Provided by: libjpeg-progs_9d-1_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, Targa, or RLE (Utah Raster Toolkit) output
format can be selected. (RLE is supported only if the URT library is available, which it isn't on most
non-Unix systems.)
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. This is provided to support applications that
don't want to cope with grayscale as a separate case.
-scale M/N
Scale the output image by a factor M/N. Currently supported scale factors are M/N with all M from
1 to 16, where N is the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
then M specifies the DCT scaled size to be applied on the given input. For baseline JPEG this is
equivalent to M/8 scaling, since the source DCT size for baseline JPEG is 8. 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.
-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).
-dct float
Use 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. Also note
that results of the floating-point method may vary slightly across machines, while the integer
methods should give the same results everywhere. The fast integer method is much less accurate
than the other two.
-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
Don't use high-quality upsampling.
-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.
-verbose
Enable debug printout. More -v's give more output. Also, version information is printed at
startup.
-debug Same as -verbose.
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
28 April 2019 DJPEG(1)