered dither is only available in -onepass mode.
367 -map FILE Quantize to the colors used in the specified image
368 file. This is useful for producing multiple files
369 with identical color maps, or for forcing a predefined
370 set of colors to be used. The FILE must be a GIF
371 or PPM file. This option overrides -colors and
372 -onepass.
374 -nosmooth Don't use high-quality upsampling.
376 -onepass Use one-pass instead of two-pass color quantization.
377 The one-pass method is faster and needs less memory,
378 but it produces a lower-quality image. -onepass is
379 ignored unless you also say -colors N. Also,
380 the one-pass method is always used for grayscale
381 output (the two-pass method is no improvement then).
383 -maxmemory N Set limit for amount of memory to use in processing
384 large images. Value is in thousands of bytes, or
385 millions of bytes if "M" is attached to the number.
386 For example, -max 4m selects 4000000 bytes. If more
387 space is needed, temporary files will be used.
389 -verbose Enable debug printout. More -v's give more printout.
390 or -debug Also, version information is printed at startup.
393 HINTS FOR CJPEG
395 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
396 compressing full-color (24-bit) images. In particular, don't try to convert
397 cartoons, line drawings, and other images that have only a few distinct
398 colors. GIF works great on these, JPEG does not. If you want to convert a
399 GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
400 to get a satisfactory conversion. -smooth 10 or so is often helpful.
402 Avoid running an image through a series of JPEG compression/decompression
403 cycles. Image quality loss will accumulate; after ten or so cycles the image
404 may be noticeably worse than it was after one cycle. It's best to use a
405 lossless format while manipulating an image, then convert to JPEG format when
406 you are ready to file the image away.
408 The -optimize option to cjpeg is worth using when you are making a "final"
409 version for posting or archiving. It's also a win when you are using low
410 quality settings to make very small JPEG files; the percentage improvement
411 is often a lot more than it is on larger files. (At present, -optimize
412 mode is always selected when generating progressive JPEG files.)
414 GIF input files are no longer supported, to avoid the Unisys LZW patent
415 (now expired).
416 (Conversion of GIF files to JPEG is usually a bad idea anyway.)
419 HINTS FOR DJPEG
421 To get a quick preview of an image, use the -grayscale and/or -scale switches.
422 "-grayscale -scale 1/8" is the fastest case.
424 Several options are available that trade off image quality to gain speed.
425 "-fast" turns on the recommended settings.
427 "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
428 When producing a color-quantized image, "-onepass -dither ordered" is fast but
429 much lower quality than the default behavior. "-dither none" may give
430 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
432 If you are fortunate enough to have very fast floating point hardware,
433 "-dct float" may be even faster than "-dct fast". But on most machines
434 "-dct float" is slower than "-dct int"; in this case it is not worth using,
435 because its theoretical accuracy advantage is too small to be significant
436 in practice.
438 Two-pass color quantization requires a good deal of memory; on MS-DOS machines
439 it may run out of memory even with -maxmemory 0. In that case you can still
440 decompress, with some loss of image quality, by specifying -onepass for
441 one-pass quantization.
443 To avoid the Unisys LZW patent (now expired), djpeg produces uncompressed GIF
444 files. These are larger than they should be, but are readable by standard GIF
445 decoders.
448 HINTS FOR BOTH PROGRAMS
450 If more space is needed than will fit in the available main memory (as
451 determined by -maxmemory), temporary files will be used. (MS-DOS versions
452 will try to get extended or expanded memory first.) The temporary files are
453 often rather large: in typical cases they occupy three bytes per pixel, for
454 example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
455 free disk space, leave out -progressive and -optimize (for cjpeg) or specify
456 -onepass (for djpeg).
458 On MS-DOS, the temporary files are created in the directory named by the TMP
459 or TEMP environment variable, or in the current directory if neither of those
460 exist. Amiga implementations put the temp files in the directory named by
461 JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
462 space.
464 The default memory usage limit (-maxmemory) is set when the software is
465 compiled. If you get an "insufficient memory" error, try specifying a smaller
466 -maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
467 may want to recompile with a smaller default value if this happens often.
469 On machines that have "environment" variables, you can define the environment
470 variable JPEGMEM to set the default memory limit. The value is specified as
471 described for the -maxmemory switch. JPEGMEM overrides the default value
472 specified when the program was compiled, and itself is overridden by an
473 explicit -maxmemory switch.
475 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
476 use. (Extended or expanded memory is also used if available.) Most
477 DOS-specific versions of this software do their own memory space estimation
478 and do not need you to specify -maxmemory.
481 JPEGTRAN
483 jpegtran performs various useful transformations of JPEG files.
484 It can translate the coded representation from one variant of JPEG to another,
485 for example from baseline JPEG to progressive JPEG or vice versa. It can also
486 perform some rearrangements of the image data, for example turning an image
487 from landscape to portrait format by rotation. For EXIF files and JPEG files
488 containing Exif data, you may prefer to use exiftran instead.
490 jpegtran works by rearranging the compressed data (DCT coefficients), without
491 ever fully decoding the image. Therefore, its transformations are lossless:
492 there is no image degradation at all, which would not be true if you used
493 djpeg followed by cjpeg to accomplish the same conversion. But by the same
494 token, jpegtran cannot perform lossy operations such as changing the image
495 quality. However, while the image data is losslessly transformed, metadata
496 can be removed. See the -copy option for specifics.
498 jpegtran uses a command line syntax similar to cjpeg or djpeg.
499 On Unix-like systems, you say:
500 jpegtran [switches] [inputfile] >outputfile
501 On most non-Unix systems, you say:
502 jpegtran [switches] inputfile outputfile
503 where both the input and output files are JPEG files.
505 To specify the coded JPEG representation used in the output file,
506 jpegtran accepts a subset of the switches recognized by cjpeg:
507 -optimize Perform optimization of entropy encoding parameters.
508 -progressive Create progressive JPEG file.
509 -arithmetic Use arithmetic coding.
510 -restart N Emit a JPEG restart marker every N MCU rows, or every
511 N MCU blocks if "B" is attached to the number.
512 -scans file Use the scan script given in the specified text file.
513 See the previous discussion of cjpeg for more details about these switches.
514 If you specify none of these switches, you get a plain baseline-JPEG output
515 file. The quality setting and so forth are determined by the input file.
517 The image can be losslessly transformed by giving one of these switches:
518 -flip horizontal Mirror image horizontally (left-right).
519 -flip vertical Mirror image vertically (top-bottom).
520 -rotate 90 Rotate image 90 degrees clockwise.
521 -rotate 180 Rotate image 180 degrees.
522 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw).
523 -transpose Transpose image (across UL-to-LR axis).
524 -transverse Transverse transpose (across UR-to-LL axis).
526 The transpose transformation has no restrictions regarding image dimensions.
527 The other transformations operate rather oddly if the image dimensions are not
528 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
529 transform complete blocks of DCT coefficient data in the desired way.
531 jpegtran's default behavior when transforming an odd-size image is designed
532 to preserve exact reversibility and mathematical consistency of the
533 transformation set. As stated, transpose is able to flip the entire image
534 area. Horizontal mirroring leaves any partial iMCU column at the right edge
535 untouched, but is able to flip all rows of the image. Similarly, vertical
536 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
537 able to flip all columns. The other transforms can be built up as sequences
538 of transpose and flip operations; for consistency, their actions on edge
539 pixels are defined to be the same as the end result of the corresponding
540 transpose-and-flip sequence.
542 For practical use, you may prefer to discard any untransformable edge pixels
543 rather than having a strange-looking strip along the right and/or bottom edges
544 of a transformed image. To do this, add the -trim switch:
545 -trim Drop non-transformable edge blocks.
546 Obviously, a transformation with -trim is not reversible, so strictly speaking
547 jpegtran with this switch is not lossless. Also, the expected mathematical
548 equivalences between the transformations no longer hold. For example,
549 "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
550 "-rot 180 -trim" trims both edges.
552 If you are only interested in perfect transformation, add the -perfect switch:
553 -perfect Fails with an error if the transformation is not
554 perfect.
555 For example you may want to do
556 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
557 to do a perfect rotation if available or an approximated one if not.
559 We also offer a lossless-crop option, which discards data outside a given
560 image region but losslessly preserves what is inside. Like the rotate and
561 flip transforms, lossless crop is restricted by the current JPEG format: the
562 upper left corner of the selected region must fall on an iMCU boundary. If
563 this does not hold for the given crop parameters, we silently move the upper
564 left corner up and/or left to make it so, simultaneously increasing the
565 region dimensions to keep the lower right crop corner unchanged. (Thus, the
566 output image covers at least the requested region, but may cover more.)
567 The adjustment of the region dimensions may be optionally disabled by
568 attaching an 'f' character ("force") to the width or height number.
570 The image can be losslessly cropped by giving the switch:
571 -crop WxH+X+Y Crop to a rectangular subarea of width W, height H
572 starting at point X,Y.
574 A complementary lossless-wipe option is provided to discard (gray out) data
575 inside a given image region while losslessly preserving what is outside:
576 -wipe WxH+X+Y Wipe (gray out) a rectangular subarea of
577 width W, height H starting at point X,Y.
579 Other not-strictly-lossless transformation switches are:
581 -grayscale Force grayscale output.
582 This option discards the chrominance channels if the input image is YCbCr
583 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
584 luminance channel is preserved exactly, so this is a better method of reducing
585 to grayscale than decompression, conversion, and recompression. This switch
586 is particularly handy for fixing a monochrome picture that was mistakenly
587 encoded as a color JPEG. (In such a case, the space savings from getting rid
588 of the near-empty chroma channels won't be large; but the decoding time for
589 a grayscale JPEG is substantially less than that for a color JPEG.)
591 -scale M/N Scale the output image by a factor M/N.
592 Currently supported scale factors are M/N with all M from 1 to 16, where N is
593 the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
594 then M specifies the DCT scaled size to be applied on the given input. For
595 baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
596 for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale
597 extension is required for this feature. SmartScale enabled JPEG is not yet
598 widely implemented, so many decoders will be unable to view a SmartScale
599 extended JPEG file at all.
601 jpegtran also recognizes these switches that control what to do with "extra"
602 markers, such as comment blocks:
603 -copy none Copy no extra markers from source file.
604 This setting suppresses all comments
605 and other metadata in the source file.
606 -copy comments Copy only comment markers.
607 This setting copies comments from the source file,
608 but discards any other metadata.
609 -copy all Copy all extra markers. This setting preserves
610 metadata found in the source file, such as JFIF
611 thumbnails, Exif data, and Photoshop settings.
612 In some files these extra markers can be sizable.
613 Note that this option will copy thumbnails as-is;
614 they will not be transformed.
615 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
616 jpegtran always did the equivalent of -copy none.)
618 Additional switches recognized by jpegtran are:
619 -outfile filename
620 -maxmemory N
621 -verbose
622 -debug
623 These work the same as in cjpeg or djpeg.
626 THE COMMENT UTILITIES
628 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
629 Although the standard doesn't actually define what COM blocks are for, they
630 are widely used to hold user-supplied text strings. This lets you add
631 annotations, titles, index terms, etc to your JPEG files, and later retrieve
632 them as text. COM blocks do not interfere with the image stored in the JPEG
633 file. The maximum size of a COM block is 64K, but you can have as many of
634 them as you like in one JPEG file.
636 We provide two utility programs to display COM block contents and add COM
637 blocks to a JPEG file.
639 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
640 standard output. The command line syntax is
641 rdjpgcom [-raw] [-verbose] [inputfilename]
642 The switch "-raw" (or just "-r") causes rdjpgcom to also output non-printable
643 characters in comments, which are normally escaped for security reasons.
644 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
645 image dimensions. If you omit the input file name from the command line,
646 the JPEG file is read from standard input. (This may not work on some
647 operating systems, if binary data can't be read from stdin.)
649 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
650 Ordinarily, the COM block is added after any existing COM blocks, but you
651 can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
652 file; it does not modify the input file. DO NOT try to overwrite the input
653 file by directing wrjpgcom's output back into it; on most systems this will
654 just destroy your file.
656 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
657 systems, it is
658 wrjpgcom [switches] [inputfilename]
659 The output file is written to standard output. The input file comes from
660 the named file, or from standard input if no input file is named.
662 On most non-Unix systems, the syntax is
663 wrjpgcom [switches] inputfilename outputfilename
664 where both input and output file names must be given explicitly.
666 wrjpgcom understands three switches:
667 -replace Delete any existing COM blocks from the file.
668 -comment "Comment text" Supply new COM text on command line.
669 -cfile name Read text for new COM block from named file.
670 (Switch names can be abbreviated.) If you have only one line of comment text
671 to add, you can provide it on the command line with -comment. The comment
672 text must be surrounded with quotes so that it is treated as a single
673 argument. Longer comments can be read from a text file.
675 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
676 text from standard input. (In this case an input image file name MUST be
677 supplied, so that the source JPEG file comes from somewhere else.) You can
678 enter multiple lines, up to 64KB worth. Type an end-of-file indicator
679 (usually control-D or control-Z) to terminate the comment text entry.
681 wrjpgcom will not add a COM block if the provided comment string is empty.
682 Therefore -replace -comment "" can be used to delete all COM blocks from a
683 file.
685 These utility programs do not depend on the IJG JPEG library. In
686 particular, the source code for rdjpgcom is intended as an illustration of
687 the minimum amount of code required to parse a JPEG file header correctly.