jpegtran.1 - third_party/libjpeg-turbo - Git at Google

 .TH JPEGTRAN 1 "18 February 2016"
 .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
 For EXIF files and JPEG files containing Exif data, you may prefer to use
 .B exiftran
 instead.
 .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.  However,
 while the image data is losslessly transformed, metadata can be removed.  See
 the
 .B \-copy
 option for specifics.
 .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).
 .PP
 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.
 .PP
 .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.
 .PP
 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.
 .TP
 .B \-perfect
 If you are only interested in perfect transformations, add the
 .B \-perfect
 switch.  This causes
 .B jpegtran
 to fail 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
 This version of \fBjpegtran\fR also offers a lossless crop option, which
 discards data outside of 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 it doesn't, then it is silently moved up and/or left to
 the nearest iMCU boundary (the lower right corner is unchanged.)  Thus, the
 output image covers at least the requested region, but it may cover more.  The
 adjustment of the region dimensions may be optionally disabled by attaching
 an 'f' character ("force") to the width or height number.

 The image can be losslessly cropped by giving the switch:
 .TP
 .B \-crop WxH+X+Y
 Crop the image to a rectangular region of width W and height H, starting at
 point X,Y.  The lossless crop feature discards data outside of 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 it
 doesn't, then it is silently moved up and/or left to the nearest iMCU boundary
 (the lower right corner is unchanged.)
 .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.)
 .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 metadata in the source file.
 .TP
 .B \-copy comments
 Copy only comment markers.  This setting copies comments from the source file
 but discards any other metadata.
 .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.  Note that this
 option will copy thumbnails as-is; they will not be transformed.
 .PP
 The default behavior is \fB-copy comments\fR.  (Note: in IJG releases v6 and
 v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.)
 .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 .
 .TP
 .B \-version
 Print version information and exit.
 .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
 .PP
 This file was modified by The libjpeg-turbo Project to include only information
 relevant to libjpeg-turbo and to wordsmith certain sections.
 .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 "18 February 2016"
	.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
	For EXIF files and JPEG files containing Exif data, you may prefer to use
	.B exiftran
	instead.
	.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. However,
	while the image data is losslessly transformed, metadata can be removed. See
	the
	.B \-copy
	option for specifics.
	.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).
	.PP
	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.
	.PP
	.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.
	.PP
	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.
	.TP
	.B \-perfect
	If you are only interested in perfect transformations, add the
	.B \-perfect
	switch. This causes
	.B jpegtran
	to fail 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
	This version of \fBjpegtran\fR also offers a lossless crop option, which
	discards data outside of 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 it doesn't, then it is silently moved up and/or left to
	the nearest iMCU boundary (the lower right corner is unchanged.) Thus, the
	output image covers at least the requested region, but it may cover more. The
	adjustment of the region dimensions may be optionally disabled by attaching
	an 'f' character ("force") to the width or height number.

	The image can be losslessly cropped by giving the switch:
	.TP
	.B \-crop WxH+X+Y
	Crop the image to a rectangular region of width W and height H, starting at
	point X,Y. The lossless crop feature discards data outside of 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 it
	doesn't, then it is silently moved up and/or left to the nearest iMCU boundary
	(the lower right corner is unchanged.)
	.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.)
	.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 metadata in the source file.
	.TP
	.B \-copy comments
	Copy only comment markers. This setting copies comments from the source file
	but discards any other metadata.
	.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. Note that this
	option will copy thumbnails as-is; they will not be transformed.
	.PP
	The default behavior is \fB-copy comments\fR. (Note: in IJG releases v6 and
	v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.)
	.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 .
	.TP
	.B \-version
	Print version information and exit.
	.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
	.PP
	This file was modified by The libjpeg-turbo Project to include only information
	relevant to libjpeg-turbo and to wordsmith certain sections.
	.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.