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From Odwiki

Global Illumination (GI) refers to the simulation of direct and indirect lighting.

GI in Mantra

mantra has support for gathering contributing indirect light from diffuse surfaces (Irradiance) and from reflected light off specular surfaces (Caustics).

The primary tools in Houdini consist of VEX functions for gathering Irradiance and accessing PhotonMaps for caustics, or for calculating total occlusion: AmbientOcclusion. mantra also has a special mode for generating the necessary photon maps for direct and indirect lighting.

Houdini also supports the use of an IrradianceCache to speed up iterations or tweaks on a scene. Used correctly, an IrradianceCache can also provide better filtering and reduce flicker.

Another technique for rendering Global Illumination in mantra is to take advantage of PointClouds to calculate the indirect energy and merely look up into the point clouds to get this typically subtle low-frequency illumination contribution. The advantage come in that you can calculate the illumination at relatively sparse points, which is often adequate for GI. Please read more on the PointCloud page.

The tools

Houdini is shipped with a shader call the VEXGlobalIlluminationSHOP which is a LightShader. This shader provides simple access to all the necessary GI components; Full Irradiance, AmbientOcclusion and PhotonMapping.

Tutorials

• There is a simple walkthrough in the documents provided with Houdini. Look under User Guide> Lighting> Global Illumination