Global illumination enhances the realism of a scene by simulating radiosity, or the interreflection of light (other than caustics) in a scene. It generates such effects as "color bleeding," where for example, a white shirt next to a red wall will appear to have a slight red tint.

To calculate global illumination, the mental ray renderer uses the photon map technique.

Important: The mental ray renderer generates global illumination without requiring you to generate a radiosity solution. A photon map is a model of global illumination in its own right.

Note: In order to use global illumination in mental ray, the photons must be able to bounce among two or more surfaces. This can be accomplished by having a single object with some concavity in its surface that’s exposed to the light source, or at least two objects, and at least one object must be set to receive global illumination (see mental ray Panel (Object Properties Dialog)). Otherwise you’ll receive error messages and no photons will be stored.

Using a photon map can cause rendering artifacts such as dark corners and low-frequency variations in the lighting. You can reduce or eliminate these artifacts by turning on final gathering, which increases the number of rays used to calculate global illumination.

You enable global illumination and final gathering on the Render Scene dialog > Indirect Illumination panel > Caustics And Global Illumination rollout. In addition, you must designate:

The settings for generating and receiving global illumination are on the Object Properties dialog > mental ray Panel.