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From Odwiki

Irradiance is a GlobalIllumination technique for gathering indirect lighting contribution by sending out rays from the sample point and averaging those samples.

The major difference between Irradiance and AmbientOcclusion is that the irradiance function will evaluate the shader attached to the surfaces which the sample rays strike. This allows the function to calculate the radiant contribution of all its samples.

VEX Shaders in mantra have the use of the builtin irradiance() function which performs this efficiently for you. Using this function also allows you to take advantage of the IrradianceCache functionality to counteract noise and improve speed.

The standard shader VEXGlobalIlluminationSHOP allows the user to select Full Irradiance.

Irradiance and AmbientOcclusion computation can be optimized using IrradianceCaching.

VexPointCloud textures can also be used to speed up GlobalIllumination computations in mantra.