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By convention, Bxdfs call the DrawSamples?DDrawSamples1D(), DrawSamples2D(), and/or DrawSamples3D() functions.  These functions do not increment the sampleids.  This means that an Integrator calling a Bxdf has to do this incrementing after the Bxdf call – otherwise multiple bxdf samples will be the same sample value, ie. the same direction.  Similar for light sampling and indirect illumination sampling: the Integrator has to increment the sampleids after the samples have been used.  This is easily done by looping over all the sample contexts in a RixRNG like this:

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