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 conver-
    sion.  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, jpeg-
    tran 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.

    -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 dimen-
    sions. The other transformations operate rather  oddly if the  image
    dimensions are not a multiple of the iMCU size (usually 8 or 16 pix-
    els), because they can only transform complete blocks of DCT coeffi-
    cient 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 con-
    sistency, 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.

    Another not-strictly-lossless transformation switch is:

    -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 recompres-
    sion.  This switch is particularly handy for fixing a monochrome pic-
    ture 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 inessential data.

    -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.

    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, ver-
    sion information is printed at startup.

    -debug Same as -verbose.

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 speci-
    fied when the program was compiled, and itself is overridden by
    an explicit -maxmemory.

SEE ALSO
   cjpeg(1),djpeg(1),rdjpgcom(1),wrjpgcom(1)
    Wallace,  Gregory K.  "The JPEG Still Picture Compression Standard",
    Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.

AUTHOR
    Independent JPEG Group

BUGS
    Arithmetic coding is not supported for legal reasons.

    The transform options can't transform odd-size images perfectly.  Use
    -trim if you don't like the results without it.

    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.