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RE: [css-compositing] some proposals

From: Alex Danilo <alex@abbra.com>
Date: Wed, 23 Nov 2011 10:38:28 +1100
Message-Id: <4O53VL.4SERV82IC612@abbra.com>
To: Cyril Concolato <Cyril.Concolato@cisra.canon.com.au>
Cc: "www-svg@w3.org" <www-svg@w3.org>, "public-fx@w3.org" <public-fx@w3.org>
Hi Cyril,

--Original Message--:
>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?


The model doesn't change but it's slightly more than syntactic
sugar. By separating out the blending from the P-D you get more
combinatorial possibilities that you couldn't do nicely if it
was one property.

>        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.


It's not obvious since there was only one property before.
So I agree it needs to be documented, and in fact it's
pretty simple once you work out what's really going on.

Rik followed up with a similar message just now and says:

The model is that you calculate the blended image first
(= f(Sc, Dc)) and then composite it with whatever mode was
specified. The current spec always assumes that you do
src-over with blending and that is changing.
So in the new spec:
- multiply becomes f(Sc,Dc) = Sc x Dc (no more x, y, z)
- src-in becomes X = 1, Y = Z = 0 (no more f(Sc, Dc))

What he's really saying is that the AB (colored top section
of the diagram) is calculated as the blending step, then the
other areas are done using P-D.

In P-D terms, the areas are _coverage_ over a pixel. The alpha
channel is a convenience masquerading as a shape channel.

If you imagine that the source and destination are two triangles
that overlap the pixel - that is what is being shown. i.e. the
area covered by source consists of a section of the pixel where
there is no destination color, and a section where there is
destination color and a section where it doesn't overlap
the pixel. Same for the destination pixel coverage.

The Porter-Duff terms refer to the part of source that has no
overlap and so no blend is possible with the destination (the
left yellow triangle); the destination color that has no
source overlapping it (the right blue triangle); the area
not covered by either source or destination (the white area
at the bottom); and finally the area covered by source and
destination color where blending of the colors happens (the
multi-colored triangle at the top).

So, what the blend modes do is alter the function of the
color mix in the top triangle and that's it. There really
isn't a conceptual first blend then second 'src-over' or
whatever since there is only one color channel which ends
up getting input from the blend function and the areas
of source and destination that don't intersect.

Clear as mud hey:-)

I'm sure we can turn this discussion into something
that is readable by general developers since that's
what would be ideal.


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Received on Tuesday, 22 November 2011 23:39:03 UTC

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