DJPEG(1) UNIX System V (22 August 1997) 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 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 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.
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DJPEG(1) UNIX System V (22 August 1997) DJPEG(1)-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 format). 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; otherwise, 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 accurate than the other two.
-dither fs
Use Floyd-Steinberg dithering in color quantization.
-dither ordered
Use ordered dithering in color quantization.
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DJPEG(1) UNIX System V (22 August 1997) DJPEG(1)-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, 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
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DJPEG(1) UNIX System V (22 August 1997) DJPEG(1)
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
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.
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