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Dees_Troy51a0e822012-09-05 15:24:24 -04001USAGE instructions for the Independent JPEG Group's JPEG software
2=================================================================
3
4This file describes usage of the JPEG conversion programs cjpeg and djpeg,
5as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See
6the other documentation files if you wish to use the JPEG library within
7your own programs.)
8
9If you are on a Unix machine you may prefer to read the Unix-style manual
10pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.
11
12
13INTRODUCTION
14
15These programs implement JPEG image compression and decompression. JPEG
16(pronounced "jay-peg") is a standardized compression method for full-color
17and gray-scale images. JPEG is designed to handle "real-world" scenes,
18for example scanned photographs. Cartoons, line drawings, and other
19non-realistic images are not JPEG's strong suit; on that sort of material
20you may get poor image quality and/or little compression.
21
22JPEG is lossy, meaning that the output image is not necessarily identical to
23the input image. Hence you should not use JPEG if you have to have identical
24output bits. However, on typical real-world images, very good compression
25levels can be obtained with no visible change, and amazingly high compression
26is possible if you can tolerate a low-quality image. You can trade off image
27quality against file size by adjusting the compressor's "quality" setting.
28
29
30GENERAL USAGE
31
32We provide two programs, cjpeg to compress an image file into JPEG format,
33and djpeg to decompress a JPEG file back into a conventional image format.
34
35On Unix-like systems, you say:
36 cjpeg [switches] [imagefile] >jpegfile
37or
38 djpeg [switches] [jpegfile] >imagefile
39The programs read the specified input file, or standard input if none is
40named. They always write to standard output (with trace/error messages to
41standard error). These conventions are handy for piping images between
42programs.
43
44On most non-Unix systems, you say:
45 cjpeg [switches] imagefile jpegfile
46or
47 djpeg [switches] jpegfile imagefile
48i.e., both the input and output files are named on the command line. This
49style is a little more foolproof, and it loses no functionality if you don't
50have pipes. (You can get this style on Unix too, if you prefer, by defining
51TWO_FILE_COMMANDLINE when you compile the programs; see install.doc.)
52
53You can also say:
54 cjpeg [switches] -outfile jpegfile imagefile
55or
56 djpeg [switches] -outfile imagefile jpegfile
57This syntax works on all systems, so it is useful for scripts.
58
59The currently supported image file formats are: PPM (PBMPLUS color format),
60PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Toolkit
61format). (RLE is supported only if the URT library is available.)
62cjpeg recognizes the input image format automatically, with the exception
63of some Targa-format files. You have to tell djpeg which format to generate.
64
65JPEG files are in the defacto standard JFIF file format. There are other,
66less widely used JPEG-based file formats, but we don't support them.
67
68All 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
70one letter. Upper and lower case are equivalent (-BMP is the same as -bmp).
71British spellings are also accepted (e.g., -greyscale), though for brevity
72these are not mentioned below.
73
74
75CJPEG DETAILS
76
77The basic command line switches for cjpeg are:
78
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.)
82
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.
89
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.
96
97 -progressive Create progressive JPEG file (see below).
98
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.
104
105The -quality switch lets you trade off compressed file size against quality of
106the reconstructed image: the higher the quality setting, the larger the JPEG
107file, and the closer the output image will be to the original input. Normally
108you want to use the lowest quality setting (smallest file) that decompresses
109into something visually indistinguishable from the original image. For this
110purpose the quality setting should be between 50 and 95; the default of 75 is
111often about right. If you see defects at -quality 75, then go up 5 or 10
112counts at a time until you are happy with the output image. (The optimal
113setting will vary from one image to another.)
114
115-quality 100 will generate a quantization table of all 1's, minimizing loss
116in the quantization step (but there is still information loss in subsampling,
117as well as roundoff error). This setting is mainly of interest for
118experimental purposes. Quality values above about 95 are NOT recommended for
119normal use; the compressed file size goes up dramatically for hardly any gain
120in output image quality.
121
122In the other direction, quality values below 50 will produce very small files
123of low image quality. Settings around 5 to 10 might be useful in preparing an
124index of a large image library, for example. Try -quality 2 (or so) for some
125amusing Cubist effects. (Note: quality values below about 25 generate 2-byte
126quantization tables, which are considered optional in the JPEG standard.
127cjpeg emits a warning message when you give such a quality value, because some
128other JPEG programs may be unable to decode the resulting file. Use -baseline
129if you need to ensure compatibility at low quality values.)
130
131The -progressive switch creates a "progressive JPEG" file. In this type of
132JPEG file, the data is stored in multiple scans of increasing quality. If the
133file is being transmitted over a slow communications link, the decoder can use
134the first scan to display a low-quality image very quickly, and can then
135improve the display with each subsequent scan. The final image is exactly
136equivalent to a standard JPEG file of the same quality setting, and the total
137file size is about the same --- often a little smaller. CAUTION: progressive
138JPEG is not yet widely implemented, so many decoders will be unable to view a
139progressive JPEG file at all.
140
141Switches for advanced users:
142
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.
153
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.
157
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.
161
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.
167
168 -verbose Enable debug printout. More -v's give more printout.
169 or -debug Also, version information is printed at startup.
170
171The -restart option inserts extra markers that allow a JPEG decoder to
172resynchronize after a transmission error. Without restart markers, any damage
173to a compressed file will usually ruin the image from the point of the error
174to the end of the image; with restart markers, the damage is usually confined
175to the portion of the image up to the next restart marker. Of course, the
176restart markers occupy extra space. We recommend -restart 1 for images that
177will be transmitted across unreliable networks such as Usenet.
178
179The -smooth option filters the input to eliminate fine-scale noise. This is
180often useful when converting dithered images to JPEG: a moderate smoothing
181factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
182in a smaller JPEG file and a better-looking image. Too large a smoothing
183factor will visibly blur the image, however.
184
185Switches for wizards:
186
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.)
193
194 -qtables file Use the quantization tables given in the specified
195 text file.
196
197 -qslots N[,...] Select which quantization table to use for each color
198 component.
199
200 -sample HxV[,...] Set JPEG sampling factors for each color component.
201
202 -scans file Use the scan script given in the specified text file.
203
204The "wizard" switches are intended for experimentation with JPEG. If you
205don't know what you are doing, DON'T USE THEM. These switches are documented
206further in the file wizard.doc.
207
208
209DJPEG DETAILS
210
211The basic command line switches for djpeg are:
212
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.)
220
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".
225
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.
229
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
234 down the output.
235
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.
240
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.
245
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.
250
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
254 PPM is emitted.
255
256 -rle Select RLE output format. (Requires URT library.)
257
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.
263
264Switches for advanced users:
265
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.
276
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.
287
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
293 -onepass.
294
295 -nosmooth Use a faster, lower-quality upsampling routine.
296
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).
303
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.
309
310 -verbose Enable debug printout. More -v's give more printout.
311 or -debug Also, version information is printed at startup.
312
313
314HINTS FOR CJPEG
315
316Color GIF files are not the ideal input for JPEG; JPEG is really intended for
317compressing full-color (24-bit) images. In particular, don't try to convert
318cartoons, line drawings, and other images that have only a few distinct
319colors. GIF works great on these, JPEG does not. If you want to convert a
320GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
321to get a satisfactory conversion. -smooth 10 or so is often helpful.
322
323Avoid running an image through a series of JPEG compression/decompression
324cycles. Image quality loss will accumulate; after ten or so cycles the image
325may be noticeably worse than it was after one cycle. It's best to use a
326lossless format while manipulating an image, then convert to JPEG format when
327you are ready to file the image away.
328
329The -optimize option to cjpeg is worth using when you are making a "final"
330version for posting or archiving. It's also a win when you are using low
331quality settings to make very small JPEG files; the percentage improvement
332is often a lot more than it is on larger files. (At present, -optimize
333mode is always selected when generating progressive JPEG files.)
334
335GIF input files are no longer supported, to avoid the Unisys LZW patent.
336Use a Unisys-licensed program if you need to read a GIF file. (Conversion
337of GIF files to JPEG is usually a bad idea anyway.)
338
339
340HINTS FOR DJPEG
341
342To get a quick preview of an image, use the -grayscale and/or -scale switches.
343"-grayscale -scale 1/8" is the fastest case.
344
345Several options are available that trade off image quality to gain speed.
346"-fast" turns on the recommended settings.
347
348"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
349When producing a color-quantized image, "-onepass -dither ordered" is fast but
350much lower quality than the default behavior. "-dither none" may give
351acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
352
353If 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,
356because its theoretical accuracy advantage is too small to be significant
357in practice.
358
359Two-pass color quantization requires a good deal of memory; on MS-DOS machines
360it may run out of memory even with -maxmemory 0. In that case you can still
361decompress, with some loss of image quality, by specifying -onepass for
362one-pass quantization.
363
364To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files. These
365are larger than they should be, but are readable by standard GIF decoders.
366
367
368HINTS FOR BOTH PROGRAMS
369
370If more space is needed than will fit in the available main memory (as
371determined by -maxmemory), temporary files will be used. (MS-DOS versions
372will try to get extended or expanded memory first.) The temporary files are
373often rather large: in typical cases they occupy three bytes per pixel, for
374example 3*800*600 = 1.44Mb for an 800x600 image. If you don't have enough
375free disk space, leave out -progressive and -optimize (for cjpeg) or specify
376-onepass (for djpeg).
377
378On MS-DOS, the temporary files are created in the directory named by the TMP
379or TEMP environment variable, or in the current directory if neither of those
380exist. Amiga implementations put the temp files in the directory named by
381JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
382space.
383
384The default memory usage limit (-maxmemory) is set when the software is
385compiled. If you get an "insufficient memory" error, try specifying a smaller
386-maxmemory value, even -maxmemory 0 to use the absolute minimum space. You
387may want to recompile with a smaller default value if this happens often.
388
389On machines that have "environment" variables, you can define the environment
390variable JPEGMEM to set the default memory limit. The value is specified as
391described for the -maxmemory switch. JPEGMEM overrides the default value
392specified when the program was compiled, and itself is overridden by an
393explicit -maxmemory switch.
394
395On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
396use. (Extended or expanded memory is also used if available.) Most
397DOS-specific versions of this software do their own memory space estimation
398and do not need you to specify -maxmemory.
399
400
401JPEGTRAN
402
403jpegtran performs various useful transformations of JPEG files.
404It can translate the coded representation from one variant of JPEG to another,
405for example from baseline JPEG to progressive JPEG or vice versa. It can also
406perform some rearrangements of the image data, for example turning an image
407from landscape to portrait format by rotation.
408
409jpegtran works by rearranging the compressed data (DCT coefficients), without
410ever fully decoding the image. Therefore, its transformations are lossless:
411there is no image degradation at all, which would not be true if you used
412djpeg followed by cjpeg to accomplish the same conversion. But by the same
413token, jpegtran cannot perform lossy operations such as changing the image
414quality.
415
416jpegtran uses a command line syntax similar to cjpeg or djpeg.
417On Unix-like systems, you say:
418 jpegtran [switches] [inputfile] >outputfile
419On most non-Unix systems, you say:
420 jpegtran [switches] inputfile outputfile
421where both the input and output files are JPEG files.
422
423To specify the coded JPEG representation used in the output file,
424jpegtran 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.
430See the previous discussion of cjpeg for more details about these switches.
431If you specify none of these switches, you get a plain baseline-JPEG output
432file. The quality setting and so forth are determined by the input file.
433
434The 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).
442
443The transpose transformation has no restrictions regarding image dimensions.
444The other transformations operate rather oddly if the image dimensions are not
445a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
446transform complete blocks of DCT coefficient data in the desired way.
447
448jpegtran's default behavior when transforming an odd-size image is designed
449to preserve exact reversibility and mathematical consistency of the
450transformation set. As stated, transpose is able to flip the entire image
451area. Horizontal mirroring leaves any partial iMCU column at the right edge
452untouched, but is able to flip all rows of the image. Similarly, vertical
453mirroring leaves any partial iMCU row at the bottom edge untouched, but is
454able to flip all columns. The other transforms can be built up as sequences
455of transpose and flip operations; for consistency, their actions on edge
456pixels are defined to be the same as the end result of the corresponding
457transpose-and-flip sequence.
458
459For practical use, you may prefer to discard any untransformable edge pixels
460rather than having a strange-looking strip along the right and/or bottom edges
461of a transformed image. To do this, add the -trim switch:
462 -trim Drop non-transformable edge blocks.
463Obviously, a transformation with -trim is not reversible, so strictly speaking
464jpegtran with this switch is not lossless. Also, the expected mathematical
465equivalences 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.
468
469Another not-strictly-lossless transformation switch is:
470 -grayscale Force grayscale output.
471This option discards the chrominance channels if the input image is YCbCr
472(ie, a standard color JPEG), resulting in a grayscale JPEG file. The
473luminance channel is preserved exactly, so this is a better method of reducing
474to grayscale than decompression, conversion, and recompression. This switch
475is particularly handy for fixing a monochrome picture that was mistakenly
476encoded as a color JPEG. (In such a case, the space savings from getting rid
477of the near-empty chroma channels won't be large; but the decoding time for
478a grayscale JPEG is substantially less than that for a color JPEG.)
479
480jpegtran also recognizes these switches that control what to do with "extra"
481markers, 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.
492The default behavior is -copy comments. (Note: in IJG releases v6 and v6a,
493jpegtran always did the equivalent of -copy none.)
494
495Additional switches recognized by jpegtran are:
496 -outfile filename
497 -maxmemory N
498 -verbose
499 -debug
500These work the same as in cjpeg or djpeg.
501
502
503THE COMMENT UTILITIES
504
505The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
506Although the standard doesn't actually define what COM blocks are for, they
507are widely used to hold user-supplied text strings. This lets you add
508annotations, titles, index terms, etc to your JPEG files, and later retrieve
509them as text. COM blocks do not interfere with the image stored in the JPEG
510file. The maximum size of a COM block is 64K, but you can have as many of
511them as you like in one JPEG file.
512
513We provide two utility programs to display COM block contents and add COM
514blocks to a JPEG file.
515
516rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
517standard output. The command line syntax is
518 rdjpgcom [-verbose] [inputfilename]
519The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
520image dimensions. If you omit the input file name from the command line,
521the JPEG file is read from standard input. (This may not work on some
522operating systems, if binary data can't be read from stdin.)
523
524wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
525Ordinarily, the COM block is added after any existing COM blocks, but you
526can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG
527file; it does not modify the input file. DO NOT try to overwrite the input
528file by directing wrjpgcom's output back into it; on most systems this will
529just destroy your file.
530
531The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like
532systems, it is
533 wrjpgcom [switches] [inputfilename]
534The output file is written to standard output. The input file comes from
535the named file, or from standard input if no input file is named.
536
537On most non-Unix systems, the syntax is
538 wrjpgcom [switches] inputfilename outputfilename
539where both input and output file names must be given explicitly.
540
541wrjpgcom 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
546to add, you can provide it on the command line with -comment. The comment
547text must be surrounded with quotes so that it is treated as a single
548argument. Longer comments can be read from a text file.
549
550If you give neither -comment nor -cfile, then wrjpgcom will read the comment
551text from standard input. (In this case an input image file name MUST be
552supplied, so that the source JPEG file comes from somewhere else.) You can
553enter 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.
555
556wrjpgcom will not add a COM block if the provided comment string is empty.
557Therefore -replace -comment "" can be used to delete all COM blocks from a
558file.
559
560These utility programs do not depend on the IJG JPEG library. In
561particular, the source code for rdjpgcom is intended as an illustration of
562the minimum amount of code required to parse a JPEG file header correctly.