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.TH CJPEG 1 "30 August 1994"
cjpeg \- compress an image file to a JPEG file
.B cjpeg
.I options
.I filename
.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), BMP, GIF, Targa, and RLE (Utah Raster
Toolkit format). (RLE is supported only if the URT library is available.)
All switch names may be abbreviated; for example,
.B \-grayscale
may be written
.B \-gray
.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.
The basic switches are:
.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.)
.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.
.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 .
.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.
For most Targa files, you won't need this switch.
.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.)
.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.
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.)
Switches for advanced users:
.B \-dct int
Use integer DCT method (default).
.B \-dct fast
Use fast integer DCT (less accurate).
.B \-dct float
Use floating-point DCT method.
The floating-point method is the most accurate, but will be the slowest 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.
.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.
.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.
.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.
.BI \-outfile " name"
Send output image to the named file, not to standard output.
.B \-verbose
Enable debug printout. More
.BR \-v 's
give more output. Also, version information is printed at startup.
.B \-debug
Same as
.BR \-verbose .
.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.
.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.
Switches for wizards:
.B \-arithmetic
Use arithmetic coding rather than Huffman coding. (Not currently
supported for legal reasons.)
.B \-baseline
Force a baseline JPEG file to be generated. This clamps quantization values
to 8 bits even at low quality settings.
.B \-nointerleave
Generate noninterleaved JPEG file (not yet supported).
.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
.BI \-quality " N"
is also specified, the values in the file are scaled according to
.BR cjpeg 's
quality scaling curve.
.BI \-qslots " N[,...]"
Select which quantization table to use for each color component. By default,
table 0 is used for luminance and table 1 for chrominance components.
.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.
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.
This example compresses the PPM file foo.ppm with a quality factor of
60 and saves the output as foo.jpg:
.B cjpeg \-quality
.I 60 foo.ppm
.B >
.I foo.jpg
Color GIF files are not the ideal input for JPEG; JPEG is really intended for
compressing full-color (24-bit) images. In particular, don't try to convert
cartoons, line drawings, and other images that have only a few distinct
colors. GIF works great on these, JPEG does not. If you want to convert a
GIF to JPEG, you should experiment with
.BR cjpeg 's
.B \-quality
.B \-smooth
options to get a satisfactory conversion.
.B \-smooth 10
or so is often helpful.
Avoid running an image through a series of JPEG compression/decompression
cycles. Image quality loss will accumulate; after ten or so cycles the image
may be noticeably worse than it was after one cycle. It's best to use a
lossless format while manipulating an image, then convert to JPEG format when
you are ready to file the image away.
.B \-optimize
option to
.B cjpeg
is worth using when you are making a "final" version for posting or archiving.
It's also a win when you are using low quality settings to make very small
JPEG files; the percentage improvement is often a lot more than it is on
larger files.
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
.B \-maxmemory
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
.BR \-maxmemory .
.BR djpeg (1),
.BR rdjpgcom (1),
.BR wrjpgcom (1)
.BR ppm (5),
.BR pgm (5)
Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
Independent JPEG Group
Arithmetic coding is not supported for legal reasons.
Not all variants of BMP and Targa file formats are supported.
.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.)
Still not as fast as we'd like.