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

On May 29, 2013, at 9:23 PM, Rik Cabanier <cabanier@gmail.com> wrote:



On Wed, May 29, 2013 at 8:35 PM, Dirk Schulze <dschulze@adobe.com> wrote:

>
> On May 29, 2013, at 7:41 PM, Rik Cabanier <cabanier@gmail.com> wrote:
>
> >
> >
> > On Wed, May 29, 2013 at 7:14 PM, Dirk Schulze <dschulze@adobe.com>
> wrote:
> >
> > On May 29, 2013, at 6:59 PM, Rik Cabanier <cabanier@gmail.com> wrote:
> >
> > >
> > >
> > > 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]
> > > > 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.
> >
> > If ar = as, why would the alpha channel would not be affected on
> blurring?
> >
> > If you do a gaussianBlur, its input regardless of it's blended or not,
> will always have the same alpha values.
> > A good question is if blurring after blending makes sense of course :-)
>
> You blur all channels, including the alpha channel for feGaussianBlur.
> Since you say ar = as, this basically means source-in, doesn't it?
>

I'm unsure what you mean.
Blending changes the color, but leaves the alpha. This result is fed into
feGaussianBlur which will change the colors and the alpha. There's no
'compositing' happening...


>From the 1.1 spec:

"For all feBlend modes, the result opacity is computed as follows:

qr = 1 - (1-qa)*(1-qb)"

(If either of the inputs is fully opaque then result is fully opaque.
Perhaps this is what Dirk means?)