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Radius

The radius controls the maximum distance within which Mental Ray considers a photon to compute the caustic and determines how much of the effect is illuminated.

The radius value can be either specified or estimated automatically by Mental Ray based on the size of the bounding box of the scene. The estimated value will be used in the render if a value of zero is entered in the radius value option. Therefore the default appears as zero but is actually a larger value.

Increasing the radius value effectively increases the size of the effect so larger values will produce larger caustics but some detail will be lost in the results.

It is best to adjust the value by small amounts. A value of 0.05 can be used to show the individual photons used to create the effect. Larger radius values will illuminate more of the caustic effect and can reduce grainiess but will diffuse the light and cause the effect to appear dimmer.

Radius Values
Further Information
Susan Windeatt