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authorJörg Frings-Fürst <debian@jff-webhosting.net>2015-11-06 07:14:47 +0100
committerJörg Frings-Fürst <debian@jff-webhosting.net>2015-11-06 07:14:47 +0100
commitd479dd1aab1c1cb907932c6595b0ef33523fc797 (patch)
treead7d454b9edaae3d8892d84cd8f8ef5c2697b79b /jpeg/djpeg.1
parent9491825ddff7a294d1f49061bae7044e426aeb2e (diff)
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-.TH DJPEG 1 "3 October 2009"
-.SH NAME
-djpeg \- decompress a JPEG file to an image file
-.SH SYNOPSIS
-.B djpeg
-[
-.I options
-]
-[
-.I filename
-]
-.LP
-.SH DESCRIPTION
-.LP
-.B djpeg
-decompresses the named JPEG file, or the standard input if no file is named,
-and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP,
-GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected.
-(RLE is supported only if the URT library is available.)
-.SH OPTIONS
-All switch names may be abbreviated; for example,
-.B \-grayscale
-may be written
-.B \-gray
-or
-.BR \-gr .
-Most of the "basic" switches can be abbreviated to as little as one letter.
-Upper and lower case are equivalent (thus
-.B \-BMP
-is the same as
-.BR \-bmp ).
-British spellings are also accepted (e.g.,
-.BR \-greyscale ),
-though for brevity these are not mentioned below.
-.PP
-The basic switches are:
-.TP
-.BI \-colors " N"
-Reduce image to at most N colors. This reduces the number of colors used in
-the output image, so that it can be displayed on a colormapped display or
-stored in a colormapped file format. For example, if you have an 8-bit
-display, you'd need to reduce to 256 or fewer colors.
-.TP
-.BI \-quantize " N"
-Same as
-.BR \-colors .
-.B \-colors
-is the recommended name,
-.B \-quantize
-is provided only for backwards compatibility.
-.TP
-.B \-fast
-Select recommended processing options for fast, low quality output. (The
-default options are chosen for highest quality output.) Currently, this is
-equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
-.TP
-.B \-grayscale
-Force gray-scale output even if JPEG file is color. Useful for viewing on
-monochrome displays; also,
-.B djpeg
-runs noticeably faster in this mode.
-.TP
-.BI \-scale " M/N"
-Scale the output image by a factor M/N. Currently supported scale factors are
-M/N with all M from 1 to 16, where N is the source DCT size, which is 8 for
-baseline JPEG. If the /N part is omitted, then M specifies the DCT scaled
-size to be applied on the given input. For baseline JPEG this is equivalent
-to M/8 scaling, since the source DCT size for baseline JPEG is 8.
-Scaling is handy if the image is larger than your screen; also,
-.B djpeg
-runs much faster when scaling down the output.
-.TP
-.B \-bmp
-Select BMP output format (Windows flavor). 8-bit colormapped format is
-emitted if
-.B \-colors
-or
-.B \-grayscale
-is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
-format is emitted.
-.TP
-.B \-gif
-Select GIF output format. Since GIF does not support more than 256 colors,
-.B \-colors 256
-is assumed (unless you specify a smaller number of colors).
-.TP
-.B \-os2
-Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
-emitted if
-.B \-colors
-or
-.B \-grayscale
-is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
-format is emitted.
-.TP
-.B \-pnm
-Select PBMPLUS (PPM/PGM) output format (this is the default format).
-PGM is emitted if the JPEG file is gray-scale or if
-.B \-grayscale
-is specified; otherwise PPM is emitted.
-.TP
-.B \-rle
-Select RLE output format. (Requires URT library.)
-.TP
-.B \-targa
-Select Targa output format. Gray-scale format is emitted if the JPEG file is
-gray-scale or if
-.B \-grayscale
-is specified; otherwise, colormapped format is emitted if
-.B \-colors
-is specified; otherwise, 24-bit full-color format is emitted.
-.PP
-Switches for advanced users:
-.TP
-.B \-dct int
-Use integer DCT method (default).
-.TP
-.B \-dct fast
-Use fast integer DCT (less accurate).
-.TP
-.B \-dct float
-Use floating-point DCT method.
-The float method is very slightly more accurate than the int method, but is
-much slower unless your machine has very fast floating-point hardware. Also
-note that results of the floating-point method may vary slightly across
-machines, while the integer methods should give the same results everywhere.
-The fast integer method is much less accurate than the other two.
-.TP
-.B \-dither fs
-Use Floyd-Steinberg dithering in color quantization.
-.TP
-.B \-dither ordered
-Use ordered dithering in color quantization.
-.TP
-.B \-dither none
-Do not use dithering in color quantization.
-By default, Floyd-Steinberg dithering is applied when quantizing colors; this
-is slow but usually produces the best results. Ordered dither is a compromise
-between speed and quality; no dithering is fast but usually looks awful. Note
-that these switches have no effect unless color quantization is being done.
-Ordered dither is only available in
-.B \-onepass
-mode.
-.TP
-.BI \-map " file"
-Quantize to the colors used in the specified image file. This is useful for
-producing multiple files with identical color maps, or for forcing a
-predefined set of colors to be used. The
-.I file
-must be a GIF or PPM file. This option overrides
-.B \-colors
-and
-.BR \-onepass .
-.TP
-.B \-nosmooth
-Don't use high-quality upsampling.
-.TP
-.B \-onepass
-Use one-pass instead of two-pass color quantization. The one-pass method is
-faster and needs less memory, but it produces a lower-quality image.
-.B \-onepass
-is ignored unless you also say
-.B \-colors
-.IR N .
-Also, the one-pass method is always used for gray-scale output (the two-pass
-method is no improvement then).
-.TP
-.BI \-maxmemory " N"
-Set limit for amount of memory to use in processing large images. Value is
-in thousands of bytes, or millions of bytes if "M" is attached to the
-number. For example,
-.B \-max 4m
-selects 4000000 bytes. If more space is needed, temporary files will be used.
-.TP
-.BI \-outfile " name"
-Send output image to the named file, not to standard output.
-.TP
-.B \-verbose
-Enable debug printout. More
-.BR \-v 's
-give more output. Also, version information is printed at startup.
-.TP
-.B \-debug
-Same as
-.BR \-verbose .
-.SH EXAMPLES
-.LP
-This example decompresses the JPEG file foo.jpg, quantizes it to
-256 colors, and saves the output in 8-bit BMP format in foo.bmp:
-.IP
-.B djpeg \-colors 256 \-bmp
-.I foo.jpg
-.B >
-.I foo.bmp
-.SH HINTS
-To get a quick preview of an image, use the
-.B \-grayscale
-and/or
-.B \-scale
-switches.
-.B \-grayscale \-scale 1/8
-is the fastest case.
-.PP
-Several options are available that trade off image quality to gain speed.
-.B \-fast
-turns on the recommended settings.
-.PP
-.B \-dct fast
-and/or
-.B \-nosmooth
-gain speed at a small sacrifice in quality.
-When producing a color-quantized image,
-.B \-onepass \-dither ordered
-is fast but much lower quality than the default behavior.
-.B \-dither none
-may give acceptable results in two-pass mode, but is seldom tolerable in
-one-pass mode.
-.PP
-If you are fortunate enough to have very fast floating point hardware,
-\fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most
-machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
-not worth using, because its theoretical accuracy advantage is too small to be
-significant in practice.
-.SH ENVIRONMENT
-.TP
-.B JPEGMEM
-If this environment variable is set, its value is the default memory limit.
-The value is specified as described for the
-.B \-maxmemory
-switch.
-.B JPEGMEM
-overrides the default value specified when the program was compiled, and
-itself is overridden by an explicit
-.BR \-maxmemory .
-.SH SEE ALSO
-.BR cjpeg (1),
-.BR jpegtran (1),
-.BR rdjpgcom (1),
-.BR wrjpgcom (1)
-.br
-.BR ppm (5),
-.BR pgm (5)
-.br
-Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
-Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
-.SH AUTHOR
-Independent JPEG Group
-.SH BUGS
-To avoid the Unisys LZW patent,
-.B djpeg
-produces uncompressed GIF files. These are larger than they should be, but
-are readable by standard GIF decoders.