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Diffuse Materials

BRDFs that model diffuse materials approximates behaviour of light when light interacts with highly scattering materials. The most well-known is obviously the Lambertian model whose BRDF is only a constant and easiest to implement. Also, Oren-Nayar models the real world objects more realistically than Lambertian model. It assumes the surface is made up of many microfacets each of which is a perfect lambertian surface. Since the model does not have a closed-form solution, an approximation can be used which is presented by the related work. It considers shadowing, masking and interreflection between the facets.

There is not a specific importance sampling strategy for BRDF parts. Samples are taken from cosine-weighted distribution which is already enough to reduce most of the variance.