Copper Hairball

The glossy Copper Hairball was deliberately placed inside of a glass cube to create pathologically difficult light transport under directional illumination by the sun. Unbiased flavors of path tracing - including bidrectional path tracing - can not sample the triple product of the BSDF, incident illumination, and cosine term well. While radiance-based path guiding offers a significant improvement over unidirectional path tracing and primary-sample-space path-sampling techniques, product-based path guiding is required to fully capture the triple product on the copper hairball itself. We demonstrate that neural networks can leverage additional features, such as the surface normal, to capture the triple product even on high-frequency geometry such as the hairball (NPG-Product).