Let's talk about Prefiltered Environment Mapping!

Environment maps are a widely used techniques to approximate reflections ininteractive rendering.
Although environment maps make the assumption that the reflected environment is far away - thus being an approximation - they often nevertheless achieve convincing reflections.
Recently environment maps have been introduced as a means to render glossy reflections.
All of these methods prefilter a given environment map with either a fixed reflection model or a certain class of BRDF's(bidirectional reflectance distribution functions).
Although these methods are similar,they have different strengths and weakness,which are worthwhile to discuss.
In order to be able to compare these methods we present a general notation of prefiltered environment maps,which allows us to classify and contrast all the well-known prefiltering techniques.