Re: Compositing and Blending

On Sun, Aug 5, 2012 at 3:10 AM, Bob Holmes <rangsynth@gmail.com> wrote:

> THESE DOCUMENTS...
>
> 1)
> https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#backdropexamples
> 2) http://www.w3.org/TR/SVGCompositing/
>
> States that ...
>
> Blending is the aspect of compositing that calculates the mixing of
> colors where the source element and backdrop overlap. Blending takes
> the colors of the source element and mixes them with the backdrop in
> areas where the source element and backdrop overlap. Conceptually, the
> colors in the source element are blended in place with the backdrop.
> After blending, the modified source element is composited with the
> backdrop. In practice, this is usually all performed in one step.
>
> I THINK WHAT IT MEANS TO SAY IS THAT "ALL THE BLENDING AND
> COMPOSITING" IS ACTUALLY REDUCED TO WELL DEFINED FUNCTIONS AS
> DESCRIBED IN THE SECOND DOCUMENT.
>
> In other words the first document is making a distinction between the
> blending and the compositing, but in theory the functions are defined
> as singular functions already in the second? So why the distinction
> between the two in the first document?
>
> Is this correct?
>

Not quite. Only blending + src-over is described in the SVG compositing
spec.
In practice, this is what will happen most of the time so I expect that
browsers (that do software rendering) will optimize that particular path.

The new document allows any composting operation to happen after blending.
This will allow effects that were not very hard or impossible to achieve
with the old spec.
For instance, the new spec allows you to blend with the backdrop, but then
composite with 'source'. This will remove the backdrop but will still show
the interaction of the backdrop with the source.
Other hard to achieve effects allow masking of the blended source with the
backdrop,

We've had a conversation on the splitting of the properties on this list.
Just search for [css-compositing].
Let me know if you have question or if you feel that the split should not
happen.

Rik