Importance Sampling of Reflection from Hair Fibers
Hair and fur are increasingly important visual features in production rendering, and physically-based light scattering models are now commonly used. In this paper, we enable efficient Monte Carlo rendering of specular reflections from hair fibers. We describe a simple and practical importance sampling strategy for the reflection term in the Marschner hair model. Our implementation enforces approximate energy conservation, including at grazing angles by modifying the samples appropriately, and includes a Box-Muller transform to effectively sample a Gaussian lobe. These ideas are simple to implement, but have not been commonly reported in standard references. Moreover, we have found them to have broader applicability in sampling surface specular BRDFs. Our method has been widely used in production for more than a year, and complete pseudocode is provided.
Christope Hery and Ravi Ramamoorthi. "Importance Sampling of Reflection from Hair Fibers". Journal of Computer Graphics Techniques, 2012.