libjpeg-turbo/cjpeg.1

.TH CJPEG 1 "4 November 1992"
.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 \-nointerleave
]
[
.BI \-qtables " file"
]
[
.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.)
.PP
Switches for advanced users:
.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 \-restart " N"
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
attached to the number.
.B \-restart 0
(the default) means no restart markers.
.TP
.BI \-smooth " N"
Smooth the input image to eliminate dithering noise.  N, ranging from 1 to
100, indicates the strength of smoothing.  0 (the default) means no smoothing.
.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 .
.PP
The
.B \-restart
option inserts extra markers that allow a JPEG decoder to resynchronize after
a transmission error.  Without restart markers, any damage to a compressed
file will usually ruin the image from the point of the error to the end of the
image; with restart markers, the damage is usually confined to the portion of
the image up to the next restart marker.  Of course, the restart markers
occupy extra space.  We recommend
.B \-restart 1
for images that will be transmitted across unreliable networks such as Usenet.
.PP
The
.B \-smooth
option filters the input to eliminate fine-scale noise.  This is often useful
when converting GIF files to JPEG: a moderate smoothing factor of 10 to 50
gets rid of dithering patterns in the input file, resulting in a smaller JPEG
file and a better-looking image.  Too large a smoothing factor will visibly
blur the image, however.
.PP
Switches for wizards:
.TP
.B \-arithmetic
Use arithmetic coding rather than Huffman coding.  (Not currently
supported for legal reasons.)
.TP
.B \-nointerleave
Generate noninterleaved JPEG file (not yet supported).
.TP
.BI \-qtables " file"
Use the quantization tables given in the specified file.  The file should
contain one to four tables (64 values each) as plain text.  Comments preceded
by '#' may be included in the file.  The tables are implicitly numbered
0,1,etc.  If
.B \-quality
N is also specified, the values in the file are scaled according to
.BR cjpeg 's
quality scaling curve.
.TP
.BI \-sample " HxV[,...]"
Set JPEG sampling factors.  If you specify fewer H/V pairs than there are
components, the remaining components are set to 1x1 sampling.  The default
setting is equivalent to \fB\-sample 2x2\fR.
.PP
The "wizard" switches are intended for experimentation with JPEG.  If you
don't know what you are doing, \fBdon't use them\fR.  You can easily produce
files with worse image quality and/or poorer compression than you'll get from
the default settings.  Furthermore, these switches should not be used when
making files intended for general use, because not all JPEG implementations
will support unusual JPEG parameter settings.
.SH EXAMPLES
.LP
This example compresses the PPM file foo.ppm with a quality factor of
60 and saves the output as foo.jpg:
.IP
.B cjpeg \-quality
.I 60 foo.ppm
.B >
.I foo.jpg
.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 djpeg (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 and interleaved output not yet supported.
.PP
Not all variants of Targa file format are supported.
.PP
The
.B -targa
switch is not a bug, it's a feature.  (It would be a bug if the Targa format
designers had not been clueless.)
.PP
Still not as fast as we'd like.