[cairo] [RFC] Cairo operators expressed in shader syntax

[Carl Worth]

On Tue, 15 May 2007 10:33:24 +0100, Bill Spitzak wrote:
> This is not as complicated as everybody is making out. ALL(*) cairo
> operators are equivalent to:
>
> (src OP dest) * mask + dest * (1-mask).

That's not correct, (not for IN, OUT, DEST_IN, nor DEST_ATOP in
addition to SATURATE).

I'm looking forward to getting the math nicely documented soon so we
can stop revisiting it in threads like this.

Here's Owen's original description that led to the current
implementation:

	http://article.gmane.org/gmane.comp.lib.cairo/4791

And here's the crux of what he 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

And yes, it is quite ugly that we basically have 3 different
fundamental equations. And it can be quite confusing that for the most
common (and default) operator of OVER, the 3 equations yield identical
results.

-Carl
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