jpegtran - Online in the Cloud

PROGRAM:

NAME

jpegtran - lossless transformation of JPEG files

SYNOPSIS

jpegtran [ options ] [ filename ]

DESCRIPTION

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.

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 djpeg followed by cjpeg to
accomplish the same conversion. But by the same token, jpegtran cannot perform lossy
operations such as changing the image quality.

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.

OPTIONS

All switch names may be abbreviated; for example, -optimize may be written -opt or -o.
Upper and lower case are equivalent. British spellings are also accepted (e.g.,
-optimise), though for brevity these are not mentioned below.

To specify the coded JPEG representation used in the output file, jpegtran accepts a
subset of the switches recognized by cjpeg:

-optimize
Perform optimization of entropy encoding parameters.

-progressive
Create progressive JPEG file.

-restart N
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
attached to the number.

-arithmetic
Use arithmetic coding.

-scans file
Use the scan script given in the specified text file.

See 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.

The image can be losslessly transformed by giving one of these switches:

-flip horizontal
Mirror image horizontally (left-right).

-flip vertical
Mirror image vertically (top-bottom).

-rotate 90
Rotate image 90 degrees clockwise.

-rotate 180
Rotate image 180 degrees.

-rotate 270
Rotate image 270 degrees clockwise (or 90 ccw).

-transpose
Transpose image (across UL-to-LR axis).

-transverse
Transverse transpose (across UR-to-LL axis).

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.

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.

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 -trim switch:

-trim Drop non-transformable edge blocks.

Obviously, a transformation with -trim is not reversible, so strictly speaking
jpegtran with this switch is not lossless. Also, the expected mathematical
equivalences between the transformations no longer hold. For example, -rot 270
-trim trims only the bottom edge, but -rot 90 -trim followed by -rot 180 -trim
trims both edges.

-perfect
If you are only interested in perfect transformations, add the -perfect switch.
This causes jpegtran to fail with an error if the transformation is not perfect.

For example, you may want to do

(jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg)

to do a perfect rotation, if available, or an approximated one if not.

-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.)

Other not-strictly-lossless transformation switches are:

-grayscale
Force grayscale output.

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.)

jpegtran also recognizes these switches that control what to do with "extra" markers, such
as comment blocks:

-copy none
Copy no extra markers from source file. This setting suppresses all comments and
other excess baggage present in the source file.

-copy comments
Copy only comment markers. This setting copies comments from the source file but
discards any other data that is inessential for image display.

-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.

The default behavior is -copy comments. (Note: in IJG releases v6 and v6a, jpegtran
always did the equivalent of -copy none.)

Additional switches recognized by jpegtran are:

-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, -max 4m selects 4000000 bytes. If more space is needed, temporary files
will be used.

-outfile name
Send output image to the named file, not to standard output.

-verbose
Enable debug printout. More -v's give more output. Also, version information is
printed at startup.

-debug Same as -verbose.

-version
Print version information and exit.

EXAMPLES

This example converts a baseline JPEG file to progressive form:

jpegtran -progressive foo.jpg > fooprog.jpg

This example rotates an image 90 degrees clockwise, discarding any unrotatable edge
pixels:

jpegtran -rot 90 -trim foo.jpg > foo90.jpg

ENVIRONMENT

JPEGMEM
If this environment variable is set, its value is the default memory limit. The
value is specified as described for the -maxmemory switch. JPEGMEM overrides the
default value specified when the program was compiled, and itself is overridden by
an explicit -maxmemory.

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