[cairo] Redoing SOURCE and CLEAR

[Owen Taylor]

On Wed, 2005-08-17 at 11:11 -0700, Carl Worth wrote:

> 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.

Here is 2) - the columns are:

 IN  OUT   DEST_IN    DEST_OUT   DEST_ATOP

I've included DEST_OUT because I think it helps elucidate how things
should work, though it isn't affected by these changes.

I'll skip adding commentary here and let the images stand on their
own.

Regards,
						Owen

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