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RLECOMP(1)							    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	 stan‐
       dard 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 inde‐
	      pendent 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)

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