286 lines
9.0 KiB
Groff
286 lines
9.0 KiB
Groff
.TH JPEGTRAN 1 "28 December 2009"
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.SH NAME
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jpegtran \- lossless transformation of JPEG files
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.SH SYNOPSIS
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.B jpegtran
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[
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.I options
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]
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[
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.I filename
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]
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.LP
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.SH DESCRIPTION
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.LP
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.B jpegtran
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performs various useful transformations of JPEG files.
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It can translate the coded representation from one variant of JPEG to another,
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for example from baseline JPEG to progressive JPEG or vice versa. It can also
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perform some rearrangements of the image data, for example turning an image
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from landscape to portrait format by rotation.
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.PP
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.B jpegtran
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works by rearranging the compressed data (DCT coefficients), without
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ever fully decoding the image. Therefore, its transformations are lossless:
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there is no image degradation at all, which would not be true if you used
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.B djpeg
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followed by
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.B cjpeg
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to accomplish the same conversion. But by the same token,
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.B jpegtran
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cannot perform lossy operations such as changing the image quality.
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.PP
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.B jpegtran
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reads the named JPEG/JFIF file, or the standard input if no file is
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named, and produces a JPEG/JFIF file on the standard output.
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.SH OPTIONS
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All switch names may be abbreviated; for example,
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.B \-optimize
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may be written
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.B \-opt
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or
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.BR \-o .
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Upper and lower case are equivalent.
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British spellings are also accepted (e.g.,
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.BR \-optimise ),
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though for brevity these are not mentioned below.
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.PP
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To specify the coded JPEG representation used in the output file,
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.B jpegtran
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accepts a subset of the switches recognized by
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.BR cjpeg :
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.TP
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.B \-optimize
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Perform optimization of entropy encoding parameters.
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.TP
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.B \-progressive
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Create progressive JPEG file.
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.TP
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.BI \-restart " N"
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Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
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attached to the number.
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.TP
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.B \-arithmetic
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Use arithmetic coding.
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.TP
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.BI \-scans " file"
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Use the scan script given in the specified text file.
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.PP
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See
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.BR cjpeg (1)
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for more details about these switches.
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If you specify none of these switches, you get a plain baseline-JPEG output
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file. The quality setting and so forth are determined by the input file.
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.PP
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The image can be losslessly transformed by giving one of these switches:
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.TP
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.B \-flip horizontal
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Mirror image horizontally (left-right).
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.TP
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.B \-flip vertical
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Mirror image vertically (top-bottom).
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.TP
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.B \-rotate 90
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Rotate image 90 degrees clockwise.
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.TP
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.B \-rotate 180
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Rotate image 180 degrees.
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.TP
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.B \-rotate 270
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Rotate image 270 degrees clockwise (or 90 ccw).
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.TP
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.B \-transpose
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Transpose image (across UL-to-LR axis).
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.TP
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.B \-transverse
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Transverse transpose (across UR-to-LL axis).
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.IP
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The transpose transformation has no restrictions regarding image dimensions.
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The other transformations operate rather oddly if the image dimensions are not
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a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
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transform complete blocks of DCT coefficient data in the desired way.
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.IP
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.BR jpegtran 's
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default behavior when transforming an odd-size image is designed
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to preserve exact reversibility and mathematical consistency of the
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transformation set. As stated, transpose is able to flip the entire image
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area. Horizontal mirroring leaves any partial iMCU column at the right edge
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untouched, but is able to flip all rows of the image. Similarly, vertical
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mirroring leaves any partial iMCU row at the bottom edge untouched, but is
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able to flip all columns. The other transforms can be built up as sequences
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of transpose and flip operations; for consistency, their actions on edge
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pixels are defined to be the same as the end result of the corresponding
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transpose-and-flip sequence.
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.IP
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For practical use, you may prefer to discard any untransformable edge pixels
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rather than having a strange-looking strip along the right and/or bottom edges
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of a transformed image. To do this, add the
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.B \-trim
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switch:
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.TP
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.B \-trim
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Drop non-transformable edge blocks.
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.IP
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Obviously, a transformation with
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.B \-trim
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is not reversible, so strictly speaking
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.B jpegtran
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with this switch is not lossless. Also, the expected mathematical
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equivalences between the transformations no longer hold. For example,
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.B \-rot 270 -trim
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trims only the bottom edge, but
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.B \-rot 90 -trim
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followed by
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.B \-rot 180 -trim
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trims both edges.
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.IP
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If you are only interested in perfect transformation, add the
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.B \-perfect
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switch:
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.TP
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.B \-perfect
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Fails with an error if the transformation is not perfect.
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.IP
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For example you may want to do
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.IP
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.B (jpegtran \-rot 90 -perfect
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.I foo.jpg
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.B || djpeg
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.I foo.jpg
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.B | pnmflip \-r90 | cjpeg)
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.IP
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to do a perfect rotation if available or an approximated one if not.
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.PP
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We also offer a lossless-crop option, which discards data outside a given
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image region but losslessly preserves what is inside. Like the rotate and
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flip transforms, lossless crop is restricted by the current JPEG format: the
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upper left corner of the selected region must fall on an iMCU boundary. If
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this does not hold for the given crop parameters, we silently move the upper
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left corner up and/or left to make it so, simultaneously increasing the region
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dimensions to keep the lower right crop corner unchanged. (Thus, the output
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image covers at least the requested region, but may cover more.)
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The image can be losslessly cropped by giving the switch:
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.TP
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.B \-crop WxH+X+Y
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Crop to a rectangular subarea of width W, height H starting at point X,Y.
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.PP
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Other not-strictly-lossless transformation switches are:
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.TP
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.B \-grayscale
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Force grayscale output.
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.IP
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This option discards the chrominance channels if the input image is YCbCr
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(ie, a standard color JPEG), resulting in a grayscale JPEG file. The
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luminance channel is preserved exactly, so this is a better method of reducing
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to grayscale than decompression, conversion, and recompression. This switch
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is particularly handy for fixing a monochrome picture that was mistakenly
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encoded as a color JPEG. (In such a case, the space savings from getting rid
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of the near-empty chroma channels won't be large; but the decoding time for
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a grayscale JPEG is substantially less than that for a color JPEG.)
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.TP
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.BI \-scale " M/N"
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Scale the output image by a factor M/N.
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.IP
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Currently supported scale factors are M/N with all M from 1 to 16, where N is
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the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
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then M specifies the DCT scaled size to be applied on the given input. For
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baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
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for baseline JPEG is 8.
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.B Caution:
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An implementation of the JPEG SmartScale extension is required for this
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feature. SmartScale enabled JPEG is not yet widely implemented, so many
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decoders will be unable to view a SmartScale extended JPEG file at all.
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.PP
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.B jpegtran
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also recognizes these switches that control what to do with "extra" markers,
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such as comment blocks:
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.TP
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.B \-copy none
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Copy no extra markers from source file. This setting suppresses all
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comments and other excess baggage present in the source file.
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.TP
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.B \-copy comments
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Copy only comment markers. This setting copies comments from the source file,
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but discards any other inessential (for image display) data.
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.TP
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.B \-copy all
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Copy all extra markers. This setting preserves miscellaneous markers
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found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
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settings. In some files these extra markers can be sizable.
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.IP
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The default behavior is
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.BR "\-copy comments" .
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(Note: in IJG releases v6 and v6a,
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.B jpegtran
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always did the equivalent of
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.BR "\-copy none" .)
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.PP
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Additional switches recognized by jpegtran are:
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.TP
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.BI \-maxmemory " N"
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Set limit for amount of memory to use in processing large images. Value is
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in thousands of bytes, or millions of bytes if "M" is attached to the
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number. For example,
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.B \-max 4m
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selects 4000000 bytes. If more space is needed, temporary files will be used.
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.TP
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.BI \-outfile " name"
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Send output image to the named file, not to standard output.
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.TP
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.B \-verbose
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Enable debug printout. More
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.BR \-v 's
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give more output. Also, version information is printed at startup.
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.TP
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.B \-debug
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Same as
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.BR \-verbose .
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.SH EXAMPLES
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.LP
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This example converts a baseline JPEG file to progressive form:
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.IP
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.B jpegtran \-progressive
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.I foo.jpg
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.B >
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.I fooprog.jpg
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.PP
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This example rotates an image 90 degrees clockwise, discarding any
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unrotatable edge pixels:
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.IP
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.B jpegtran \-rot 90 -trim
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.I foo.jpg
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.B >
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.I foo90.jpg
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.SH ENVIRONMENT
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.TP
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.B JPEGMEM
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If this environment variable is set, its value is the default memory limit.
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The value is specified as described for the
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.B \-maxmemory
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switch.
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.B JPEGMEM
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overrides the default value specified when the program was compiled, and
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itself is overridden by an explicit
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.BR \-maxmemory .
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.SH SEE ALSO
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.BR cjpeg (1),
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.BR djpeg (1),
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.BR rdjpgcom (1),
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.BR wrjpgcom (1)
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.br
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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.SH AUTHOR
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Independent JPEG Group
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.SH BUGS
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The transform options can't transform odd-size images perfectly. Use
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.B \-trim
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or
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.B \-perfect
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if you don't like the results.
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.PP
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The entire image is read into memory and then written out again, even in
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cases where this isn't really necessary. Expect swapping on large images,
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especially when using the more complex transform options.
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