A LARGE PART OF RENDERING HAS TRADITIONALLY been concerned withphotorealistic rendering, where the goal is to mimic the appearance of reality as closely as possible. Many of these methods are based on physical models of light transport, especially global illumination methods such as ray tracing [79], radiosity [5], or photon mapping [114]. These physically based methods for generating realistic images have become more and more sophisticated over the years [55]. However, even empirical localillumination models such as the Blinn-Phong model [12] try to emulate the appearance of real light reflection with very simple means, even though they are not completely based on actual physics.