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authorJörg Frings-Fürst <debian@jff-webhosting.net>2015-11-06 07:14:47 +0100
committerJörg Frings-Fürst <debian@jff-webhosting.net>2015-11-06 07:14:47 +0100
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-.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.