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RLECOMP(1)             DragonFly General Commands Manual            RLECOMP(1)

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.

4th Berkeley Distribution      December 20, 1986                    RLECOMP(1)