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USAGE
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1992-11-16
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USAGE instructions for the Independent JPEG Group's JPEG software
=================================================================
INTRODUCTION
This distribution contains software to implement JPEG image compression and
decompression. JPEG (pronounced "jay-peg") is a standardized compression
method for full-color and gray-scale images. JPEG is designed to handle
"real-world" scenes, for example scanned photographs. Cartoons, line
drawings, and other non-realistic images are not JPEG's strong suit; on this
sort of material you may get poor image quality and/or little compression.
JPEG is lossy, meaning that the output image is not necessarily identical to
the input image. Hence you should not use JPEG if you have to have identical
output bits. However, on typical real-world images, very good compression
levels can be obtained with no visible change, and amazingly high compression
is possible if you can tolerate a low-quality image. You can trade off image
quality against file size by adjusting the compressor's "quality" setting.
This file describes usage of the standard programs "cjpeg" and "djpeg" that
can be built directly from the distributed C code. See the README file for
hints on incorporating the JPEG software into other programs.
If you are on a Unix machine you may prefer to read the Unix-style manual
pages in files cjpeg.1 and djpeg.1. But also see the HINTS section below,
which is not present in either manual page.
NOTE: the switch syntax has been redesigned since the v3 release of
cjpeg/djpeg. Switch names are now words instead of single letters.
GENERAL USAGE
We provide two programs, cjpeg to compress an image file into JPEG format,
and djpeg to decompress a JPEG file back into a conventional image format.
On Unix-like systems, you say:
cjpeg [switches] [imagefile] >jpegfile
or
djpeg [switches] [jpegfile] >imagefile
The programs read the specified input file, or standard input if none is
named. They always write to standard output (with trace/error messages to
standard error). These conventions are handy for piping images between
programs.
On most non-Unix systems, you say:
cjpeg [switches] imagefile jpegfile
or
djpeg [switches] jpegfile imagefile
i.e., both the input and output files are named on the command line. This
style is a little more foolproof, and it loses no functionality if you don't
have pipes. (You can get this style on Unix too, if you prefer, by defining
TWO_FILE_COMMANDLINE when you compile the programs; see SETUP.)
The currently supported image 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.)
cjpeg recognizes the input image format automatically, with the exception
of some Targa-format files. You have to tell djpeg which format to generate.
The only JPEG file format currently supported is the JFIF format. Support for
the TIFF 6.0 JPEG format will probably be added at some future date.
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 (-GIF is the same as -gif).
British spellings are also accepted (e.g., -greyscale), though for brevity
these are not mentioned below.
CJPEG DETAILS
The basic command line switches for cjpeg 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 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.
-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.
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, 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 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 commercial JPEG programs may be unable to decode the resulting file.)
Switches for advanced users:
-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.
-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.
-verbose Enable debug printout. More -v's give more printout.
or -debug 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 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:
-arithmetic Use arithmetic coding rather than Huffman coding.
(Not currently supported for legal reasons.)
-nointerleave Generate noninterleaved JPEG file (not yet supported).
-qtables file Use the quantization tables given in the specified
file. The file should contain one to four tables