1CJPEG(1) General Commands Manual CJPEG(1)
2
6 cjpeg - compress an image file to a JPEG file
7
9 cjpeg [ options ] [ filename ]
10
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.)
17
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 -rgb Create RGB JPEG file. Using this switch suppresses the conver‐
40 sion from RGB colorspace input to the default YCbCr JPEG col‐
41 orspace.
42 -optimize
44 Perform optimization of entropy encoding parameters. Without
45 this, default encoding parameters are used. -optimize usually
46 makes the JPEG file a little smaller, but cjpeg runs somewhat
47 slower and needs much more memory. Image quality and speed of
48 decompression are unaffected by -optimize.
49 -progressive
51 Create progressive JPEG file (see below).
52 -targa Input file is Targa format. Targa files that contain an "iden‐
54 tification" field will not be automatically recognized by cjpeg;
55 for such files you must specify -targa to make cjpeg treat the
56 input as Targa format. For most Targa files, you won't need
57 this switch.
58 The -quality switch lets you trade off compressed file size against
60 quality of the reconstructed image: the higher the quality setting, the
61 larger the JPEG file, and the closer the output image will be to the
62 original input. Normally you want to use the lowest quality setting
63 (smallest file) that decompresses into something visually indistin‐
64 guishable from the original image. For this purpose the quality set‐
65 ting should be between 50 and 95; the default of 75 is often about
66 right. If you see defects at -quality 75, then go up 5 or 10 counts at
67 a time until you are happy with the output image. (The optimal setting
68 will vary from one image to another.)
69 -quality 100 will generate a quantization table of all 1's, minimizing
71 loss in the quantization step (but there is still information loss in
72 subsampling, as well as roundoff error). This setting is mainly of
73 interest for experimental purposes. Quality values above about 95 are
74 not recommended for normal use; the compressed file size goes up dra‐
75 matically for hardly any gain in output image quality.
76 In the other direction, quality values below 50 will produce very small
78 files of low image quality. Settings around 5 to 10 might be useful in
79 preparing an index of a large image library, for example. Try -quality
80 2 (or so) for some amusing Cubist effects. (Note: quality values below
81 about 25 generate 2-byte quantization tables, which are considered
82 optional in the JPEG standard. cjpeg emits a warning message when you
83 give such a quality value, because some other JPEG programs may be
84 unable to decode the resulting file. Use -baseline if you need to
85 ensure compatibility at low quality values.)
86 The -quality option has been extended in this version of cjpeg to sup‐
88 port separate quality settings for luminance and chrominance (or, in
89 general, separate settings for every quantization table slot.) The
90 principle is the same as chrominance subsampling: since the human eye
91 is more sensitive to spatial changes in brightness than spatial changes
92 in color, the chrominance components can be quantized more than the
93 luminance components without incurring any visible image quality loss.
94 However, unlike subsampling, this feature reduces data in the frequency
95 domain instead of the spatial domain, which allows for more fine-
96 grained control. This option is useful in quality-sensitive applica‐
97 tions, for which the artifacts generated by subsampling may be unac‐
98 ceptable.
99 The -quality option accepts a comma-separated list of parameters, which
101 respectively refer to the quality levels that should be assigned to the
102 quantization table slots. If there are more q-table slots than parame‐
103 ters, then the last parameter is replicated. Thus, if only one quality
104 parameter is given, this is used for both luminance and chrominance
105 (slots 0 and 1, respectively), preserving the legacy behavior of cjpeg
106 v6b and prior. More (or customized) quantization tables can be set
107 with the -qtables option and assigned to components with the -qslots
108 option (see the "wizard" switches below.)
109 JPEG files generated with separate luminance and chrominance quality
111 are fully compliant with standard JPEG decoders.
112 CAUTION: For this setting to be useful, be sure to pass an argument of
114 -sample 1x1 to cjpeg to disable chrominance subsampling. Otherwise,
115 the default subsampling level (2x2, AKA "4:2:0") will be used.
116 The -progressive switch creates a "progressive JPEG" file. In this
118 type of JPEG file, the data is stored in multiple scans of increasing
119 quality. If the file is being transmitted over a slow communications
120 link, the decoder can use the first scan to display a low-quality image
121 very quickly, and can then improve the display with each subsequent
122 scan. The final image is exactly equivalent to a standard JPEG file of
123 the same quality setting, and the total file size is about the same ---
124 often a little smaller.
125 Switches for advanced users:
127 -arithmetic
129 Use arithmetic coding. Caution: arithmetic coded JPEG is not
130 yet widely implemented, so many decoders will be unable to view
131 an arithmetic coded JPEG file at all.
132 -dct int
134 Use integer DCT method (default).
135 -dct fast
137 Use fast integer DCT (less accurate).
138 -dct float
140 Use floating-point DCT method. The float method is very
141 slightly more accurate than the int method, but is much slower
142 unless your machine has very fast floating-point hardware. Also
143 note that results of the floating-point method may vary slightly
144 across machines, while the integer methods should give the same
145 results everywhere. The fast integer method is much less accu‐
146 rate than the other two.
147 -restart N
149 Emit a JPEG restart marker every N MCU rows, or every N MCU
150 blocks if "B" is attached to the number. -restart 0 (the
151 default) means no restart markers.
152 -smooth N
154 Smooth the input image to eliminate dithering noise. N, ranging
155 from 1 to 100, indicates the strength of smoothing. 0 (the
156 default) means no smoothing.
157 -maxmemory N
159 Set limit for amount of memory to use in processing large
160 images. Value is in thousands of bytes, or millions of bytes if
161 "M" is attached to the number. For example, -max 4m selects
162 4000000 bytes. If more space is needed, temporary files will be
163 used.
164 -outfile name
166 Send output image to the named file, not to standard output.
167 -memdst
169 Compress to memory instead of a file. This feature was imple‐
170 mented mainly as a way of testing the in-memory destination man‐
171 ager (jpeg_mem_dest()), but it is also useful for benchmarking,
172 since it reduces the I/O overhead.
173 -verbose
175 Enable debug printout. More -v's give more output. Also, ver‐
176 sion information is printed at startup.
177 -debug Same as -verbose.
179 The -restart option inserts extra markers that allow a JPEG decoder to
181 resynchronize after a transmission error. Without restart markers, any
182 damage to a compressed file will usually ruin the image from the point
183 of the error to the end of the image; with restart markers, the damage
184 is usually confined to the portion of the image up to the next restart
185 marker. Of course, the restart markers occupy extra space. We recom‐
186 mend -restart 1 for images that will be transmitted across unreliable
187 networks such as Usenet.
188 The -smooth option filters the input to eliminate fine-scale noise.
190 This is often useful when converting dithered images to JPEG: a moder‐
191 ate smoothing factor of 10 to 50 gets rid of dithering patterns in the
192 input file, resulting in a smaller JPEG file and a better-looking
193 image. Too large a smoothing factor will visibly blur the image, how‐
194 ever.
195 Switches for wizards:
197 -baseline
199 Force baseline-compatible quantization tables to be generated.
200 This clamps quantization values to 8 bits even at low quality
201 settings. (This switch is poorly named, since it does not
202 ensure that the output is actually baseline JPEG. For example,
203 you can use -baseline and -progressive together.)
204 -qtables file
206 Use the quantization tables given in the specified text file.
207 -qslots N[,...]
209 Select which quantization table to use for each color component.
210 -sample HxV[,...]
212 Set JPEG sampling factors for each color component.
213 -scans file
215 Use the scan script given in the specified text file.
216 The "wizard" switches are intended for experimentation with JPEG. If
218 you don't know what you are doing, don't use them. These switches are
219 documented further in the file wizard.txt.
220
222 This example compresses the PPM file foo.ppm with a quality factor of
223 60 and saves the output as foo.jpg:
224 cjpeg -quality 60 foo.ppm > foo.jpg
226
228 Color GIF files are not the ideal input for JPEG; JPEG is really
229 intended for compressing full-color (24-bit) images. In particular,
230 don't try to convert cartoons, line drawings, and other images that
231 have only a few distinct colors. GIF works great on these, JPEG does
232 not. If you want to convert a GIF to JPEG, you should experiment with
233 cjpeg's -quality and -smooth options to get a satisfactory conversion.
234 -smooth 10 or so is often helpful.
235 Avoid running an image through a series of JPEG compression/decompres‐
237 sion cycles. Image quality loss will accumulate; after ten or so
238 cycles the image may be noticeably worse than it was after one cycle.
239 It's best to use a lossless format while manipulating an image, then
240 convert to JPEG format when you are ready to file the image away.
241 The -optimize option to cjpeg is worth using when you are making a
243 "final" version for posting or archiving. It's also a win when you are
244 using low quality settings to make very small JPEG files; the percent‐
245 age improvement is often a lot more than it is on larger files. (At
246 present, -optimize mode is always selected when generating progressive
247 JPEG files.)
248
250 JPEGMEM
251 If this environment variable is set, its value is the default
252 memory limit. The value is specified as described for the
253 -maxmemory switch. JPEGMEM overrides the default value speci‐
254 fied when the program was compiled, and itself is overridden by
255 an explicit -maxmemory.
256
258 djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
259 ppm(5), pgm(5)
260 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
261 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
262
264 Independent JPEG Group
265 This file was modified by The libjpeg-turbo Project to include only
267 information relevant to libjpeg-turbo, to wordsmith certain sections,
268 and to describe features not present in libjpeg.
269
271 Support for GIF input files was removed in cjpeg v6b due to concerns
272 over the Unisys LZW patent. Although this patent expired in 2006,
273 cjpeg still lacks GIF support, for these historical reasons. (Conver‐
274 sion of GIF files to JPEG is usually a bad idea anyway.)
275 Not all variants of BMP and Targa file formats are supported.
277 The -targa switch is not a bug, it's a feature. (It would be a bug if
279 the Targa format designers had not been clueless.)
280 18 January 2013 CJPEG(1)