About vectorization and planar channels in Krita

Boudewijn Rempt boud at valdyas.org
Thu Oct 25 14:18:09 UTC 2012

On Wednesday 24 October 2012 Oct, Andre Gemuend wrote:
> Hello everyone,
> sorry for suddenly dropping in here, until now I was only reading this list. I hope you don't mind
> that I comment on this issue, because it interests me.

No, all insights are very welcome!

> > We have one (and the only one) division in the composite over: it scales
> > the result by the new alpha value (srcBlend), so it looks like it cannot be
> > transformed to multiplication. This single division takes about 20% of
> > composing time.
> I just wanted to say that this is a disadvantage of non-premultiplied alpha. You actually
> divide out the new alpha value from the new colors, because the result of the composition is by
> definition a color premultiplied with alpha.
> Moreover, you do this for every composition, iow a pixel undergoes N divisions,
> whereas in the premultiplied case you would only have one division at the very last step, iff
> you really need the non-premultiplied value. Its also much easier to implement with premultiplied alpha.

The previous maintainer of Krita always intended to make Krita work with pre-multiplied alpha -- but I never really understood how to make that properly work given lots of independent layers. I don't think other open source graphics apps use pre-multiplied alpha either, but I'm not completely sure of that. Do you know of one?

Boudewijn Rempt
http://www.valdyas.org, http://www.krita.org, http://www.boudewijnrempt.nl

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