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	-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
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.