USAGE instructions for the Independent JPEG Group's JPEG software
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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 MS-DOS distribution DOES NOT use the same user interface as the regular
Unix-like programs.  If you've used cjpeg/djpeg on other machines, please read
carefully.  NOTE: at some point we will probably redesign the user interface,
so the command line switches described here will change.


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.

The basic command line is:
	cjpeg [switches] list of image files
or
	djpeg [switches] list of jpeg files

For example, to convert xxx.jpg to xxx.gif and yyy.jpg to yyy.gif, just say:
	djpeg xxx yyy

The usual DOS wildcards (* and ?) can be used in the file lists.  If you omit
any extension from an input file name, .GIF or .JPG is assumed, respectively.
The output file name is the same as the input file name, except for extension.
cjpeg writes to .JPG; djpeg writes to .GIF, .PPM, or .TGA depending on which
output format you've selected.  If a selected output file name already exists,
you'll be asked whether or not to overwrite it.

The currently supported image file formats are: GIF, Targa, PPM (PBMPLUS color
format), and PGM (PBMPLUS gray-scale format).
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/JPEG format will probably be added at some future date.


CJPEG DETAILS

The command line switches for cjpeg are:

	-Q quality	Scale quantization tables to adjust image quality.
			Quality is 0 (worst) to 100 (best); default is 75.
			(See below for more info.)

	-o		Perform optimization of entropy encoding parameters.
			Without this, default encoding parameters are used.
			-o 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 -o.

	-T		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
			-T to force cjpeg to treat the input as Targa format.

	-d		Enable debug printout.  More -d's give more printout.
			Also, version information is printed at startup.

The -Q switch lets you trade off compressed file size against quality of the
reconstructed image: the higher the -Q 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 -Q setting (smallest file) that decompresses into something
visually indistinguishable from the original image.  For this purpose the -Q
setting should be between 50 and 95; the default of 75 is often about right.
If you see defects at -Q 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.)

-Q 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.  -Q 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, -Q 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 -Q 2 (or so) for some
amusing Cubist effects.  (Note: -Q 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 -Q value, because some
commercial JPEG programs may be unable to decode the resulting file.)


DJPEG DETAILS

The command line switches for djpeg are:

	-G		Select GIF output format (implies -q, with default
			of 256 colors).  This is the default output format.

	-P		Select PPM or PGM output format.
			PGM is emitted if the JPEG file is gray-scale or if -g
			is specified.

	-T		Select Targa output format.  Gray-scale format is
			emitted if the JPEG file is gray-scale or if -g is
			specified; otherwise, colormapped format is emitted
			if -q is specified; otherwise, 24-bit full-color
			format is emitted.

	-g		Force gray-scale output even if input is color.

	-q N		Quantize to N colors.  This reduces the number of
			colors in the output image so that it can be displayed
			on a colormapped display or stored in a colormapped
			file format.  For example, if you have an 8-bit
			display, you'd need to quantize to 256 or fewer colors.

	-D		Do not use dithering in color quantization.
			By default, Floyd-Steinberg dithering is applied when
			quantizing colors, but on some images dithering may
			result in objectionable "graininess".  If that
			happens, you can turn off dithering with -D.
			-D is ignored unless you also say -q or -G.

	-1		Use one-pass instead of two-pass color quantization.
			The one-pass method is faster and needs less memory,
			but it produces a lower-quality image.
			-1 is ignored unless you also say -q or -G.  Also,
			the one-pass method is always used for gray-scale
			output (the two-pass method is no improvement then).

	-b		Perform cross-block smoothing.  This is quite
			memory-intensive and only seems to improve the image
			at very low quality settings (-Q 10 to 20 or so).
			At normal -Q settings it may make the image worse.

	-d		Enable debug printout.  More -d's give more printout.
			Also, version information is printed at startup.


HINTS

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 -o 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 -Q settings
to make very small JPEG files; the percentage improvement is often a lot more
than it is on larger files.

djpeg with two-pass color quantization requires a good deal of space; it may
run out of memory on large images.  If you don't want to enlarge the virtual
memory region (as described below), you can still decompress, with some loss
of image quality, by specifying -1 for one-pass quantization.


VIRTUAL MEMORY NOTES

These programs use the virtual-memory scheme provided by Intel's Code Builder
C compiler.  The programs' working space consists of whatever extended memory
is available, plus space in a temporary "swap file".  The total working space
is called the "region".  If the programs run out of space when processing a
large image, you can increase the region size as explained below.  Conversely,
if you need to make the swap file smaller, you can decrease the region size.

As distributed, the programs have a region size of 4 MB.  Thus, if you have
2 MB of free extended memory, the programs will create a 2 MB swap file upon
startup.  If there isn't enough free disk space to create the swap file,
you'll get an error message like this:
	DOS Extender: Error X0130:  Failure detected during program load
If this happens, you must free up some disk space or reduce the region size
in order to run the program.  Another possibility is to put the swap file on a
different disk drive that has more free space.  The default swap file is
	C:\XMSWAP.TMP
You can select a different swap file name/location by setting the SWAP
environment variable, for example "set SWAP=D:\XMSWAP.TMP".

If you get an error message that looks like this:
	Insufficient memory (case N)
then you need to increase the region size in order to process your image.
(Another possibility is to use a processing mode that doesn't need as much
workspace.  For cjpeg, omit -o if you were using it; for djpeg, specify -1.)

The region size for each program is encoded in the program's .EXE file.
To change the region size, run the provided MODXCONF program, for example
	modxconf cjpeg.exe
Follow the on-screen directions to change the region size.  (The stored region
size is ignored if it is less than the available extended memory; in that case
the programs always use all available extended memory.)

It is possible to pre-allocate a swap file.  This makes cjpeg and djpeg start
up faster, since they don't have to create and delete the swap file.  However,
the swap file then takes up space even when you are not using the programs.
If you want to create a pre-allocated swap file, run modxconf and follow its
directions, or use any other method to create a file of the right name and
size.  Note: when a pre-allocated swap file is present, the region size is
determined by the available extended memory plus the swap file size; the
region size stored in the .EXE file is ignored.

If you are running the programs in a DOS window under Windows 3.x (in 386
enhanced mode), the programs do not allocate a swap file.  Instead, the amount
of extended memory indicated by the region size is requested from Windows, and
any swapping needed is handled by Windows using its swap file.
