1 .TH CJPEG 1 "20 March 1998"
3 cjpeg \- compress an image file to a JPEG file
16 compresses the named image file, or the standard input if no file is
17 named, and produces a JPEG/JFIF file on the standard output.
18 The currently supported input file formats are: PPM (PBMPLUS color
19 format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster
20 Toolkit format). (RLE is supported only if the URT library is available.)
22 All switch names may be abbreviated; for example,
28 Most of the "basic" switches can be abbreviated to as little as one letter.
29 Upper and lower case are equivalent (thus
33 British spellings are also accepted (e.g.,
35 though for brevity these are not mentioned below.
37 The basic switches are:
40 Scale quantization tables to adjust image quality. Quality is 0 (worst) to
41 100 (best); default is 75. (See below for more info.)
44 Create monochrome JPEG file from color input. Be sure to use this switch when
45 compressing a grayscale BMP file, because
47 isn't bright enough to notice whether a BMP file uses only shades of gray.
50 you'll get a smaller JPEG file that takes less time to process.
53 Perform optimization of entropy encoding parameters. Without this, default
54 encoding parameters are used.
56 usually makes the JPEG file a little smaller, but
58 runs somewhat slower and needs much more memory. Image quality and speed of
59 decompression are unaffected by
63 Create progressive JPEG file (see below).
66 Input file is Targa format. Targa files that contain an "identification"
67 field will not be automatically recognized by
69 for such files you must specify
73 treat the input as Targa format.
74 For most Targa files, you won't need this switch.
78 switch lets you trade off compressed file size against quality of the
79 reconstructed image: the higher the quality setting, the larger the JPEG file,
80 and the closer the output image will be to the original input. Normally you
81 want to use the lowest quality setting (smallest file) that decompresses into
82 something visually indistinguishable from the original image. For this
83 purpose the quality setting should be between 50 and 95; the default of 75 is
84 often about right. If you see defects at
86 75, then go up 5 or 10 counts at a time until you are happy with the output
87 image. (The optimal setting will vary from one image to another.)
90 100 will generate a quantization table of all 1's, minimizing loss in the
91 quantization step (but there is still information loss in subsampling, as well
92 as roundoff error). This setting is mainly of interest for experimental
93 purposes. Quality values above about 95 are
95 recommended for normal use; the compressed file size goes up dramatically for
96 hardly any gain in output image quality.
98 In the other direction, quality values below 50 will produce very small files
99 of low image quality. Settings around 5 to 10 might be useful in preparing an
100 index of a large image library, for example. Try
102 2 (or so) for some amusing Cubist effects. (Note: quality
103 values below about 25 generate 2-byte quantization tables, which are
104 considered optional in the JPEG standard.
106 emits a warning message when you give such a quality value, because some
107 other JPEG programs may be unable to decode the resulting file. Use
109 if you need to ensure compatibility at low quality values.)
113 switch creates a "progressive JPEG" file. In this type of JPEG file, the data
114 is stored in multiple scans of increasing quality. If the file is being
115 transmitted over a slow communications link, the decoder can use the first
116 scan to display a low-quality image very quickly, and can then improve the
117 display with each subsequent scan. The final image is exactly equivalent to a
118 standard JPEG file of the same quality setting, and the total file size is
119 about the same --- often a little smaller.
121 progressive JPEG is not yet widely implemented, so many decoders will be
122 unable to view a progressive JPEG file at all.
124 Switches for advanced users:
127 Use integer DCT method (default).
130 Use fast integer DCT (less accurate).
133 Use floating-point DCT method.
134 The float method is very slightly more accurate than the int method, but is
135 much slower unless your machine has very fast floating-point hardware. Also
136 note that results of the floating-point method may vary slightly across
137 machines, while the integer methods should give the same results everywhere.
138 The fast integer method is much less accurate than the other two.
141 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
142 attached to the number.
144 (the default) means no restart markers.
147 Smooth the input image to eliminate dithering noise. N, ranging from 1 to
148 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
151 Set limit for amount of memory to use in processing large images. Value is
152 in thousands of bytes, or millions of bytes if "M" is attached to the
155 selects 4000000 bytes. If more space is needed, temporary files will be used.
157 .BI \-outfile " name"
158 Send output image to the named file, not to standard output.
161 Enable debug printout. More
163 give more output. Also, version information is printed at startup.
171 option inserts extra markers that allow a JPEG decoder to resynchronize after
172 a transmission error. Without restart markers, any damage to a compressed
173 file will usually ruin the image from the point of the error to the end of the
174 image; with restart markers, the damage is usually confined to the portion of
175 the image up to the next restart marker. Of course, the restart markers
176 occupy extra space. We recommend
178 for images that will be transmitted across unreliable networks such as Usenet.
182 option filters the input to eliminate fine-scale noise. This is often useful
183 when converting dithered images to JPEG: a moderate smoothing factor of 10 to
184 50 gets rid of dithering patterns in the input file, resulting in a smaller
185 JPEG file and a better-looking image. Too large a smoothing factor will
186 visibly blur the image, however.
188 Switches for wizards:
191 Force baseline-compatible quantization tables to be generated. This clamps
192 quantization values to 8 bits even at low quality settings. (This switch is
193 poorly named, since it does not ensure that the output is actually baseline
194 JPEG. For example, you can use
200 .BI \-qtables " file"
201 Use the quantization tables given in the specified text file.
203 .BI \-qslots " N[,...]"
204 Select which quantization table to use for each color component.
206 .BI \-sample " HxV[,...]"
207 Set JPEG sampling factors for each color component.
210 Use the scan script given in the specified text file.
212 The "wizard" switches are intended for experimentation with JPEG. If you
213 don't know what you are doing, \fBdon't use them\fR. These switches are
214 documented further in the file wizard.doc.
217 This example compresses the PPM file foo.ppm with a quality factor of
218 60 and saves the output as foo.jpg:
225 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
226 compressing full-color (24-bit) images. In particular, don't try to convert
227 cartoons, line drawings, and other images that have only a few distinct
228 colors. GIF works great on these, JPEG does not. If you want to convert a
229 GIF to JPEG, you should experiment with
234 options to get a satisfactory conversion.
236 or so is often helpful.
238 Avoid running an image through a series of JPEG compression/decompression
239 cycles. Image quality loss will accumulate; after ten or so cycles the image
240 may be noticeably worse than it was after one cycle. It's best to use a
241 lossless format while manipulating an image, then convert to JPEG format when
242 you are ready to file the image away.
248 is worth using when you are making a "final" version for posting or archiving.
249 It's also a win when you are using low quality settings to make very small
250 JPEG files; the percentage improvement is often a lot more than it is on
251 larger files. (At present,
253 mode is always selected when generating progressive JPEG files.)
257 If this environment variable is set, its value is the default memory limit.
258 The value is specified as described for the
262 overrides the default value specified when the program was compiled, and
263 itself is overridden by an explicit
274 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
275 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
277 Independent JPEG Group
279 Arithmetic coding is not supported for legal reasons.
281 GIF input files are no longer supported, to avoid the Unisys LZW patent.
282 Use a Unisys-licensed program if you need to read a GIF file. (Conversion
283 of GIF files to JPEG is usually a bad idea anyway.)
285 Not all variants of BMP and Targa file formats are supported.
289 switch is not a bug, it's a feature. (It would be a bug if the Targa format
290 designers had not been clueless.)
292 Still not as fast as we'd like.
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