[Bf-committers] About Final Gathering

[Yves Poissant]

From: <echelon at infomail.upr.edu.cu>
Sent: Monday, April 06, 2009 12:37 PM


>  for the photon mapping build I have implemented now a first draft of
> final gathering according to the algorithm definition but I have still
> several doubts about it.
>   with FG you shoot random rays to gather illumination samples from the
> photon map (done), but that way you get high frecyuency noise and that
> is not observed in other softwares final gather algoritms.
>  did they use some sort of interpolation?

Brecht covered the issue quite well.

Usually, Final Gather (FG) is combined with Irradiance Caching (IC). That 
would be the interpolation scheme you are looking for I guess. But as 
already mentioned, without sufficient samples, the variations will either 
produce high frequency noise without IC or low frequency noise with IC. One 
way to avoid shooting a too high number of rays is to do Importance Sampling 
for the FG. When you gather the irradiance for each photons during the 
irradiance precomputation phase, you can use the photon directions to build 
an importance map. Find papers by Per H Christensen for Importance Sampling 
techniques for Photon Mapping. And of course, A good low discrepancy sampler 
is your definitive friend but I'm pretty sure you already know that.

And if you use FG, you need to store each photon first hit in the photon map 
contrary to the usual wisdon of not storing it. Otherwise, your FG will be 
off.

Yves