131 lines
3.7 KiB
Groff
131 lines
3.7 KiB
Groff
.TH CJPEG 1 "11 December 1991"
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.SH NAME
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cjpeg \- compress an image file to a JPEG file
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.SH SYNOPSIS
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.B cjpeg
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[
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.BI \-Q " quality"
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]
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[
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.B \-oTIad
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]
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[
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.I filename
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]
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.LP
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.SH DESCRIPTION
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.LP
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.B cjpeg
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compresses the named image file, or the standard input if no file is
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named, and produces a JPEG/JFIF file on the standard output.
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The currently supported image file formats are: PPM (PBMPLUS color
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format), PGM (PBMPLUS gray-scale format), GIF, Targa, and RLE (Utah Raster
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Toolkit format). (RLE is supported only if the URT library is available.)
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.SH OPTIONS
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.TP
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.BI \-Q " quality"
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Scale quantization tables to adjust image quality. Quality is 0 (worst) to
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100 (best); default is 75. (See below for more info.)
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.TP
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.B \-o
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Perform optimization of entropy encoding parameters. Without this, default
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encoding parameters are used.
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.B \-o
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usually makes the JPEG file a little smaller, but
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.B cjpeg
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runs much slower. Image quality and speed of decompression are unaffected by
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.BR \-o .
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.TP
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.B \-T
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Input file is Targa format. Targa files that contain an "identification"
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field will not be automatically recognized by
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.BR cjpeg ;
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for such files you must specify
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.B \-T
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to force
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.B cjpeg
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to treat the input as Targa format.
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.TP
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.B \-I
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Generate noninterleaved JPEG file (not yet supported).
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.TP
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.B \-a
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Use arithmetic coding rather than Huffman coding (not currently
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supported for legal reasons).
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.TP
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.B \-d
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Enable debug printout. More
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.BR \-d 's
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give more output. Also, version information is printed at startup.
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.PP
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The
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.B \-Q
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switch lets you trade off compressed file size against quality of the
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reconstructed image: the higher the
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.B \-Q
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setting, the larger the JPEG file, and the closer the output image will be to
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the original input. Normally you want to use the lowest
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.B \-Q
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setting (smallest file) that decompresses into something visually
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indistinguishable from the original image. For this purpose the
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.B \-Q
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setting should be between 50 and 95; the default of 75 is often about right.
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If you see defects at
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.B \-Q
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75, then go up 5 or 10 counts at a time until you are happy with the output
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image. (The optimal setting will vary from one image to another.)
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.PP
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.B \-Q
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100 will generate a quantization table of all 1's, eliminating loss in the
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quantization step (but there is still information loss in subsampling, as well
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as roundoff error). This setting is mainly of interest for experimental
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purposes.
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.B \-Q
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values above about 95 are
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.B not
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recommended for normal use; the compressed file size goes up dramatically for
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hardly any gain in output image quality.
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.PP
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In the other direction,
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.B \-Q
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values below 50 will produce very small files of low image quality. Settings
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around 5 to 10 might be useful in preparing an index of a large image library,
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for example. Try
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.B \-Q
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2 (or so) for some amusing Cubist effects. (Note:
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.B \-Q
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values below about 25 generate 2-byte quantization tables, which are
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considered optional in the JPEG standard.
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.B cjpeg
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emits a warning message when you give such a
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.B \-Q
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value, because some commercial JPEG programs may be unable to decode the
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resulting file.)
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.SH EXAMPLES
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.LP
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This example compresses the PPM file foo.ppm with a quality factor of
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60 and saves the output as foo.jpg:
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.IP
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.B cjpeg \-Q
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.I 60 foo.ppm
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.B >
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.I foo.jpg
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.SH SEE ALSO
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.BR djpeg (1)
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.br
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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.SH AUTHOR
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Independent JPEG Group
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.SH BUGS
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Arithmetic coding and interleaved output not yet supported.
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.PP
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Not all variants of Targa file format are supported.
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.PP
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The
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.B -T
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switch is not a bug, it's a feature. (It would be a bug if the Targa format
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designers had not been clueless.)
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.PP
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Not as fast as we'd like.
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