VisibilityChunk: average directional visibility for importance sampling

Monte Carlo ray tracing could suffer from noisy results due to large variance. Importance sampling is an effective strategy for reducing such variance. It requires to efficiently approximate the integral kernel of the rendering equation, a triple product of incident lighting, material properties and visibility. Among them, visibility is often ignored because its estimation demands expensive shadow ray casting or shadow map construction. Such neglect limits the effectiveness of importance sampling. This paper proposes an efficient method for estimating average visibility to improve the effectiveness of importance sampling.