PBRT_V2 总结记录 <33> Microfacet 和 MicrofacetDistribution

概述

（这里 模拟一个金属表面，这个面是由 很多 很细的十分平滑的microfacets(微小的面)组成，这些microfacets 有自己的法线，对于整一个 表面的BRDF来说，需要传入 wi 和wo, 那么，只有那些法线是 wh 的 microfacets ，才会 从wi 方向 反射 光到 wo 方向上）

One of the first microfacet models for computer graphics was developed by Torrance
and Sparrow (1967) to model metallic(金属) surfaces. They modeled surfaces as collections of
perfectly smooth mirrored microfacets. The surface is statistically described by a distribution
function D(ωh) that gives the probability that a microfacet has orientation ωh
(recall Figure 8.12, which shows how roughness and the microfacet normal distribution
function are related).

Figure 8.12: Microfacet surface models are often described by a function that gives the distribution
of microfacet normals nf with respect to the surface normal n. (a) The greater the variation of
microfacet normals, the rougher the surface is. (b) Smooth surfaces have relatively little variation
of microfacet normals.

Because the microfacets are perfectly specular, only those with a normal equal to the halfangle
vector,

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