Mathematical Structures for Computer Graphics

9.3 Global Illumination

The lighting models developed so far are local models considering only the direct effect of light sources in illuminating a specified point. Yet, there are many complex lighting interactions in a scene and a physically correct global illumination model is generally computationally intractable. Instead, we move up one more step from the local models and try to incorporate a little more of light interaction in the scene. Ray tracing, which was introduced in an earlier chapter, takes a geometric approach to this goal and systematically follows light rays in the scene trying to track sources of illumination for each point. A more physics-oriented method, radiosity, focuses on light energy and attempts to balance that energy across all the object faces in a scene. Both of these methods are incremental improvements on the local models.

9.3.1 Ray Tracing

Ray tracing follows light rays as if they travel from the eye into the scene. We could stop at the first intersection with an object, say point , and use the elementary lighting model to calculate the color at . If we did this, then ray tracing simply becomes a visibility test and we gain nothing over some of the other elementary lighting models. However, some of the light sources may not reach , or light reflected ...