www.pudn.com > jpegsr6b.zip > djpeg.1

.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.