Christophe Hery, Ravi Ramamoorthi
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 reflections from hair fibers by describing 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.
A revision of this work was accepted as the first paper in the Journal of Computer Graphics Techniques, and available as http://jcgt.org/published/0001/01/01/.
Available as Pixar Technical Memo #11-07