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 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 over-
              lap.

       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 transforma-
              tions.


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.



1                      December 20, 1986             RLECOMP(1)
