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List:       kde-kimageshop
Subject:    Re: About vectorization and planar channels in Krita
From:       Martin Renold <martinxyz () gmx ! ch>
Date:       2012-10-27 19:43:21
Message-ID: 20121027194321.GA18503 () old ! homeip ! net
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On Thu, Oct 25, 2012 at 04:18:09PM +0200, Boudewijn Rempt wrote:
> On Wednesday 24 October 2012 Oct, Andre Gemuend wrote:
> > 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?

Using premultiplied alpha as an image model doesn't seem to be very common,
but MyPaint does it.  I mentioned it in this (very old) blogpost:
http://mypaint.intilinux.com/?p=19

What I like: when you do all the math premultiplied, you are not tempted to
use color information that is barely visible (e.g. un-erasing, and you will
think twice about picking a color from an almost transparent layer).

The main drawback is that save/load always is un-premultiplied, and you
cannot roundtrip the invisible colors.  And some color transformations
become harder (you have to un-premultiply first and come up with some random
color if the alpha is zero).

-- 
Martin Renold
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