All Rix plugins are descendants of this class.
Describe various conventions that are shared amonst RixBxdf, RixIntegrator, RixProjection etc.
Must describe InstanceData.
important class! must describe EvalParam and how classes use it!
even more important class!
numPts
scTraits
integratorCtxIndex, rayId, etc etc
GetBuiltinVar: why
GetPrimVar: why
EvalParam: how to pull on upstream pattern nodes
when to SetBuiltinVar
GetNearestHits: when to trace rays
Memory allocation