get the pdf of the sampled direction

@brief Evalute the pdf of an existance direction given the Incident direction. If one implements customized sample_f for the brdf, it needs to have c

Importance sampling for catmull rom

Importance sampling for fourier interpolation Refer these two wiki pages for further detail: Bisection method : https://en.wikipedia.org/wiki/Bisect

sample a direction randomly

@brief Importance sampling for the bxdf. This method is not pure virtual and it has a default implementation, which sample out-going directions that

sample ray from light

sample a ray

sample ray from light