Provided by: libjpeg-turbo-progs_2.1.5-3ubuntu2_amd64 
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, 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 output. (The default
options are chosen for highest quality output.) 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 inclusive, 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 format). 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; otherwise, 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 mainstream 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 noticeable. On modern x86/x86-64 CPUs that support AVX2
instructions, 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 images that were compressed using quality levels above 85,
however, 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 images 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 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, an error will
occur.
-maxscans N
Abort if the JPEG image contains more than N scans. This feature 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 represented 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 dimensions.
-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 instigated 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, version 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 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
This file was modified by The libjpeg-turbo Project to include only information relevant
to libjpeg-turbo, to wordsmith certain sections, and to describe features not present in
libjpeg.
4 November 2020 DJPEG(1)