DJPEG(1) 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.
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 [legacy feature]. This
reduces the number of colors used in the output image so that
it can be stored in a colormapped file format. This feature
cannot be used when decompressing lossless JPEG images.

-quantize N
Same as -colors. -colors is the recommended name. -quantize
is provided only for backward compatibility.

-fast Select recommended processing options for low-quality output
[legacy feature]. (The default options are chosen for
highest-quality output.) Currently, this is equivalent to
-dct fast -nosmooth -onepass -dither ordered. On modern CPUs,
these settings have little or no performance benefit and are
retained solely for backward compatibility.

-grayscale
Force grayscale output even if JPEG file is full-color. This
feature cannot be used when decompressing full-color lossless
JPEG images.

-rgb Force RGB output even if JPEG file is grayscale. This feature
cannot be used when decompressing grayscale lossless JPEG
images.

-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. This feature cannot be used when decompressing
lossless JPEG images.

-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. This format can only be used when
decompressing 8-bit-per-sample JPEG images.

-gif Select GIF output format (LZW-compressed) [legacy feature].
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.
This format can only be used when decompressing 8-bit-per-
sample or 12-bit-per-sample lossy JPEG images.

-gif0 Select GIF output format (uncompressed) [legacy feature].
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.
This format can only be used when decompressing 8-bit-per-
sample or 12-bit-per-sample lossy JPEG images.

-os2 Select BMP output format (OS/2 1.x flavor) [legacy feature].
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. This format can only be
used when decompressing 8-bit-per-sample JPEG images.

-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 [legacy feature]. 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.
This format can only be used when decompressing 8-bit-per-
sample JPEG images.

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 when quantizing colors [legacy
feature].

-dither ordered
Use ordered dithering when quantizing colors [legacy feature].

-dither none
Do not use dithering when quantizing colors [legacy feature].
By default, Floyd-Steinberg dithering is applied when
quantizing colors. This is slower but usually produces the
best results. Ordered dithering is a compromise between speed
and quality. No dithering is faster but usually looks awful.
Note that these switches have no effect unless color
quantization is being done. Ordered dithering 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
[legacy feature]. 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 [legacy
feature]. The one-pass method needs less memory, but it
produces a lower-quality image. -onepass is ignored unless
you also specify -colors N. Also, the one-pass method is
always used for grayscale output. (The two-pass method has no
improvement in that case.)

-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 and saves the output
in 8-bit BMP format in foo.bmp:

djpeg -bmp foo.jpg > foo.bmp

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.

17 June 2024 DJPEG(1)