We introduce a new class of primitive functions with non-linear parametersfor representing light reflectance functions. The functionsare reciprocal, energy-conserving and expressive. They can captureimportant phenomena such as off-specular reflection, increasing reflectanceand retro-reflection. We demonstrate this by fitting sumsof primitive functions to a physically-based model and to actualmeasurements. The resulting representation is simple, compact anduniform. It can be applied...