[cairo] Clip region problems

Keith Packard keithp at keithp.com
Wed May 18 15:18:22 PDT 2005

On Wed, 2005-05-18 at 17:30 -0400, Owen Taylor wrote:
> Do we have any reason to think that source clips are useful? If not,
> then putting them in the pattern object sounds like overcomplication.
> (And causes huge implementation difficulties for many backends.)

Not really, and we can always add them if necessary, for now a
consistent definition of not clipping from source patterns seems like a
reasonable plan.  For the xlib backend, this just means creating
separate picture objects for surfaces used as patterns; that's easy to
detect and can be entirely hidden within the backend.  I think that is
also separable from the surface-level clipping changes that I've barely
started, so it can be done in parallel or at another time.

> The whole idea of a source clip has some funny interaction with
> the idea of "extend" modes as well. I think what we decided for
> RENDER is that we first determine the source pixel in the source 
> image and then use the source clip to determine original vs. 
> transparent for that. Which is logically consistent, but sort of
> violates the reasons that you want extend modes at all.

I suggest we revisit this whole issue when (and if) we decided to add
clipping to patterns.


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