Computation of Polarized Subsurface BRDF for Rendering

Abstract

Interest in polarization properties of the rendered materials is growing, but so far discussions on polarization have been restricted only to surface reflection, and the reflection due to subsurface scattering is assumed to be unpolarized. Findings from other field (e.g. optics and atmospheric science) show that volumetric interaction of light can contribute to polarization. So we investigated the polarized nature of the radiance field due to subsurface scattering as a function of the thickness of the material layer for various types of materials. Though our computations shows negligible polarization for material layers of high thickness, thin layered materials show significant degree of polarization. That means polarization cannot be ignored for subsurface component of reflection from painted surfaces (particularly painted metal surfaces) or from coated materials. In this paper we employ the vector radiative transfer equation (VRTE), which is the polarized version of the radiative transfer equation inside the material. We use a discrete ordinate based method to solve the VRTE and compute the polarized radiance field at the surface of the material layer. We generate the polarimetric BRDF from the solutions of the VRTE for incident irradiance with different polarizations. We validate our VRTE solution against a benchmark and demonstrate our results through renderings using the computed BRDF.