Re: [compositing] Add plus-darker blend mode

On Mon, Jan 19, 2015 at 4:31 PM, Dean Jackson <dino@apple.com> wrote:

> Our designers at Apple often request a blending operation that is not
> supported in the CSS Compositing and Blending specification: plus-darker.
> You can find many examples and discussion of this mode if you Bing on the
> Google.
>
> We’d like to add this mode to Level 2.
>
> Details are here: http://en.wikipedia.org/wiki/Blend_modes#Addition
>
> The math is pretty simple:
>
> R = min(0, (1 - D) + (1 - S))
>

Doesn't Core Graphics implement this as a compositing operation as opposed
to a real blend?

Per the spec [1],  a blending operation calculates the result as follows:

αo x Co = αs x (1 - αb) x Cs + αs x αb x B(Cb, Cs) + (1 - αs) x αb x Cb

with B being the blend mode formula.
I believe CG implements plus-darker as:

αo x Co = min(0, (1 - αb x Cb) + (1 - αs x Cs))


1: http://dev.w3.org/fxtf/compositing-1/#blending