Alien-FreeImage

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src/Source/LibJPEG/usage.txt  view on Meta::CPAN

			JPEG file at all.

	-rgb1		Create RGB JPEG file with reversible color transform.
			Works like the -rgb switch (see above) and inserts a
			simple reversible color transform into the processing
			which significantly improves the compression.
			Use this switch in combination with the -block N
			switch (see above) for lossless JPEG coding.
			CAUTION: A decoder with inverse color transform
			support is required for this feature.  Reversible
			color transform support is not yet widely implemented,
			so many decoders will be unable to view a reversible
			color transformed JPEG file at all.

	-bgycc		Create big gamut YCC JPEG file.
			In this type of encoding the color difference
			components are quantized further by a factor of 2
			compared to the normal Cb/Cr values, thus creating
			space to allow larger color values with higher
			saturation than the normal gamut limits to be encoded.
			In order to compensate for the loss of color fidelity
			compared to a normal YCC encoded file, the color
			quantization tables can be adjusted accordingly.
			For example, cjpeg -bgycc -quality 80,90 will give
			similar results as cjpeg -quality 80.
			CAUTION: For correct decompression a decoder with big
			gamut YCC support (JFIF version 2) is required.
			An old decoder may or may not display a big gamut YCC
			encoded JPEG file, depending on JFIF version check
			and corresponding warning/error configuration.
			In case of a granted decompression the old decoder
			will display the image with half saturated colors.

	-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.

	-nosmooth	Don't use high-quality downsampling.

	-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.

	-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 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:

	-baseline	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
			-baseline and -progressive together.)

	-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.

The "wizard" switches are intended for experimentation with JPEG.  If you
don't know what you are doing, DON'T USE THEM.  These switches are documented
further in the file wizard.txt.


DJPEG DETAILS

The basic command line switches for djpeg are:

	-colors N	Reduce image to at most N colors.  This reduces the
	or -quantize N	number of colors used 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 reduce to 256 or fewer
			colors.  (-colors is the recommended name, -quantize
			is provided only for backwards compatibility.)

	-fast		Select recommended processing options for fast, low
			quality output.  (The default options are chosen for
			highest quality output.)  Currently, this is equivalent
			to "-dct fast -nosmooth -onepass -dither ordered".

	-grayscale	Force gray-scale output even if JPEG file is color.
			Useful for viewing on monochrome displays; also,
			djpeg runs noticeably faster in this mode.

src/Source/LibJPEG/usage.txt  view on Meta::CPAN

	-pnm		Select PBMPLUS (PPM/PGM) output format (this is the
			default format).  PGM is emitted if the JPEG file is
			gray-scale or if -grayscale is specified; otherwise
			PPM is emitted.

	-rle		Select RLE output format.  (Requires URT library.)

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

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.

	-dither fs	Use Floyd-Steinberg dithering in color quantization.
	-dither ordered	Use ordered dithering in color quantization.
	-dither none	Do not use dithering in color quantization.
			By default, Floyd-Steinberg dithering is applied when
			quantizing colors; this is slow but usually produces
			the best results.  Ordered dither is a compromise
			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	Don't use high-quality upsampling.

	-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
(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

src/Source/LibJPEG/usage.txt  view on Meta::CPAN

We also offer a lossless-crop option, which discards data outside a given
image region but losslessly preserves what is inside.  Like the rotate and
flip transforms, lossless crop is restricted by the current JPEG format: the
upper left corner of the selected region must fall on an iMCU boundary.  If
this does not hold for the given crop parameters, we silently move the upper
left corner up and/or left to make it so, simultaneously increasing the
region dimensions to keep the lower right crop corner unchanged.  (Thus, the
output image covers at least the requested region, but may cover more.)
The adjustment of the region dimensions may be optionally disabled.

The image can be losslessly cropped by giving the switch:
	-crop WxH+X+Y	Crop to a rectangular subarea of width W, height H
			starting at point X,Y.

A complementary lossless-wipe option is provided to discard (gray out) data
inside a given image region while losslessly preserving what is outside:
	-wipe WxH+X+Y	Wipe (gray out) a rectangular subarea of
			width W, height H starting at point X,Y.

Other not-strictly-lossless transformation switches are:

	-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.)

	-scale M/N	Scale the output image by a factor M/N.
Currently supported scale factors are M/N with all M from 1 to 16, where N is
the source DCT size, which is 8 for baseline JPEG.  If the /N part is omitted,
then M specifies the DCT scaled size to be applied on the given input.  For
baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
for baseline JPEG is 8.  CAUTION: An implementation of the JPEG SmartScale
extension is required for this feature.  SmartScale enabled JPEG is not yet
widely implemented, so many decoders will be unable to view a SmartScale
extended JPEG file at all.

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 (for image display) data.
	-copy all	Copy all extra markers.  This setting preserves
			miscellaneous markers found in the source file, such
			as JFIF thumbnails, Exif data, 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 [-raw] [-verbose] [inputfilename]
The switch "-raw" (or just "-r") causes rdjpgcom to also output non-printable
characters in comments, which are normally escaped for security reasons.
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