Re: Compositing math in SVG

That looks correct.
Try to debug your code with solid color bitmaps (the top one should be 75%
alpha) and see if your output matches the formulas and photoshop's/gimp's
rendering.

Rik

On Wed, Jul 25, 2012 at 5:59 PM, Calculemus <calculemus1988@gmail.com>wrote:

> Hmm, actually I do take care of that.
>
> These are the formulas I use:
>
> Notation:
>
> a - premultiplied color
> alpha_a - alpha for a
> same notation for b
>
> color = a*(1-alpha_b) + b*(1-alpha_a) + alpha_a*alpha_b*Blend(a/alpha_a,
> b/alpha_b)
> alpha = alpha_a + alpha_b*(1-alpha_a)
>
> where Blend is replaced with the proper blend formula as needed.
>
> I don't divide 'color' with 'alpha' because we use pre-multiplied values.
> But I do provide Blend with non-premultiplied values, that is why I divide
> there.
>
>
>