About vectorization and planar channels in Krita

Andre Gemuend scroogie at scroogie.de
Wed Oct 24 17:31:49 UTC 2012

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.

> 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.


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