cjpeg
compress an image file to a JPEG file
see also :
djpeg - jpegtran - rdjpgcom - wrjpgcom
Synopsis
cjpeg [
options ] [ filename ]
add an example, a script, a trick and tips
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
source
for image in *.gif
do
cjpeg $image > ${image%%gif}jpg
done
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 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,
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
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 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
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
other JPEG programs may be unable to decode the resulting
file. Use -baseline if you need to ensure
compatibility at low quality values.)
The
-quality option has been extended in this version of
cjpeg to support separate quality settings for
luminance and chrominance (or, in general, separate settings
for every quantization table slot.) The principle is the
same as chrominance subsampling: since the human eye is more
sensitive to spatial changes in brightness than spatial
changes in color, the chrominance components can be
quantized more than the luminance components without
incurring any visible image quality loss. However, unlike
subsampling, this feature reduces data in the frequency
domain instead of the spatial domain, which allows for more
fine-grained control. This option is useful in
quality-sensitive applications, for which the artifacts
generated by subsampling may be unacceptable.
The
-quality option accepts a comma-separated list of
parameters, which respectively refer to the quality levels
that should be assigned to the quantization table slots. If
there are more q-table slots than parameters, then the last
parameter is replicated. Thus, if only one quality parameter
is given, this is used for both luminance and chrominance
(slots 0 and 1, respectively), preserving the legacy
behavior of cjpeg v6b and prior. More (or customized)
quantization tables can be set with the -qtables
option and assigned to components with the -qslots
option (see the "wizard" switches below.)
JPEG files
generated with separate luminance and chrominance quality
are fully compliant with standard JPEG decoders.
CAUTION:
For this setting to be useful, be sure to pass an argument
of -sample 1x1 to cjpeg to disable chrominance
subsampling. Otherwise, the default subsampling level (2x2,
AKA "4:2:0") will be used.
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 transmitted 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.
Switches for
advanced users:
-arithmetic
Use arithmetic coding.
Caution: arithmetic coded JPEG is not yet widely
implemented, so many decoders will be unable to view an
arithmetic coded JPEG file at all.
-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 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 standard output.
-verbose
Enable debug printout. More
-v’s give more output. Also, version
information is printed at startup.
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.txt.
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 compiled, and itself is
overridden by an explicit -maxmemory.
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 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. (At present, -optimize mode is
always selected when generating progressive JPEG files.)
bugs
Support for GIF
input files was removed in cjpeg v6b due to concerns over
the Unisys LZW patent. Although this patent expired in 2006,
cjpeg still lacks GIF support, for these historical reasons.
(Conversion of GIF files to JPEG is usually a bad idea
anyway.)
Not all
variants of BMP and Targa file formats are supported.
The
-targa switch is not a bug, it’s a
feature. (It would be a bug if the Targa format designers
had not been clueless.)
see also
djpeg ,
jpegtran , rdjpgcom , wrjpgcom
ppm, pgm
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