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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
(now expired).
(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 (now expired), 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.
	-arithmetic	Use arithmetic coding.
	-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