Applicability of back-vector retro-reflection to dielectric microfacets

Ascertain whether the back-vector modification used in the minimal retro-reflective microfacet model should be applied to both conductor and dielectric microfacet BRDFs in OpenPBR, or restricted to conductors, given plausibility concerns for dielectric retro-reflection.

Background

The notes evaluate a retro-reflection model that replaces the half-vector with a back-vector in the microfacet formulation, yielding an efficient, energy-conserving, and reciprocal BRDF. This approach matches measured retro-reflective materials reasonably well and can be mixed with standard microfacet reflection via a weight parameter.

While visually plausible for conductors, the suitability for dielectrics is questioned because typical retro-reflective materials are not dielectric microfacet surfaces. The authors explicitly flag the decision of applying the modification to dielectrics as an unresolved question.