253 lines
7.4 KiB
Groff
253 lines
7.4 KiB
Groff
.TH DJPEG 1 "3 October 2009"
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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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.I options
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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), BMP,
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GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected.
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(RLE is 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 \-BMP
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is the same as
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.BR \-bmp ).
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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 \-fast
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Select recommended processing options for fast, low quality output. (The
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default options are chosen for highest quality output.) Currently, this is
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equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
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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. Useful for viewing on
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monochrome displays; also,
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.B djpeg
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runs noticeably faster in this mode.
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.TP
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.BI \-scale " M/N"
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Scale the output image by a factor M/N. Currently supported scale factors are
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M/N with all M from 1 to 16, where N is the source DCT size, which is 8 for
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baseline JPEG. If the /N part is omitted, then M specifies the DCT scaled
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size to be applied on the given input. For baseline JPEG this is equivalent
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to M/8 scaling, since the source DCT size for baseline JPEG is 8.
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Scaling is handy if the image is larger than your screen; also,
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.B djpeg
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runs much faster when scaling down the output.
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.TP
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.B \-bmp
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Select BMP output format (Windows flavor). 8-bit colormapped format is
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emitted if
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.B \-colors
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or
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.B \-grayscale
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is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
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format is emitted.
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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 \-os2
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Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
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emitted if
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.B \-colors
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or
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.B \-grayscale
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is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
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format is emitted.
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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 \-dct int
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Use integer DCT method (default).
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.TP
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.B \-dct fast
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Use fast integer DCT (less accurate).
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.TP
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.B \-dct float
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Use floating-point DCT method.
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The float method is very slightly more accurate than the int method, but is
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much slower unless your machine has very fast floating-point hardware. Also
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note that results of the floating-point method may vary slightly across
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machines, while the integer methods should give the same results everywhere.
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The fast integer method is much less accurate than the other two.
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.TP
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.B \-dither fs
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Use Floyd-Steinberg dithering in color quantization.
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.TP
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.B \-dither ordered
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Use ordered dithering in color quantization.
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.TP
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.B \-dither none
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Do not use dithering in color quantization.
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By default, Floyd-Steinberg dithering is applied when quantizing colors; this
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is slow but usually produces the best results. Ordered dither is a compromise
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between speed and quality; no dithering is fast but usually looks awful. Note
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that these switches have no effect unless color quantization is being done.
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Ordered dither is only available in
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.B \-onepass
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mode.
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.TP
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.BI \-map " file"
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Quantize to the colors used in the specified image file. This is useful for
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producing multiple files with identical color maps, or for forcing a
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predefined set of colors to be used. The
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.I file
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must be a GIF or PPM file. This option overrides
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.B \-colors
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and
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.BR \-onepass .
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.TP
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.B \-nosmooth
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Don't use high-quality upsampling.
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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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.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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.BI \-outfile " name"
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Send output image to the named file, not to standard output.
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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, quantizes it to
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256 colors, and saves the output in 8-bit BMP format in foo.bmp:
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.IP
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.B djpeg \-colors 256 \-bmp
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.I foo.jpg
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.B >
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.I foo.bmp
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.SH HINTS
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To get a quick preview of an image, use the
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.B \-grayscale
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and/or
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.B \-scale
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switches.
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.B \-grayscale \-scale 1/8
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is the fastest case.
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.PP
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Several options are available that trade off image quality to gain speed.
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.B \-fast
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turns on the recommended settings.
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.PP
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.B \-dct fast
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and/or
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.B \-nosmooth
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gain speed at a small sacrifice in quality.
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When producing a color-quantized image,
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.B \-onepass \-dither ordered
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is fast but much lower quality than the default behavior.
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.B \-dither none
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may give acceptable results in two-pass mode, but is seldom tolerable in
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one-pass mode.
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.PP
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If you are fortunate enough to have very fast floating point hardware,
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\fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most
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machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
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not worth using, because its theoretical accuracy advantage is too small to be
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significant in practice.
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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 jpegtran (1),
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.BR rdjpgcom (1),
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.BR wrjpgcom (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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To avoid the Unisys LZW patent,
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.B djpeg
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produces uncompressed GIF files. These are larger than they should be, but
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are readable by standard GIF decoders.
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