Re: [Devel] Freetype2 & gamma correction

[Artur Zaprzala]

Vincent Caron wrote:
> Keith Packard said:
>
> > So, we really need two fixes -- one fix to adjust the partial coverage  values 
> > to
> > accomodate the output device pixel characteristics, and a gamma  correction to
> > adjust for output device luminosity behaviour.
>
>
> Keith said it all, the GRAYS_USE_GAMMA is a bad hack that works by chance in a
> peculiar environment, it should go away. The 'gamma code' in the ftstring demo
> implements such a hack, but it's wired to the [monochrome] rendering code here 
> (thus
> it failed horribly when the colors were inverted).
>
> I have implemented a gamma-corrected alpha blending to play with (in GTK+ widh 
> GdkRGB,
> I think I lost the code) : http://zerodeux.net/misc/ft/aa/
>
> Basically, you do the linear interpolation in the 'gamma corrected space', so 
> it goes
> like :
>
>   dest = GammaInverse( coverage * Gamma(src) + (1 - coverage) * Gamma(dest) )
I think that physically correct is to do alpha blending in linear space. 
For instance, notice that when you turn two light bulbs on instead of 
one you get twice as much light (energy simply sums up in linear space) 
but it is only 50% (roughly) brighter (the response of the eye is 
nonlinear).


--
Artur Zaprzala