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Wood coated with transparent finish has a beautiful and distinctive appearance that is familiar to everyone. Woods with unusual grain patterns. such as tiger, burl, and birdseye figures, have a strikingly unusual directional reflectance that is prized for decorative applications. With new, high resolution measurements of spatially varying BRDFs. we show that this distinctive appearance is due to light scattering that does not conform to the usual notion of anisotropic surface reflection. The behavior can be explained by scattering from the matrix of wood fibers below the surface, resulting in a subsurface highlight that occurs on a cone with an out-of-plane axis. We propose a new shading model component to handle reflection from subsurface fibers, which is combined with the standard diffuse and specular components to make a complete shading model. Rendered results from fits of our model to the measurement data demonstrate that this new model captures the distinctive appearance of wood.
Marschner et al. (Fri,) studied this question.