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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 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 dis-
play, 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 speci-
fied, 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 spec-
ified, 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 spec-
ified; 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 col-
ors; this is slow but usually produces the best results. Ordered dither is a compromise between speed and qual-
ity; no dithering is fast but usually looks awful. Note that these switches have no effect unless color quantiza-
tion 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 identi-
cal 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
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
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.
11 October 2010 DJPEG(1)
