should be the relative IOR of the outgoing media to the incoming media, depending on `bsdf->ior` and whether the interaction is refraction. Reference paper: [Robust Fitting of Parallax-Aware Mixtures for Path Guiding](https://uni-tuebingen.de/fakultaeten/mathematisch-naturwissenschaftliche-fakultaet/fachbereiche/informatik/lehrstuehle/computergrafik/lehrstuhl/veroeffentlichungen/robust-fitting-of-parallax-aware-mixtures-for-path-guiding/) Eq (35) Pull Request: https://projects.blender.org/blender/blender/pulls/112157