Planetary Shadow-Aware Distance Sampling

Abstract

Dusk and dawn scenes have been difficult for brute force path tracers to handle. We identify that a major source of the inefficiency in explicitly path tracing the atmosphere in such conditions stems from wasting samples on the denser lower parts of atmosphere that get shadowed by the planet before the upper, thinner parts when the star sets below the horizon. We present a technique that overcomes this issue by sampling the star only from the unshadowed segments along rays based on boundaries found by intersecting a cylinder fit to the planet's shadow. We also sample the transmittance by mapping the distances of the boundaries to opacities and sampling the visible segments uniformly in opacity space. Our technique can achieve similar quality compared to brute-force path tracing at around a 60th of the time in such conditions.