W3C home > Mailing lists > Public > www-svg@w3.org > November 2011

RE: [css-compositing] some proposals

From: Cyril Concolato <Cyril.Concolato@cisra.canon.com.au>
Date: Tue, 22 Nov 2011 21:50:40 +0000
To: Alex Danilo <alex@abbra.com>, "www-svg@w3.org" <www-svg@w3.org>, "public-fx@w3.org" <public-fx@w3.org>
Message-ID: <54EF1DA3171CEE48AD59D7FF0DE043C3D5AAC2@exm01-wvp.cisra.canon.com.au>
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;-(
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.

The information contained in this email message and any attachments may be confidential and may also be the subject to legal professional privilege. If you are not the intended recipient, any use, interference with, disclosure or copying of this material is unauthorised and prohibited. If you have received this email in error, please immediately advise the sender by return email and delete the information from your system.
Received on Tuesday, 22 November 2011 21:51:13 UTC

This archive was generated by hypermail 2.4.0 : Friday, 17 January 2020 22:54:33 UTC