Flash Cache: Reducing Bias in Radiance Cache Based Inverse Rendering

State-of-the-art techniques for 3D reconstruction are largely based on volumetric scene representations, which require sampling multiple points to compute the color arriving along a ray. Using these representations for more general inverse rendering --- reconstructing geometry, materials, and lighting from observed images --- is challenging because recursively path-tracing such volumetric representations is prohibitively expensive. Recent works alleviate this issue through the use of radiance caches: data structures that store the steady-state, infinite-bounce radiance arriving at any point from any direction. However, these solutions rely on approximations that introduce bias into the renderings and, more importantly, into the gradients used for optimization. We present a method that avoids these approximations, while remaining computationally efficient. In particular, we leverage two techniques to reduce variance for unbiased estimators of the rendering equation: (1) a trainable, occlusion-aware importance sampler for incoming illumination and (2) a fast cache architecture that can be used as a control variate for the radiance from a high-quality, but more expensive, volumetric cache. We show that our approach, and removing these biases, improves the quality of recovered geometry and materials, especially in the presence of effects like specular reflections.