Caustics occur when focused
specular light reflects or refracts onto a diffuse surface. Most
surfaces are usually diffuse or partially diffuse and all objects in
the scene were assigned diffuse materials in order to produce
caustics effects. Diffuse surfaces reflect light uniformly in all
directions and indirect illumination of diffuse surfaces is
generally calculated with radiosity.
Photon shaders store
photons only if the surface has a diffuse component since indirect
illumination of completely specular surfaces wihtout a diffuse
component is computed using ray tracing. The room surfaces such as
the window ledges were given a phong shader with the specular turned
off in order to best show up the caustic effects.
the diffuse color value of the receiving surfaces affects the
caustics. By decreasing this value the caustics appear brighter.
Giving a diffuse value of 0 allows no surfaces for the photons to be
stored and therefore no caustics appear. If the specular is turned
on the caustics do not appear in the area of specular highlight.