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.TH DJPEG 1 "22 August 1997"

.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 gray-scale output even if JPEG file is color.  Useful for viewing on

monochrome displays; also,

.B djpeg

runs noticeably faster in this mode.

.TP

.BI \-scale " M/N"

Scale the output image by a factor M/N.  Currently the scale factor must be

1/1, 1/2, 1/4, or 1/8.  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 gray-scale; 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 gray-scale; 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 gray-scale 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.  Gray-scale format is emitted if the JPEG file is

gray-scale 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 integer DCT method (default).

.TP

.B \-dct fast

Use fast integer DCT (less accurate).

.TP

.B \-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.

.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 \-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 gray-scale 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, temporary files will be used.

.TP

.BI \-outfile " name"

Send output image to the named file, not to standard output.

.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 .

.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.

.PP

If you are fortunate enough to have very fast floating point hardware,

\fB\-dct float\fR may be even faster than \fB\-dct fast\fR.  But on most

machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is

not worth using, because its theoretical accuracy advantage is too small to be

significant in practice.

.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

.SH BUGS

Arithmetic coding is not supported for legal reasons.

.PP

To avoid the Unisys LZW patent,

.B djpeg

produces uncompressed GIF files.  These are larger than they should be, but

are readable by standard GIF decoders.

.PP

Still not as fast as we'd like.