Re: [css-compositing] new Editor's draft posted -> update

On Thu, May 23, 2013 at 4:02 PM, Rik Cabanier <cabanier@gmail.com> wrote:

> After talking this over with our engineers, it turns out that the
> invisible 'layers' that browsers create, are not actually a problem.
> This is because they are composited in order and not as a tree (at least
> on webkit and blink).
> For instance, if you have content like this:
>
> <video>...</video>
> <div>
>
> <p>...</p>
>
> <p style="mix-blend-mode">...
>
> there will be 3 layers on the back-end: one for video, one for the <div>
> and for the <p> with the blending.
> This content will be composited as a list so <p> will composite and blend
> with the composited result of <video> + <div> which is the desired behavior.
>

Well no, it depends.  Sometimes it will render in the way you say here, but
sometimes we may blend the <p> together with the <div> before blending that
with the <video> or we may put different subsets of these layers into
separate intermediate surfaces.  We may be forced to blend the <div> and
the <p> together without involving the <video>'s contents if the <div> has
a transform-style: preserve3D, for example.

Whether this produces any observable difference in the final output depends
on exactly why you are depending on the draw order - in general, you really
shouldn't.

- James


>
> I updated the spec and removed that particular issue. I also worded it so
> blending will happen between stacking context (which is what David Baron
> suggested in an earlier email)
> There will still be work needed on the implementation side to plumb this
> i, but I think this will suffice for the specification.
>
> Rik
>
>