What is Phong-Shading

Phong shading interpolates the surface normal, rather than the intensity as is the case with gouraud shading. At each point on the surface the normal vector is calculated as an interpolation of the normal vectors at the vertices of the surfaces. Each vector is then normalized and for each point an illumation model is used to determine the color at that point.

This shading method looks much better than gouraud shading, but is also a lot more CPU-intensitive and therefor not usable for realtime animation of 3D objects on a PC. Fortunatly there is a way to simplify the calculation a lot, without really affecting the nice properties and looks of Phong shading. With that method Phong shading is only a little bit more CPU-intensitive than gouraud shading.