1 .TH CJPEG 1 "29 June 2023"
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 grayscale format), BMP, GIF, and Targa.
21 All switch names may be abbreviated; for example,
27 Most of the "basic" switches can be abbreviated to as little as one letter.
28 Upper and lower case are equivalent (thus
32 British spellings are also accepted (e.g.,
34 though for brevity these are not mentioned below.
36 The basic switches are:
38 .BI \-quality " N[,...]"
39 Scale quantization tables to adjust image quality. Quality is 0 (worst) to
40 100 (best); default is 75. (See below for more info.)
43 Create monochrome JPEG file from color input. By saying
45 you'll get a smaller JPEG file that takes less time to process.
49 Using this switch suppresses the conversion from RGB
50 colorspace input to the default YCbCr JPEG colorspace.
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 Input file is Targa format. Targa files that contain an "identification"
67 field will not be automatically recognized by
69 for such files you must specify
73 treat the input as Targa format.
74 For most Targa files, you won't need this switch.
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 generally be between 50 and 95 (the default
84 is 75) for photographic images. If you see defects at
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.)
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.) For most images, specifying a quality value above
93 about 95 will increase the size of the compressed file dramatically, and while
94 the quality gain from these higher quality values is measurable (using metrics
95 such as PSNR or SSIM), it is rarely perceivable by human vision.
97 In the other direction, quality values below 50 will produce very small files
98 of low image quality. Settings around 5 to 10 might be useful in preparing an
99 index of a large image library, for example. Try
101 2 (or so) for some amusing Cubist effects. (Note: quality
102 values below about 25 generate 2-byte quantization tables, which are
103 considered optional in the JPEG standard.
105 emits a warning message when you give such a quality value, because some
106 other JPEG programs may be unable to decode the resulting file. Use
108 if you need to ensure compatibility at low quality values.)
110 The \fB-quality\fR option has been extended in this version of \fBcjpeg\fR to
111 support separate quality settings for luminance and chrominance (or, in
112 general, separate settings for every quantization table slot.) The principle
113 is the same as chrominance subsampling: since the human eye is more sensitive
114 to spatial changes in brightness than spatial changes in color, the chrominance
115 components can be quantized more than the luminance components without
116 incurring any visible image quality loss. However, unlike subsampling, this
117 feature reduces data in the frequency domain instead of the spatial domain,
118 which allows for more fine-grained control. This option is useful in
119 quality-sensitive applications, for which the artifacts generated by
120 subsampling may be unacceptable.
122 The \fB-quality\fR option accepts a comma-separated list of parameters, which
123 respectively refer to the quality levels that should be assigned to the
124 quantization table slots. If there are more q-table slots than parameters,
125 then the last parameter is replicated. Thus, if only one quality parameter is
126 given, this is used for both luminance and chrominance (slots 0 and 1,
127 respectively), preserving the legacy behavior of cjpeg v6b and prior.
128 More (or customized) quantization tables can be set with the \fB-qtables\fR
129 option and assigned to components with the \fB-qslots\fR option (see the
130 "wizard" switches below.)
132 JPEG files generated with separate luminance and chrominance quality are fully
133 compliant with standard JPEG decoders.
136 For this setting to be useful, be sure to pass an argument of \fB-sample 1x1\fR
137 to \fBcjpeg\fR to disable chrominance subsampling. Otherwise, the default
138 subsampling level (2x2, AKA "4:2:0") will be used.
142 switch creates a "progressive JPEG" file. In this type of JPEG file, the data
143 is stored in multiple scans of increasing quality. If the file is being
144 transmitted over a slow communications link, the decoder can use the first
145 scan to display a low-quality image very quickly, and can then improve the
146 display with each subsequent scan. The final image is exactly equivalent to a
147 standard JPEG file of the same quality setting, and the total file size is
148 about the same --- often a little smaller.
150 Switches for advanced users:
153 Create JPEG file with N-bit data precision. N is 8, 12, or 16; default is 8.
156 must also be specified.
158 12-bit and 16-bit JPEG is not yet widely implemented, so many decoders will be
159 unable to view a 12-bit or 16-bit JPEG file at all.
161 .BI \-lossless " psv[,Pt]"
162 Create a lossless JPEG file using the specified predictor selection value
163 (1 through 7) and optional point transform (0 through
171 is the JPEG data precision in bits). A point transform value of 0 (the
172 default) is necessary in order to create a fully lossless JPEG file. (A
173 non-zero point transform value right-shifts the input samples by the specified
174 number of bits, which is effectively a form of lossy color quantization.)
176 lossless JPEG is not yet widely implemented, so many decoders will be unable to
177 view a lossless JPEG file at all. Note that the following features will be
178 unavailable when compressing or decompressing a lossless JPEG file:
180 - Quality/quantization table selection
182 - Color space conversion (the JPEG image will use the same color space as the
187 - DCT/IDCT algorithm selection
191 - Downsampling/upsampling
195 - Partial image decompression
197 - Transformations using
200 Any switches used to enable or configure those features will be ignored.
203 Use arithmetic coding.
205 arithmetic coded JPEG is not yet widely implemented, so many decoders will be
206 unable to view an arithmetic coded JPEG file at all.
209 Use accurate integer DCT method (default).
212 Use less accurate integer DCT method [legacy feature].
213 When the Independent JPEG Group's software was first released in 1991, the
214 compression time for a 1-megapixel JPEG image on a mainstream PC was measured
215 in minutes. Thus, the \fBfast\fR integer DCT algorithm provided noticeable
216 performance benefits. On modern CPUs running libjpeg-turbo, however, the
217 compression time for a 1-megapixel JPEG image is measured in milliseconds, and
218 thus the performance benefits of the \fBfast\fR algorithm are much less
219 noticeable. On modern x86/x86-64 CPUs that support AVX2 instructions, the
220 \fBfast\fR and \fBint\fR methods have similar performance. On other types of
221 CPUs, the \fBfast\fR method is generally about 5-15% faster than the \fBint\fR
224 For quality levels of 90 and below, there should be little or no perceptible
225 quality difference between the two algorithms. For quality levels above 90,
226 however, the difference between the \fBfast\fR and \fBint\fR methods becomes
227 more pronounced. With quality=97, for instance, the \fBfast\fR method incurs
228 generally about a 1-3 dB loss in PSNR relative to the \fBint\fR method, but
229 this can be larger for some images. Do not use the \fBfast\fR method with
230 quality levels above 97. The algorithm often degenerates at quality=98 and
231 above and can actually produce a more lossy image than if lower quality levels
232 had been used. Also, in libjpeg-turbo, the \fBfast\fR method is not fully
233 accelerated for quality levels above 97, so it will be slower than the
237 Use floating-point DCT method [legacy feature].
238 The \fBfloat\fR method does not produce significantly more accurate results
239 than the \fBint\fR method, and it is much slower. The \fBfloat\fR method may
240 also give different results on different machines due to varying roundoff
241 behavior, whereas the integer methods should give the same results on all
245 Embed ICC color management profile contained in the specified file.
248 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks (samples in
249 lossless mode) if "B" is attached to the number.
251 (the default) means no restart markers.
254 Smooth the input image to eliminate dithering noise. N, ranging from 1 to
255 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
258 Set limit for amount of memory to use in processing large images. Value is
259 in thousands of bytes, or millions of bytes if "M" is attached to the
262 selects 4000000 bytes. If more space is needed, an error will occur.
264 .BI \-outfile " name"
265 Send output image to the named file, not to standard output.
268 Compress to memory instead of a file. This feature was implemented mainly as a
269 way of testing the in-memory destination manager (jpeg_mem_dest()), but it is
270 also useful for benchmarking, since it reduces the I/O overhead.
273 Report compression progress.
276 Treat all warnings as fatal. Enabling this option will cause the compressor to
277 abort if an LZW-compressed GIF input image contains incomplete or corrupt image
281 Enable debug printout. More
283 give more output. Also, version information is printed at startup.
290 Print version information and exit.
294 option inserts extra markers that allow a JPEG decoder to resynchronize after
295 a transmission error. Without restart markers, any damage to a compressed
296 file will usually ruin the image from the point of the error to the end of the
297 image; with restart markers, the damage is usually confined to the portion of
298 the image up to the next restart marker. Of course, the restart markers
299 occupy extra space. We recommend
301 for images that will be transmitted across unreliable networks such as Usenet.
305 option filters the input to eliminate fine-scale noise. This is often useful
306 when converting dithered images to JPEG: a moderate smoothing factor of 10 to
307 50 gets rid of dithering patterns in the input file, resulting in a smaller
308 JPEG file and a better-looking image. Too large a smoothing factor will
309 visibly blur the image, however.
311 Switches for wizards:
314 Force baseline-compatible quantization tables to be generated. This clamps
315 quantization values to 8 bits even at low quality settings. (This switch is
316 poorly named, since it does not ensure that the output is actually baseline
317 JPEG. For example, you can use
323 .BI \-qtables " file"
324 Use the quantization tables given in the specified text file.
326 .BI \-qslots " N[,...]"
327 Select which quantization table to use for each color component.
329 .BI \-sample " HxV[,...]"
330 Set JPEG sampling factors for each color component.
333 Use the scan script given in the specified text file.
335 The "wizard" switches are intended for experimentation with JPEG. If you
336 don't know what you are doing, \fBdon't use them\fR. These switches are
337 documented further in the file wizard.txt.
340 This example compresses the PPM file foo.ppm with a quality factor of
341 60 and saves the output as foo.jpg:
348 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
349 compressing full-color (24-bit) images. In particular, don't try to convert
350 cartoons, line drawings, and other images that have only a few distinct
351 colors. GIF works great on these, JPEG does not. If you want to convert a
352 GIF to JPEG, you should experiment with
357 options to get a satisfactory conversion.
359 or so is often helpful.
361 Avoid running an image through a series of JPEG compression/decompression
362 cycles. Image quality loss will accumulate; after ten or so cycles the image
363 may be noticeably worse than it was after one cycle. It's best to use a
364 lossless format while manipulating an image, then convert to JPEG format when
365 you are ready to file the image away.
371 is worth using when you are making a "final" version for posting or archiving.
372 It's also a win when you are using low quality settings to make very small
373 JPEG files; the percentage improvement is often a lot more than it is on
374 larger files. (At present,
376 mode is always selected when generating progressive JPEG files.)
380 If this environment variable is set, its value is the default memory limit.
381 The value is specified as described for the
385 overrides the default value specified when the program was compiled, and
386 itself is overridden by an explicit
397 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
398 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
400 Independent JPEG Group
402 This file was modified by The libjpeg-turbo Project to include only information
403 relevant to libjpeg-turbo, to wordsmith certain sections, and to describe
404 features not present in libjpeg.
406 Not all variants of BMP and Targa file formats are supported.
410 switch is not a bug, it's a feature. (It would be a bug if the Targa format
411 designers had not been clueless.)
projects / platform / upstream / libjpeg-turbo.git / blob