Improved shading performance by avoiding vector normalization

Phongs illumination model requires unit length vectors. The surface normal has to be normalized due to the linear interpolation, and if we use single point light sources or a fixed view point, we have to normalize the vectors pointing to the light source and to the viewer. Unfortunately, normalization is a relatively costly operation. One of the main reasons for this is the square root involved. But when we calculate the reflection vector, we actually do not need a normalized normal. This fact can be used in order to get an approximation for the vector we want when we interpolate between normals. The result is faster Phong shading and faster lighting calculations when we are using a single point light source or having a viewer which is not placed at infinity