Analyzing and predicting anisotropic effects of BRDFs

The majority of the materials we encounter in the real-world have variable reflectance when rotated along a surface normal. This view and illumination azimuthally-variable behavior is known as visual anisotropy. Such behavior can be represented by a four-dimensional anisotropic BRDF that characterizes the anisotropic appearance of homogeneous materials. Unfortunately, most past research has been devoted to simplistic three dimensional isotropic BRDFs. In this paper, we analyze and categorize basic types of BRDF anisotropy, use a psychophysical study to assess at which conditions can isotropic appearance be used without loss of details in material appearance. To this end, we tested the human impression of material anisotropy on various shapes and under two illuminations. We conclude that subjects sensitivity to anisotropy declines with increasing complexity of 3D geometry and increasing uniformity of illumination environment. Finally, we derive and perceptually validate a computationally efficient measure of material visual anisotropy.