Practical Multiple Scattering for Rough Surfaces

Joo Ho Lee

Abstract:

Microfacet theory concisely models light transport over rough surfaces.
Specular reflection is the result of single mirror reflections on each facet,

while exact computation of multiple scattering is either neglected, or mod-
eled using costly importance sampling techniques. Practical but accurate

simulation of multiple scattering in microfacet theory thus remains an open
challenge. In this work, we revisit the traditional V-groove cavity model
and derive an analytical, cost-effective solution for multiple scattering in

rough surfaces. Our kaleidoscopic model is made up of both real and virtual V-
grooves, and allows us to calculate higher-order scattering in the microfacets

in an analytical fashion. We then extend our model to include nonsymmetric
grooves, allowing for additional degrees of freedom on the surface geometry,

improving multiple reflections at grazing angles with backward compatibil-
ity to traditional normal distribution functions. We validate the accuracy of

our model against ground-truth Monte Carlo simulations, and demonstrate
its flexibility on anisotropic and textured materials. Our model is analytical,
does not introduce significant cost and variance, can be seamless integrated
in any rendering engine, preserves reciprocity and energy conservation, and
is suitable for bidirectional methods.