Global illumination overrides and Caustics overrides let you turn on or off the photon casting or receiving capabilities of all objects in your scene at once (with the exception of any flags you’ve set on a per-object basis, see Flag objects to cast and receive photons).

You can further define the look of your scene and reduce the rendering load (especially good for test renders) by defining to what degree all objects should or should not participate in global illumination and caustics.

Global illumination overrides and Caustics overrides always work in conjunction with local (per-object) global illumination and caustics settings (see Flag objects to cast and receive photons). Per-object flags always take precedent over global overrides, giving you a great amount of control over the how photons are distributed. You can, for example, flag specific objects to receive global illumination, while the rest of the objects in your scene don’t.

To set scene-wide (global) caustics / global illumination overrides

1. Select the Rendering menu set.
2. Open the Render Settings window.

   Select Window > Rendering Editors > Render Settings or click the Render Settings window icon.

3. Select the mental ray for Maya render.
4. Click the Options tab.
5. Expand the mental ray Overrides> Global Illumination/Caustics section.
6. Set the Global Illum Generating settings and, or the Global Illum Receiving settings. See Caustics/Global Illumination in the Rendering guide for more information.
7. Make sure the appropriate per-object global illumination flags have been set for the look you want to achieve.

   See Flag objects to cast and receive photons.