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Re: [css-compositing] some proposals

From: Rik Cabanier <cabanier@gmail.com>
Date: Sun, 20 Nov 2011 22:26:54 -0800
Message-ID: <CAGN7qDDY+yW6JDMf1tz=+rbNCC7qX5Y2aC0TJ+D+h7XUvmjZiQ@mail.gmail.com>
To: Cyril Concolato <Cyril.Concolato@cisra.canon.com.au>
Cc: www-svg <www-svg@w3.org>, "public-fx@w3.org" <public-fx@w3.org>, "Alex Danilo (alex@abbra.com)" <alex@abbra.com>
> On Sun, Nov 20, 2011 at 4:29 PM, Cyril Concolato <
> Cyril.Concolato@cisra.canon.com.au> wrote:
> Hi Rik, Alex,
> Just to make sure I understand the proposal. What would happen if you
> apply the ‘dst-atop’ compositing operator with partially transparent
> objects together with say a ‘plus’ blending operator. I understand that the
> ‘plus’ would only apply on the intersection but I’m not sure about the
> maths. Would you compose the equations, in which order? It doesn’t look
> like you can consider that the result of the (alpha-)compositing becomes
> the source (or destination) of the (color-compositing) blending ,does it?
> No, blending/color-compositing happens first, followed by
> alpha-compositing.
> [Cyril] How can you do the blending/color-compositing on the intersection
> of the two objects if you haven’t done alpha-compositing? You might not
> even have an intersection if you apply xor for instance. Anyway, I think
> the two operations should actually happen at the same time. I’m just trying
> to see what would be the result of applying both ‘dst-atop’ and ‘plus’
> because both take care of opacity.

That is how blending is done. The background is calculated and then
'blended' with the source.
For an in-depth explanation, you can consult the PDF reference spec:

Chapter 11.3.3 has the information.

the Filter spec also describes how this is done:
Of importance here is:
For each Ai (from i=n to 1), initialize BUFi to transparent black. Render
all children of Ai up to but not including Ai-1 into BUFi. The children are
painted, then filtered, clipped, masked and composited using the various
painting, filtering, clipping, masking and object opacity settings on the
given child. Any filter effects, masking and* group opacity* that might be
set on Ai do *not* apply when rendering the children of Ai into BUFi.

Received on Monday, 21 November 2011 06:27:41 UTC

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