I would like to ask a few questions about the way the layered material system is handled in pbrt (atleast with respect to the path tracing integrator).
1. A material contains a BSDF which is nothing but a stack of bxdfs. During monte-carlo path tracing, at the intersection point, we can choose any layer of the stack to generate the incoming direction. My question is that is this similar to Erich Veach's 'One Sample Model'?
2. My other question is that are we doing a MIS of the bsdf itself. I know that once the incoming direction is sampled with respect to one bsdf, we compute the probabilities of how the same direction could be sampled with respect to other similar bxdfs. The probabilities are averaged over the number of similar bxdfs and the brdfs are added up (over similar bxdfs). But this doesn't look like the way MIS (multi sample model in Eric Veach) is formulated. I mean we don't compute any weights nor evaluate any functions that we'd like to do an MIS(The MIS evaluation EstimateDirect function is pretty clear though). Can anyone explain to me the logic of how this system was actually formulated and the logic behind it.
Practical and theoretical implementation discussion.
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