cjpeg - Unix, Linux Command
cjpeg - compress an image file to a JPEG file
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
-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:
The "wizard" switches are intended for experimentation with JPEG. If you
dont know what you are doing, dont use them. These switches are
documented further in the file wizard.doc.
-quality N |
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 isnt bright enough to notice whether a BMP file uses only shades of gray.
-grayscale, youll 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
Create progressive JPEG file (see below).
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 wont need this switch.
-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 1s, 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.)
-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.
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 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.
Enable debug printout. More
-vs give more output. Also, version information is printed at startup.
-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.
-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:
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
-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.
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 |
Color GIF files are not the ideal input for JPEG; JPEG is really intended for
compressing full-color (24-bit) images. In particular, dont 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
cjpegs -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. Its best to use a
lossless format while manipulating an image, then convert to JPEG format when
you are ready to file the image away.
-optimize option to
cjpeg is worth using when you are making a "final" version for posting or archiving.
Its 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.)
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
JPEGMEM overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
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 supported.
-targa switch is not a bug, its a feature. (It would be a bug if the Targa format
designers had not been clueless.)
Still not as fast as wed like.