[cairo] [PATCH] [API] Support component-alpha.

[Chris Wilson]

Excerpts from Soeren Sandmann's message of Tue Oct 20 16:29:05 +0100 2009:
> Well, there are two subtly different concepts in play here:
> 
> 1. The concept of a 'three times horizontally supersampled image'
> 
> 2. The concept of 'values to be presented to an output device'
> 
> It seems to me that (1) is the only thing a user can really be
> expected to provide, because while (2) can be computed from (1), to do
> a good job of it, you have to know about device characteristics and
> human vision, then apply appropriate filtering and correction. You
> also need to control which color space compositing takes place in.
> 
> If the component_alpha pattern property uses interpretation (2), then
> we are effectively asking users to use their understanding of cairo
> internals to compute something quite non-trivial.

Hmm, I was approaching this from the "how can I generate sub-pixel
geometry masks to perform CAIRO_ANTIALIAS_SUBPIXEL with Render". Which
as you point out requires the knowledge (as provided by Render) of the
sub-pixel layout. That knowledge can only be sanely handled internally,
as it is meaningless for the rest of cairo and indeed is out of scope.

Ok, I'm convinced that the antialias property that we currently expose
on the context (hence target surface) is the way to go here and that
exposing per-component alpha is insane.
-ickle
-- 
Chris Wilson, Intel Open Source Technology Centre