1CJPEG(1) General Commands Manual CJPEG(1)
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6 cjpeg - compress an image file to a JPEG file
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9 cjpeg [ options ] [ filename ]
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12 cjpeg compresses the named image file, or the standard input if no file
13 is named, and produces a JPEG/JFIF file on the standard output. The
14 currently supported input file formats are: PPM (PBMPLUS color format),
15 PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Tool‐
16 kit format). (RLE is supported only if the URT library is available.)
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19 All switch names may be abbreviated; for example, -grayscale may be
20 written -gray or -gr. Most of the "basic" switches can be abbreviated
21 to as little as one letter. Upper and lower case are equivalent (thus
22 -BMP is the same as -bmp). British spellings are also accepted (e.g.,
23 -greyscale), though for brevity these are not mentioned below.
24 The basic switches are:
26 -quality N[,...]
28 Scale quantization tables to adjust image quality. Quality is 0
29 (worst) to 100 (best); default is 75. (See below for more
30 info.)
31 -grayscale
33 Create monochrome JPEG file from color input. Be sure to use
34 this switch when compressing a grayscale BMP file, because cjpeg
35 isn't bright enough to notice whether a BMP file uses only
36 shades of gray. By saying -grayscale, you'll get a smaller JPEG
37 file that takes less time to process.
38 -optimize
40 Perform optimization of entropy encoding parameters. Without
41 this, default encoding parameters are used. -optimize usually
42 makes the JPEG file a little smaller, but cjpeg runs somewhat
43 slower and needs much more memory. Image quality and speed of
44 decompression are unaffected by -optimize.
45 -progressive
47 Create progressive JPEG file (see below).
48 -targa Input file is Targa format. Targa files that contain an "iden‐
50 tification" field will not be automatically recognized by cjpeg;
51 for such files you must specify -targa to make cjpeg treat the
52 input as Targa format. For most Targa files, you won't need
53 this switch.
54 The -quality switch lets you trade off compressed file size against
56 quality of the reconstructed image: the higher the quality setting, the
57 larger the JPEG file, and the closer the output image will be to the
58 original input. Normally you want to use the lowest quality setting
59 (smallest file) that decompresses into something visually indistin‐
60 guishable from the original image. For this purpose the quality set‐
61 ting should be between 50 and 95; the default of 75 is often about
62 right. If you see defects at -quality 75, then go up 5 or 10 counts at
63 a time until you are happy with the output image. (The optimal setting
64 will vary from one image to another.)
65 -quality 100 will generate a quantization table of all 1's, minimizing
67 loss in the quantization step (but there is still information loss in
68 subsampling, as well as roundoff error). This setting is mainly of
69 interest for experimental purposes. Quality values above about 95 are
70 not recommended for normal use; the compressed file size goes up dra‐
71 matically for hardly any gain in output image quality.
72 In the other direction, quality values below 50 will produce very small
74 files of low image quality. Settings around 5 to 10 might be useful in
75 preparing an index of a large image library, for example. Try -quality
76 2 (or so) for some amusing Cubist effects. (Note: quality values below
77 about 25 generate 2-byte quantization tables, which are considered
78 optional in the JPEG standard. cjpeg emits a warning message when you
79 give such a quality value, because some other JPEG programs may be
80 unable to decode the resulting file. Use -baseline if you need to
81 ensure compatibility at low quality values.)
82 The -quality option has been extended in this version of cjpeg to sup‐
84 port separate quality settings for luminance and chrominance (or, in
85 general, separate settings for every quantization table slot.) The
86 principle is the same as chrominance subsampling: since the human eye
87 is more sensitive to spatial changes in brightness than spatial changes
88 in color, the chrominance components can be quantized more than the
89 luminance components without incurring any visible image quality loss.
90 However, unlike subsampling, this feature reduces data in the frequency
91 domain instead of the spatial domain, which allows for more fine-
92 grained control. This option is useful in quality-sensitive applica‐
93 tions, for which the artifacts generated by subsampling may be unac‐
94 ceptable.
95 The -quality option accepts a comma-separated list of parameters, which
97 respectively refer to the quality levels which should be assigned to
98 the quantization table slots. If there are more q-table slots than
99 parameters, then the last parameter is replicated. Thus, if only one
100 quality parameter is given, this is used for both luminance and chromi‐
101 nance (slots 0 and 1, respectively), preserving the legacy behavior of
102 cjpeg v6b and prior. More (or customized) quantization tables can be
103 set with the -qtables option and assigned to components with the
104 -qslots option (see the "wizard" switches below.)
105 JPEG files generated with separate luminance and chrominance quality
107 are fully compliant with standard JPEG decoders.
108 CAUTION: For this setting to be useful, be sure to pass an argument of
110 -sample 1x1 to cjpeg to disable chrominance subsampling. Otherwise,
111 the default subsampling level (2x2, AKA "4:2:0") will be used.
112 The -progressive switch creates a "progressive JPEG" file. In this
114 type of JPEG file, the data is stored in multiple scans of increasing
115 quality. If the file is being transmitted over a slow communications
116 link, the decoder can use the first scan to display a low-quality image
117 very quickly, and can then improve the display with each subsequent
118 scan. The final image is exactly equivalent to a standard JPEG file of
119 the same quality setting, and the total file size is about the same ---
120 often a little smaller.
121 Switches for advanced users:
123 -dct int
125 Use integer DCT method (default).
126 -dct fast
128 Use fast integer DCT (less accurate).
129 -dct float
131 Use floating-point DCT method. The float method is very
132 slightly more accurate than the int method, but is much slower
133 unless your machine has very fast floating-point hardware. Also
134 note that results of the floating-point method may vary slightly
135 across machines, while the integer methods should give the same
136 results everywhere. The fast integer method is much less accu‐
137 rate than the other two.
138 -restart N
140 Emit a JPEG restart marker every N MCU rows, or every N MCU
141 blocks if "B" is attached to the number. -restart 0 (the
142 default) means no restart markers.
143 -smooth N
145 Smooth the input image to eliminate dithering noise. N, ranging
146 from 1 to 100, indicates the strength of smoothing. 0 (the
147 default) means no smoothing.
148 -maxmemory N
150 Set limit for amount of memory to use in processing large
151 images. Value is in thousands of bytes, or millions of bytes if
152 "M" is attached to the number. For example, -max 4m selects
153 4000000 bytes. If more space is needed, temporary files will be
154 used.
155 -outfile name
157 Send output image to the named file, not to standard output.
158 -verbose
160 Enable debug printout. More -v's give more output. Also, ver‐
161 sion information is printed at startup.
162 -debug Same as -verbose.
164 The -restart option inserts extra markers that allow a JPEG decoder to
166 resynchronize after a transmission error. Without restart markers, any
167 damage to a compressed file will usually ruin the image from the point
168 of the error to the end of the image; with restart markers, the damage
169 is usually confined to the portion of the image up to the next restart
170 marker. Of course, the restart markers occupy extra space. We recom‐
171 mend -restart 1 for images that will be transmitted across unreliable
172 networks such as Usenet.
173 The -smooth option filters the input to eliminate fine-scale noise.
175 This is often useful when converting dithered images to JPEG: a moder‐
176 ate smoothing factor of 10 to 50 gets rid of dithering patterns in the
177 input file, resulting in a smaller JPEG file and a better-looking
178 image. Too large a smoothing factor will visibly blur the image, how‐
179 ever.
180 Switches for wizards:
182 -arithmetic
184 Use arithmetic coding. Caution: arithmetic coded JPEG is not
185 yet widely implemented, so many decoders will be unable to view
186 an arithmetic coded JPEG file at all.
187 -baseline
189 Force baseline-compatible quantization tables to be generated.
190 This clamps quantization values to 8 bits even at low quality
191 settings. (This switch is poorly named, since it does not
192 ensure that the output is actually baseline JPEG. For example,
193 you can use -baseline and -progressive together.)
194 -qtables file
196 Use the quantization tables given in the specified text file.
197 -qslots N[,...]
199 Select which quantization table to use for each color component.
200 -sample HxV[,...]
202 Set JPEG sampling factors for each color component.
203 -scans file
205 Use the scan script given in the specified text file.
206 The "wizard" switches are intended for experimentation with JPEG. If
208 you don't know what you are doing, don't use them. These switches are
209 documented further in the file wizard.txt.
210
212 This example compresses the PPM file foo.ppm with a quality factor of
213 60 and saves the output as foo.jpg:
214 cjpeg -quality 60 foo.ppm > foo.jpg
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218 Color GIF files are not the ideal input for JPEG; JPEG is really
219 intended for compressing full-color (24-bit) images. In particular,
220 don't try to convert cartoons, line drawings, and other images that
221 have only a few distinct colors. GIF works great on these, JPEG does
222 not. If you want to convert a GIF to JPEG, you should experiment with
223 cjpeg's -quality and -smooth options to get a satisfactory conversion.
224 -smooth 10 or so is often helpful.
225 Avoid running an image through a series of JPEG compression/decompres‐
227 sion cycles. Image quality loss will accumulate; after ten or so
228 cycles the image may be noticeably worse than it was after one cycle.
229 It's best to use a lossless format while manipulating an image, then
230 convert to JPEG format when you are ready to file the image away.
231 The -optimize option to cjpeg is worth using when you are making a
233 "final" version for posting or archiving. It's also a win when you are
234 using low quality settings to make very small JPEG files; the percent‐
235 age improvement is often a lot more than it is on larger files. (At
236 present, -optimize mode is always selected when generating progressive
237 JPEG files.)
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240 JPEGMEM
241 If this environment variable is set, its value is the default
242 memory limit. The value is specified as described for the
243 -maxmemory switch. JPEGMEM overrides the default value speci‐
244 fied when the program was compiled, and itself is overridden by
245 an explicit -maxmemory.
246
248 djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
249 ppm(5), pgm(5)
250 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
251 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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254 Independent JPEG Group
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257 Support for GIF input files was removed in cjpeg v6b due to concerns
258 over the Unisys LZW patent. Although this patent expired in 2006,
259 cjpeg still lacks GIF support, for these historical reasons. (Conver‐
260 sion of GIF files to JPEG is usually a bad idea anyway.)
261 Not all variants of BMP and Targa file formats are supported.
263 The -targa switch is not a bug, it's a feature. (It would be a bug if
265 the Targa format designers had not been clueless.)
266 11 October 2010 CJPEG(1)