Lots of posts today, sorry for that. This one will be short.
So, Final Gather is slow as molasses. Furthermore it seems to be not working right in my code. But, it is very important for smoothing out noise in the photon map. Enter the irradiance cache.
The irradiance cache is a concept introduced by Greg Ward and co. back in 1988 and I believe is part of Ward's Radiance renderer. The paper that describes it in detail (and which is surprisingly easy to read) can be found here (hint to Scott, check this out):
The basic idea is that diffuse inter-reflections are very hard to compute using Monte Carlo methods (and Final Gather is a Monte Carlo method) but by-and-large they are very uniform and slowly changing across a surface (i.e. a good candidate for interpolation). So we would like to reduce the number of times it needs to be computed and interpolate everywhere else.
Ward describes a two tiered method: the primary method is the standard Monte Carlo method and the secondary method will interpolate cached values from previous computations. The secondary method needs to know when it can interpolate and how to weight the cached values. This is done by approximating the gradient of irradiance between the cached point and the point we need irradiance for. If the gradient estimate is too high a new value is needed. Otherwise we can weight the cached value by the inverse of the error inherent in the estimate and get a very good approximation that is considerably cheaper than the Monte Carlo method.
The devil's in the details and I won't bore you with it (other than to say it involves an octree based around the valid distance of each cached sample). But with Jensen's summary of Ward's paper (and the paper itself) I think I've got the Irradiance Cache implemented. It needs some testing and such (still to come) but hopefully this will help with noise and speed ... provided I can then figure out what is wrong with the Final Gather!
I'd kill for smaller unit testing right now but we'll see if we need it first.
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Info about progress on my Google Summer of Code 2009 project on Advanced Lighting & Shading in CrystalSpace.
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