Re: [pygame] Pre-multiplied alpha

["René Dudfield" <renesd@xxxxxxxxx>]

ah, that's interesting.  Thanks for sharing :)

On 8/4/07, Brian Fisher <brian@xxxxxxxxxxxxxxxxxxx> wrote:
> If you are just talking about alpha blending, you can blend raw color
> values at exactly the same speed as pre-multiplied color values, when
> you do a little trick where you blend by the difference between source
> and dest ((dest - src)*alpha + dest for no pre-mult vs. dest*(1-alpha)
> + src with pre-mult), so there is no speed up at all for
> pre-multiplied for the most common cases
>
> Pre-multiplied only really helps on the performance front a little bit
> with additive blending (you can save a multiply per pixel per
> channel), or a whole lot when doing accurate color interpolation (like
> bilinear filtering or gaussian blur where you end up saving a divide
> per pixel channel)
>
>
> ... on a slightly related note, many bilinear filter implementations
> don't do interpolation correctly (they interpolate color without
> considering alpha), probably cause it's fast not to do it right - but
> using pre-multiplied alpha color values works correctly in those cases
>
>
> On 8/3/07, René Dudfield <renesd@xxxxxxxxx> wrote:
> > Hi,
> >
> > on a slightly related note, a pre-multiplied alpha blitter would be a
> > great a great speed up.
> >
> >
> > On 8/4/07, Will McGugan <will@xxxxxxxxxxxxxxx> wrote:
> > > Hi,
> > >
> > > Is there any way to pre-multiply a surface with its alpha channel? I
> > > guess I could to it with surfarray, but I was wondering if there was
> > > another way.
> > >
> > > Regards,
> > >
> > > Will McGugan
> > >
> >
>