ABSTRACT
Lighting or illumination models are divided into two categories: local illumination and global illumination. The Phong model is the representative of local illumination models, where the illumination of an object depends solely on the properties of the object and the light sources. Even though light sources are invisible from a particular point in the scene, light can still be transferred indirectly to the point through reflections from other object surfaces. The ambient term of the Phong model accounts for such indirect lighting but it is overly simplified. When the irradiance values are pre-computed, there is no real-time constraint. Therefore, we can use an expensive global illumination model. Environment mapping is often taken as an effort toward global illumination, and in fact it adds a global illumination look to the images generated by a local illumination model. However, it is just a small step out of local illumination and does not sufficiently overcome its limitations.
