.TH DJPEG 1 "2 August 1992"
.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 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.