DJPEG(1) General Commands Manual 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. PBM-
PLUS (PPM/PGM), BMP, GIF, or Targa output format can be selected.
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 out-
put. (The default options are chosen for highest quality out-
put.) Currently, this is equivalent to -dct fast -nosmooth
-onepass -dither ordered.
-grayscale
Force grayscale output even if JPEG file is color. Useful for
viewing on monochrome displays; also, djpeg runs noticeably
faster in this mode.
-rgb Force RGB output even if JPEG file is grayscale.
-scale M/N
Scale the output image by a factor M/N. Currently the scale
factor must be M/8, where M is an integer between 1 and 16 in-
clusive, or any reduced fraction thereof (such as 1/2, 3/4,
etc.) 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 grayscale; otherwise, 24-bit full-color format
is emitted.
-gif Select GIF output format (LZW-compressed). Since GIF does not
support more than 256 colors, -colors 256 is assumed (unless you
specify a smaller number of colors). If you specify -fast, the
default number of colors is 216.
-gif0 Select GIF output format (uncompressed). Since GIF does not
support more than 256 colors, -colors 256 is assumed (unless you
specify a smaller number of colors). If you specify -fast, the
default number of colors is 216.
-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 grayscale; otherwise, 24-bit full-color format
is emitted.
-pnm Select PBMPLUS (PPM/PGM) output format (this is the default for-
mat). PGM is emitted if the JPEG file is grayscale or if
-grayscale is specified; otherwise PPM is emitted.
-targa Select Targa output format. Grayscale format is emitted if the
JPEG file is grayscale or if -grayscale is specified; otherwise,
colormapped format is emitted if -colors is specified; other-
wise, 24-bit full-color format is emitted.
Switches for advanced users:
-dct int
Use accurate integer DCT method (default).
-dct fast
Use less accurate integer DCT method [legacy feature]. When the
Independent JPEG Group's software was first released in 1991,
the decompression time for a 1-megapixel JPEG image on a main-
stream PC was measured in minutes. Thus, the fast integer DCT
algorithm provided noticeable performance benefits. On modern
CPUs running libjpeg-turbo, however, the decompression time for
a 1-megapixel JPEG image is measured in milliseconds, and thus
the performance benefits of the fast algorithm are much less no-
ticeable. On modern x86/x86-64 CPUs that support AVX2 instruc-
tions, the fast and int methods have similar performance. On
other types of CPUs, the fast method is generally about 5-15%
faster than the int method.
If the JPEG image was compressed using a quality level of 85 or
below, then there should be little or no perceptible quality
difference between the two algorithms. When decompressing im-
ages that were compressed using quality levels above 85, how-
ever, the difference between the fast and int methods becomes
more pronounced. With images compressed using quality=97, for
instance, the fast method incurs generally about a 4-6 dB loss
in PSNR relative to the int method, but this can be larger for
some images. If you can avoid it, do not use the fast method
when decompressing images that were compressed using quality
levels above 97. The algorithm often degenerates for such im-
ages and can actually produce a more lossy output image than if
the JPEG image had been compressed using lower quality levels.
-dct float
Use floating-point DCT method [legacy feature]. The float
method does not produce significantly more accurate results than
the int method, and it is much slower. The float method may
also give different results on different machines due to varying
roundoff behavior, whereas the integer methods should give the
same results on all machines.
-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.
-icc file
Extract ICC color management profile to the specified file.
-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
grayscale output (the two-pass method is no improvement then).
-maxmemory N
Set limit for amount of memory to use in processing large im-
ages. 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, an error will occur.
-maxscans N
Abort if the JPEG image contains more than N scans. This fea-
ture demonstrates a method by which applications can guard
against denial-of-service attacks instigated by specially-
crafted malformed JPEG images containing numerous scans with
missing image data or image data consisting only of "EOB runs"
(a feature of progressive JPEG images that allows potentially
hundreds of thousands of adjoining zero-value pixels to be rep-
resented using only a few bytes.) Attempting to decompress such
malformed JPEG images can cause excessive CPU activity, since
the decompressor must fully process each scan (even if the scan
is corrupt) before it can proceed to the next scan.
-outfile name
Send output image to the named file, not to standard output.
-memsrc
Load input file into memory before decompressing. This feature
was implemented mainly as a way of testing the in-memory source
manager (jpeg_mem_src().)
-report
Report decompression progress.
-skip Y0,Y1
Decompress all rows of the JPEG image except those between Y0
and Y1 (inclusive.) Note that if decompression scaling is being
used, then Y0 and Y1 are relative to the scaled image dimen-
sions.
-crop WxH+X+Y
Decompress only a rectangular subregion of the image, starting
at point X,Y with width W and height H. If necessary, X will be
shifted left to the nearest iMCU boundary, and the width will be
increased accordingly. Note that if decompression scaling is
being used, then X, Y, W, and H are relative to the scaled image
dimensions. Currently this option only works with the PBMPLUS
(PPM/PGM), GIF, and Targa output formats.
-strict
Treat all warnings as fatal. This feature also demonstrates a
method by which applications can guard against attacks insti-
gated by specially-crafted malformed JPEG images. Enabling this
option will cause the decompressor to abort if the JPEG image
contains incomplete or corrupt image data.
-verbose
Enable debug printout. More -v's give more output. Also, ver-
sion information is printed at startup.
-debug Same as -verbose.
-version
Print version information and exit.
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.
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 speci-
fied 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
This file was modified by The libjpeg-turbo Project to include only in-
formation relevant to libjpeg-turbo, to wordsmith certain sections, and
to describe features not present in libjpeg.
4 November 2020 DJPEG(1)
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