[cairo] defining CLAMP extend mode

[Bill Spitzak]

Owen Taylor wrote:
> On Mon, 2006-10-23 at 16:15 -0700, Bill Spitzak wrote:
> 
>>> The behavior you are asking for is not implementable as sampling if you
>>> are doing antialiasing by any means other than super-sampling.
>> Not sure I understand this. If my plan cannot be done then multiplying 
>> two masks cannot be done.
> 
> You can't generally multiply two masks. We get away with it a lot in
> Cairo because it works OK as long as the two masks don't share an
> anti-aliased contour.

But in this case one of the images does NOT have an anti-aliased 
contour, as it is the EXTEND_PAD sampled image. By definition this image 
goes out to infinity and thus has no edge, antialiased or not. Therefore 
the multiplication will work fine.