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

Re: [css-compositing] split up the spec (was: some proposals)

From: Alex Danilo <alex@abbra.com>
Date: Wed, 09 Nov 2011 16:22:55 +1100
Message-Id: <7AODUL.08A8N4Z6EW6@abbra.com>
To: Rik Cabanier <cabanier@gmail.com>
Cc: www-svg <www-svg@w3.org>, public-fx@w3.org
Hi Rik,

--Original Message--:
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>>a. split up the spec in 2 sections: Porter-Duff & Blending. 
>
>Agreed, it separates the two concepts properly.
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>>Porter-Duff are compositing primitives that describes how 2 images (src+dst) can be merged.
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>Here, I'd agree with David Baron's comment and change the names to 'source' and 'dest' in
>line with CSS spec. readability.
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>That sounds fine.
> 
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>>Blending describes how the top image will 'blend' with its underlying colors. The result of this is a new blended image. So far, we assume that after blending, this image is composited with Porter-Duff src-over.
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>No it doesn't.
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>The SVG compositing spec does.
>Some of the blending modes talk about doing 'src-over' or 'dst-over' depending on the color, but I believe that's a bug.

That is a bug for sure.

>If the blending was designed to be compatible with Adobe's model, src-over IS implied.
>If you look at basic compositing formula in the PDF reference manual (https://www.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/PDF32000_2008.pdf section 11.3.3), you will see that it is using 'src-over'.

I am intimately knowledgable with those functions and yes PDF is using src-over. But there's
no reason to limit it to just that.

>
>Blending describes how the colour channels mix. It is independent of the P-D operators.
>The diagram in the existing spec. attempts to describe that. i.e. the one that has a square
>with 4 sections - one multi-coloured, one blue, one yellow and one white.
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>Blending applies only to the multi-colour area whilst P-D manages how the different
>'regions' combine. There is no reason that a blending mode can't apply to a different
>P-D operator.
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>>I am tempted to split the spec, but keep the comp-op keyword the same for both PD and blending.
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>I think this is a mistake. We should split them.
>
>For example, I have two circles as part of a Venn diagram and I want to apply a blend
>to their intersection - I can P-D 'src-in' the objects and apply comp-op="overlay" or similar.
>The result of that is an area which has the 'overlay' applied to just the intersection of
>the objects.
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>I can't remember if 'Shake' was able to do that, but it did handle P-D and blending IIRC.
>
>>This does imply that you won't be able to do xor or src-in with blended content. Does anyone believe that this is a use case?
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>Yes it's a use case. I think you'll find that sort of thing used in high-end video editing etc,
>
> 
>OK. Unless anyone objects, let's make them 2 separate properties. It is cleaner that way.
>
>The 'comp-op' keyword will only apply to the PD blend modes. 'src-over' will be the default.
>The 'blend' keyword will specify what blending mode to use. 'Normal' will be the default.

In ASV the blend keyword was 'adobe-blending-mode' and it handles all the examples
in the spec dealing with blending if you go:

s/comp-op/adobe-blending-mode/g

in a text editor.

So, I'd suggest 'blend-mode' or 'blending-mode' may be nicer.

Alex

>Rik
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Received on Wednesday, 9 November 2011 05:23:32 GMT

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