The irradiance particles algorithm provides a method of calculating global illumination using importons. Irradiance particles are easy to set up in your scene and with just the default settings, you can reach a desirable illumination quality much faster than the existing solutions of global illumination photon tracing combined with final gathering.
Computing global illumination with irradiance particles is performed in multiple preprocessing steps prior to rendering:
Importons are shot from the camera into the scene.
Importons are collected in several passes as irradiance particles.
Irradiance particles hold information about the amount of direct and indirect illumination coming in at the particle positions.
Irradiance particles can also be interpolated from precomputed values at the particle positions so that mental ray does not need to estimate the irradiance by shooting rays at every shading point.
After the preprocessing is complete, the irradiance particles are stored in a particle map file.
During rendering, the stored particles in the particle map are used to estimate the indirect illumination (the irradiance) for every shading point.
Irradiance particles simulate some (although not all) of the indirect lighting interactions of traditional global illumination. For this reason, if you enable irradiance particles, then global illumination photon tracing and final gathering are automatically turned off. Caustics can still be used together with irradiance particles to capture indirect lighting effects that irradiance particles cannot simulate.
