[Bf-committers] About Final Gathering
Yves Poissant
ypoissant2 at videotron.ca
Tue Apr 7 01:48:06 CEST 2009
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
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