Illumination models model the interaction of light with the surface and range from simple to very complex. In Computer Graphics, we use physics to derive an illumination model and then fudge it to make the picture look good. More realistic and complex models adhere more closely to the underlying physics. In this section, we will look at only a simple illumination model and consider approximations for two types of light (ambient and point light sources) and two types of light reflection (diffuse and specular). Remember that we have to handle both specular and diffuse reflections and to solve the "pixel equation" as given below:
A local illumination model must handle diffuse and specular reflections. Diffuse reflections are fairly simple and easily handled. Much research has gone into the specular reflection component. Below, two different models are discussed. The Phong model is not based in physics, but on empirical observation. Despite its lack of a theoretical grounding, it produces quite good results and has been the basis for most computer graphics imagery produced since it was developed in 1975. The Blinn model, developed in 1977, is based on research results from Physics and corrects some of the deficiencies of the Phong model. Both of these models assume that the specular highlights are the color of the light source and make no attempt to estimate the color contribution from the material itself. An even more advanced model by Cook and Torrance does estimate the color contribution from the material and is more asccurate for some substances, especially metals.
Phong Model for Specular Reflection
Blinn Model for Specular Reflection
Phong Illumination Model Demonstration program
Main reflection page.
HyperGraph home page.