Premultiplied alpha handle for driver

[Jonathan Morton]

On Tue, 2010-06-01 at 19:26 +0300, Siarhei Siamashka wrote:
> On Tuesday 01 June 2010, Jonathan Morton wrote:
> > On Mon, 2010-05-31 at 10:42 +0800, Huang, FrankR wrote:
> > > We know the premultiplied alpha will be used sometimes in blend. Is
> > > that used in the graphics driver?
> >
> > XRender always expects premultiplied alpha in images.  Your previous
> > example, with ARGB values of (100,255,0,0) is therefore an invalid pixel
> > vector, since the R channel is greater than the A channel.
> 
> This is not necessarily invalid. It's more like "additive blending" feature
> which is even used by some people:
> http://home.comcast.net/~tom_forsyth/blog.wiki.html#[[Premultiplied%20alpha]]

Well, that's a corner case, but I was trying to get a conceptual point
across.

-- 
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From: Jonathan Morton
      jonathan.morton at movial.com