Re: Utility of background-composite and background-blend-mode?

On Mar 14, 2013, at 10:43 AM, Rik Cabanier <cabanier@gmail.com> wrote:

> On Thu, Mar 14, 2013 at 10:21 AM, L. David Baron <dbaron@dbaron.org> wrote:
> On Thursday 2013-03-14 09:55 -0700, Rik Cabanier wrote:
> > On Thu, Mar 14, 2013 at 9:19 AM, L. David Baron <dbaron@dbaron.org> wrote:
> >
> > > On Thursday 2013-03-14 05:44 -0700, Dirk Schulze wrote:
> > > > No, not WebKit's rules. And I do not think that we want to specify
> > > > buffering. A behavior in situations like scrolling for blending
> > > > should be specified and browser need to follow. I see that this
> > > > can be challenging but would be most desireabale. After all,
> > > > scrolling should not affect the browser experience of the user on
> > > > the visual side - especially for blending.
> > >
> > > As I said in http://dbaron.org/log/20130306-compositing-blending , I
> > > think there's a lot less to specify and a lot less to drive towards
> > > interoperability if compositing and blending operations are limited
> > > to things that create stacking contexts.  This limitation would be
> > > present if background-blend-mode and background-composite are
> > > dropped, which I think should be done.
> >
> >
> > David,
> > the exact same issue will happen if blending applies to elements (in which
> > case stacking contexts are created).
> > For instance, an element with blending that is a child of an element that
> > uses fixed positioning will render differently today in FF and WK.
> >
> > Dropping background-blend-mode will not solve this problem.
> 
> Dropping background-blend-mode simplifies it a lot, because you only
> have to consider elements that form stacking contexts when
> addressing it.
> 
> Maybe I'm not seeing the problem that makes background-blend-mode harder.
> What would be the difference between:
> <div style="position: fixed">
> <img src="ducky.png" style="mix-blend-mode: multiply">
> 
> and
> <div style="position: fixed;backgound:url('ducky.png');background-blend-mode:multiply">
> 
> Both will render differently because 'position:fixed' creates an offscreen buffer in WK but not in FF

I'm confused about background-blend-mode, and the spec <https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#background-blend-mode> is not helping.

Does it blend with anything in other elements, or only between the background images/background color of the element to which it's applied?

The spec's use of "the background images at a lower z-index." is confusing here. Is that "the background images of elements at a lower z-index", or "later  background images of the element"?

If it's only within elements and not between them, I don't see why it matters whether position:fixed creates a group.

Simon