Computer Science
DJPEG(1) DJPEG(1)
NAME
djpeg - 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
All switch names may be abbreviated; for example,
-grayscale may be written -gray or -gr. Most of the
"basic" switches can be abbreviated to as little as one
letter. Upper and lower case are equivalent (thus -BMP is
the same as -bmp). British spellings are also accepted
(e.g., -greyscale), though for brevity these are not men-
tioned below.
The basic switches are:
-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.
-quantize N
Same as -colors. -colors is the recommended name,
-quantize is provided only for backwards compati-
bility.
-fast Select recommended processing options for fast, low
quality output. (The default options are chosen
for highest quality output.) Currently, this is
equivalent to -dct fast -nosmooth -onepass -dither
ordered.
-grayscale
Force gray-scale output even if JPEG file is color.
Useful for viewing on monochrome displays; also,
djpeg runs noticeably faster in this mode.
-scale M/N
Scale the output image by a factor M/N. Currently
the scale factor must be 1/1, 1/2, 1/4, or 1/8.
Scaling is handy if the image is larger than your
screen; also, djpeg runs much faster when scaling
down the output.
-bmp Select BMP output format (Windows flavor). 8-bit
colormapped format is emitted if -colors or
-grayscale is specified, or if the JPEG file is
gray-scale; otherwise, 24-bit full-color format is
emitted.
-gif Select GIF output format. Since GIF does not sup-
port more than 256 colors, -colors 256 is assumed
(unless you specify a smaller number of colors).
-os2 Select BMP output format (OS/2 1.x flavor). 8-bit
colormapped format is emitted if -colors or
-grayscale is specified, or if the JPEG file is
gray-scale; otherwise, 24-bit full-color format is
emitted.
-pnm Select PBMPLUS (PPM/PGM) output format (this is the
default format). PGM is emitted if the JPEG file
is gray-scale or if -grayscale is specified; other-
wise PPM is emitted.
-rle Select RLE output format. (Requires URT library.)
-targa Select Targa output format. Gray-scale format is
emitted if the JPEG file is gray-scale or if
-grayscale is specified; otherwise, colormapped
format is emitted if -colors is specified; other-
wise, 24-bit full-color format is emitted.
Switches for advanced users:
-dct int
Use integer DCT method (default).
-dct fast
Use fast integer DCT (less accurate).
-dct float
Use floating-point DCT method. The float method is
very slightly more accurate than the int method,
but is much slower unless your machine has very
fast floating-point hardware. Also note that
results of the floating-point method may vary
slightly across machines, while the integer methods
should give the same results everywhere. The fast
integer method is much less accurate than the other
two.
-dither fs
Use Floyd-Steinberg dithering in color quantiza-
tion.
-dither ordered
Use ordered dithering in color quantization.
-dither none
Do not use dithering in color quantization. By
default, Floyd-Steinberg dithering is applied when
quantizing colors; this is slow but usually pro-
duces the best results. Ordered dither is a com-
promise between speed and quality; no dithering is
fast but usually looks awful. Note that these
switches have no effect unless color quantization
is being done. Ordered dither is only available in
-onepass mode.
-map file
Quantize to the colors used in the specified image
file. This is useful for producing multiple files
with identical color maps, or for forcing a prede-
fined set of colors to be used. The file must be a
GIF or PPM file. This option overrides -colors and
-onepass.
-nosmooth
Use a faster, lower-quality upsampling routine.
-onepass
Use one-pass instead of two-pass color quantiza-
tion. The one-pass method is faster and needs less
memory, but it produces a lower-quality image.
-onepass is ignored unless you also say -colors N.
Also, the one-pass method is always used for gray-
scale output (the two-pass method is no improvement
then).
-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, -max 4m selects 4000000 bytes. If
more space is needed, temporary files will be used.
-outfile name
Send output image to the named file, not to stan-
dard output.
-verbose
Enable debug printout. More -v's give more output.
Also, version information is printed at startup.
-debug Same as -verbose.
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 accept-
able 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 theo-
retical 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 pro-
gram was compiled, and itself is overridden by an
explicit -maxmemory.
SEE ALSO
cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
ppm(5), pgm(5)
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
Arithmetic coding is not supported for legal reasons.
To avoid the Unisys LZW patent, djpeg produces uncom-
pressed GIF files. These are larger than they should be,
but are readable by standard GIF decoders.
Still not as fast as we'd like.
22 August 1997 1
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