[cairo] Road to 1.10

[Owen Taylor]

On Mon, 2009-02-16 at 10:27 -0500, Kristian Høgsberg wrote:
> On Mon, Feb 16, 2009 at 6:30 AM, Emmanuele Bassi <ebassi at linux.intel.com> wrote:
> > On Mon, 2009-02-16 at 10:06 +0000, Chris Wilson wrote:
> >
> >> On a more wider topic, having played with wayland and creating a
> >> cairo-based compositor, we sorely lack the ability to use a
> >> non-premultiplied source (such as an OpenGL window, or SDL surface as
> >> above). CAIRO_FORMAT_ARGB32_NON_PREMULTIPLIED_SOURCE - hideous, but
> >> fairly trivial to handle in a shader. (Slightly more work to add the
> >> extra code paths to pixman though!)
> >
> > one of the hits we get when using Cairo in Clutter comes from the
> > unpremultiplication of the alpha channel when uploading an image surface
> > into a GL texture. it's a one-off operation, but done for a lot of
> > textures, and if the textures are changing their size (e.g. during an
> > animation involving width/height properties) then it causes performance
> > issues -- on top of the usual performance implications of using an image
> > surface.
> 
> Is there a reason you can't just set
> 
>   glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
> 
> Or am I missing something?

No, not missing anything.

 http://bugzilla.openedhand.com/show_bug.cgi?id=1406

Has details about fixing the handling of premultiplication in Clutter
and a patch.

- Owen