djpeg(1) User Commands djpeg(1)NAMEdjpeg - decompress a JPEG file to an image file
SYNOPSISdjpeg [options] [filename]
DESCRIPTIONdjpeg decompresses the named JPEG file, or the standard input if no
file is named, and produces an image file on the standard output. The
following output file formats are currently supported:
· BMP
· GIF
· PGM, the PBMPLUS gray-scale format
· PPM, the PBMPLUS color format
· RLE, the Utah Raster Toolkit format
· Targa
RLE is supported only if the URT library is available.
OPTIONS
All options may be abbreviated. For example, -grayscale may be written
-gray or -gr. Upper and lower case are equivalent. For example, -BMP
is the same as -bmp. British spellings are also accepted. For example,
-greyscale.
Basic Options
The following basic options are supported:
-bmp Specify that the output file is in BMP format, Windows
flavor. 8-bit colormapped format is displayed if -col‐
ors or -grayscale is specified, or if the JPEG file is
grayscale. Otherwise, 24-bit full-color format is dis‐
played.
-colors N Reduce the image to at most N colors. This option
reduces the number of colors used in the output image,
so that the output image can be displayed on a col‐
ormapped display or stored in a colormapped file for‐
mat. For example, if you have an 8-bit display, you
must reduce to 256 colors or less.
You can also use -quantize to specify this option. How‐
ever, -colors is the recommended option name. The
-quantize option is provided only for backwards compat‐
ibility.
-fast Select the 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.
-gif Specify that the output file is in GIF format. GIF does
not support more than 256 colors, so -colors 256 is
assumed unless you specify a smaller number of colors.
-grayscale Create a monochrome image file even if the JPEG file is
a color file. This option is useful for viewing images
on monochrome displays. djpeg runs noticeably faster in
this mode.
-os2 Specify that the output file is in BMP format, OS/2 1.x
flavor. 8-bit colormapped format is displayed if -col‐
ors or -grayscale is specified, or if the JPEG file is
grayscale. Otherwise, 24-bit full-color format is dis‐
played.
-pnm Specify that the output file is in PBMPLUS format. PGM
format is displayed if -grayscale is specified, or if
the JPEG file is grayscale. Otherwise, PPM format is
displayed.
-rle Specify that the output file is in RLE format. This
option requires the URT library.
-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
useful if the image is larger than your screen. djpeg
runs much faster when scaling down the output.
-targa Specify that the output file is in Targa format.
Grayscale format is displayed if -grayscale is speci‐
fied, or if the JPEG file is grayscale. Colormapped
format is displayed if -colors is specified. Otherwise,
24-bit full-color format is displayed.
Intermediate Options
The following intermediate options are supported:
-dct fast Use the fast integer DCT method. This method is less
accurate than the integer DCT method or the floating-
point DCT method.
-dct float Use the 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. The results of the floating-point
method may vary slightly across machines, while the
integer methods should give the same results every‐
where.
-dct int Use the integer DCT method. This is the default method.
-dither fs Use Floyd-Steinberg dithering in color quantization. By
default, Floyd-Steinberg dithering is applied when
quantizing colors. This process is slow but usually
produces the best results. This option has no effect
unless color quantization is being done.
-dither none Do not use dithering in color quantization. No dither‐
ing is fast but is usually of poor quality. This option
has no effect unless color quantization is being done.
-dither ordered Use ordered dithering in color quantization. Ordered
dither is a compromise between speed and quality.
Ordered dither is only available in -onepass mode. This
option has no effect unless color quantization is being
done.
-map file Quantize to the colors used in the specified image
file. This option is useful for producing multiple
files with identical color maps, or for forcing a pre‐
defined set of colors to be used. file must be a GIF or
PPM file. This option overrides the -colors and
-onepass options.
-maxmemory N Set the limit for the amount of memory to use in pro‐
cessing large images. N is specified in thousands of
bytes, or in millions of bytes if "M" is specified with
the number. For example, -max 4m selects 4000000 bytes.
If more space is needed, temporary files are used.
-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 requires less memory,
but produces a lower-quality image. The -onepass option
is ignored unless you also specify the -colors N
option. The one-pass method is always used for
grayscale output, the two-pass method provides no
improvement for such output.
-outfile name Send the output image to the named file, instead of to
the standard output.
-verbose Display version information at startup, and enable
debug printout. The -vv option displays more verbose
output than the -v option. The -vvv option displays the
most verbose output. You can also use -debug to spec‐
ify the verbose option.
OPERANDS
The following operands are supported:
filename The name of the JPEG file to be decompressed.
EXTENDED DESCRIPTION
Hints
To get a quick preview of an image, use the -grayscale or -scale
options, or a combination of both options. For example, -grayscale
-scale 1/8 is the fastest case.
Several options trade image quality to gain speed. The -fast option
configures the recommended settings.
The -dct fast and -nosmooth options gain speed for 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 sel‐
dom tolerable in one-pass mode.
If you have very fast floating point hardware, -dct float may be even
faster than -dct fast. However, on most machines, -dct float is slower
than -dct int. In such cases, do not use -dct float, because the theo‐
retical accuracy advantage is too small to be significant in practice.
EXAMPLES
Example 1: Decompressing the JPEG File test.jpg, Quantizing to 256 Col‐
ors, and Saving the Output in 8-bit BMP Format as test.bmp
example% djpeg-colors 256 -bmp test.jpg > test.bmp
ENVIRONMENT VARIABLESdjpeg uses the following environment variables:
JPEGMEM The value of this environment variable, if set,
is the default memory limit. The value is spec‐
ified as described for the -maxmemory option.
JPEGMEM overrides the default value specified
when the program was compiled, and is in turn
overridden by an explicit -maxmemory option.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Availability │image/library/libjpeg │
├─────────────────────────────┼─────────────────────────────┤
│Interface stability │Uncommitted │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSO
Wallace, Gregory K., The JPEG Still Picture Compression Standard Commu‐
nications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)NOTES
Arithmetic coding is not supported. djpeg produces uncompressed GIF
files. These large files are readable by standard GIF decoders.
This man page was originally written by the Independent JPEG Group.
Updated by Breda McColgan, Sun Microsystems Inc., 2004.
SunOS 5.11 26 Mar 2004 djpeg(1)