# Ugh... Qt4 porting

Ingo KlĂ¶cker kloecker at kde.org
Mon May 16 16:00:28 BST 2005

```On Monday 16 May 2005 15:06, Lars Knoll wrote:
> On Monday 16 May 2005 14:55, Mosfet wrote:
> > >Lars wrote:
> > >> I'm also curious about what they are going to do with the new
> > >> Premultiplied format. As far as I can tell it's a 32bpp format
> > >> with the source part of the alpha already calculated. Good if
> > >> your going to alphablend the image a lot, bad if you want to
> > >> modify the original color channels.
> > >
> > >The image paint engine uses it. All drawing operations are a lot
> > > faster when using premultiplied alpha. It's also the format used
> > > by RENDER, so it matches fine there as well.
> > >
> > >And I don't see why you shouldn't be able to modify the color
> > > channels in a premultiplied image more or less the same way then
> > > in a non premultiplied one.
> >
> > Because a pixel's RGB values aren't the actual RGB values, they are
> > averaged with the alpha value. So if your doing something like
> > interpolating a color you'd be interpolating with the wrong color,
> > no?
>
> As long as you don't take the premulitplication into account, yes.
> But it's trivial to do it right.
>
> > Even
> > simple things like the gray level would be wrong. A red value of 25
> > may actually be 100 but with 1/4th the opacity. Most effects won't
> > work properly with this.
>
> The math behind is trivial: Instead of the value being R directly,
> it's A*R. As long as you know it's premultiplied it should be trivial

The problem is that the precision of the premultiplied values is less
than the precision of the non-premultiplied values due to integer
arithmetic being used. In general
( A*R ) / A != R
So you can't recover the original values from the premultiplied values.
Now if you do several iterations of this then the precision loss might
increase dramatically
(http://en.wikipedia.org/wiki/Propagation_of_errors_resulting_from_algebraic_manipulations).
And this is far from being trivial math.

Regards,
Ingo
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