SCEA programmer Naty Hoffman pointed us to a useful technical report from researchers Michael Ashikhmin and Simon Premoze on bidirectional reflectance distribution function, a formalism often used for representing surface reflection properties in computer graphics. Titled Distribution-based BRDFs, the paper is now available online.

Ashikhmin and Premoze outline several characteristics desirable in a useful BRDF representation:

• represent a significant number of real-world materials with sufficient accuracy for visual applications;
• use measured data and allow acquisition of the necessary information for existing materials quickly and easily;
• be able to model new materials from scratch, i.e., not rely exclusively on measured data;
• respect basic physical properties of non-negativity, reciprocity and energy conservation;
• allow efficient sampling in a Monte-Carlo rendering system;
• allow straightforward hardware implementation;
• be compact;
• have at least a semi-intuitive interpretation and be simple to use for non-BRDF experts.

Though many BRDF models have been proposed, the report notes that most of those models do not have "at least some of the desirable practical properties." Distribution-based BRDFs aims to present a simple, flexible model satisfying many of the above requirements: ]"We show that the proposed model provides a good approximation for many real world materials, obeys basic physical restrictions, allows straightforward hardware implementation and provides for efficient sampling in a Monte-Carlo rendering system. A procedure to fit the model to BRDF measurement data is presented which suggests a simplified way of measuring surface reflection."