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NAME
cjpeg - compress an image file to a JPEG file
SYNOPSIS
cjpeg [ options ] [ filename ]
DESCRIPTION
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, Targa, and RLE (Utah Raster Toolkit format).
(RLE is supported only if the URT library is available.)
OPTIONS
All switch names may be abbreviated; for example, -grayscale
may be written -gray or -gr. Most of the "basic" switches
can be abbreviated to as little as one letter. Upper and
lower case are equivalent (thus -BMP is the same as -bmp).
British spellings are also accepted (e.g., -greyscale),
though for brevity these are not mentioned below.
The basic switches are:
-quality N
Scale quantization tables to adjust image quality.
Quality is 0 (worst) to 100 (best); default is 75.
(See below for more info.)
-grayscale
Create monochrome JPEG file from color input. Be sure
to use this switch when compressing a grayscale BMP
file, because cjpeg isn't bright enough to notice
whether a BMP file uses only shades of gray. By saying
-grayscale, you'll get a smaller JPEG file that takes
less time to process.
-optimize
Perform optimization of entropy encoding parameters.
Without this, default encoding parameters are used.
-optimize usually makes the JPEG file a little smaller,
but cjpeg runs somewhat slower and needs much more
memory. Image quality and speed of decompression are
unaffected by -optimize.
-progressive
Create progressive JPEG file (see below).
-targa
Input file is Targa format. Targa files that contain
an "identification" field will not be automatically
recognized by cjpeg; for such files you must specify
-targa to make cjpeg treat the input as Targa format.
For most Targa files, you won't need this switch.
The -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 set-
ting should be between 50 and 95; the default of 75 is often
about right. If you see defects at -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.)
-quality 100 will generate a quantization table of all 1's,
minimizing 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 pur-
poses. Quality values above about 95 are not recommended
for normal use; the compressed file size goes up dramati-
cally 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 -quality 2 (or so) for some amus-
ing Cubist effects. (Note: quality values below about 25
generate 2-byte quantization tables, which are considered
optional in the JPEG standard. cjpeg emits a warning mes-
sage when you give such a quality value, because some other
JPEG programs may be unable to decode the resulting file.
Use -baseline if you need to ensure compatibility at low
quality values.)
The -progressive switch creates a "progressive JPEG" file.
In this type of JPEG file, the data is stored in multiple
scans of increasing quality. If the file is being transmit-
ted over a slow communications link, the decoder can use the
first scan to display a low-quality image very quickly, and
can then improve the display with each subsequent scan. The
final image is exactly equivalent to a standard JPEG file of
the same quality setting, and the total file size is about
the same --- often a little smaller. Caution: progressive
JPEG is not yet widely implemented, so many decoders will be
unable to view a progressive JPEG file at all.
Switches for advanced users:
-dct int
Use integer DCT method (default).
-dct fast
Use fast integer DCT (less accurate).
-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.
-restart N
Emit a JPEG restart marker every N MCU rows, or every N
MCU blocks if "B" is attached to the number. -restart
0 (the default) means no restart markers.
-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.
-maxmemory N
Set limit for amount of memory to use in processing
large images. Value is in thousands of bytes, or mil-
lions of bytes if "M" is attached to the number. For
example, -max 4m selects 4000000 bytes. If more space
is needed, temporary files will be used.
-outfile name
Send output image to the named file, not to standard
output.
-verbose
Enable debug printout. More -v's give more output.
Also, version information is printed at startup.
-debug
Same as -verbose.
The -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 -restart 1 for images that will be
transmitted across unreliable networks such as Usenet.
The -smooth option filters the input to eliminate fine-scale
noise. This is often useful when converting dithered images
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:
-baseline
Force baseline-compatible quantization tables to be
generated. This clamps quantization values to 8 bits
even at low quality settings. (This switch is poorly
named, since it does not ensure that the output is
actually baseline JPEG. For example, you can use
-baseline and -progressive together.)
-qtables file
Use the quantization tables given in the specified text
file.
-qslots N[,...]
Select which quantization table to use for each color
component.
-sample HxV[,...]
Set JPEG sampling factors for each color component.
-scans file
Use the scan script given in the specified text file.
The "wizard" switches are intended for experimentation with
JPEG. If you don't know what you are doing, don't use them.
These switches are documented further in the file
wizard.doc.
EXAMPLES
This example compresses the PPM file foo.ppm with a quality
factor of 60 and saves the output as foo.jpg:
cjpeg -quality 60 foo.ppm > foo.jpg
HINTS
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 cjpeg's -quality
and -smooth options to get a satisfactory conversion.
-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 notice-
ably 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.
The -optimize option to 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. (At present, -optimize
mode is always selected when generating progressive JPEG
files.)
ENVIRONMENT
JPEGMEM
If this environment variable is set, its value is the
default memory limit. The value is specified as
described for the -maxmemory switch. JPEGMEM overrides
the default value specified when the program was com-
piled, and itself is overridden by an explicit -max-
memory.
SEE ALSO
djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
ppm(5), pgm(5)
Wallace, Gregory K. "The JPEG Still Picture Compression
Standard", Communications of the ACM, April 1991 (vol. 34,
no. 4), pp. 30-44.
AUTHOR
Independent JPEG Group
BUGS
Arithmetic coding is not supported for legal reasons.
GIF input files are no longer supported, to avoid the Unisys
LZW patent. Use a Unisys-licensed program if you need to
read a GIF file. (Conversion of GIF files to JPEG is usu-
ally a bad idea anyway.)
Not all variants of BMP and Targa file formats are sup-
ported.
The -targa switch is not a bug, it's a feature. (It would
be a bug if the Targa format designers had not been clue-
less.)
Still not as fast as we'd like.
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