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cjpeg.man
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CJPEG(1) CJPEG(1)
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, GIF, 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 -GIF is
the same as -gif). British spellings are also accepted
(e.g., -greyscale), though for brevity these are not men
tioned 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 GIF file, because cjpeg isn't bright
enough to notice whether a GIF 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 parame
ters. 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.
-targa Input file is Targa format. Targa files that con
tain an "identification" field will not be automat
ically 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.
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CJPEG(1) CJPEG(1)
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 pur
pose the quality setting 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, 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
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 pro
duce 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 amusing 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 message when you give such a quality value,
because some commercial JPEG programs may be unable to
decode the resulting file. Use -baseline if you need to
ensure compatibility at low quality values.)
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 floating-point
method is the most accurate, but will be the slow
est 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
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CJPEG(1) CJPEG(1)
every N MCU blocks if "B" is attached to the num
ber. -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
millions 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 stan
dard 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 GIF
files to JPEG: a moderate smoothing factor of 10 to 50
gets rid of dithering patterns in the input file, result
ing in a smaller JPEG file and a better-looking image.
Too large a smoothing factor will visibly blur the image,
however.
Switches for wizards:
-arithmetic
Use arithmetic coding rather than Huffman coding.
(Not currently supported for legal reasons.)
-baseline
Force a baseline JPEG file to be generated. This
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CJPEG(1) CJPEG(1)
clamps quantization values to 8 bits even at low
quality settings.
-nointerleave
Generate noninterleaved JPEG file (not yet sup
ported).
-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 -quality
N
is also
specified, the values in the file are scaled
according to cjpeg's quality scaling curve.
-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.
-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 -sample 2x2.
The "wizard" switches are intended for experimentation
with JPEG. If you don't know what you are doing, don't
use them. 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.
EXAMPLES
This example compresses the PPM file foo.ppm with a qual
ity 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 dis
tinct colors. GIF works great on these, JPEG does not.
If you want to convert a GIF to JPEG, you should experi
ment with cjpeg's -quality and -smooth options to get a
satisfactory conversion. -smooth 10 or so is often help
ful.
Avoid running an image through a series of JPEG
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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.
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.
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 pro
gram was compiled, and itself is overridden by an
explicit -maxmemory.
SEE ALSO
djpeg(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.
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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