RE: [css-compositing] some proposals

Hi Alex,

Thank you for your answer. See my comments below.

-----Original Message-----
From: Alex Danilo [mailto:alex@abbra.com]
Sent: Tuesday, 22 November 2011 1:01 PM
To: www-svg@w3.org; public-fx@w3.org
Subject: RE: [css-compositing] some proposals

--
NB: Sorry for the spam www-svg (note to self: debug mailer agent;-(
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Hi Cyril & Rik,

        Sorry I've been a bit busy of late so following up now.

        First, one thing that's been missed is that in Porter-Duff the alpha channel represents a mask. The crux of the original paper is representing what happens with shape overlap on a pixel and so in the compositing spec. we reproduce the 'square with 4 areas'
that has the top section multi-coloured, the left yellow, the right blue and the bottom white.

        Those areas represent 0, A, B, and AB in the original P-D paper.

        In the spec. further down where it says:
"The operation used to place objects onto the background is as follows"

there are these equations:
Dca' = f(Sc, Dc) × Sa × Da  + Y × Sca × (1-Da)  + Z × Dca × (1-Sa)
Da'  =         X × Sa × Da  + Y × Sa × (1-Da)   + Z × Da × (1-Sa)

        The Porter-Duff mode chosen affects X, Y, and Z.

        The blend mode is f(Sc, Dc).
[Cyril] So I understand that the proposal to split the modes into two parts (P-D and blending) is purely syntactic sugar and it does not change the model, right?

        So for example if you chose blend-mode:multiply, comp-op:src-in; you would get:

f(Sc,Dc) = Sc × Dc
X        = 1
Y        = 0
Z        = 0
[Cyril] According to the current draft: src-in  is f(Sc,Dc)=Sc and X=1 and Y=Z=0. Multiply is f(Sc,Dc)=Sc x Dc and X=Y=Z=1. So you are saying that when you use both the compositing and the blending modes, you keep the value of f(Sc,Dc) from the blending and you keep the X, Y, Z values from the compositing mode. It make sense but it's not obvious. I would have thought that you would somehow compose them. Anyway, I think that if we split the spec, the merging of equation needs to be documented.

Regards,
Cyril
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Received on Tuesday, 22 November 2011 21:51:24 UTC