Defining final gathering in a scene is more straightforward than applying caustics or global illumination. Typically, all of the objects in a scene participate in final gathering calculations, so it's less likely that you'll want to limit the numbers of casters and receivers (although you can).
The following is an overview of the process of setting up a final gathering render. The final gathering options listed below and others that are not discussed in this section are all described in the mental ray Render Options Property Editor > Final Gathering [Properties Reference].
To create a final gathering effect
Optional: Specify which scene objects can cast final gathering rays, which objects are sampled by those rays, and/or which objects are invisible to final gathering rays. See Setting Final Gathering Visibility Options.
Make sure that your scene contains at least one light with an Intensity value greater than 0. With final gathering you can light a scene without actually using any active lights, but it helps to have a least one low-intensity light in your scene to allow you to see the final gathering effect while you set it up. See Lighting a Scene without Using Lights.
On the Optimization tab, make sure that Raytracing is selected for Primary Rays.
On the Final Gathering tab, select the Enable option to activate final gathering.
Select an Accuracy - Mode option to determine which parameters are displayed on this page and their default values. See Selecting a Final Gathering Mode.
Adjust the Accuracy values as described in Setting Final Gathering Accuracy Options.
If desired, activate the Falloff and set the Start and Stop values to limit the effect of distant objects on the final gathering calculation. See Setting the Final Gathering Falloff.
If your scene contains transparent and/or reflective objects, set the Trace Depth settings to define the number of times final gathering rays are reflected and/or refracted, or bounced. See Setting the Final Gathering Trace Depth.
If desired, set the final gathering Map File options to use the map and specify its behavior. See Using the Final Gathering Map.
If you are rendering final gathering with global illumination, you may want to activate the Fast Lookup - Irradiance from Photon Map option. See Using Final Gathering with Global Illumination.
Final gathering is now rendered when you render your scene or draw a render region.
