.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.