188 lines
4.4 KiB
Groff
188 lines
4.4 KiB
Groff
.TH DJPEG 1 "17 February 1993"
|
|
.SH NAME
|
|
djpeg \- decompress a JPEG file to an image file
|
|
.SH SYNOPSIS
|
|
.B djpeg
|
|
[
|
|
.BI \-colors " N"
|
|
]
|
|
[
|
|
.B \-gif
|
|
]
|
|
[
|
|
.B \-pnm
|
|
]
|
|
[
|
|
.B \-rle
|
|
]
|
|
[
|
|
.B \-targa
|
|
]
|
|
[
|
|
.B \-blocksmooth
|
|
]
|
|
[
|
|
.B \-grayscale
|
|
]
|
|
[
|
|
.BI \-maxmemory " N"
|
|
]
|
|
[
|
|
.B \-nodither
|
|
]
|
|
[
|
|
.B \-onepass
|
|
]
|
|
[
|
|
.B \-verbose
|
|
]
|
|
[
|
|
.B \-debug
|
|
]
|
|
[
|
|
.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), 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 \-GIF
|
|
is the same as
|
|
.BR \-gif ).
|
|
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 \-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 \-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 \-blocksmooth
|
|
Perform cross-block smoothing. This is slow, quite memory-intensive, and only
|
|
seems to improve the image at very low quality settings (\fB\-quality\fR 10 to
|
|
20 or so). At normal quality settings it may make the image worse.
|
|
.TP
|
|
.B \-grayscale
|
|
Force gray-scale output even if JPEG file is color.
|
|
Useful for viewing on monochrome displays.
|
|
.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
|
|
.B \-nodither
|
|
Do not use dithering in color quantization. By default, Floyd-Steinberg
|
|
dithering is applied when quantizing colors, but on some images dithering may
|
|
result in objectionable "graininess". If that happens, you can turn off
|
|
dithering with
|
|
.BR \-nodither .
|
|
.B \-nodither
|
|
is ignored unless you also say
|
|
.B \-colors
|
|
.IR N .
|
|
.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
|
|
.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, automatically quantizes to
|
|
256 colors, and saves the output in GIF format in foo.gif:
|
|
.IP
|
|
.B djpeg \-gif
|
|
.I foo.jpg
|
|
.B >
|
|
.I foo.gif
|
|
.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
|
|
.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
|
|
Arithmetic coding is not supported for legal reasons.
|
|
.PP
|
|
Still not as fast as we'd like.
|