  Importance sampling the distribution of visible GGX normals requires sampling
those of a hemisphere. In this work, we introduce a novel method for sampling
such visible normals. Our method builds upon the insight that a hemispherical
mirror reflects parallel light rays uniformly within a solid angle shaped as a
spherical cap. This spherical cap has the same apex as the hemispherical
mirror, and its aperture given by the angle formed by the orientation of that
apex and the direction of incident light rays. Based on this insight, we sample
GGX visible normals as halfway vectors between a given incident direction and
directions drawn from its associated spherical cap. Our resulting
implementation is even simpler than that of Heitz and leads to up to systematic
speed-ups in our benchmarks.
