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 compression and decompression. JPEG
16 (pronounced "jay-peg") is a standardized compression method for full-color
17 and gray-scale images. JPEG is designed to handle "real-world" scenes,
18 for example scanned photographs. Cartoons, line drawings, and other
19 non-realistic images are not JPEG's strong suit; on that sort of material
20 you may get poor image quality and/or little compression.
22 JPEG is lossy, meaning that the output image is not necessarily identical to
23 the input image. Hence you should not use JPEG if you have to have identical
24 output bits. However, on typical real-world images, very good compression
25 levels can be obtained with no visible change, and amazingly high compression
26 is possible if you can tolerate a low-quality image. You can trade off image
27 quality against file size by adjusting the compressor's "quality" setting.
32 We provide two programs, cjpeg to compress an image file into JPEG format,
33 and djpeg to decompress a JPEG file back into a conventional image format.
35 On Unix-like systems, you say:
36 cjpeg [switches] [imagefile] >jpegfile
38 djpeg [switches] [jpegfile] >imagefile
39 The programs read the specified input file, or standard input if none is
40 named. They always write to standard output (with trace/error messages to
41 standard error). These conventions are handy for piping images between
44 On most non-Unix systems, you say:
45 cjpeg [switches] imagefile jpegfile
47 djpeg [switches] jpegfile imagefile
48 i.e., both the input and output files are named on the command line. This
49 style is a little more foolproof, and it loses no functionality if you don't
50 have pipes. (You can get this style on Unix too, if you prefer, by defining
51 TWO_FILE_COMMANDLINE when you compile the programs; see install.doc.)
54 cjpeg [switches] -outfile jpegfile imagefile
56 djpeg [switches] -outfile imagefile jpegfile
57 This syntax works on all systems, so it is useful for scripts.
59 The currently supported image file formats are: PPM (PBMPLUS color format),
60 PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Toolkit
61 format). (RLE is supported only if the URT library is available.)
62 cjpeg recognizes the input image format automatically, with the exception
63 of some Targa-format files. You have to tell djpeg which format to generate.
65 JPEG files are in the defacto standard JFIF file format. There are other,
66 less widely used JPEG-based file formats, but we don't support them.
68 All switch names may be abbreviated; for example, -grayscale may be written
69 -gray or -gr. Most of the "basic" switches can be abbreviated to as little as
70 one letter. Upper and lower case are equivalent (-BMP is the same as -bmp).
71 British spellings are also accepted (e.g., -greyscale), though for brevity
72 these are not mentioned below.
77 The basic command line switches for cjpeg are:
79 -quality N Scale quantization tables to adjust image quality.
80 Quality is 0 (worst) to 100 (best); default is 75.
81 (See below for more info.)
83 -grayscale Create monochrome JPEG file from color input.
84 Be sure to use this switch when compressing a grayscale
85 BMP file, because cjpeg isn't bright enough to notice
86 whether a BMP file uses only shades of gray. By
87 saying -grayscale, you'll get a smaller JPEG file that
88 takes less time to process.
90 -optimize Perform optimization of entropy encoding parameters.
91 Without this, default encoding parameters are used.
92 -optimize usually makes the JPEG file a little smaller,
93 but cjpeg runs somewhat slower and needs much more
94 memory. Image quality and speed of decompression are
95 unaffected by -optimize.
97 -progressive Create progressive JPEG file (see below).
99 -targa Input file is Targa format. Targa files that contain
100 an "identification" field will not be automatically
101 recognized by cjpeg; for such files you must specify
102 -targa to make cjpeg treat the input as Targa format.
103 For most Targa files, you won't need this switch.
105 The -quality switch lets you trade off compressed file size against quality of
106 the reconstructed image: the higher the quality setting, the larger the JPEG
107 file, and the closer the output image will be to the original input. Normally
108 you want to use the lowest quality setting (smallest file) that decompresses
109 into something visually indistinguishable from the original image. For this
110 purpose the quality setting should be between 50 and 95; the default of 75 is
111 often about right. If you see defects at -quality 75, then go up 5 or 10
112 counts at a time until you are happy with the output image. (The optimal
113 setting will vary from one image to another.)
115 -quality 100 will generate a quantization table of all 1's, minimizing loss
116 in the quantization step (but there is still information loss in subsampling,
117 as well as roundoff error). This setting is mainly of interest for
118 experimental purposes. Quality values above about 95 are NOT recommended for
119 normal use; the compressed file size goes up dramatically for hardly any gain
120 in output image quality.
122 In the other direction, quality values below 50 will produce very small files
123 of low image quality. Settings around 5 to 10 might be useful in preparing an
124 index of a large image library, for example. Try -quality 2 (or so) for some
125 amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
126 quantization tables, which are considered optional in the JPEG standard.
127 cjpeg emits a warning message when you give such a quality value, because some
128 other JPEG programs may be unable to decode the resulting file. Use -baseline
129 if you need to ensure compatibility at low quality values.)
131 The -progressive switch creates a "progressive JPEG" file. In this type of
132 JPEG file, the data is stored in multiple scans of increasing quality. If the
133 file is being transmitted over a slow communications link, the decoder can use
134 the first scan to display a low-quality image very quickly, and can then
135 improve the display with each subsequent scan. The final image is exactly
136 equivalent to a standard JPEG file of the same quality setting, and the total
137 file size is about the same --- often a little smaller. CAUTION: progressive
138 JPEG is not yet widely implemented, so many decoders will be unable to view a
139 progressive JPEG file at all.
141 Switches for advanced users:
143 -dct int Use integer DCT method (default).
144 -dct fast Use fast integer DCT (less accurate).
145 -dct float Use floating-point DCT method.
146 The float method is very slightly more accurate than
147 the int method, but is much slower unless your machine
148 has very fast floating-point hardware. Also note that
149 results of the floating-point method may vary slightly
150 across machines, while the integer methods should give
151 the same results everywhere. The fast integer method
152 is much less accurate than the other two.
154 -restart N Emit a JPEG restart marker every N MCU rows, or every
155 N MCU blocks if "B" is attached to the number.
156 -restart 0 (the default) means no restart markers.
158 -smooth N Smooth the input image to eliminate dithering noise.
159 N, ranging from 1 to 100, indicates the strength of
160 smoothing. 0 (the default) means no smoothing.
162 -maxmemory N Set limit for amount of memory to use in processing
163 large images. Value is in thousands of bytes, or
164 millions of bytes if "M" is attached to the number.
165 For example, -max 4m selects 4000000 bytes. If more
166 space is needed, temporary files will be used.
168 -verbose Enable debug printout. More -v's give more printout.
169 or -debug Also, version information is printed at startup.
171 The -restart option inserts extra markers that allow a JPEG decoder to
172 resynchronize after a transmission error. Without restart markers, any damage
173 to a compressed file will usually ruin the image from the point of the error
174 to the end of the image; with restart markers, the damage is usually confined
175 to the portion of the image up to the next restart marker. Of course, the
176 restart markers occupy extra space. We recommend -restart 1 for images that
177 will be transmitted across unreliable networks such as Usenet.
179 The -smooth option filters the input to eliminate fine-scale noise. This is
180 often useful when converting dithered images to JPEG: a moderate smoothing
181 factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
182 in a smaller JPEG file and a better-looking image. Too large a smoothing
183 factor will visibly blur the image, however.
185 Switches for wizards:
187 -baseline Force baseline-compatible quantization tables to be
188 generated. This clamps quantization values to 8 bits
189 even at low quality settings. (This switch is poorly
190 named, since it does not ensure that the output is
191 actually baseline JPEG. For example, you can use
192 -baseline and -progressive together.)
194 -qtables file Use the quantization tables given in the specified
197 -qslots N[,...] Select which quantization table to use for each color
200 -sample HxV[,...] Set JPEG sampling factors for each color component.
202 -scans file Use the scan script given in the specified text file.
204 The "wizard" switches are intended for experimentation with JPEG. If you
205 don't know what you are doing, DON'T USE THEM. These switches are documented
206 further in the file wizard.doc.
211 The basic command line switches for djpeg are:
213 -colors N Reduce image to at most N colors. This reduces the
214 or -quantize N number of colors used in the output image, so that it
215 can be displayed on a colormapped display or stored in
216 a colormapped file format. For example, if you have
217 an 8-bit display, you'd need to reduce to 256 or fewer
218 colors. (-colors is the recommended name, -quantize
219 is provided only for backwards compatibility.)
221 -fast Select recommended processing options for fast, low
222 quality output. (The default options are chosen for
223 highest quality output.) Currently, this is equivalent
224 to "-dct fast -nosmooth -onepass -dither ordered".
226 -grayscale Force gray-scale output even if JPEG file is color.
227 Useful for viewing on monochrome displays; also,
228 djpeg runs noticeably faster in this mode.
230 -scale M/N Scale the output image by a factor M/N. Currently
231 the scale factor must be 1/1, 1/2, 1/4, or 1/8.
232 Scaling is handy if the image is larger than your
233 screen; also, djpeg runs much faster when scaling
236 -bmp Select BMP output format (Windows flavor). 8-bit
237 colormapped format is emitted if -colors or -grayscale
238 is specified, or if the JPEG file is gray-scale;
239 otherwise, 24-bit full-color format is emitted.
241 -gif Select GIF output format. Since GIF does not support
242 more than 256 colors, -colors 256 is assumed (unless
243 you specify a smaller number of colors). If you
244 specify -fast, the default number of colors is 216.
246 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit
247 colormapped format is emitted if -colors or -grayscale
248 is specified, or if the JPEG file is gray-scale;
249 otherwise, 24-bit full-color format is emitted.
251 -pnm Select PBMPLUS (PPM/PGM) output format (this is the
252 default format). PGM is emitted if the JPEG file is
253 gray-scale or if -grayscale is specified; otherwise
256 -rle Select RLE output format. (Requires URT library.)
258 -targa Select Targa output format. Gray-scale format is
259 emitted if the JPEG file is gray-scale or if
260 -grayscale is specified; otherwise, colormapped format
261 is emitted if -colors is specified; otherwise, 24-bit
262 full-color format is emitted.
264 Switches for advanced users:
266 -dct int Use integer DCT method (default).
267 -dct fast Use fast integer DCT (less accurate).
268 -dct float Use floating-point DCT method.
269 The float method is very slightly more accurate than
270 the int method, but is much slower unless your machine
271 has very fast floating-point hardware. Also note that
272 results of the floating-point method may vary slightly
273 across machines, while the integer methods should give
274 the same results everywhere. The fast integer method
275 is much less accurate than the other two.
277 -dither fs Use Floyd-Steinberg dithering in color quantization.
278 -dither ordered Use ordered dithering in color quantization.
279 -dither none Do not use dithering in color quantization.
280 By default, Floyd-Steinberg dithering is applied when
281 quantizing colors; this is slow but usually produces
282 the best results. Ordered dither is a compromise
283 between speed and quality; no dithering is fast but
284 usually looks awful. Note that these switches have
285 no effect unless color quantization is being done.
286 Ordered dither is only available in -onepass mode.
288 -map FILE Quantize to the colors used in the specified image
289 file. This is useful for producing multiple files
290 with identical color maps, or for forcing a predefined
291 set of colors to be used. The FILE must be a GIF
292 or PPM file. This option overrides -colors and
295 -nosmooth Use a faster, lower-quality upsampling routine.
297 -onepass Use one-pass instead of two-pass color quantization.
298 The one-pass method is faster and needs less memory,
299 but it produces a lower-quality image. -onepass is
300 ignored unless you also say -colors N. Also,
301 the one-pass method is always used for gray-scale
302 output (the two-pass method is no improvement then).
304 -maxmemory N Set limit for amount of memory to use in processing
305 large images. Value is in thousands of bytes, or
306 millions of bytes if "M" is attached to the number.
307 For example, -max 4m selects 4000000 bytes. If more
308 space is needed, temporary files will be used.
310 -verbose Enable debug printout. More -v's give more printout.
311 or -debug Also, version information is printed at startup.
316 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
317 compressing full-color (24-bit) images. In particular, don't try to convert
318 cartoons, line drawings, and other images that have only a few distinct
319 colors. GIF works great on these, JPEG does not. If you want to convert a
320 GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
321 to get a satisfactory conversion. -smooth 10 or so is often helpful.
323 Avoid running an image through a series of JPEG compression/decompression
324 cycles. Image quality loss will accumulate; after ten or so cycles the image
325 may be noticeably worse than it was after one cycle. It's best to use a
326 lossless format while manipulating an image, then convert to JPEG format when
327 you are ready to file the image away.
329 The -optimize option to cjpeg is worth using when you are making a "final"
330 version for posting or archiving. It's also a win when you are using low
331 quality settings to make very small JPEG files; the percentage improvement
332 is often a lot more than it is on larger files. (At present, -optimize
333 mode is always selected when generating progressive JPEG files.)
335 GIF input files are no longer supported, to avoid the Unisys LZW patent.
336 Use a Unisys-licensed program if you need to read a GIF file. (Conversion
337 of GIF files to JPEG is usually a bad idea anyway.)
342 To get a quick preview of an image, use the -grayscale and/or -scale switches.
343 "-grayscale -scale 1/8" is the fastest case.
345 Several options are available that trade off image quality to gain speed.
346 "-fast" turns on the recommended settings.
348 "-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
349 When producing a color-quantized image, "-onepass -dither ordered" is fast but
350 much lower quality than the default behavior. "-dither none" may give
351 acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
353 If you are fortunate enough to have very fast floating point hardware,
354 "-dct float" may be even faster than "-dct fast". But on most machines
355 "-dct float" is slower than "-dct int"; in this case it is not worth using,
356 because its theoretical accuracy advantage is too small to be significant
359 Two-pass color quantization requires a good deal of memory; on MS-DOS machines
360 it may run out of memory even with -maxmemory 0. In that case you can still
361 decompress, with some loss of image quality, by specifying -onepass for
362 one-pass quantization.
364 To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These
365 are larger than they should be, but are readable by standard GIF decoders.
368 HINTS FOR BOTH PROGRAMS
370 If more space is needed than will fit in the available main memory (as
371 determined by -maxmemory), temporary files will be used. (MS-DOS versions
372 will try to get extended or expanded memory first.) The temporary files are
373 often rather large: in typical cases they occupy three bytes per pixel, for
374 example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
375 free disk space, leave out -progressive and -optimize (for cjpeg) or specify
376 -onepass (for djpeg).
378 On MS-DOS, the temporary files are created in the directory named by the TMP
379 or TEMP environment variable, or in the current directory if neither of those
380 exist. Amiga implementations put the temp files in the directory named by
381 JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
384 The default memory usage limit (-maxmemory) is set when the software is
385 compiled. If you get an "insufficient memory" error, try specifying a smaller
386 -maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
387 may want to recompile with a smaller default value if this happens often.
389 On machines that have "environment" variables, you can define the environment
390 variable JPEGMEM to set the default memory limit. The value is specified as
391 described for the -maxmemory switch. JPEGMEM overrides the default value
392 specified when the program was compiled, and itself is overridden by an
393 explicit -maxmemory switch.
395 On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
396 use. (Extended or expanded memory is also used if available.) Most
397 DOS-specific versions of this software do their own memory space estimation
398 and do not need you to specify -maxmemory.
403 jpegtran performs various useful transformations of JPEG files.
404 It can translate the coded representation from one variant of JPEG to another,
405 for example from baseline JPEG to progressive JPEG or vice versa. It can also
406 perform some rearrangements of the image data, for example turning an image
407 from landscape to portrait format by rotation.
409 jpegtran works by rearranging the compressed data (DCT coefficients), without
410 ever fully decoding the image. Therefore, its transformations are lossless:
411 there is no image degradation at all, which would not be true if you used
412 djpeg followed by cjpeg to accomplish the same conversion. But by the same
413 token, jpegtran cannot perform lossy operations such as changing the image
416 jpegtran uses a command line syntax similar to cjpeg or djpeg.
417 On Unix-like systems, you say:
418 jpegtran [switches] [inputfile] >outputfile
419 On most non-Unix systems, you say:
420 jpegtran [switches] inputfile outputfile
421 where both the input and output files are JPEG files.
423 To specify the coded JPEG representation used in the output file,
424 jpegtran accepts a subset of the switches recognized by cjpeg:
425 -optimize Perform optimization of entropy encoding parameters.
426 -progressive Create progressive JPEG file.
427 -restart N Emit a JPEG restart marker every N MCU rows, or every
428 N MCU blocks if "B" is attached to the number.
429 -scans file Use the scan script given in the specified text file.
430 See the previous discussion of cjpeg for more details about these switches.
431 If you specify none of these switches, you get a plain baseline-JPEG output
432 file. The quality setting and so forth are determined by the input file.
434 The image can be losslessly transformed by giving one of these switches:
435 -flip horizontal Mirror image horizontally (left-right).
436 -flip vertical Mirror image vertically (top-bottom).
437 -rotate 90 Rotate image 90 degrees clockwise.
438 -rotate 180 Rotate image 180 degrees.
439 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw).
440 -transpose Transpose image (across UL-to-LR axis).
441 -transverse Transverse transpose (across UR-to-LL axis).
443 The transpose transformation has no restrictions regarding image dimensions.
444 The other transformations operate rather oddly if the image dimensions are not
445 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
446 transform complete blocks of DCT coefficient data in the desired way.
448 jpegtran's default behavior when transforming an odd-size image is designed
449 to preserve exact reversibility and mathematical consistency of the
450 transformation set. As stated, transpose is able to flip the entire image
451 area. Horizontal mirroring leaves any partial iMCU column at the right edge
452 untouched, but is able to flip all rows of the image. Similarly, vertical
453 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
454 able to flip all columns. The other transforms can be built up as sequences
455 of transpose and flip operations; for consistency, their actions on edge
456 pixels are defined to be the same as the end result of the corresponding
457 transpose-and-flip sequence.
459 For practical use, you may prefer to discard any untransformable edge pixels
460 rather than having a strange-looking strip along the right and/or bottom edges
461 of a transformed image. To do this, add the -trim switch:
462 -trim Drop non-transformable edge blocks.
463 Obviously, a transformation with -trim is not reversible, so strictly speaking
464 jpegtran with this switch is not lossless. Also, the expected mathematical
465 equivalences between the transformations no longer hold. For example,
466 "-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
467 "-rot 180 -trim" trims both edges.
469 Another not-strictly-lossless transformation switch is:
470 -grayscale Force grayscale output.
471 This option discards the chrominance channels if the input image is YCbCr
472 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
473 luminance channel is preserved exactly, so this is a better method of reducing
474 to grayscale than decompression, conversion, and recompression. This switch
475 is particularly handy for fixing a monochrome picture that was mistakenly
476 encoded as a color JPEG. (In such a case, the space savings from getting rid
477 of the near-empty chroma channels won't be large; but the decoding time for
478 a grayscale JPEG is substantially less than that for a color JPEG.)
480 jpegtran also recognizes these switches that control what to do with "extra"
481 markers, such as comment blocks:
482 -copy none Copy no extra markers from source file. This setting
483 suppresses all comments and other excess baggage
484 present in the source file.
485 -copy comments Copy only comment markers. This setting copies
486 comments from the source file, but discards
487 any other inessential data.
488 -copy all Copy all extra markers. This setting preserves
489 miscellaneous markers found in the source file, such
490 as JFIF thumbnails and Photoshop settings. In some
491 files these extra markers can be sizable.
492 The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
493 jpegtran always did the equivalent of -copy none.)
495 Additional switches recognized by jpegtran are:
500 These work the same as in cjpeg or djpeg.
503 THE COMMENT UTILITIES
505 The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
506 Although the standard doesn't actually define what COM blocks are for, they
507 are widely used to hold user-supplied text strings. This lets you add
508 annotations, titles, index terms, etc to your JPEG files, and later retrieve
509 them as text. COM blocks do not interfere with the image stored in the JPEG
510 file. The maximum size of a COM block is 64K, but you can have as many of
511 them as you like in one JPEG file.
513 We provide two utility programs to display COM block contents and add COM
514 blocks to a JPEG file.
516 rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
517 standard output. The command line syntax is
518 rdjpgcom [-verbose] [inputfilename]
519 The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
520 image dimensions. If you omit the input file name from the command line,
521 the JPEG file is read from standard input. (This may not work on some
522 operating systems, if binary data can't be read from stdin.)
524 wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
525 Ordinarily, the COM block is added after any existing COM blocks, but you
526 can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
527 file; it does not modify the input file. DO NOT try to overwrite the input
528 file by directing wrjpgcom's output back into it; on most systems this will
529 just destroy your file.
531 The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
533 wrjpgcom [switches] [inputfilename]
534 The output file is written to standard output. The input file comes from
535 the named file, or from standard input if no input file is named.
537 On most non-Unix systems, the syntax is
538 wrjpgcom [switches] inputfilename outputfilename
539 where both input and output file names must be given explicitly.
541 wrjpgcom understands three switches:
542 -replace Delete any existing COM blocks from the file.
543 -comment "Comment text" Supply new COM text on command line.
544 -cfile name Read text for new COM block from named file.
545 (Switch names can be abbreviated.) If you have only one line of comment text
546 to add, you can provide it on the command line with -comment. The comment
547 text must be surrounded with quotes so that it is treated as a single
548 argument. Longer comments can be read from a text file.
550 If you give neither -comment nor -cfile, then wrjpgcom will read the comment
551 text from standard input. (In this case an input image file name MUST be
552 supplied, so that the source JPEG file comes from somewhere else.) You can
553 enter multiple lines, up to 64KB worth. Type an end-of-file indicator
554 (usually control-D or control-Z) to terminate the comment text entry.
556 wrjpgcom will not add a COM block if the provided comment string is empty.
557 Therefore -replace -comment "" can be used to delete all COM blocks from a
560 These utility programs do not depend on the IJG JPEG library. In
561 particular, the source code for rdjpgcom is intended as an illustration of
562 the minimum amount of code required to parse a JPEG file header correctly.
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