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List:       kde-kimageshop
Subject:    About vectorization and planar channels in Krita
From:       Andre Gemuend <scroogie () scroogie ! de>
Date:       2012-10-24 17:31:49
Message-ID: 50882605.8090901 () scroogie ! de
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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 t=
he composition is by
definition a color premultiplied with alpha.

Moreover, you do this for every composition, iow a pixel undergoes N divisi=
ons,
whereas in the premultiplied case you would only have one division at the v=
ery last step, iff
you really need the non-premultiplied value. Its also much easier to implem=
ent with premultiplied alpha.


Greetings
Andr=E9

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