Table of Contents
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
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.
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.
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