A Practical Model for Subsurface Light Transport
In computer graphics, a bidirectional reflectance distribution function (BRDF) is used to model light reflectance properties at a surface, and is defined as the ratio of the radiance (incident light) to the irradiance (reflected light) per unit surface area. All BRDF models operate on surface scattering, i.e., the assumption that “light scatters at one surface point” and that light enters and exits a material at the same position, and do not model subsurface transport of incident light. Although this assumption stands valid for metals, translucent surfaces modeled using BRDF exhibit a distinct hard and computer-generated appearance and poor blending of local color and geometry features. While there have been works to model subsurface transport of light, the existing methods are either slow or inefficient for anisotropic or highly scattering translucent media (such as skin and milk). This papers attempts to address this shortcoming by proposing a model for subsurface light transport in translucent materials using bidirectional surface scattering reflectance distribution function (BSSRDF). BSSRDFs are a generalization of BRDFs and, unlike the latter, can model light transport between any two rays that hit a surface. Since the exact BSSRDF derivation is quite involved, we only present a brief summary here, followed by its extension to a model for rendering computer graphics. ...