It would take many thousands of samples per pixel to properly integrate light coming from such a concentrated source. The key to fast convergence is in deciding what to sample by removing those parts of the integral we can compute deterministically and gauging the importance of the rest so as to maximize the payback from our ray calculations. In the case of the outdoor scene just described, we would want to consider the sun as an important contribution to be sampled separately, thus removing the biggest source of variance from our integral. Instead of relying on random samples over the hemisphere, we send a single sample ray towards the sun, and if it arrives unobstructed, we use a deterministic calculation of the total solar contribution based on the known size and luminosity of the sun as a whole. We are making the assumption that the sun is not partially occluded, but such an assumption would only be in error within the penumbra of a solar shadow region, and we know these regions to represent a very small portion of our scene.
Light sources cause peaks in the incident radiance distribution.
Directional reflection and transmission cause peaks in the scattering function. Removing such peak reflection and transmission angles by sending separate samples reduces the variance of our integral at a comparatively modest cost. Electric Lighting 4.2 Daylighting 5. Software Availability 8.