Table of Contents 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 -GIF is
the same as -gif). 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.
- -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.
- -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 onepass 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,
automatically quantizes to 256 colors, and saves the output in GIF format in foo.gif:
djpeg -gif foo.jpg > foo.gif
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
onepass 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.
Still not as fast as we'd like.
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