usage.doc

game engine, which is useful for everyone who is interested in it. I hope you can enjoy it.

			between speed and quality; no dithering is fast but
			usually looks awful.  Note that these switches have
			no effect unless color quantization is being done.
			Ordered dither is only available in -onepass mode.

	-map FILE	Quantize to the colors used in the specified image
			file.  This is useful for producing multiple files
			with identical color maps, or for forcing a predefined
			set of colors to be used.  The FILE must be a GIF
			or PPM file.  This option overrides -colors and
			-onepass.

	-nosmooth	Use a faster, lower-quality upsampling routine.

	-onepass	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.  -onepass is
			ignored unless you also say -colors N.  Also,
			the one-pass method is always used for gray-scale
			output (the two-pass method is no improvement then).

	-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.

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


HINTS FOR CJPEG

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 distinct
colors.  GIF works great on these, JPEG does not.  If you want to convert a
GIF to JPEG, you should experiment with cjpeg's -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.  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 progressive JPEG files.)

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.)


HINTS FOR DJPEG

To get a quick preview of an image, use the -grayscale and/or -scale switches.
"-grayscale -scale 1/8" is the fastest case.

Several options are available that trade off image quality to gain speed.
"-fast" turns on the recommended settings.

"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
When producing a color-quantized image, "-onepass -dither ordered" is fast but
much lower quality than the default behavior.  "-dither none" may give
acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.

If you are fortunate enough to have very fast floating point hardware,
"-dct float" may be even faster than "-dct fast".  But on most machines
"-dct float" is slower than "-dct int"; in this case it is not worth using,
because its theoretical accuracy advantage is too small to be significant
in practice.

Two-pass color quantization requires a good deal of memory; on MS-DOS machines
it may run out of memory even with -maxmemory 0.  In that case you can still
decompress, with some loss of image quality, by specifying -onepass for
one-pass quantization.

To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files.  These
are larger than they should be, but are readable by standard GIF decoders.


HINTS FOR BOTH PROGRAMS

If more space is needed than will fit in the available main memory (as
determined by -maxmemory), temporary files will be used.  (MS-DOS versions
will try to get extended or expanded memory first.)  The temporary files are
often rather large: in typical cases they occupy three bytes per pixel, for
example 3*800*600 = 1.44Mb for an 800x600 image.  If you don't have enough
free disk space, leave out -progressive and -optimize (for cjpeg) or specify
-onepass (for djpeg).

On MS-DOS, the temporary files are created in the directory named by the TMP
or TEMP environment variable, or in the current directory if neither of those
exist.  Amiga implementations put the temp files in the directory named by
JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
space.

The default memory usage limit (-maxmemory) is set when the software is
compiled.  If you get an "insufficient memory" error, try specifying a smaller
-maxmemory value, even -maxmemory 0 to use the absolute minimum space.  You
may want to recompile with a smaller default value if this happens often.

On machines that have "environment" variables, you can define the environment
variable JPEGMEM to set the default memory limit.  The value is specified as
described for the -maxmemory switch.  JPEGMEM overrides the default value
specified when the program was compiled, and itself is overridden by an
explicit -maxmemory switch.

On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
use.  (Extended or expanded memory is also used if available.)  Most
DOS-specific versions of this software do their own memory space estimation
and do not need you to specify -maxmemory.


JPEGTRAN

jpegtran performs various useful transformations of JPEG files.
It can translate the coded representation from one variant of JPEG to another,
for example from baseline JPEG to progressive JPEG or vice versa.  It can also
perform some rearrangements of the image data, for example turning an image
from landscape to portrait format by rotation.

jpegtran works by rearranging the compressed data (DCT coefficients), without
ever fully decoding the image.  Therefore, its transformations are lossless:
there is no image degradation at all, which would not be true if you used
djpeg followed by cjpeg to accomplish the same conversion.  But by the same
token, jpegtran cannot perform lossy operations such as changing the image
quality.

jpegtran uses a command line syntax similar to cjpeg or djpeg.
On Unix-like systems, you say:
	jpegtran [switches] [inputfile] >outputfile
On most non-Unix systems, you say:
	jpegtran [switches] inputfile outputfile
where both the input and output files are JPEG files.

To specify the coded JPEG representation used in the output file,
jpegtran accepts a subset of the switches recognized by cjpeg:
	-optimize	Perform optimization of entropy encoding parameters.
	-progressive	Create progressive JPEG file.
	-restart N	Emit a JPEG restart marker every N MCU rows, or every
			N MCU blocks if "B" is attached to the number.
	-scans file	Use the scan script given in the specified text file.
See the previous discussion of cjpeg for more details about these switches.
If you specify none of these switches, you get a plain baseline-JPEG output
file.  The quality setting and so forth are determined by the input file.

The image can be losslessly transformed by giving one of these switches:
	-flip horizontal	Mirror image horizontally (left-right).
	-flip vertical		Mirror image vertically (top-bottom).
	-rotate 90		Rotate image 90 degrees clockwise.
	-rotate 180		Rotate image 180 degrees.
	-rotate 270		Rotate image 270 degrees clockwise (or 90 ccw).
	-transpose		Transpose image (across UL-to-LR axis).
	-transverse		Transverse transpose (across UR-to-LL axis).

The transpose transformation has no restrictions regarding image dimensions.
The other transformations operate rather oddly if the image dimensions are not
a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
transform complete blocks of DCT coefficient data in the desired way.

jpegtran's default behavior when transforming an odd-size image is designed
to preserve exact reversibility and mathematical consistency of the
transformation set.  As stated, transpose is able to flip the entire image
area.  Horizontal mirroring leaves any partial iMCU column at the right edge
untouched, but is able to flip all rows of the image.  Similarly, vertical
mirroring leaves any partial iMCU row at the bottom edge untouched, but is
able to flip all columns.  The other transforms can be built up as sequences
of transpose and flip operations; for consistency, their actions on edge
pixels are defined to be the same as the end result of the corresponding
transpose-and-flip sequence.

For practical use, you may prefer to discard any untransformable edge pixels
rather than having a strange-looking strip along the right and/or bottom edges
of a transformed image.  To do this, add the -trim switch:
	-trim		Drop non-transformable edge blocks.
Obviously, a transformation with -trim is not reversible, so strictly speaking
jpegtran with this switch is not lossless.  Also, the expected mathematical
equivalences between the transformations no longer hold.  For example,
"-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
"-rot 180 -trim" trims both edges.

Another not-strictly-lossless transformation switch is:
	-grayscale	Force grayscale output.
This option discards the chrominance channels if the input image is YCbCr
(ie, a standard color JPEG), resulting in a grayscale JPEG file.  The
luminance channel is preserved exactly, so this is a better method of reducing
to grayscale than decompression, conversion, and recompression.  This switch
is particularly handy for fixing a monochrome picture that was mistakenly
encoded as a color JPEG.  (In such a case, the space savings from getting rid
of the near-empty chroma channels won't be large; but the decoding time for
a grayscale JPEG is substantially less than that for a color JPEG.)

jpegtran also recognizes these switches that control what to do with "extra"
markers, such as comment blocks:
	-copy none	Copy no extra markers from source file.  This setting
			suppresses all comments and other excess baggage
			present in the source file.
	-copy comments	Copy only comment markers.  This setting copies
			comments from the source file, but discards
			any other inessential data. 
	-copy all	Copy all extra markers.  This setting preserves
			miscellaneous markers found in the source file, such
			as JFIF thumbnails and Photoshop settings.  In some
			files these extra markers can be sizable.
The default behavior is -copy comments.  (Note: in IJG releases v6 and v6a,
jpegtran always did the equivalent of -copy none.)

Additional switches recognized by jpegtran are:
	-outfile filename
	-maxmemory N
	-verbose
	-debug
These work the same as in cjpeg or djpeg.


THE COMMENT UTILITIES

The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
Although the standard doesn't actually define what COM blocks are for, they
are widely used to hold user-supplied text strings.  This lets you add
annotations, titles, index terms, etc to your JPEG files, and later retrieve
them as text.  COM blocks do not interfere with the image stored in the JPEG
file.  The maximum size of a COM block is 64K, but you can have as many of
them as you like in one JPEG file.

We provide two utility programs to display COM block contents and add COM
blocks to a JPEG file.

rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
standard output.  The command line syntax is
	rdjpgcom [-verbose] [inputfilename]
The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
image dimensions.  If you omit the input file name from the command line,
the JPEG file is read from standard input.  (This may not work on some
operating systems, if binary data can't be read from stdin.)

wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
Ordinarily, the COM block is added after any existing COM blocks, but you
can delete the old COM blocks if you wish.  wrjpgcom produces a new JPEG
file; it does not modify the input file.  DO NOT try to overwrite the input
file by directing wrjpgcom's output back into it; on most systems this will
just destroy your file.

The command line syntax for wrjpgcom is similar to cjpeg's.  On Unix-like
systems, it is
	wrjpgcom [switches] [inputfilename]
The output file is written to standard output.  The input file comes from
the named file, or from standard input if no input file is named.

On most non-Unix systems, the syntax is
	wrjpgcom [switches] inputfilename outputfilename
where both input and output file names must be given explicitly.

wrjpgcom understands three switches:
	-replace		 Delete any existing COM blocks from the file.
	-comment "Comment text"	 Supply new COM text on command line.
	-cfile name		 Read text for new COM block from named file.
(Switch names can be abbreviated.)  If you have only one line of comment text
to add, you can provide it on the command line with -comment.  The comment
text must be surrounded with quotes so that it is treated as a single
argument.  Longer comments can be read from a text file.

If you give neither -comment nor -cfile, then wrjpgcom will read the comment
text from standard input.  (In this case an input image file name MUST be
supplied, so that the source JPEG file comes from somewhere else.)  You can
enter multiple lines, up to 64KB worth.  Type an end-of-file indicator
(usually control-D or control-Z) to terminate the comment text entry.

wrjpgcom will not add a COM block if the provided comment string is empty.
Therefore -replace -comment "" can be used to delete all COM blocks from a
file.

These utility programs do not depend on the IJG JPEG library.  In
particular, the source code for rdjpgcom is intended as an illustration of
the minimum amount of code required to parse a JPEG file header correctly.