.TH DJPEG 1 "4 November 2020"
.SH NAME
djpeg \- decompress a JPEG file to an image file
.SH SYNOPSIS
.B djpeg
[
.I options
]
[
.I filename
]
.LP
.SH DESCRIPTION
.LP
.B 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.)
.SH OPTIONS
All switch names may be abbreviated; for example,
.B \-grayscale
may be written
.B \-gray
or
.BR \-gr .
Most of the "basic" switches can be abbreviated to as little as one letter.
Upper and lower case are equivalent (thus
.B \-BMP
is the same as
.BR \-bmp ).
British spellings are also accepted (e.g.,
.BR \-greyscale ),
though for brevity these are not mentioned below.
.PP
The basic switches are:
.TP
.BI \-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.
.TP
.BI \-quantize " N"
Same as
.BR \-colors .
.B \-colors
is the recommended name,
.B \-quantize
is provided only for backwards compatibility.
.TP
.B \-fast
Select recommended processing options for fast, low quality output.  (The
default options are chosen for highest quality output.)  Currently, this is
equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
.TP
.B \-grayscale
Force grayscale output even if JPEG file is color.  Useful for viewing on
monochrome displays; also,
.B djpeg
runs noticeably faster in this mode.
.TP
.B \-rgb
Force RGB output even if JPEG file is grayscale.
.TP
.BI \-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,
.B djpeg
runs much faster when scaling down the output.
.TP
.B \-bmp
Select BMP output format (Windows flavor).  8-bit colormapped format is
emitted if
.B \-colors
or
.B \-grayscale
is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
format is emitted.
.TP
.B \-gif
Select GIF output format.  Since GIF does not support more than 256 colors,
.B \-colors 256
is assumed (unless you specify a smaller number of colors).
.TP
.B \-os2
Select BMP output format (OS/2 1.x flavor).  8-bit colormapped format is
emitted if
.B \-colors
or
.B \-grayscale
is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
format is emitted.
.TP
.B \-pnm
Select PBMPLUS (PPM/PGM) output format (this is the default format).
PGM is emitted if the JPEG file is grayscale or if
.B \-grayscale
is specified; otherwise PPM is emitted.
.TP
.B \-rle
Select RLE output format.  (Requires URT library.)
.TP
.B \-targa
Select Targa output format.  Grayscale format is emitted if the JPEG file is
grayscale or if
.B \-grayscale
is specified; otherwise, colormapped format is emitted if
.B \-colors
is specified; otherwise, 24-bit full-color format is emitted.
.PP
Switches for advanced users:
.TP
.B \-dct int
Use accurate integer DCT method (default).
.TP
.B \-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 \fBfast\fR 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 \fBfast\fR algorithm are much less
noticeable.  On modern x86/x86-64 CPUs that support AVX2 instructions, the
\fBfast\fR and \fBint\fR methods have similar performance.  On other types of
CPUs, the \fBfast\fR method is generally about 5-15% faster than the \fBint\fR
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 \fBfast\fR and \fBint\fR
methods becomes more pronounced.  With images compressed using quality=97, for
instance, the \fBfast\fR method incurs generally about a 4-6 dB loss in PSNR
relative to the \fBint\fR method, but this can be larger for some images.  If
you can avoid it, do not use the \fBfast\fR 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.
.TP
.B \-dct float
Use floating-point DCT method [legacy feature].
The \fBfloat\fR method does not produce significantly more accurate results
than the \fBint\fR method, and it is much slower.  The \fBfloat\fR 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.
.TP
.B \-dither fs
Use Floyd-Steinberg dithering in color quantization.
.TP
.B \-dither ordered
Use ordered dithering in color quantization.
.TP
.B \-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
.B \-onepass
mode.
.TP
.BI \-icc " file"
Extract ICC color management profile to the specified file.
.TP
.BI \-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
.I file
must be a GIF or PPM file. This option overrides
.B \-colors
and
.BR \-onepass .
.TP
.B \-nosmooth
Use a faster, lower-quality upsampling routine.
.TP
.B \-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.
.B \-onepass
is ignored unless you also say
.B \-colors
.IR N .
Also, the one-pass method is always used for grayscale output (the two-pass
method is no improvement then).
.TP
.BI \-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,
.B \-max 4m
selects 4000000 bytes.  If more space is needed, an error will occur.
.TP
.BI \-outfile " name"
Send output image to the named file, not to standard output.
.TP
.BI \-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().)
.TP
.BI \-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.
.TP
.BI \-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.
.TP
.B \-verbose
Enable debug printout.  More
.BR \-v 's
give more output.  Also, version information is printed at startup.
.TP
.B \-debug
Same as
.BR \-verbose .
.TP
.B \-version
Print version information and exit.
.SH EXAMPLES
.LP
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:
.IP
.B djpeg \-colors 256 \-bmp
.I foo.jpg
.B >
.I foo.bmp
.SH HINTS
To get a quick preview of an image, use the
.B \-grayscale
and/or
.B \-scale
switches.
.B \-grayscale \-scale 1/8
is the fastest case.
.PP
Several options are available that trade off image quality to gain speed.
.B \-fast
turns on the recommended settings.
.PP
.B \-dct fast
and/or
.B \-nosmooth
gain speed at a small sacrifice in quality.
When producing a color-quantized image,
.B \-onepass \-dither ordered
is fast but much lower quality than the default behavior.
.B \-dither none
may give acceptable results in two-pass mode, but is seldom tolerable in
one-pass mode.
.SH ENVIRONMENT
.TP
.B JPEGMEM
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
.B \-maxmemory
switch.
.B JPEGMEM
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
.BR \-maxmemory .
.SH SEE ALSO
.BR cjpeg (1),
.BR jpegtran (1),
.BR rdjpgcom (1),
.BR wrjpgcom (1)
.br
.BR ppm (5),
.BR pgm (5)
.br
Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
.SH AUTHOR
Independent JPEG Group
.PP
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.
.SH 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.