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

>
>
> >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.

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'.


>
> 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.

Rik