Guided path tracing using clustered virtual point lights

Monte Carlo path tracing has been increasingly popular in movie production recently. It is a general and unbiased rendering technique that can easily handle diffuse and glossy surfaces. To trace light paths, most of existing path tracers rely on surface BRDFs for directional sampling. This works well for glossy appearance, but tends to be not effective for diffuse surfaces because in such cases, the rendering integral is mostly driven by the incoming radiance distribution, not the BRDFs. Therefore, with the same number of samples, it is more favorable to sample the incoming radiance distribution to achieve better effectiveness for diffuse scenes. [Vorba et al. 2014] addressed this sampling problem by using photons to estimate incoming radiance distributions which can then be compactly represented using Gaussian mixture functions.