

Am 11.03.2011 22:52, schrieb Trevor G Quayle:
> However, I am not fully convinced the light fading model being used is correct.
> For inverse square, I would think the genral function would be:
>
> a= FD/(x^2)
>
> There is a difference between this function and the one used: at x=1, the
> functions are equal, however as x increases, the function used overcalculates
> the value (eg, x=1.3 +25%, x=2.0 +60%, x=inf +100%). Below x=1, the function
> undercalculates the value (eg, x=0.7 34%, x=0.5 60%).
The formula is defined as it is to account for the fact that real light
sources are not point light sources.
> I have posted a graphic in p.b.i to illustrate the differences. Perhaps
> generally the differences in a scene may seem subtle, but there may be
> circumstances (eg, my Matinee scene) where it can make a difference. The light
> fading of the projector 'looks' ok, as we don't have any reference, but in
> reality, the decay should occur quicker. One way to get closer I suppose, is to
> use a higher fade power in the formula (oddly, using exp(1) for the power gives
> a reasonable balance between x=1 and x=4 with a variance from +12% to 28%),
> however it is still not technically correct (perhaps that just bothers me as a
> perfectionist...)
The proper way to do it would be to reduce fade_distance to a very small
value, and increase the light intensity to compensate.
Maybe things become a bit clearer by writing the attenuation function
differently:
a = 2 / ( 1 + (d/FD)^FP )
= 2 / ( FD^FP / FD^FP + d^FP / FD^FP )
= 2 / ( ( FD^FP + d^FP) / FD^FP )
= FD^FP / ( FD^FP + d^FP )
= c / ( c + d^FP )
a ~ 1 / ( c + d^FP )
If you'd choose c=0 and FD=2, you'd theoretically get the attenuation
law you're asking for  except that you'd need an infinitely bright
light source to compensate for the proportionality factor. So choose a
very, very small FD (giving a very, very small c in the above formula),
multiply the light source intensity by FD^FP (presuming you previously
used FD=1), and you'll be fine. Even theater projectors aren't perfect
in simulating a point light source, so it's also realistic.
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