The paper presents new, simple and physically plausible, but not physically based reflectance models for metals and other specular materials. The most important member of this family is the modification of the Phong model, which can eliminate its non-metallic characteristics. The new model gives back the ideal mirror in the limit case, is easier to compute than other known models and is particularly suitable for importance sampling to efficiently generate reflected directions in Monte-Carlo ray-tracing algorithms. Due to its simplicity, the new model can become an integral part of commercial CAD systems to describe the specular part or metallic behavior. The paper also examines the energy balance of the previously known and the newly proposed BRDFs concentrating on the behavior at grazing angles, and comes to the conclusion that the Ward and the Cook-Torrance models cannot guarantee energy balance and thus are not physically plausible. Finally, the generated images demonstrate how the metallic impression can be provided by the new models.
Reflectance function, BRDF representation, albedo function, importance