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jpegtran(1)                                                        jpegtran(1)




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.

       For  EXIF  files and JPEG files containing Exif data, you may prefer to
       use exiftran instead.

       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.  However, while the image data is
       losslessly transformed, metadata can be removed.  See the -copy  option
       for specifics.

       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.

       -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  (usu-
              ally  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 trans-
              formations 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.

              If you are only interested in perfect  transformation,  add  the
              -perfect switch:

       -perfect
              Fails 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.

       We  also  offer  a  lossless-crop option, which discards data outside 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 this does not hold for the given crop parameters,
       we silently move the upper left corner up and/or left to  make  it  so,
       simultaneously increasing the region dimensions to keep the lower right
       crop corner unchanged.  (Thus, the output image  covers  at  least  the
       requested  region,  but  may cover more.)  The adjustment of the region
       dimensions may be optionally disabled by  attaching  an  'f'  character
       ("force") to the width or height number.

       The image can be losslessly cropped by giving the switch:

       -crop WxH+X+Y
              Crop  to  a rectangular subarea of width W, height H starting at
              point X,Y.

       Crop extension: The width or height parameters can be made larger  than
       the  source  image.  In this case the extra area is filled in with zero
       (neutral gray).   A  larger  width  parameter  has  two  more  options:
       Attaching an 'f' character ("flatten") to the width number will fill in
       the extra area with the DC of the adjacent block, instead of gray  out.
       Attaching an 'r' character ("reflect") to the width number will fill in
       the extra area with repeated reflections of the source region,  instead
       of gray out.

       A  complementary lossless-wipe option is provided to discard (gray out)
       data inside a given image region while losslessly  preserving  what  is
       outside:

       -wipe WxH+X+Y
              Wipe  (gray  out)  a  rectangular  subarea  of width W, height H
              starting at point X,Y.

       Attaching an 'f' character ("flatten") to the width  number  will  fill
       the  region  with  the average of adjacent blocks, instead of gray out.
       In case the wipe region and outside area form two horizontally adjacent
       rectangles,  attaching an 'r' character ("reflect") to the width number
       will fill the region with repeated reflections  of  the  outside  area,
       instead of gray out.

       Another  option  is lossless-drop, which replaces data at a given image
       position by another image:

       -drop +X+Y filename
              Drop another image

       Both source images must have the same subsampling values.  It  is  best
       if  they  also have the same quantization, otherwise quantization adap-
       tion occurs.  The trim option can be  used  with  the  drop  option  to
       requantize the drop file to the source file.

       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 substan-
              tially less than that for a color JPEG.)

       -scale M/N
              Scale the output image by a factor M/N.

              Currently supported scale factors are M/N with all M from  1  to
              16,  where  N  is  the  source DCT size, which is 8 for baseline
              JPEG.  If the /N part is  omitted,  then  M  specifies  the  DCT
              scaled size to be applied on the given input.  For baseline JPEG
              this is equivalent to M/8 scaling, since the source DCT size for
              baseline  JPEG  is  8.   Caution:  An implementation of the JPEG
              SmartScale extension is required for this  feature.   SmartScale
              enabled  JPEG  is  not  yet widely implemented, so many decoders
              will be unable to view a SmartScale extended JPEG file at all.

       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 metadata in the source file.

       -copy comments
              Copy only comment markers.  This setting  copies  comments  from
              the source file, but discards any other metadata.

       -copy all
              Copy  all  extra markers.  This setting preserves metadata found
              in the source file, such as JFIF thumbnails, Exif data, and Pho-
              toshop  settings.  In some files these extra markers can be siz-
              able.  Note that this option will copy  thumbnails  as-is;  they
              will not be transformed.

              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

       The  transform  options can't transform odd-size images perfectly.  Use
       -trim or -perfect if you don't like the results.

       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.



                                28 August 2019                     jpegtran(1)

jpeg 9d - Generated Fri Jan 17 07:18:19 CST 2020
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