About vectorization and planar channels in Krita

Sven Langkamp sven.langkamp at gmail.com
Thu Oct 25 17:53:39 UTC 2012

On Wed, Oct 24, 2012 at 7:31 PM, Andre Gemuend <scroogie at scroogie.de> 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.
>  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.

It doesn't do N division per pixel currently, just one.

Beside that using premultiplied alpha causes a loss in precision.
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