JPEGView/Independent JPEG Group/cjpeg.1

.TH CJPEG 1 "24 April 1994"
.SH NAME
cjpeg \- compress an image file to a JPEG file
.SH SYNOPSIS
.B cjpeg
[
.BI \-quality " N"
]
[
.B \-grayscale
]
[
.B \-optimize
]
[
.B \-targa
]
[
.BI \-maxmemory " N"
]
[
.BI \-restart " N"
]
[
.BI \-smooth " N"
]
[
.B \-verbose
]
[
.B \-debug
]
[
.B \-arithmetic
]
[
.B \-baseline
]
[
.B \-nointerleave
]
[
.BI \-qtables " file"
]
[
.BI \-qslots " N[,...]"
]
[
.BI \-sample " HxV[,...]"
]
[
.I filename
]
.LP
.SH DESCRIPTION
.LP
.B cjpeg
compresses the named image file, or the standard input if no file is
named, and produces a JPEG/JFIF file on the standard output.
The currently supported input file formats are: PPM (PBMPLUS color
format), PGM (PBMPLUS gray-scale format), GIF, Targa, and RLE (Utah Raster
Toolkit format).  (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 \-GIF
is the same as
.BR \-gif ).
British spellings are also accepted (e.g.,
.BR \-greyscale ),
though for brevity these are not mentioned below.
.PP
The basic switches are:
.TP
.BI \-quality " N"
Scale quantization tables to adjust image quality.  Quality is 0 (worst) to
100 (best); default is 75.  (See below for more info.)
.TP
.B \-grayscale
Create monochrome JPEG file from color input.  Be sure to use this switch when
compressing a grayscale GIF file, because
.B cjpeg
isn't bright enough to notice whether a GIF file uses only shades of gray.
By saying
.BR \-grayscale ,
you'll get a smaller JPEG file that takes less time to process.
.TP
.B \-optimize
Perform optimization of entropy encoding parameters.  Without this, default
encoding parameters are used.
.B \-optimize
usually makes the JPEG file a little smaller, but
.B cjpeg
runs somewhat slower and needs much more memory.  Image quality and speed of
decompression are unaffected by
.BR \-optimize .
.TP
.B \-targa
Input file is Targa format.  Targa files that contain an "identification"
field will not be automatically recognized by
.BR cjpeg ;
for such files you must specify
.B \-targa
to make
.B cjpeg
treat the input as Targa format.
.PP
The
.B \-quality
switch lets you trade off compressed file size against quality of the
reconstructed image: the higher the quality setting, the larger the JPEG file,
and the closer the output image will be to the original input.  Normally you
want to use the lowest quality setting (smallest file) that decompresses into
something visually indistinguishable from the original image.  For this
purpose the quality setting should be between 50 and 95; the default of 75 is
often about right.  If you see defects at
.B \-quality
75, then go up 5 or 10 counts at a time until you are happy with the output
image.  (The optimal setting will vary from one image to another.)
.PP
.B \-quality
100 will generate a quantization table of all 1's, eliminating loss in the
quantization step (but there is still information loss in subsampling, as well
as roundoff error).  This setting is mainly of interest for experimental
purposes.  Quality values above about 95 are
.B not
recommended for normal use; the compressed file size goes up dramatically for
hardly any gain in output image quality.
.PP
In the other direction, quality values below 50 will produce very small files
of low image quality.  Settings around 5 to 10 might be useful in preparing an
index of a large image library, for example.  Try
.B \-quality
2 (or so) for some amusing Cubist effects.  (Note: quality
values below about 25 generate 2-byte quantization tables, which are
considered optional in the JPEG standard.
.B cjpeg
emits a warning message when you give such a quality value, because some
commercial JPEG programs may be unable to decode the resulting file.  Use
.B \-baseline
if you need to ensure compatibility at low quality values.)
.PP
Switches for advanced users:
.TP
.BI \-maxmemory " N"
Set limit for amount of memory to use in processing large