Re: alpha support for colors

[Eli Zaretskii <eliz@gnu.org>]

> From: "Stephen J. Turnbull" <stephen@xemacs.org>
> Cc: aaditya sood <aaditya@sood.net.in>,
>     emacs-devel@gnu.org
> Date: Wed, 31 Oct 2012 11:06:11 +0900
> 
> Eli Zaretskii writes:
> 
>  > What do you mean by "composition of all the faces"?  Why "all"?
> 
> I suspect aaditya is thinking in terms of bitmap composition
> operations on graphics layers, not Emacs faces.

Maybe.  I just don't understand clearly enough what exactly was the
intent.

As you know, Emacs doesn't operate on the bitmap level, except very
low in the terminal-specific code.  At that level, there are no faces
anymore, just colors and other graphics-related stuff.

> So he'd like to put an overlay on some text, give that overlay a 'face
> property of '(+ red), and turn all the blue text in the overlay to
> violet (the RGB sum of #F00 and #00F == #F0F).

Maybe I don't get something, but (a) this _is_ about faces and their
combination, '(+ red) being a kind of "syntactic sugar"; and (b) all
it needs is the ability to combine colors when merging faces, instead
of just selecting one color of the two.  By contrast, alpha-blending,
AFAIU, is about transparency, not about combining any 2 colors.  So
I'm probably still missing something.