[Bf-cycles] Lightprobe importance sampling

[jumpjump88]

Hi Brecht and Mike, 
I am having trouble understanding this part:
"It is not needed that for every Light sample ray, there is a matching 
BSDF sample ray, all that is needed is that the pdf's match. Avoiding 
that extra BSDF sample and instead doing it using the BSDF sample that 
is already done for the next path vertex gives a nice speedup."
I learned MIS from PBRT, and as Mike said, PBRT takes one sample generated
from Light and one sample generated from BSDF, and then use power heuristic
to weight their contribution. My understanding is, for MIS to work, there
needs to have probabilities to sample from both light and BSDF. But if we
are only generating samples from BSDF, how will the light samples weigh in?
To be more specific, I am looking at the code in function
indirect_background(KernelGlobals *kg, Ray *ray, int path_flag, float
bsdf_pdf, int bounce), before calling indirect_background, the ray direction
is already determined by importance sampling the BSDF, it seems that there
is no chance to generate a ray direction by importance sampling the
background as a light source. Could you please point out what I am missing?

Thanks a lot,
Zhanpeng



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