1 .TH CJPEG 1 "28 August 2011"
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.
54 Using this switch suppresses the conversion from RGB
55 colorspace input to the default YCbCr JPEG colorspace.
56 Use this switch in combination with the
58 switch (see below) for lossless JPEG coding.
61 Perform optimization of entropy encoding parameters. Without this, default
62 encoding parameters are used.
64 usually makes the JPEG file a little smaller, but
66 runs somewhat slower and needs much more memory. Image quality and speed of
67 decompression are unaffected by
71 Create progressive JPEG file (see below).
74 Scale the output image by a factor M/N. Currently supported scale factors are
75 M/N with all N from 1 to 16, where M is the destination DCT size, which is 8
81 Input file is Targa format. Targa files that contain an "identification"
82 field will not be automatically recognized by
84 for such files you must specify
88 treat the input as Targa format.
89 For most Targa files, you won't need this switch.
93 switch lets you trade off compressed file size against quality of the
94 reconstructed image: the higher the quality setting, the larger the JPEG file,
95 and the closer the output image will be to the original input. Normally you
96 want to use the lowest quality setting (smallest file) that decompresses into
97 something visually indistinguishable from the original image. For this
98 purpose the quality setting should be between 50 and 95; the default of 75 is
99 often about right. If you see defects at
101 75, then go up 5 or 10 counts at a time until you are happy with the output
102 image. (The optimal setting will vary from one image to another.)
105 100 will generate a quantization table of all 1's, minimizing loss in the
106 quantization step (but there is still information loss in subsampling, as well
107 as roundoff error). This setting is mainly of interest for experimental
108 purposes. Quality values above about 95 are
110 recommended for normal use; the compressed file size goes up dramatically for
111 hardly any gain in output image quality.
113 In the other direction, quality values below 50 will produce very small files
114 of low image quality. Settings around 5 to 10 might be useful in preparing an
115 index of a large image library, for example. Try
117 2 (or so) for some amusing Cubist effects. (Note: quality
118 values below about 25 generate 2-byte quantization tables, which are
119 considered optional in the JPEG standard.
121 emits a warning message when you give such a quality value, because some
122 other JPEG programs may be unable to decode the resulting file. Use
124 if you need to ensure compatibility at low quality values.)
128 option has been extended in IJG version 7 for support of separate quality
129 settings for luminance and chrominance (or in general, for every provided
130 quantization table slot). This feature is useful for high-quality
131 applications which cannot accept the damage of color data by coarse
132 subsampling settings. You can now easily reduce the color data amount more
133 smoothly with finer control without separate subsampling. The resulting file
134 is fully compliant with standard JPEG decoders.
137 ratings refer to the quantization table slots, and that the last value is
138 replicated if there are more q-table slots than parameters. The default
139 q-table slots are 0 for luminance and 1 for chrominance with default tables as
140 given in the JPEG standard. This is compatible with the old behaviour in case
141 that only one parameter is given, which is then used for both luminance and
142 chrominance (slots 0 and 1). More or custom quantization tables can be set
145 and assigned to components with
147 parameter (see the "wizard" switches below).
149 You must explicitly add
151 for efficient separate color
152 quality selection, since the default value used by library is 2x2!
156 switch creates a "progressive JPEG" file. In this type of JPEG file, the data
157 is stored in multiple scans of increasing quality. If the file is being
158 transmitted over a slow communications link, the decoder can use the first
159 scan to display a low-quality image very quickly, and can then improve the
160 display with each subsequent scan. The final image is exactly equivalent to a
161 standard JPEG file of the same quality setting, and the total file size is
162 about the same --- often a little smaller.
164 Switches for advanced users:
167 Use arithmetic coding.
169 arithmetic coded JPEG is not yet widely implemented, so many decoders will be
170 unable to view an arithmetic coded JPEG file at all.
173 Set DCT block size. All N from 1 to 16 are possible.
174 Default is 8 (baseline format).
175 Larger values produce higher compression,
176 smaller values produce higher quality
177 (exact DCT stage possible with 1 or 2; with the default quality of 75 and
178 default Luminance qtable the DCT+Quantization stage is lossless for N=1).
180 An implementation of the JPEG SmartScale extension is required for this
181 feature. SmartScale enabled JPEG is not yet widely implemented, so many
182 decoders will be unable to view a SmartScale extended JPEG file at all.
185 Use integer DCT method (default).
188 Use fast integer DCT (less accurate).
191 Use floating-point DCT method.
192 The float method is very slightly more accurate than the int method, but is
193 much slower unless your machine has very fast floating-point hardware. Also
194 note that results of the floating-point method may vary slightly across
195 machines, while the integer methods should give the same results everywhere.
196 The fast integer method is much less accurate than the other two.
199 Don't use high-quality downsampling.
202 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
203 attached to the number.
205 (the default) means no restart markers.
208 Smooth the input image to eliminate dithering noise. N, ranging from 1 to
209 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
212 Set limit for amount of memory to use in processing large images. Value is
213 in thousands of bytes, or millions of bytes if "M" is attached to the
216 selects 4000000 bytes. If more space is needed, temporary files will be used.
218 .BI \-outfile " name"
219 Send output image to the named file, not to standard output.
222 Enable debug printout. More
224 give more output. Also, version information is printed at startup.
232 option inserts extra markers that allow a JPEG decoder to resynchronize after
233 a transmission error. Without restart markers, any damage to a compressed
234 file will usually ruin the image from the point of the error to the end of the
235 image; with restart markers, the damage is usually confined to the portion of
236 the image up to the next restart marker. Of course, the restart markers
237 occupy extra space. We recommend
239 for images that will be transmitted across unreliable networks such as Usenet.
243 option filters the input to eliminate fine-scale noise. This is often useful
244 when converting dithered images to JPEG: a moderate smoothing factor of 10 to
245 50 gets rid of dithering patterns in the input file, resulting in a smaller
246 JPEG file and a better-looking image. Too large a smoothing factor will
247 visibly blur the image, however.
249 Switches for wizards:
252 Force baseline-compatible quantization tables to be generated. This clamps
253 quantization values to 8 bits even at low quality settings. (This switch is
254 poorly named, since it does not ensure that the output is actually baseline
255 JPEG. For example, you can use
261 .BI \-qtables " file"
262 Use the quantization tables given in the specified text file.
264 .BI \-qslots " N[,...]"
265 Select which quantization table to use for each color component.
267 .BI \-sample " HxV[,...]"
268 Set JPEG sampling factors for each color component.
271 Use the scan script given in the specified text file.
273 The "wizard" switches are intended for experimentation with JPEG. If you
274 don't know what you are doing, \fBdon't use them\fR. These switches are
275 documented further in the file wizard.txt.
278 This example compresses the PPM file foo.ppm with a quality factor of
279 60 and saves the output as foo.jpg:
286 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
287 compressing full-color (24-bit) images. In particular, don't try to convert
288 cartoons, line drawings, and other images that have only a few distinct
289 colors. GIF works great on these, JPEG does not. If you want to convert a
290 GIF to JPEG, you should experiment with
295 options to get a satisfactory conversion.
297 or so is often helpful.
299 Avoid running an image through a series of JPEG compression/decompression
300 cycles. Image quality loss will accumulate; after ten or so cycles the image
301 may be noticeably worse than it was after one cycle. It's best to use a
302 lossless format while manipulating an image, then convert to JPEG format when
303 you are ready to file the image away.
309 is worth using when you are making a "final" version for posting or archiving.
310 It's also a win when you are using low quality settings to make very small
311 JPEG files; the percentage improvement is often a lot more than it is on
312 larger files. (At present,
314 mode is always selected when generating progressive JPEG files.)
318 If this environment variable is set, its value is the default memory limit.
319 The value is specified as described for the
323 overrides the default value specified when the program was compiled, and
324 itself is overridden by an explicit
335 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
336 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
338 Independent JPEG Group
340 GIF input files are no longer supported, to avoid the Unisys LZW patent.
341 (Conversion of GIF files to JPEG is usually a bad idea anyway.)
343 Not all variants of BMP and Targa file formats are supported.
347 switch is not a bug, it's a feature. (It would be a bug if the Targa format
348 designers had not been clueless.)