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RLECOMP(1)		    General Commands Manual		    RLECOMP(1)

       rlecomp - Digital image compositor

       rlecomp [ -o outfile ] Afile operator Bfile

       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

       over   The result will be the union of the two image shapes, with A ob-
	      scuring 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 be-
	      cause the	portion	of A outside B's shape does not	appear in  the

       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).

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

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

       Rod Bogart and John W. Peterson

       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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