DJPEG
Section: User Commands (1)
Updated: 11 December 1991
Index
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NAME
djpeg - decompress a JPEG file to an image file
SYNOPSIS
djpeg
[
-GPRTbgD2d
]
[
-q N
]
[
filename
]
DESCRIPTION
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.)
The color quantization algorithm is currently shoddy. Because of this, the
GIF output mode is not recommended in the current release, except for
gray-scale output (obtained with
-g).
OPTIONS
- -G
-
Select GIF output format (implies
-q,
with default of 256 colors).
Currently the color quantization uses a shoddy algorithm and external
quantization (e.g.
ppmquant,
rlequant)
is recommended before conversion to GIF format.
- -P
-
Select PPM or PGM output format (this is the default). PGM is emitted if the
JPEG file is gray-scale or if
-g
is specified.
- -R
-
Select RLE output format. Requires URT library.
- -T
-
Select Targa output format. Gray-scale format is emitted if the JPEG file is
gray-scale or if
-g
is specified; otherwise, colormapped format is emitted if
-q
is specified; otherwise, 24-bit full-color format is emitted.
- -b
-
Perform cross-block smoothing. This is quite memory-intensive and only seems
to improve the image at low quality settings (-Q 10 to 20 or so).
At normal
-Q
settings it may make the image worse.
- -g
-
Force gray-scale output even if input is color.
- -q N
-
Quantize to N colors.
- -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
-D.
- -2
-
Use two-pass color quantization (not yet supported).
- -d
-
Enable debug printout. More
-d's
give more output. Also, version information is printed at startup.
EXAMPLES
This example decompresses the JPEG file foo.jpg and saves the output
as a gray-scale image in foo.pgm:
-
djpeg -g
foo.jpg
>
foo.pgm
SEE ALSO
cjpeg(1)
ppmquant(1)
[From the PBMplus distribution]
rlequant(1)
[From the Utah Raster Toolkit distribution]
Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
AUTHOR
Independent JPEG Group
BUGS
djpeg
currently uses a shoddy color quantization algorithm. This leads to
poor GIF file output. Two-pass color quantization is not yet
supported.
Arithmetic coding is not supported for legal reasons.
Not as fast as we'd like.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- EXAMPLES
-
- SEE ALSO
-
- AUTHOR
-
- BUGS
-
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