An extended version of the [ray tracing|Whitted Ray Tracing] algorithm from [Siggraph] 1981, enhancing the [render|rendering] of diffuse surfaces.

Background

The [ray tracing|Whitted Ray Trace] starts shooting rays from the camera. Once the ray hits a diffuse surface it stops, resulting in that the light variations on these surfaces are lost.

An example read from 3D Computer Graphics, [Allan Watt] mentions the reflexes from the sun, that shows on the bottom the swimming pool. When ray traced, these reflexes will be lost, since the bottom of the pool is a diffuse surface.

Two pass tracing

Let us start in the other end. Once a ray is cast -- this time from the light source -- it will [reflect] on and [refract] through [specular] surfaces until hitting a diffuse surface. All as usual; except that this time, the information about the light energy is stored in a [light map] which is associated with the surface.

Now, when doing a normal Whitted Ray Trace and the ray stops at a diffuse surface, the information from the [light map] will be added. The result will be that the bottom of our swimming pool will be full of beautiful reflexes, just as it should be.

Two pass tracing (idea)