Index of Section 1 Manual Pages
|Interix / SUA||cjpeg.1||Interix / SUA
cjpeg - compress an image file to a JPEG file
cjpeg [ options ] [ 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), BMP, 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 -BMP is
the same as -bmp). British spellings are also accepted
(e.g., -greyscale), though for brevity these are not men-
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 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.
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.
Create progressive JPEG file (see below).
-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
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, 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
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 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 trans-
mitted 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 subse-
quent 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:
Use integer DCT method (default).
Use fast integer DCT (less accurate).
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
Emit a JPEG restart marker every N MCU rows, or
every N MCU blocks if "B" is attached to the num-
ber. -restart 0 (the default) means no restart
Smooth the input image to eliminate dithering
noise. N, ranging from 1 to 100, indicates the
strength of smoothing. 0 (the default) means no
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.
Send output image to the named file, not to stan-
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
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
Switches for wizards:
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.)
Use the quantization tables given in the specified
Select which quantization table to use for each
Set JPEG sampling factors for each color component.
Use the scan script given in the specified text
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
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
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-
Avoid running an image through a series of JPEG compres-
sion/decompression cycles. Image quality loss will accu-
mulate; 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 progres-
sive JPEG files.)
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
djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
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.
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 usually a bad idea anyway.)
Not all variants of BMP and Targa file formats are sup-
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.
20 March 1998 CJPEG(1)