libjpeg-turbo/djpeg.1

.TH DJPEG 1 "28 February 1992"
.SH NAME
djpeg \- decompress a JPEG file to an image file
.SH SYNOPSIS
.B djpeg
[
.B \-GPRTgD1bd
]
[
.BI \-q " colors"
]
[
.BI \-m " memory"
]
[
.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.  PPM, GIF, Targa, or RLE
output format can be selected.  (RLE is supported only if the URT library is
available.)
.SH OPTIONS
.TP
.B \-G
Select GIF output format (implies
.BR \-q ,
with default of 256 colors).
.TP
.B \-P
Select PPM or PGM output format (this is the default).  PGM is emitted if the
JPEG file is gray-scale or if
.B \-g
is specified.
.TP
.B \-R
Select RLE output format.  Requires URT library.
.TP
.B \-T
Select Targa output format.  Gray-scale format is emitted if the JPEG file is
gray-scale or if
.B \-g
is specified; otherwise, colormapped format is emitted if
.B \-q
is specified; otherwise, 24-bit full-color format is emitted.
.TP
.B \-g
Force gray-scale output even if input is color.
.TP
.BI \-q " N"
Quantize to N colors.  This reduces the number of colors 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 quantize to 256 or fewer colors.
.TP
.B \-D
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 \-D .
.B \-D
is ignored unless you also say
.B \-q
or
.BR \-G .
.TP
.B \-1
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 \-1
is ignored unless you also say
.B \-q
or
.BR \-G .
Also, the one-pass method is always used for gray-scale output (the two-pass
method is no improvement then).
.TP
.B \-b
Perform cross-block smoothing.  This is quite memory-intensive and only seems
to improve the image at low quality settings (\fB\-Q\fR 10 to 20 or so).
At normal
.B \-Q
settings it may make the image worse.
.TP
.B \-d
Enable debug printout.  More
.BR \-d 's
give more output.  Also, version information is printed at startup.
.TP
.BI \-m " memory"
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 \-m 4m
selects 4000000 bytes.  If more space is needed, temporary files will be used.
.SH EXAMPLES
.LP
This example decompresses the JPEG file foo.jpg, quantizes to 256 colors,
and saves the output in GIF format in foo.gif:
.IP
.B djpeg \-G
.I foo.jpg
.B >
.I foo.gif
.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
Not as fast as we'd like.