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1 .TH CJPEG 1 "11 October 2010"
2 .SH NAME
3 cjpeg \- compress an image file to a JPEG file
4 .SH SYNOPSIS
5 .B cjpeg
6 [
7 .I options
8 ]
9 [
10 .I filename
11 ]
12 .LP
13 .SH DESCRIPTION
14 .LP
15 .B cjpeg
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.)
21 .SH OPTIONS
22 All switch names may be abbreviated; for example,
23 .B \-grayscale
24 may be written
25 .B \-gray
26 or
27 .BR \-gr .
28 Most of the "basic" switches can be abbreviated to as little as one letter.
29 Upper and lower case are equivalent (thus
30 .B \-BMP
31 is the same as
32 .BR \-bmp ).
33 British spellings are also accepted (e.g.,
34 .BR \-greyscale ),
35 though for brevity these are not mentioned below.
36 .PP
37 The basic switches are:
38 .TP
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.)
42 .TP
43 .B \-grayscale
44 Create monochrome JPEG file from color input. Be sure to use this switch when
45 compressing a grayscale BMP file, because
46 .B cjpeg
47 isn't bright enough to notice whether a BMP file uses only shades of gray.
48 By saying
49 .BR \-grayscale ,
50 you'll get a smaller JPEG file that takes less time to process.
51 .TP
52 .B \-optimize
53 Perform optimization of entropy encoding parameters. Without this, default
54 encoding parameters are used.
55 .B \-optimize
56 usually makes the JPEG file a little smaller, but
57 .B cjpeg
58 runs somewhat slower and needs much more memory. Image quality and speed of
59 decompression are unaffected by
60 .BR \-optimize .
61 .TP
62 .B \-progressive
63 Create progressive JPEG file (see below).
64 .TP
65 .B \-targa
66 Input file is Targa format. Targa files that contain an "identification"
67 field will not be automatically recognized by
68 .BR cjpeg ;
69 for such files you must specify
70 .B \-targa
71 to make
72 .B cjpeg
73 treat the input as Targa format.
74 For most Targa files, you won't need this switch.
75 .PP
76 The
77 .B \-quality
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
85 .B \-quality
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.)
88 .PP
89 .B \-quality
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
94 .B not
95 recommended for normal use; the compressed file size goes up dramatically for
96 hardly any gain in output image quality.
97 .PP
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
101 .B \-quality
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.
105 .B cjpeg
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
108 .B \-baseline
109 if you need to ensure compatibility at low quality values.)
110 .PP
111 The \fB-quality\fR option has been extended in this version of \fBcjpeg\fR to
112 support separate quality settings for luminance and chrominance (or, in
113 general, separate settings for every quantization table slot.) The principle
114 is the same as chrominance subsampling: since the human eye is more sensitive
115 to spatial changes in brightness than spatial changes in color, the chrominance
116 components can be quantized more than the luminance components without
117 incurring any visible image quality loss. However, unlike subsampling, this
118 feature reduces data in the frequency domain instead of the spatial domain,
119 which allows for more fine-grained control. This option is useful in
120 quality-sensitive applications, for which the artifacts generated by
121 subsampling may be unacceptable.
122 .PP
123 The \fB-quality\fR option accepts a comma-separated list of parameters, which
124 respectively refer to the quality levels which should be assigned to the
125 quantization table slots. If there are more q-table slots than parameters,
126 then the last parameter is replicated. Thus, if only one quality parameter is
127 given, this is used for both luminance and chrominance (slots 0 and 1,
128 respectively), preserving the legacy behavior of cjpeg v6b and prior.
129 More (or customized) quantization tables can be set with the \fB-qtables\fR
130 option and assigned to components with the \fB-qslots\fR option (see the
131 "wizard" switches below.)
132 .PP
133 JPEG files generated with separate luminance and chrominance quality are fully
134 compliant with standard JPEG decoders.
135 .PP
136 .BR CAUTION:
137 For this setting to be useful, be sure to pass an argument of \fB-sample 1x1\fR
138 to \fBcjpeg\fR to disable chrominance subsampling. Otherwise, the default
139 subsampling level (2x2, AKA "4:2:0") will be used.
140 .PP
141 The
142 .B \-progressive
143 switch creates a "progressive JPEG" file. In this type of JPEG file, the data
144 is stored in multiple scans of increasing quality. If the file is being
145 transmitted over a slow communications link, the decoder can use the first
146 scan to display a low-quality image very quickly, and can then improve the
147 display with each subsequent scan. The final image is exactly equivalent to a
148 standard JPEG file of the same quality setting, and the total file size is
149 about the same --- often a little smaller.
150 .PP
151 Switches for advanced users:
152 .TP
153 .B \-dct int
154 Use integer DCT method (default).
155 .TP
156 .B \-dct fast
157 Use fast integer DCT (less accurate).
158 .TP
159 .B \-dct float
160 Use floating-point DCT method.
161 The float method is very slightly more accurate than the int method, but is
162 much slower unless your machine has very fast floating-point hardware. Also
163 note that results of the floating-point method may vary slightly across
164 machines, while the integer methods should give the same results everywhere.
165 The fast integer method is much less accurate than the other two.
166 .TP
167 .BI \-restart " N"
168 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
169 attached to the number.
170 .B \-restart 0
171 (the default) means no restart markers.
172 .TP
173 .BI \-smooth " N"
174 Smooth the input image to eliminate dithering noise. N, ranging from 1 to
175 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
176 .TP
177 .BI \-maxmemory " N"
178 Set limit for amount of memory to use in processing large images. Value is
179 in thousands of bytes, or millions of bytes if "M" is attached to the
180 number. For example,
181 .B \-max 4m
182 selects 4000000 bytes. If more space is needed, temporary files will be used.
183 .TP
184 .BI \-outfile " name"
185 Send output image to the named file, not to standard output.
186 .TP
187 .B \-verbose
188 Enable debug printout. More
189 .BR \-v 's
190 give more output. Also, version information is printed at startup.
191 .TP
192 .B \-debug
193 Same as
194 .BR \-verbose .
195 .PP
196 The
197 .B \-restart
198 option inserts extra markers that allow a JPEG decoder to resynchronize after
199 a transmission error. Without restart markers, any damage to a compressed
200 file will usually ruin the image from the point of the error to the end of the
201 image; with restart markers, the damage is usually confined to the portion of
202 the image up to the next restart marker. Of course, the restart markers
203 occupy extra space. We recommend
204 .B \-restart 1
205 for images that will be transmitted across unreliable networks such as Usenet.
206 .PP
207 The
208 .B \-smooth
209 option filters the input to eliminate fine-scale noise. This is often useful
210 when converting dithered images to JPEG: a moderate smoothing factor of 10 to
211 50 gets rid of dithering patterns in the input file, resulting in a smaller
212 JPEG file and a better-looking image. Too large a smoothing factor will
213 visibly blur the image, however.
214 .PP
215 Switches for wizards:
216 .TP
217 .B \-arithmetic
218 Use arithmetic coding.
219 .B Caution:
220 arithmetic coded JPEG is not yet widely implemented, so many decoders will be
221 unable to view an arithmetic coded JPEG file at all.
222 .TP
223 .B \-baseline
224 Force baseline-compatible quantization tables to be generated. This clamps
225 quantization values to 8 bits even at low quality settings. (This switch is
226 poorly named, since it does not ensure that the output is actually baseline
227 JPEG. For example, you can use
228 .B \-baseline
229 and
230 .B \-progressive
231 together.)
232 .TP
233 .BI \-qtables " file"
234 Use the quantization tables given in the specified text file.
235 .TP
236 .BI \-qslots " N[,...]"
237 Select which quantization table to use for each color component.
238 .TP
239 .BI \-sample " HxV[,...]"
240 Set JPEG sampling factors for each color component.
241 .TP
242 .BI \-scans " file"
243 Use the scan script given in the specified text file.
244 .PP
245 The "wizard" switches are intended for experimentation with JPEG. If you
246 don't know what you are doing, \fBdon't use them\fR. These switches are
247 documented further in the file wizard.txt.
248 .SH EXAMPLES
249 .LP
250 This example compresses the PPM file foo.ppm with a quality factor of
251 60 and saves the output as foo.jpg:
252 .IP
253 .B cjpeg \-quality
254 .I 60 foo.ppm
255 .B >
256 .I foo.jpg
257 .SH HINTS
258 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
259 compressing full-color (24-bit) images. In particular, don't try to convert
260 cartoons, line drawings, and other images that have only a few distinct
261 colors. GIF works great on these, JPEG does not. If you want to convert a
262 GIF to JPEG, you should experiment with
263 .BR cjpeg 's
264 .B \-quality
265 and
266 .B \-smooth
267 options to get a satisfactory conversion.
268 .B \-smooth 10
269 or so is often helpful.
270 .PP
271 Avoid running an image through a series of JPEG compression/decompression
272 cycles. Image quality loss will accumulate; after ten or so cycles the image
273 may be noticeably worse than it was after one cycle. It's best to use a
274 lossless format while manipulating an image, then convert to JPEG format when
275 you are ready to file the image away.
276 .PP
277 The
278 .B \-optimize
279 option to
280 .B cjpeg
281 is worth using when you are making a "final" version for posting or archiving.
282 It's also a win when you are using low quality settings to make very small
283 JPEG files; the percentage improvement is often a lot more than it is on
284 larger files. (At present,
285 .B \-optimize
286 mode is always selected when generating progressive JPEG files.)
287 .SH ENVIRONMENT
288 .TP
289 .B JPEGMEM
290 If this environment variable is set, its value is the default memory limit.
291 The value is specified as described for the
292 .B \-maxmemory
293 switch.
294 .B JPEGMEM
295 overrides the default value specified when the program was compiled, and
296 itself is overridden by an explicit
297 .BR \-maxmemory .
298 .SH SEE ALSO
299 .BR djpeg (1),
300 .BR jpegtran (1),
301 .BR rdjpgcom (1),
302 .BR wrjpgcom (1)
303 .br
304 .BR ppm (5),
305 .BR pgm (5)
306 .br
307 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
308 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
309 .SH AUTHOR
310 Independent JPEG Group
311 .SH BUGS
312 Support for GIF input files was removed in cjpeg v6b due to concerns over
313 the Unisys LZW patent. Although this patent expired in 2006, cjpeg still
314 lacks GIF support, for these historical reasons. (Conversion of GIF files to
315 JPEG is usually a bad idea anyway.)
316 .PP
317 Not all variants of BMP and Targa file formats are supported.
318 .PP
319 The
320 .B \-targa
321 switch is not a bug, it's a feature. (It would be a bug if the Targa format
322 designers had not been clueless.)
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