Re: [filter-effects] feBlend filter (was: Re: [css-filters] feBlend filter)

On Wed, May 29, 2013 at 5:33 PM, Dirk Schulze <dschulze@adobe.com> wrote:

>
> On May 29, 2013, at 5:05 PM, Rik Cabanier <cabanier@gmail.com> wrote:
>
> [snip]
>
> >> yes. An 'feComposite' with the backdrop will always be followed with
> source-over.
> >>
> >> Also, todays use of feBlend and feComposite is limited to blend and
> composite intermediate filter effects results with each other (due to the
> lack of BackgroundImage support on browsers). I think for these operations
> the compositing step in feBlend makes sense, even so you still have the
> compositing of the filter result to the canvas.
> >>
> >> That's true.
> >>
> >> I do not understand how a new filter primitive can solve this problem
> if you still have the compositing step at the end. Can you elaborate more
> on your suggestion?
> >>
> >> My issue is that the formulas for feBlend always composite, so we need
> formulas that don't do this.
> >>
> >> These formulas don't composite and are in non premultiplied alpha:
> >> Normal:  Cr = Cs
> >> Multiply: Cr = (1 - αb) x Cs + αb x B(Cb, Cs) = (1 - αb) x Cs + αb x Cb
> x Cs
> >> Screen:  Cr = (1 - αb) x Cs + αb x B(Cb, Cs) = (1 - αb) x Cs + αb x (Cb
> + Cs - (Cb x Cs)) = Cs + αb x (Cb - (Cb x Cs))
> >> Darken:  Cr = (1 - αb) x Cs + αb x B(Cb, Cs) = (1 - αb) x Cs + αb x
> min(Cb, Cs)
> >> Lighten:  Cr = (1 - αb) x Cs + αb x B(Cb, Cs) = (1 - αb) x Cs + αb x
> max(Cb, Cs)
> >>
> >> For premultiplied alpha, they become:
> >> Normal:  cr = cs
> >> Multiply: cr = (1 - αb) x cs + cb x cs
> >> Screen:  cr = (1 - αb) x cs + αs x αb x (Cb + Cs - (Cb x Cs)) = cs + αs
> x cb - cb x cs
> >> Darken:  cr = (1 - αb) x cs + αs x αb x min(Cb, Cs) = (1 - αb) x cs +
> min(αs x cb, αb x cs)
> >> Lighten:  cr = (1 - αb) x cs + αs x αb x B(Cb, Cs) = (1 - αb) x cs +
> max(αs x cb, αb x cs)
> >> Mu
> >> These formulas either replace the ones for feBlend, or become the ones
> for a new filter primitive.
> >
> > If we replace the formulas with the one without compositing, what would
> it mean for blending two intermediate results?
> > The result would exclude compositing completely. It is unclear how this
> can be done.
> >
> > You'd have to explicitly call feComposite on the blended result.
>
> feComposite uses the alpha channel of the two input primitives. feBlend
> the same, just that you loose the alpha channel on the result of your
> feBlend (since the state is unclear).


Why would you lose the alpha channel? Blending doesn't affect source alpha
(αr = αs)


> If the next primitive after feBlend is feGaussianBlur, what would be
> blurred on the alpha channel? I do not think that you can easily separate
> blending from compositing inside a SVG filter tree.
>

Sure you can. feGaussianBlur would work the same except that the source
colors are now blended and alpha is unaffected.


>
> >
> > Note that feBlend and feComposite is not always the last primitive, so
> even for BackgroundImage there are problems on the next filter primitive in
> the chain.
> >
> > If you want to a blend and then a blur, you don't want to composite.
> (Michael posted another example of the backdrop becoming blurred)
> > I'm unsure what you mean with "there are problems on the next filter
> primitive in the chain."
>
> See question above.
>
> Greetings,
> Dirk