libjpeg (9.5.0)
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VersionsDJPEG(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. PBMPLUS (PPM/PGM), BMP, GIF, Targa, or RLE (Utah Raster Toolkit) output format can be se‐
lected. (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 men‐
tioned 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 spec‐
ifies 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 col‐
ors 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 quan‐
tizing 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 mil‐
lions 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 set‐
tings.
-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 accept‐
able 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 de‐
scribed 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)
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 se‐
lected. (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 men‐
tioned 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 spec‐
ifies 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 col‐
ors 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 quan‐
tizing 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 mil‐
lions 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 set‐
tings.
-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 accept‐
able 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 de‐
scribed 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)