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. PBM-
PLUS (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 mentioned 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 compatibility.
-fast Select recommended processing options for fast, low quality out-
put. (The default options are chosen for highest quality out-
put.) 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 support 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 for-
mat). PGM is emitted if the JPEG file is gray-scale or if
-grayscale is specified; otherwise 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; other-
wise, colormapped format is emitted if -colors is specified;
otherwise, 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 accu-
rate than the other two.
-dither fs
Use Floyd-Steinberg dithering in color quantization.
-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 produces the best results. Ordered dither is a
compromise 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 predefined 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 quantization. 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 standard output.
-verbose
Enable debug printout. More -v's give more output. Also, ver-
sion 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 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 signifi-
cant 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 speci-
fied when the program 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 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.
22 August 1997 DJPEG(1)
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