cjpeg - compress an image file to a JPEG file
cjpeg [ -quality N ] [ -grayscale ] [ -optimize ] [ -targa ]
[ -maxmemory N ] [ -restart N ] [ -smooth N ] [ -verbose ] [
-debug ] [ -arithmetic ] [ -nointerleave ] [ -qtables file ]
[ -sample HxV[,...] ] [ filename ]
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.)
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 mentioned below.
The basic switches are:
Scale quantization tables to adjust image quality.
Quality is 0 (worst) to 100 (best); default is 75.
(See below for more info.)
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.
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.
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.
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
-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 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 com-
mercial JPEG programs may be unable to decode the resulting
Switches for advanced users:
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.
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 the input image to eliminate dithering noise.
N, ranging from 1 to 100, indicates the strength of
smoothing. 0 (the default) means no smoothing.
Enable debug printout. More -v's give more output.
Also, version information is printed at startup.
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, 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:
Use arithmetic coding rather than Huffman coding. (Not
currently supported for legal reasons.)
Generate noninterleaved JPEG file (not yet supported).
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.
Set JPEG sampling factors. If you specify fewer H/V
pairs than there are components, the remaining com-
ponents 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
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
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-
Independent JPEG Group
Arithmetic coding and interleaved output not yet supported.
Not all variants of Targa file format 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 clue-
Still not as fast as we'd like.