115 lines
2.8 KiB
Groff
115 lines
2.8 KiB
Groff
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.TH DJPEG 1 "11 December 1991"
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.SH NAME
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djpeg \- decompress a JPEG file to an image file
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.SH SYNOPSIS
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.B djpeg
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[
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.B \-GPRTbgD2d
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]
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[
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.BI \-q " N"
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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 djpeg
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decompresses the named JPEG file, or the standard input if no file is named,
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and produces an image file on the standard output. PPM, GIF, Targa, or RLE
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output format can be selected. (RLE is supported only if the URT library is
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available.)
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.LP
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The color quantization algorithm is currently shoddy. Because of this, the
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GIF output mode is not recommended in the current release, except for
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gray-scale output (obtained with
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.BR \-g ).
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.SH OPTIONS
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.TP
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.B \-G
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Select GIF output format (implies
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.BR \-q ,
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with default of 256 colors).
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Currently the color quantization uses a shoddy algorithm and external
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quantization (e.g.
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.IR ppmquant ,
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.IR rlequant )
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is recommended before conversion to GIF format.
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.TP
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.B \-P
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Select PPM or PGM output format (this is the default). PGM is emitted if the
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JPEG file is gray-scale or if
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.B \-g
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is specified.
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.TP
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.B \-R
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Select RLE output format. Requires URT library.
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.TP
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.B \-T
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Select Targa output format. Gray-scale format is emitted if the JPEG file is
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gray-scale or if
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.B \-g
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is specified; otherwise, colormapped format is emitted if
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.B \-q
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is specified; otherwise, 24-bit full-color format is emitted.
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.TP
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.B \-b
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Perform cross-block smoothing. This is quite memory-intensive and only seems
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to improve the image at low quality settings (\fB\-Q\fR 10 to 20 or so).
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At normal
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.B \-Q
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settings it may make the image worse.
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.TP
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.B \-g
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Force gray-scale output even if input is color.
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.TP
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.BI \-q " N"
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Quantize to N colors.
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.TP
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.B \-D
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Do not use dithering in color quantization. By default, Floyd-Steinberg
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dithering is applied when quantizing colors, but on some images dithering may
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result in objectionable "graininess". If that happens, you can turn off
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dithering with
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.BR \-D .
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.TP
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.B \-2
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Use two-pass color quantization (not yet supported).
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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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.SH EXAMPLES
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.LP
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This example decompresses the JPEG file foo.jpg and saves the output
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as a gray-scale image in foo.pgm:
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.IP
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.B djpeg \-g
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.I foo.jpg
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.B >
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.I foo.pgm
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.SH SEE ALSO
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.BR cjpeg (1)
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.br
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.BR ppmquant (1)
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[From the PBMplus distribution]
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.br
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.BR rlequant (1)
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[From the Utah Raster Toolkit distribution]
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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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.B djpeg
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currently uses a shoddy color quantization algorithm. This leads to
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poor GIF file output. Two-pass color quantization is not yet
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supported.
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.PP
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Arithmetic coding is not supported for legal reasons.
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.PP
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Not as fast as we'd like.
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