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.TH JPEGTRAN 1 "3 August 1997"

.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

.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.

.PP

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.

.PP

Another not-strictly-lossless transformation switch is:

.TP

.B \-grayscale

Force grayscale output.

.PP

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 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 data.

.TP

.B \-copy all

Copy all extra markers.  This setting preserves miscellaneous markers

found in the source file, such as JFIF thumbnails and Photoshop settings.

In some files these extra markers can be sizable.

.PP

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

Arithmetic coding is not supported for legal reasons.

.PP

The transform options can't transform odd-size images perfectly.  Use

.B \-trim

if you don't like the results without it.

.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.