DJPEG (1)
decompress a JPEG file to an image file
SYNOPSIS
djpeg
[
options
]
[
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. 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.)
OPTIONS
EXAMPLES
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:
djpeg -colors 256 -bmp
foo.jpg
>
foo.bmp
HINTS
To get a quick preview of an image, use the
-grayscale
and/or
-scale
switches.
-grayscale -scale 1/8
is the fastest case.
Several options are available that trade off image quality to gain speed.
-fast
turns on the recommended settings.
-dct fast
and/or
-nosmooth
gain speed at a small sacrifice in quality.
When producing a color-quantized image,
-onepass -dither ordered
is fast but much lower quality than the default behavior.
-dither none
may give acceptable results in two-pass mode, but is seldom tolerable in
one-pass mode.
If you are fortunate enough to have very fast floating point hardware,
-dct float may be even faster than -dct fast. But on most
machines -dct float is slower than -dct int; in this case it is
not worth using, because its theoretical accuracy advantage is too small to be
significant in practice.
ENVIRONMENT
JPEGMEM
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
-maxmemory
switch.
JPEGMEM
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
-maxmemory .
SEE ALSO
AUTHOR
BUGS
Arithmetic coding is not supported for legal reasons.
To avoid the Unisys LZW patent,
djpeg
produces uncompressed GIF files. These are larger than they should be, but
are readable by standard GIF decoders.
Still not as fast as we'd like.
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