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RE: SVG 2 rendering model

From: Rik Cabanier <cabanier@adobe.com>
Date: Thu, 2 Aug 2012 16:33:03 -0700
To: "'Nikos Andronikos'" <nikos.andronikos@cisra.canon.com.au>, "public-svg-wg@w3.org" <public-svg-wg@w3.org>
Message-ID: <83F37C1A4497B54589EAEDC750D03A9473B7E692@nambx09.corp.adobe.com>
Hi Nikos, 

There is indeed a problem, but I think it's with the wording in the spec.
I've updated the description of isolated and knockout in the CSS properties section, but we need to redo the one is group compositing as well.

In short, we shouldn't be talking about 'buffers' like 'initial backdrop' and 'group backdrop'. Those are implementation specific (i.e. when compositing on the GPU, there is no need for buffers)

What the spec should say is how a pixel calculates it's backdrop.
Normally, it does it by doing a normal blend and source-over composite with no filters of all its parent groups, starting with the page group. If one of its parent groups is isolated, it should start there.

I think that is clearer and leaves it up to the browser vendor on how they want to implement this.
It's also easier to read. :-)


> -----Original Message-----
> From: Nikos Andronikos [mailto:nikos.andronikos@cisra.canon.com.au]
> Sent: Wednesday, August 01, 2012 6:27 PM
> To: public-svg-wg@w3.org
> Subject: SVG 2 rendering model
> Hi guys,
> While reworking the rendering model section of SVG 2, I came across an
> incompatibility between filter effects and the required method of rendering
> compositing groups from the Compositing and Blending specification.
> In SVG versions < 2, the process for rendering a group is 1. Create a new
> buffer initialised to transparent black 2. Composite elements within the
> group into this buffer using src-over and the normal blend mode (aka simple
> alpha blending) 3. Apply filter effects to the buffer 4. Composite buffer with
> the group backdrop using src-over and normal blend mode
> When following the Compositing and Blending specification, as it currently
> stands, the method will be 1. Create a new buffer initialised with the initial
> backdrop* 2. Composite elements within the group into the buffer using the
> compositing operator and blend mode specified 3. Apply filter effects to the
> buffer (buffer contains elements from initial backdrop so these will be
> included in the filter effect) 4. Composite buffer with the group backdrop
> using the compositing operator and blend mode specified - compensating for
> double contribution of group backdrop
> * The initial backdrop is dependent on the isolated property.
> In the default case (isolated=false) the group backdrop == the initial
> backdrop.
> When isolated=true, the initial backdrop is transparent black.
> The group backdrop contains everything that has been drawn up to but not
> including the first element in the group.
> So why do we have to do it the new way?
> In SVG 1.1 we used src-over and normal, and this combination of operator
> and blend mode is group invariant, i.e. A + (B + C) == A + B + C, where A is the
> group backdrop and B and C are grouped.
> But now we have lots of new compositing operators and blend modes and
> some of them aren't group invariant - colour burn for example, or the src-
> atop compositing operator.
> So now we MUST do A + B + C when isolated==false
> But this means we bring the group backdrop into the group and then when
> we apply a filter effect to the group, the filter effect includes the group
> backdrop.
> To avoid bringing the group backdrop into the group, we can make the group
> isolated, but this is not currently the default behaviour in the Compositing
> and Blending specification.
> So at the moment, the default behaviour in SVG 2 is different to the default
> behaviour in other versions of SVG.
> So, as far as I can see, the options are:
> 1. make isolated the default - but this isn't consistent with other technologies
> - like PDF 2. state that for invariant groups, the contents of the groups are
> composited together first - we'd have to list all the combinations that are
> group invariant but at least it would be transparent to the author - although
> they would likely be confused when a non invariant group includes the
> backdrop in the filter effect.
> 3. not be backwards compatible - generally unpleasant
> I'm leaning towards making isolated the default, but I'd like to hear if anyone
> else has ideas for options, or other opinions.
> I hope that was all clear =)
> I can make an example if people feel it would help their understanding - but I
> won't be able to find the time this week.
> Cheers,
> Nikos
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Received on Thursday, 2 August 2012 23:35:25 UTC

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