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, Targa, or RLE (Utah Raster Toolkit) output format can be selected. (RLE is supported only if the URT library is available.)
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. Since GIF does not support more than
256 colors, -colors 256 is assumed (unless you specify a smaller
number of colors).
-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.
-rle Select RLE output format. (Requires URT library.)
-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 integer DCT method (default).
-dct fast
Use fast integer DCT (less accurate). In libjpeg-turbo, the
fast method is generally about 5-15% faster than the int method
when using the x86/x86-64 SIMD extensions (results may vary with
other SIMD implementations, or when using libjpeg-turbo without
SIMD extensions.) If the JPEG image was compressed using a
quality level of 85 or below, then there should be little or no
perceptible 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. The float method is mainly a
legacy feature. It 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.
-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, temporary files will be
used.
-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().)
-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.
-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. If you are fortunate enough to have very fast floating point hardware, -dct float may be even faster than -dct fast. But on most machines -dct float is slower than -dct int; in this case it is not worth using, because its theoretical accuracy advantage is too small to be significant in practice.
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.
ISSUES
Support for compressed GIF output files was removed in djpeg v6b due to
concerns over the Unisys LZW patent. Although this patent expired in
2006, djpeg still lacks compressed GIF support, for these historical
reasons. (Conversion of JPEG files to GIF is usually a bad idea
anyway, since GIF is a 256-color format.) The uncompressed GIF files
that djpeg generates are larger than they should be, but they are
readable by standard GIF decoders.
18 February 2016 DJPEG(1)
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