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User Commands 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. The following output file formats are
currently supported:
o BMP
o GIF
o PGM, the PBMPLUS gray-scale format
o PPM, the PBMPLUS color format
o RLE, the Utah Raster Toolkit format
o 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 for-
mat, Windows flavor. 8-bit colormapped for-
mat is displayed if -colors or -grayscale is
specified, or if the JPEG file is grayscale.
Otherwise, 24-bit full-color format is
displayed.
-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 colormapped display or
stored in a colormapped file format. For
example, if you have an 8-bit display, you
must reduce to 256 colors or less.
SunOS 5.10 Last change: 26 Mar 2004 1
User Commands djpeg(1)
You can also use -quantize to specify this
option. However, -colors is the recommended
option name. The -quantize option is pro-
vided only for backwards compatibility.
-fast Select the recommended processing options
for fast, low-quality output. The default
options are chosen for highest quality out-
put. Currently, this is equivalent to -dct
fast -nosmooth -onepass -dither ordered.
-gif Specify that the output file is in GIF for-
mat. 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 for-
mat, OS/2 1.x flavor. 8-bit colormapped for-
mat is displayed if -colors or -grayscale is
specified, or if the JPEG file is grayscale.
Otherwise, 24-bit full-color format is
displayed.
-pnm Specify that the output file is in PBMPLUS
format. PGM format is displayed if -grays-
cale is specified, or if the JPEG file is
grayscale. Otherwise, PPM format is
displayed.
-rle Specify that the output file is in RLE for-
mat. This option requires the URT library.
SunOS 5.10 Last change: 26 Mar 2004 2
User Commands djpeg(1)
-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 out-
put.
-targa Specify that the output file is in Targa
format. Grayscale format is displayed if
-grayscale is specified, or if the JPEG file
is grayscale. Colormapped format is
displayed if -colors is specified. Other-
wise, 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 everywhere.
-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.
SunOS 5.10 Last change: 26 Mar 2004 3
User Commands djpeg(1)
-dither none Do not use dithering in color quantization.
No dithering 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 avail-
able 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 pro-
ducing multiple files with identical color
maps, or for forcing a predefined 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 processing large images. N is speci-
fied 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 rou-
tine.
-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.
SunOS 5.10 Last change: 26 Mar 2004 4
User Commands djpeg(1)
-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
specify 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 exam-
ple, -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 qual-
ity than the default behavior. -dither none may give accept-
able results in two-pass mode, but is seldom 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 theoretical accuracy
advantage is too small to be significant in practice.
EXAMPLES
Example 1: Decompressing the JPEG File test.jpg, Quantizing
to 256 Colors, and Saving the Output in 8-bit BMP Format as
test.bmp
example% djpeg -colors 256 -bmp test.jpg > test.bmp
SunOS 5.10 Last change: 26 Mar 2004 5
User Commands djpeg(1)
ENVIRONMENT VARIABLES
djpeg uses the following environment variables:
JPEGMEM The value of this environment vari-
able, if set, is the default memory
limit. The value is specified as
described for the -maxmemory option.
JPEGMEM overrides the default value
specified when the program was com-
piled, and is in turn overridden by
an explicit -maxmemory option.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Availability | SUNWjpg |
|_____________________________|_____________________________|
| Interface stability | External |
|_____________________________|_____________________________|
SEE ALSO
Wallace, Gregory K., The JPEG Still Picture Compression
Standard Communications 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 5.10 Last change: 26 Mar 2004 6
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