188 lines
4.4 KiB
Groff
188 lines
4.4 KiB
Groff
.TH DJPEG 1 "2 August 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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.BI \-colors " N"
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]
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[
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.B \-gif
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]
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[
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.B \-pnm
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]
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[
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.B \-rle
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]
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[
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.B \-targa
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]
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[
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.B \-blocksmooth
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]
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[
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.B \-grayscale
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]
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[
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.BI \-maxmemory " N"
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]
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[
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.B \-nodither
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]
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[
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.B \-onepass
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]
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[
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.B \-verbose
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]
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[
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.B \-debug
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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. PBMPLUS (PPM/PGM), GIF,
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Targa, or RLE (Utah Raster Toolkit) output format can be selected. (RLE is
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supported only if the URT library is available.)
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.SH OPTIONS
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All switch names may be abbreviated; for example,
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.B \-grayscale
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may be written
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.B \-gray
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or
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.BR \-gr .
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Most of the "basic" switches can be abbreviated to as little as one letter.
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Upper and lower case are equivalent (thus
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.B \-GIF
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is the same as
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.BR \-gif ).
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British spellings are also accepted (e.g.,
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.BR \-greyscale ),
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though for brevity these are not mentioned below.
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.PP
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The basic switches are:
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.TP
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.BI \-colors " N"
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Reduce image to at most N colors. This reduces the number of colors used in
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the output image, so that it can be displayed on a colormapped display or
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stored in a colormapped file format. For example, if you have an 8-bit
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display, you'd need to reduce to 256 or fewer colors.
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.TP
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.BI \-quantize " N"
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Same as
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.BR \-colors .
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.B \-colors
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is the recommended name,
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.B \-quantize
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is provided only for backwards compatibility.
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.TP
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.B \-gif
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Select GIF output format. Since GIF does not support more than 256 colors,
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.B \-colors 256
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is assumed (unless you specify a smaller number of colors).
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.TP
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.B \-pnm
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Select PBMPLUS (PPM/PGM) output format (this is the default format).
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PGM is emitted if the JPEG file is gray-scale or if
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.B \-grayscale
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is specified; otherwise PPM is emitted.
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.TP
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.B \-rle
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Select RLE output format. (Requires URT library.)
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.TP
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.B \-targa
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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 \-grayscale
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is specified; otherwise, colormapped format is emitted if
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.B \-colors
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is specified; otherwise, 24-bit full-color format is emitted.
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.PP
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Switches for advanced users:
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.TP
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.B \-blocksmooth
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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 very low quality settings (\fB\-quality\fR 10 to 20 or
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so). At normal quality settings it may make the image worse.
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.TP
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.B \-grayscale
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Force gray-scale output even if JPEG file is color.
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Useful for viewing on monochrome displays.
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.TP
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.BI \-maxmemory " N"
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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 \-max 4m
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selects 4000000 bytes. If more space is needed, temporary files will be used.
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.TP
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.B \-nodither
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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 \-nodither .
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.B \-nodither
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is ignored unless you also say
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.B \-colors
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.IR N .
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.TP
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.B \-onepass
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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 \-onepass
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is ignored unless you also say
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.B \-colors
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.IR N .
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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 \-verbose
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Enable debug printout. More
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.BR \-v 's
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give more output. Also, version information is printed at startup.
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.TP
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.B \-debug
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Same as
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.BR \-verbose .
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.SH EXAMPLES
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.LP
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This example decompresses the JPEG file foo.jpg, automatically quantizes to
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256 colors, and saves the output in GIF format in foo.gif:
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.IP
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.B djpeg \-gif
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.I foo.jpg
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.B >
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.I foo.gif
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.SH ENVIRONMENT
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.TP
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.B JPEGMEM
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If this environment variable is set, its value is the default memory limit.
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The value is specified as described for the
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.B \-maxmemory
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switch.
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.B JPEGMEM
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overrides the default value specified when the program was compiled, and
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itself is overridden by an explicit
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.BR \-maxmemory .
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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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Still not as fast as we'd like.
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