1 .TH CJPEG 1 "30 December 2009"
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:
39 .BI \-quality " N[,...]"
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 Scale the output image by a factor M/N. Currently supported scale factors are
67 8/N with all N from 1 to 16.
70 Input file is Targa format. Targa files that contain an "identification"
71 field will not be automatically recognized by
73 for such files you must specify
77 treat the input as Targa format.
78 For most Targa files, you won't need this switch.
82 switch lets you trade off compressed file size against quality of the
83 reconstructed image: the higher the quality setting, the larger the JPEG file,
84 and the closer the output image will be to the original input. Normally you
85 want to use the lowest quality setting (smallest file) that decompresses into
86 something visually indistinguishable from the original image. For this
87 purpose the quality setting should be between 50 and 95; the default of 75 is
88 often about right. If you see defects at
90 75, then go up 5 or 10 counts at a time until you are happy with the output
91 image. (The optimal setting will vary from one image to another.)
94 100 will generate a quantization table of all 1's, minimizing loss in the
95 quantization step (but there is still information loss in subsampling, as well
96 as roundoff error). This setting is mainly of interest for experimental
97 purposes. Quality values above about 95 are
99 recommended for normal use; the compressed file size goes up dramatically for
100 hardly any gain in output image quality.
102 In the other direction, quality values below 50 will produce very small files
103 of low image quality. Settings around 5 to 10 might be useful in preparing an
104 index of a large image library, for example. Try
106 2 (or so) for some amusing Cubist effects. (Note: quality
107 values below about 25 generate 2-byte quantization tables, which are
108 considered optional in the JPEG standard.
110 emits a warning message when you give such a quality value, because some
111 other JPEG programs may be unable to decode the resulting file. Use
113 if you need to ensure compatibility at low quality values.)
117 option has been extended in IJG version 7 for support of separate quality
118 settings for luminance and chrominance (or in general, for every provided
119 quantization table slot). This feature is useful for high-quality
120 applications which cannot accept the damage of color data by coarse
121 subsampling settings. You can now easily reduce the color data amount more
122 smoothly with finer control without separate subsampling. The resulting file
123 is fully compliant with standard JPEG decoders.
126 ratings refer to the quantization table slots, and that the last value is
127 replicated if there are more q-table slots than parameters. The default
128 q-table slots are 0 for luminance and 1 for chrominance with default tables as
129 given in the JPEG standard. This is compatible with the old behaviour in case
130 that only one parameter is given, which is then used for both luminance and
131 chrominance (slots 0 and 1). More or custom quantization tables can be set
134 and assigned to components with
136 parameter (see the "wizard" switches below).
138 You must explicitly add
140 for efficient separate color
141 quality selection, since the default value used by library is 2x2!
145 switch creates a "progressive JPEG" file. In this type of JPEG file, the data
146 is stored in multiple scans of increasing quality. If the file is being
147 transmitted over a slow communications link, the decoder can use the first
148 scan to display a low-quality image very quickly, and can then improve the
149 display with each subsequent scan. The final image is exactly equivalent to a
150 standard JPEG file of the same quality setting, and the total file size is
151 about the same --- often a little smaller.
153 Switches for advanced users:
156 Use integer DCT method (default).
159 Use fast integer DCT (less accurate).
162 Use floating-point DCT method.
163 The float method is very slightly more accurate than the int method, but is
164 much slower unless your machine has very fast floating-point hardware. Also
165 note that results of the floating-point method may vary slightly across
166 machines, while the integer methods should give the same results everywhere.
167 The fast integer method is much less accurate than the other two.
170 Don't use high-quality downsampling.
173 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
174 attached to the number.
176 (the default) means no restart markers.
179 Smooth the input image to eliminate dithering noise. N, ranging from 1 to
180 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
183 Set limit for amount of memory to use in processing large images. Value is
184 in thousands of bytes, or millions of bytes if "M" is attached to the
187 selects 4000000 bytes. If more space is needed, temporary files will be used.
189 .BI \-outfile " name"
190 Send output image to the named file, not to standard output.
193 Enable debug printout. More
195 give more output. Also, version information is printed at startup.
203 option inserts extra markers that allow a JPEG decoder to resynchronize after
204 a transmission error. Without restart markers, any damage to a compressed
205 file will usually ruin the image from the point of the error to the end of the
206 image; with restart markers, the damage is usually confined to the portion of
207 the image up to the next restart marker. Of course, the restart markers
208 occupy extra space. We recommend
210 for images that will be transmitted across unreliable networks such as Usenet.
214 option filters the input to eliminate fine-scale noise. This is often useful
215 when converting dithered images to JPEG: a moderate smoothing factor of 10 to
216 50 gets rid of dithering patterns in the input file, resulting in a smaller
217 JPEG file and a better-looking image. Too large a smoothing factor will
218 visibly blur the image, however.
220 Switches for wizards:
223 Use arithmetic coding.
225 arithmetic coded JPEG is not yet widely implemented, so many decoders will be
226 unable to view an arithmetic coded JPEG file at all.
229 Force baseline-compatible quantization tables to be generated. This clamps
230 quantization values to 8 bits even at low quality settings. (This switch is
231 poorly named, since it does not ensure that the output is actually baseline
232 JPEG. For example, you can use
238 .BI \-qtables " file"
239 Use the quantization tables given in the specified text file.
241 .BI \-qslots " N[,...]"
242 Select which quantization table to use for each color component.
244 .BI \-sample " HxV[,...]"
245 Set JPEG sampling factors for each color component.
248 Use the scan script given in the specified text file.
250 The "wizard" switches are intended for experimentation with JPEG. If you
251 don't know what you are doing, \fBdon't use them\fR. These switches are
252 documented further in the file wizard.txt.
255 This example compresses the PPM file foo.ppm with a quality factor of
256 60 and saves the output as foo.jpg:
263 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
264 compressing full-color (24-bit) images. In particular, don't try to convert
265 cartoons, line drawings, and other images that have only a few distinct
266 colors. GIF works great on these, JPEG does not. If you want to convert a
267 GIF to JPEG, you should experiment with
272 options to get a satisfactory conversion.
274 or so is often helpful.
276 Avoid running an image through a series of JPEG compression/decompression
277 cycles. Image quality loss will accumulate; after ten or so cycles the image
278 may be noticeably worse than it was after one cycle. It's best to use a
279 lossless format while manipulating an image, then convert to JPEG format when
280 you are ready to file the image away.
286 is worth using when you are making a "final" version for posting or archiving.
287 It's also a win when you are using low quality settings to make very small
288 JPEG files; the percentage improvement is often a lot more than it is on
289 larger files. (At present,
291 mode is always selected when generating progressive JPEG files.)
295 If this environment variable is set, its value is the default memory limit.
296 The value is specified as described for the
300 overrides the default value specified when the program was compiled, and
301 itself is overridden by an explicit
312 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
313 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
315 Independent JPEG Group
317 GIF input files are no longer supported, to avoid the Unisys LZW patent.
318 (Conversion of GIF files to JPEG is usually a bad idea anyway.)
320 Not all variants of BMP and Targa file formats are supported.
324 switch is not a bug, it's a feature. (It would be a bug if the Targa format
325 designers had not been clueless.)
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