Modeling subsurface scattering with spatially varying layer properties

Determine a physically accurate and practical method to model subsurface scattering when the exit point encounters different local layer properties than the entry point, within the OpenPBR layered material framework for spatially varying materials.

Background

In OpenPBR, the subsurface component models dense volumetric scattering beneath a dielectric interface, with parameters controlling mean free path, anisotropy, and perceived color. Light that enters the medium may emerge at a different surface location after multiple scattering events.

Because material properties (e.g., coat presence, fuzz, base composition) can vary across the surface, the exit point may encounter a different layer configuration than the entry point. The notes acknowledge that a correct treatment should account for this spatial variation, but a concrete method for incorporating it into rendering is not yet established.