5 - General material models

Abstract

This chapter presents the basic models for how materials scatter light. One approach for representation of material models is to store values of the reflectance for densely sampled values of position, wavelength, and direction. A table of such densely sampled models would require an enormous amount of storage for even simple objects. In addition to computational inefficiency, it is not a useful representation for a user to specify materials. A number of different criteria can be used to guide the construction of a good representation. Criteria include capturing the visible effects, obeying the physical constraints of conservation of energy, and reciprocity, compactness, complying with the electromagnetic wave theory of light, ease of evaluation, and meaningful parameters for user input. Different approaches choose to emphasize some subset of these criteria and ignore others. The most basic material models are bidirectional reflectance distribution functions (BRDFs) that describe the spectral and directional characteristics of a material at a spatial location. Although BRDFs often have complicated derivations and form, they generally reduce to evaluating an expression. It traces the development of the various named reflectance models such as Phong, Blinn, Cook–Torrance, and Oren–Nayar that appear in most graphics systems. It also puts in context more complex models that account for interference, diffraction, and volumetric scattering.