Sampling Projected Spherical Caps in Real Time

Stochastic shading with area lights requires methods to sample the light sources. For diffuse materials, the best strategy is to sample proportionally to projected solid angle. Recent work in offline rendering has addressed this problem for spherical light sources, but the solution is unsuitable for a GPU implementation. We present a far more efficient solution. It offers results without noteworthy noise for diffuse surfaces lit by an unoccluded spherical light source while being only two to three times more costly than simple sampling of the solid angle. The core insight of the technique is that a projected spherical cap can be decomposed into, or at least approximated by, cut disks. We present an efficient method to sample cut disks and show how to use it to sample projected spherical caps. In some cases, our method does not sample exactly proportionally to projected solid angle but the deviation is provably bounded

Slope-space integrals for specular next event estimation

International audienceMonte Carlo light transport simulations often lack robustness in scenes containing specular or near-specular materials. Widely used uni- and bidirectional sampling strategies tend to find light paths involving such materials with insufficient probability, producing unusable images that are contaminated by significant variance.This article addresses the problem of sampling a light path connecting two given scene points via a single specular reflection or refraction, extending the range of scenes that can be robustly handled by unbiased path sampling techniques. Our technique enables efficient rendering of challenging transport phenomena caused by such paths, such as underwater caustics or caustics involving glossy metallic objects.We derive analytic expressions that predict the total radiance due to a single reflective or refractive triangle with a microfacet BSDF and we show that this reduces to the well known Lambert boundary integral for irradiance. We subsequently show how this can be leveraged to efficiently sample connections on meshes comprised of vast numbers of triangles.Our derivation builds on the theory of off-center microfacets and involves integrals in the space of surface slopes.Our approach straightforwardly applies to the related problem of rendering glints with high-resolution normal maps describing specular microstructure. Our formulation alleviates problems raised by singularities in filtering integrals and enables a generalization of previous work to perfectly specular materials. We also extend previous work to the case of GGX distributions and introduce new techniques to improve accuracy and performance