Re: [AGG] Blending colors, associated alpha, etc

[Stephan A. <sup...@gm...>]

Hi Pierre,

> Your explanation seems to be adequately describing what I had 
> observed.
> 
> > When blending pixel J over pixel I, you could say that J allows (1-
> > b)
> > percent of pixel I to shine through. Thus, the formular for correct
> > alpha blending is this:
> > 
> > C' = B' + (1-b)A'
> > 
> > C' = b*B + (1-b) * a*A
> > 
> > C is optained by dividing C' by the resulting alpha c which is 
> > optained
> > by
> > 
> > c = b + (1-b) * a
> 
> I finally got what I wanted by pretending that the output pixmap
> computed by AGG was alpha-premultiplied, and now my code in Win32
> works. I don't remember too clearly how I got to that result, though.

This will work for completely transparent backgrounds only though.

-Stephan