123 lines
3.0 KiB
Groff
123 lines
3.0 KiB
Groff
.TH DJPEG 1 "28 February 1992"
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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 \-GPRTgD1bd
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]
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[
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.BI \-q " colors"
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]
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[
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.BI \-m " memory"
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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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.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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.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 \-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. This reduces the number of colors in the output image
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so that it can be displayed on a colormapped display or stored in a
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colormapped file format. For example, if you have an 8-bit display, you'd
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need to quantize to 256 or fewer 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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.B \-D
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is ignored unless you also say
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.B \-q
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or
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.BR \-G .
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.TP
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.B \-1
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Use one-pass instead of two-pass color quantization. The one-pass method is
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faster and needs less memory, but it produces a lower-quality image.
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.B \-1
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is ignored unless you also say
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.B \-q
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or
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.BR \-G .
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Also, the one-pass method is always used for gray-scale output (the two-pass
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method is no improvement then).
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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 \-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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.TP
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.BI \-m " memory"
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Set limit for amount of memory to use in processing large images. Value is
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in thousands of bytes, or millions of bytes if "M" is attached to the
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number. For example,
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.B \-m 4m
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selects 4000000 bytes. If more space is needed, temporary files will be used.
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.SH EXAMPLES
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.LP
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This example decompresses the JPEG file foo.jpg, quantizes to 256 colors,
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and saves the output in GIF format in foo.gif:
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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.gif
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.SH SEE ALSO
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.BR cjpeg (1)
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.br
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.BR ppm (5),
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.BR pgm (5)
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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 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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