9.8.1. Normal Distribution Functions

The normal distribution function has a significant effect on the appearance of the rendered surface. The shape of the NDF, as plotted on the sphere of microfacet normals, determines the width and shape of the cone of reflected rays (the specular lobe), which in turn determines the size and shape of specular highlights. The NDF affects the overall perception of surface roughness, as well as more subtle visual aspects such as whether highlights have a distinct edge or are surrounded by haze.

However, the specular lobe is not a simple copy of the NDF shape. It, and thus the highlight shape, is distorted to a greater or lesser degree depending on surface curvature and view angle. This distortion is especially ...

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