This algorithm is a novel approach to compute global illumination based on importance sampling, which tends to converge much faster to a desirable quality than the existing solutions like global illumination photon tracing combined with final gathering.
Before rendering starts, importons are shot from the camera into the scene and collected as a new kind of particle, called an irradiance particle. They carry information about the amount of direct illumination coming in at their position (hence the name "irradiance") and, optionally, the amount of indirect irradiance incident at their position (if indirect passes are enabled). During rendering, the stored particles are used to estimate the irradiance at a shading point: if just direct illumination was collected for irradiance particles, this is equivalent to one bounce of indirect lighting.
The irradiance can also be interpolated from precomputed values at the particle positions.
The irradiance particle algorithm simulates some but not all of the indirect lighting interactions of the traditional global illumination algorithms in mental ray. For this reason, if irradiance particles are enabled then mental ray will turn off the global illumination photon tracing automatically if it was activated. This is a common situation when external applications are asked to generate mental ray scenes with photon shaders attached, which are needed for importons. Caustics can be used together with irradiance particles because they are used to capture indirect lighting effects that irradiance particles cannot simulate. If both final gathering and irradiance particles are enabled then final gathering is preferred and irradiance particles will be switched off automatically.
Irradiance particles support a special IBL-style functionality which can be enabled by setting the number of indirect passes to -1. In this case only the environment map lighting but not diffuse bounces are taken into account. If interpolation is disabled then only environment presampling map is build and no further precomputation steps are required. If interpolation is enabled then particles are emitted in the precomputation pass in a usual way, but used as interpolation points only.
The irradiance particles feature is controlled by scene options and command line arguments of a standalone mental ray.
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