Yet Another Box
Yet Another Box is an intentionally difficult scene for path guiding techniques. Like in the Indirectly Lit Cornell Box, the light source is facing upward. However, in this scene the light source is much tinier and the ceiling is modeled as a highly glossy material. This makes the scene almost impossible to render with unidirectional path tracing. Path guiding techniques have to learn a radiance field which is high-frequency both in the spatial and directional domain. PPG [Müller et al. 2017] is outperformed by the bidirectionally trained GMMs of Vorba et al.  and our unidirectionally-trained radiance-driven neural path guiding (NPG-Radiance) at equal sample counts. Interestingly, our product-driven approach (NPG-Product) performs slightly worse than the radiance-driven approach, suggesting that the larger number of dimensions makes it more difficult to optimize our neural networks. Improving upon this limitation is an interesting avenue for future work. Primary-sample-space path-sampling techniques are able to learn some components of the light transport, but fail to provide a consistent improvement over unidirectional path tracing. Our NPS slightly outperforms PSSPS [Guo et al. 2018] with an equal number of samples.