94c64ead85
- "bits per component" = "bits per sample"
Describing the data precision of a JPEG image using "bits per
component" is technically correct, but "bits per sample" is the
terminology that the JPEG-1 spec uses. Also, "bits per component" is
more commonly used to describe the precision of packed-pixel formats
(as opposed to "bits per pixel") rather than planar formats, in which
all components are grouped together.
- Unmention legacy display technologies. Colormapped and monochrome
displays aren't a thing anymore, and even when they were still a
thing, it was possible to display full-color images to them. In 1991,
when JPEG decompression time was measured in minutes per megapixel, it
made sense to keep a decompressed copy of JPEG images on disk, in a
format that could be displayed without further color conversion (since
color conversion was slow and memory-intensive.) In 2024, JPEG
decompression time is measured in milliseconds per megapixel, and
color conversion is even faster. Thus, JPEG images can be
decompressed, displayed, and color-converted (if necessary) "on the
fly" at speeds too fast for human vision to perceive. (In fact, your
TV performs much more complicated decompression algorithms at least 60
times per second.)
- Document that color quantization (and associated features), GIF
input/output, Targa input/output, and OS/2 BMP input/output are legacy
features. Legacy status doesn't necessarily mean that the features
are deprecated. Rather, it is meant to discourage users from using
features that may be of little or no benefit on modern machines (such
as low-quality modes that had significant performance advantages in
the early 1990s but no longer do) and that are maintained on a
break/fix basis only.
- General wordsmithing, grammar/punctuation policing, and formatting
tweaks
- Clarify which data precisions each cjpeg input format and each djpeg
output format supports.
- cjpeg.1: Remove unnecessary and impolitic statement about the -targa
switch.
- Adjust or remove performance claims to reflect the fact that:
* On modern machines, the djpeg "-fast" switch has a negligible effect
on performance.
* There is a measurable difference between the performance of Floyd-
Steinberg dithering and no dithering, but it is not likely
perceptible to most users.
* There is a measurable difference between the performance of 1-pass
and 2-pass color quantization, but it is not likely perceptible to
most users.
* There is a measurable difference between the performance of
full-color and grayscale output when decompressing a full-color JPEG
image, but it is not likely perceptible to most users.
* IDCT scaling does not necessarily improve performance. (It
generally does if the scaling factor is <= 1/2 and generally doesn't
if the scaling factor is > 1/2, at least on my machine. The
performance claim made in jpeg-6b was probably invalidated when we
merged the additional scaling factors from jpeg-7.)
- Clarify which djpeg switches/output formats cannot be used when
decompressing lossless JPEG images.
- Remove djpeg hints, since those involve quality vs. speed tradeoffs
that are no longer relevant for modern machines.
- Remove documentation regarding using color quantization with 16-bit
data precision. (Color quantization requires lossy mode.)
- Java: Fix typos in TJDecompressor.decompress12() and
TJDecompressor.decompress16() documentation.
- jpegtran.1: Fix truncated paragraph
In a man page, a single quote at the start of a line is interpreted as
a macro.
Closes #775
- libjpeg.txt:
* Mention J16SAMPLE data type (oversight.)
* Remove statement about extending jdcolor.c. (libjpeg-turbo is not
quite as DIY as libjpeg once was.)
* Remove paragraph about tweaking the various typedefs in jmorecfg.h.
It is no longer relevant for modern machines.
* Remove caveat regarding systems with ints less than 16 bits wide.
(ANSI/ISO C requires an int to be at least 16 bits wide, and
libjpeg-turbo has never supported non-ANSI compilers.)
- usage.txt:
* Add copyright header.
* Document cjpeg -icc, -memdst, -report, -strict, and -version
switches.
* Document djpeg -icc, -maxscans, -memsrc, -report, -skip, -crop,
-strict, and -version switches.
* Document jpegtran -icc, -maxscans, -report, -strict, and -version
switches.
363 lines
13 KiB
Groff
363 lines
13 KiB
Groff
.TH JPEGTRAN 1 "17 June 2024"
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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 lossy (DCT-based) 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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For EXIF files and JPEG files containing Exif data, you may prefer to use
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.B exiftran
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instead.
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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. However,
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while the image data is losslessly transformed, metadata can be removed. See
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the
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.B \-copy
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option for specifics.
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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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.B \-arithmetic
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Use arithmetic coding.
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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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.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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.PP
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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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.PP
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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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.PP
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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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.TP
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.B \-perfect
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If you are only interested in perfect transformations, add the
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.B \-perfect
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switch. This causes
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.B jpegtran
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to fail 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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This version of \fBjpegtran\fR also offers a lossless crop option, which
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discards data outside of a given image region but losslessly preserves what is
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inside. Like the rotate and flip transforms, lossless crop is restricted by
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the current JPEG format; the upper left corner of the selected region must fall
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on an iMCU boundary. If it doesn't, then it is silently moved up and/or left
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to the nearest iMCU boundary (the lower right corner is unchanged.) Thus, the
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output image covers at least the requested region, but it may cover more. The
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adjustment of the region dimensions may be optionally disabled by attaching
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an 'f' character ("force") to the width or height number.
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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 the image to a rectangular region of width W and height H, starting at
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point X,Y. The lossless crop feature discards data outside of a given image
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region but losslessly preserves what is inside. Like the rotate and flip
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transforms, lossless crop is restricted by the current JPEG format; the upper
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left corner of the selected region must fall on an iMCU boundary. If it
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doesn't, then it is silently moved up and/or left to the nearest iMCU boundary
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(the lower right corner is unchanged.)
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.PP
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If W or H is larger than the width/height of the input image, then the output
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image is expanded in size, and the expanded region is filled in with zeros
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(neutral gray). Attaching an 'f' character ("flatten") to the width number
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will cause each block in the expanded region to be filled in with the DC
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coefficient of the nearest block in the input image rather than grayed out.
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Attaching an 'r' character ("reflect") to the width number will cause the
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expanded region to be filled in with repeated reflections of the input image
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rather than grayed out.
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.PP
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A complementary lossless wipe option is provided to discard (gray out) data
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inside a given image region while losslessly preserving what is outside:
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.TP
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.B \-wipe WxH+X+Y
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Wipe (gray out) a rectangular region of width W and height H from the input
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image, starting at point X,Y.
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.PP
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Attaching an 'f' character ("flatten") to the width number will cause the
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region to be filled with the average of adjacent blocks rather than grayed out.
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If the wipe region and the region outside the wipe region, when adjusted to the
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nearest iMCU boundary, form two horizontally adjacent rectangles, then
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attaching an 'r' character ("reflect") to the width number will cause the wipe
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region to be filled with repeated reflections of the outside region rather than
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grayed out.
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.PP
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A lossless drop option is also provided, which allows another JPEG image to be
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inserted ("dropped") into the input image data at a given position, replacing
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the existing image data at that position:
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.TP
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.B \-drop +X+Y filename
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Drop (insert) another image at point X,Y
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.PP
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Both the input image and the drop image must have the same subsampling level.
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It is best if they also have the same quantization (quality.) Otherwise, the
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quantization of the output image will be adapted to accommodate the higher of
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the input image quality and the drop image quality. The trim option can be
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used with the drop option to requantize the drop image to match the input
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image. Note that a grayscale image can be dropped into a full-color image or
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vice versa, as long as the full-color image has no vertical subsampling. If
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the input image is grayscale and the drop image is full-color, then the
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chrominance channels from the drop image will be discarded.
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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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.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 metadata 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 metadata.
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.TP
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.B \-copy icc
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Copy only ICC profile markers. This setting copies the ICC profile from the
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source file but discards any other metadata.
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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. Note that this
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option will copy thumbnails as-is; they will not be transformed.
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.PP
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The default behavior is \fB-copy comments\fR. (Note: in IJG releases v6 and
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v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.)
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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 \-icc " file"
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Embed ICC color management profile contained in the specified file. Note that
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this will cause \fBjpegtran\fR to ignore any APP2 markers in the input file,
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even if \fB-copy all\fR or \fB-copy icc\fR is specified.
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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, an error will occur.
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.TP
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.BI \-maxscans " N"
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Abort if the input image contains more than
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.I N
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scans. This feature demonstrates a method by which applications can guard
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against denial-of-service attacks instigated by specially-crafted malformed
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JPEG images containing numerous scans with missing image data or image data
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consisting only of "EOB runs" (a feature of progressive JPEG images that allows
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potentially hundreds of thousands of adjoining zero-value pixels to be
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represented using only a few bytes.) Attempting to transform such malformed
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JPEG images can cause excessive CPU activity, since the decompressor must fully
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process each scan (even if the scan is corrupt) before it can proceed to the
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next scan.
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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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.BI \-report
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Report transformation progress.
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.TP
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.BI \-strict
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Treat all warnings as fatal. This feature also demonstrates a method by which
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applications can guard against attacks instigated by specially-crafted
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malformed JPEG images. Enabling this option will cause the decompressor to
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abort if the input image contains incomplete or corrupt image data.
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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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.TP
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.B \-version
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Print version information and exit.
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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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.PP
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This file was modified by The libjpeg-turbo Project to include only information
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relevant to libjpeg-turbo and to wordsmith certain sections.
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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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