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jpegtran (1)
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    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 coef-
         ficients),  without  ever  fully decoding the image.  There-
         fore, 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 opera-
         tions 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.
    
         -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.
    
         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  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  com-
              ments  from  the  source  file,  but discards any other
              inessential data.
    
         -copy all
              Copy all extra markers.  This setting preserves miscel-
              laneous  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 mil-
              lions 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.
    
    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, discard-
         ing 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 com-
              piled, and itself is overridden by  an  explicit  -max-
              memory.
    
    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  per-
         fectly.  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.
    
    
    
    


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