[cairo] Redoing SOURCE and CLEAR

[Carl Worth]

On Wed, 17 Aug 2005 11:45:19 -0400, Owen Taylor wrote:
> So, the proposal here is to logically divide the operators into
> 4 groups by what definition we use:
> 
>  OVER, ATOP, DEST, DEST_OVER, DEST_OUT, XOR, ADD
> 
>    All definitions equivalent
> 
>  SOURCE, CLEAR
> 
>    (mask IN clip) ? src OP dest : dest
> 
>  IN, OUT, DEST_IN, DEST_ATOP
> 
>    clip ? (src IN mask) OP dest : dest
> 
>  SATURATE
>    
>    (src IN mask IN clip) OP dest

Another proposal is to also implement IN, OUT, DEST_IN, and DEST_ATOP
with:

    (mask IN clip) ? src OP dest : dest

which would make this the fundamental equation and leave only SATURATE
as an exception.

That's appealing as far as having consistent mathematical grounding
for the rendering equation.

It's not obvious to me what impact this change would have on usability
or learnability of the behavior of these four operators. I think there
are at least two pieces of evidence that would help in this decision:

 1) Use cases for these operators. That is, English descriptions of
    things that would like to be achieved, along with corresponding
    cairo code using one model or the other.

 2) Two images much like those that Owen previously attached, but
    showing these 4 operators with both equations.

I believe Owen is working on #2 right now.

-Carl
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