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

(Sorry for the delayed reply. I was out for a long weekend.)

On Thu, Mar 14, 2013 at 1:42 PM, Simon Fraser <smfr@me.com> wrote:

> On Mar 14, 2013, at 11:21 AM, Rik Cabanier <cabanier@gmail.com> wrote:
>
>
> On Thu, Mar 14, 2013 at 11:04 AM, Simon Fraser <smfr@me.com> wrote:
>
> From the spec:
>
> Each background image will blend with the element's background and the
> background images at a lower z-index
>
>
> So, it will blend between elements.
>
>
> Wait, so it will blend with only the background images of other elements,
> not their foreground?
>

The background images is drawn as specified by the CSS painting order [1]
7.2.1.2 states when background images are drawn.
As David stated in his blog post [2], 'grouping' is not specified in this
spec. For the current behavior of simple source-over compositing, there is
no need to specify this since that alpha compositing is associative [3].
Blending unfortunately is not so it needs this.

Because of this, you never know if a stacking context creates an offscreen
bitmap so it that context contains a blended element, that element's
appearance is undefined.

What we need to do, is come up with a set of conditions that should create
offscreen bitmaps and then enforce them if absence of an offscreen bitmap
would create a visual difference.

1: http://www.w3.org/TR/CSS21/zindex.html
2: http://dbaron.org/log/20130306-compositing-blending
3: http://en.wikipedia.org/wiki/Alpha_compositing