Global Photon Map (indirect illumination)

The basic ideas are:

• We launch photons from the light sources¹ in all directions (or towards the scene, if the light source is far from it).
• We store a photon (position, power and direction) in each intersection of the photon with the scene.
• We compute the kind of interaction with the surface by using the russian roulette. The types of interactions are:
  • Diffussion: the photon is reflected in a random direction (projected hemisphere) or taking into account a BRDF.
  • Reflection: Perfect reflection of the photon.
  • Refraction: The photon is refracted using Snell's law.
  • Absortion: We don't launch that photon again.
  The russian roulette method decides which interaction to consider in function of the material: for example, it is more likely that a black object absorbs a photon than a white one. This is in general desirable over reducing the power of the photons, because concentrates the computations due to a large number of weak photons into a small set of powerful photons, and allows for further variance reduction in the illumination calculation.