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

On Tue, Nov 8, 2011 at 9:22 PM, Alex Danilo <alex@abbra.com> wrote:

> Hi Rik,
>
> --Original Message--:
> >
> >
> >>a. split up the spec in 2 sections: Porter-Duff & Blending.
> >
> >Agreed, it separates the two concepts properly.
> >
> >>Porter-Duff are compositing primitives that describes how 2 images
> (src+dst) can be merged.
> >
> >Here, I'd agree with David Baron's comment and change the names to
> 'source' and 'dest' in
> >line with CSS spec. readability.
> >
> >That sounds fine.
> >
> >
> >
> >>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.
> >
> >No it doesn't.
> >
> >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.
>

Great! I believe we're in agreement.


>
> >
> >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.
> >
> >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.
> >
> >>I am tempted to split the spec, but keep the comp-op keyword the same
> for both PD and blending.
> >
> >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.
> >
> >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?
> >
> >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.
>

OK. 'blend-mode' sounds better.


>
> Alex
>
> >Rik
> >
> >
> >
> >
>
>