jpegtran.1 - platform/external/libjpeg-turbo - Gitiles

 .TH JPEGTRAN 1 "28 December 2009"
 .SH NAME
 jpegtran \- lossless transformation of JPEG files
 .SH SYNOPSIS
 .B jpegtran
 [
 .I options
 ]
 [
 .I filename
 ]
 .LP
 .SH DESCRIPTION
 .LP
 .B 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.
 .PP
 .B 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
 .B djpeg
 followed by
 .B cjpeg
 to accomplish the same conversion.  But by the same token,
 .B jpegtran
 cannot perform lossy operations such as changing the image quality.
 .PP
 .B jpegtran
 reads the named JPEG/JFIF file, or the standard input if no file is
 named, and produces a JPEG/JFIF file on the standard output.
 .SH OPTIONS
 All switch names may be abbreviated; for example,
 .B \-optimize
 may be written
 .B \-opt
 or
 .BR \-o .
 Upper and lower case are equivalent.
 British spellings are also accepted (e.g.,
 .BR \-optimise ),
 though for brevity these are not mentioned below.
 .PP
 To specify the coded JPEG representation used in the output file,
 .B jpegtran
 accepts a subset of the switches recognized by
 .BR cjpeg :
 .TP
 .B \-optimize
 Perform optimization of entropy encoding parameters.
 .TP
 .B \-progressive
 Create progressive JPEG file.
 .TP
 .BI \-restart " N"
 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
 attached to the number.
 .TP
 .B \-arithmetic
 Use arithmetic coding.
 .TP
 .BI \-scans " file"
 Use the scan script given in the specified text file.
 .PP
 See
 .BR cjpeg (1)
 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.
 .PP
 The image can be losslessly transformed by giving one of these switches:
 .TP
 .B \-flip horizontal
 Mirror image horizontally (left-right).
 .TP
 .B \-flip vertical
 Mirror image vertically (top-bottom).
 .TP
 .B \-rotate 90
 Rotate image 90 degrees clockwise.
 .TP
 .B \-rotate 180
 Rotate image 180 degrees.
 .TP
 .B \-rotate 270
 Rotate image 270 degrees clockwise (or 90 ccw).
 .TP
 .B \-transpose
 Transpose image (across UL-to-LR axis).
 .TP
 .B \-transverse
 Transverse transpose (across UR-to-LL axis).
 .IP
 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.
 .IP
 .BR 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.
 .IP
 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
 .B \-trim
 switch:
 .TP
 .B \-trim
 Drop non-transformable edge blocks.
 .IP
 Obviously, a transformation with
 .B \-trim
 is not reversible, so strictly speaking
 .B jpegtran
 with this switch is not lossless.  Also, the expected mathematical
 equivalences between the transformations no longer hold.  For example,
 .B \-rot 270 -trim
 trims only the bottom edge, but
 .B \-rot 90 -trim
 followed by
 .B \-rot 180 -trim
 trims both edges.
 .IP
 If you are only interested in perfect transformation, add the
 .B \-perfect
 switch:
 .TP
 .B \-perfect
 Fails with an error if the transformation is not perfect.
 .IP
 For example you may want to do
 .IP
 .B (jpegtran \-rot 90 -perfect
 .I foo.jpg
 .B || djpeg
 .I foo.jpg
 .B | pnmflip \-r90 | cjpeg)
 .IP
 to do a perfect rotation if available or an approximated one if not.
 .PP
 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 image can be losslessly cropped by giving the switch:
 .TP
 .B \-crop WxH+X+Y
 Crop to a rectangular subarea of width W, height H starting at point X,Y.
 .PP
 Other not-strictly-lossless transformation switches are:
 .TP
 .B \-grayscale
 Force grayscale output.
 .IP
 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.)
 .TP
 .BI \-scale " M/N"
 Scale the output image by a factor M/N.
 .IP
 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.
 .B 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.
 .PP
 .B jpegtran
 also recognizes these switches that control what to do with "extra" markers,
 such as comment blocks:
 .TP
 .B \-copy none
 Copy no extra markers from source file.  This setting suppresses all
 comments and other excess baggage present in the source file.
 .TP
 .B \-copy comments
 Copy only comment markers.  This setting copies comments from the source file,
 but discards any other inessential (for image display) data.
 .TP
 .B \-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.
 .IP
 The default behavior is
 .BR "\-copy comments" .
 (Note: in IJG releases v6 and v6a,
 .B jpegtran
 always did the equivalent of
 .BR "\-copy none" .)
 .PP
 Additional switches recognized by jpegtran are:
 .TP
 .BI \-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,
 .B \-max 4m
 selects 4000000 bytes.  If more space is needed, temporary files will be used.
 .TP
 .BI \-outfile " name"
 Send output image to the named file, not to standard output.
 .TP
 .B \-verbose
 Enable debug printout.  More
 .BR \-v 's
 give more output.  Also, version information is printed at startup.
 .TP
 .B \-debug
 Same as
 .BR \-verbose .
 .SH EXAMPLES
 .LP
 This example converts a baseline JPEG file to progressive form:
 .IP
 .B jpegtran \-progressive
 .I foo.jpg
 .B >
 .I fooprog.jpg
 .PP
 This example rotates an image 90 degrees clockwise, discarding any
 unrotatable edge pixels:
 .IP
 .B jpegtran \-rot 90 -trim
 .I foo.jpg
 .B >
 .I foo90.jpg
 .SH ENVIRONMENT
 .TP
 .B JPEGMEM
 If this environment variable is set, its value is the default memory limit.
 The value is specified as described for the
 .B \-maxmemory
 switch.
 .B JPEGMEM
 overrides the default value specified when the program was compiled, and
 itself is overridden by an explicit
 .BR \-maxmemory .
 .SH SEE ALSO
 .BR cjpeg (1),
 .BR djpeg (1),
 .BR rdjpgcom (1),
 .BR wrjpgcom (1)
 .br
 Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
 .SH AUTHOR
 Independent JPEG Group
 .SH BUGS
 The transform options can't transform odd-size images perfectly.  Use
 .B \-trim
 or
 .B \-perfect
 if you don't like the results.
 .PP
 The entire image is read into memory and then written out again, even in
 cases where this isn't really necessary.  Expect swapping on large images,
 especially when using the more complex transform options.
	.TH JPEGTRAN 1 "28 December 2009"
	.SH NAME
	jpegtran \- lossless transformation of JPEG files
	.SH SYNOPSIS
	.B jpegtran
	[
	.I options
	]
	[
	.I filename
	]
	.LP
	.SH DESCRIPTION
	.LP
	.B 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.
	.PP
	.B 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
	.B djpeg
	followed by
	.B cjpeg
	to accomplish the same conversion. But by the same token,
	.B jpegtran
	cannot perform lossy operations such as changing the image quality.
	.PP
	.B jpegtran
	reads the named JPEG/JFIF file, or the standard input if no file is
	named, and produces a JPEG/JFIF file on the standard output.
	.SH OPTIONS
	All switch names may be abbreviated; for example,
	.B \-optimize
	may be written
	.B \-opt
	or
	.BR \-o .
	Upper and lower case are equivalent.
	British spellings are also accepted (e.g.,
	.BR \-optimise ),
	though for brevity these are not mentioned below.
	.PP
	To specify the coded JPEG representation used in the output file,
	.B jpegtran
	accepts a subset of the switches recognized by
	.BR cjpeg :
	.TP
	.B \-optimize
	Perform optimization of entropy encoding parameters.
	.TP
	.B \-progressive
	Create progressive JPEG file.
	.TP
	.BI \-restart " N"
	Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
	attached to the number.
	.TP
	.B \-arithmetic
	Use arithmetic coding.
	.TP
	.BI \-scans " file"
	Use the scan script given in the specified text file.
	.PP
	See
	.BR cjpeg (1)
	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.
	.PP
	The image can be losslessly transformed by giving one of these switches:
	.TP
	.B \-flip horizontal
	Mirror image horizontally (left-right).
	.TP
	.B \-flip vertical
	Mirror image vertically (top-bottom).
	.TP
	.B \-rotate 90
	Rotate image 90 degrees clockwise.
	.TP
	.B \-rotate 180
	Rotate image 180 degrees.
	.TP
	.B \-rotate 270
	Rotate image 270 degrees clockwise (or 90 ccw).
	.TP
	.B \-transpose
	Transpose image (across UL-to-LR axis).
	.TP
	.B \-transverse
	Transverse transpose (across UR-to-LL axis).
	.IP
	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.
	.IP
	.BR 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.
	.IP
	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
	.B \-trim
	switch:
	.TP
	.B \-trim
	Drop non-transformable edge blocks.
	.IP
	Obviously, a transformation with
	.B \-trim
	is not reversible, so strictly speaking
	.B jpegtran
	with this switch is not lossless. Also, the expected mathematical
	equivalences between the transformations no longer hold. For example,
	.B \-rot 270 -trim
	trims only the bottom edge, but
	.B \-rot 90 -trim
	followed by
	.B \-rot 180 -trim
	trims both edges.
	.IP
	If you are only interested in perfect transformation, add the
	.B \-perfect
	switch:
	.TP
	.B \-perfect
	Fails with an error if the transformation is not perfect.
	.IP
	For example you may want to do
	.IP
	.B (jpegtran \-rot 90 -perfect
	.I foo.jpg
	.B \|\| djpeg
	.I foo.jpg
	.B \| pnmflip \-r90 \| cjpeg)
	.IP
	to do a perfect rotation if available or an approximated one if not.
	.PP
	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 image can be losslessly cropped by giving the switch:
	.TP
	.B \-crop WxH+X+Y
	Crop to a rectangular subarea of width W, height H starting at point X,Y.
	.PP
	Other not-strictly-lossless transformation switches are:
	.TP
	.B \-grayscale
	Force grayscale output.
	.IP
	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.)
	.TP
	.BI \-scale " M/N"
	Scale the output image by a factor M/N.
	.IP
	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.
	.B 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.
	.PP
	.B jpegtran
	also recognizes these switches that control what to do with "extra" markers,
	such as comment blocks:
	.TP
	.B \-copy none
	Copy no extra markers from source file. This setting suppresses all
	comments and other excess baggage present in the source file.
	.TP
	.B \-copy comments
	Copy only comment markers. This setting copies comments from the source file,
	but discards any other inessential (for image display) data.
	.TP
	.B \-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.
	.IP
	The default behavior is
	.BR "\-copy comments" .
	(Note: in IJG releases v6 and v6a,
	.B jpegtran
	always did the equivalent of
	.BR "\-copy none" .)
	.PP
	Additional switches recognized by jpegtran are:
	.TP
	.BI \-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,
	.B \-max 4m
	selects 4000000 bytes. If more space is needed, temporary files will be used.
	.TP
	.BI \-outfile " name"
	Send output image to the named file, not to standard output.
	.TP
	.B \-verbose
	Enable debug printout. More
	.BR \-v 's
	give more output. Also, version information is printed at startup.
	.TP
	.B \-debug
	Same as
	.BR \-verbose .
	.SH EXAMPLES
	.LP
	This example converts a baseline JPEG file to progressive form:
	.IP
	.B jpegtran \-progressive
	.I foo.jpg
	.B >
	.I fooprog.jpg
	.PP
	This example rotates an image 90 degrees clockwise, discarding any
	unrotatable edge pixels:
	.IP
	.B jpegtran \-rot 90 -trim
	.I foo.jpg
	.B >
	.I foo90.jpg
	.SH ENVIRONMENT
	.TP
	.B JPEGMEM
	If this environment variable is set, its value is the default memory limit.
	The value is specified as described for the
	.B \-maxmemory
	switch.
	.B JPEGMEM
	overrides the default value specified when the program was compiled, and
	itself is overridden by an explicit
	.BR \-maxmemory .
	.SH SEE ALSO
	.BR cjpeg (1),
	.BR djpeg (1),
	.BR rdjpgcom (1),
	.BR wrjpgcom (1)
	.br
	Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
	Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
	.SH AUTHOR
	Independent JPEG Group
	.SH BUGS
	The transform options can't transform odd-size images perfectly. Use
	.B \-trim
	or
	.B \-perfect
	if you don't like the results.
	.PP
	The entire image is read into memory and then written out again, even in
	cases where this isn't really necessary. Expect swapping on large images,
	especially when using the more complex transform options.