NAME
      rlecomp - Digital image compositor

 SYNOPSIS
      rlecomp [ -o outfile ] Afile operator Bfile

 DESCRIPTION
      rlecomp implements an image compositor based
      on presence of an alpha, or matte channel the
      image.  This extra channel usually defines a
      mask which represents a sort of a cookie-
      cutter for the image.  This is the case when
      alpha is 255 (full coverage) for pixels
      inside the shape, zero outside, and between
      zero and 255 on the boundary. If Afile or
      Bfile is just a single -, then rlecomp reads
      that file from the standard input.

      The operations behave as follows (assuming
      the operation is "A operator B"):

      over The result will be the union of the two
           image shapes, with A obscuring B in the
           region of overlap.

      in   The result is simply the image A cut by
           the shape of B.  None of the image data
           of B will be in the result.

      atop The result is the same shape as image B,
           with A obscuring B where the image
           shapes overlap.  Note this differs from
           over because the portion of A outside
           B's shape does not appear in the result.

      out  The result image is image A with the
           shape of B cut out.

      xor  The result is the image data from both
           images that is outside the overlap
           region.  The overlap region will be
           blank.

      plus The result is just the sum of the image
           data.  Output values are clipped to 255
           (no overflow).  This operation is
           actually independent of the alpha
           channels.

      minus
           The result of A - B, with underflow
           clipped to zero.  The alpha channel is
           ignored (set to 255, full coverage).

      diff The result of abs(A - B).  This is
           useful for comparing two very similar
           images.

      add  The result of A + B, with overflow
           wrapping around (mod 256).

      subtract
           The result of A - B, with underflow
           wrapping around (mod 256).  The add and
           subtract operators can be used to
           perform reversible transformations.

 SEE ALSO
      urt(1), RLE(5),
      "Compositing Digital Images", Porter and
      Duff, Proceedings of SIGGRAPH '84 p.255

 AUTHORS
      Rod Bogart and John W. Peterson

 BUGS
      The other operations could be optimized as
      much as over is.

      Rlecomp assumes both input files have the
      same number of channels.




















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