High-Fidelity Specular SVBRDF Acquisition From Flash Photographs

Obtaining accurate SVBRDFs from 2D photographs of shiny, heterogeneous 3D objects is a highly sought-after goal for domains like cultural heritage archiving, where it is critical to document color appearance in high fidelity. In prior work such as the promising framework by Nam et al., the problem is simplified by assuming that specular highlights exhibit symmetry and isotropy about an estimated surface normal. The present work builds on this foundation with several significant modifications. Recognizing the importance of the surface normal as an axis of symmetry, we compare nonlinear optimization for normals with a linear approximation proposed by Nam et al. and find that nonlinear optimization is superior to the linear approximation, while noting that the surface normal estimates generally have a very significant impact on the reconstructed color appearance of the object. We also examine the use of a monotonicity constraint for reflectance and develop a generalization that also enforces continuity and smoothness when optimizing continuous monotonic functions like a microfacet distribution. Finally, we explore the impact of simplifying from an arbitrary 1D basis function to a traditional parametric microfacet distribution (GGX), and we find this to be a reasonable approximation that trades some fidelity for practicality in certain applications. Both representations can be used in existing rendering architectures like game engines or online 3D viewers, while retaining accurate color appearance for fidelity-critical applications like cultural heritage or online sales.