You can use the scene objects' Visibility Property Editor to precisely control how each object participates in final gathering calculations. Every object has the following three final gathering visibility parameters:
Caster: specifies whether or not an object casts final gathering rays into the scene.
Visible to Sampling: specifies whether the object is visible to final gathering rays cast by other objects. Turning this option off causes final gathering rays to pass through the object.
Turning this option off makes the Sampled option unavailable, since an object that is not visible to sampling rays cannot be sampled.
Sampled: specifies whether an object's surface is actually sampled by final gathering rays cast by other objects. Turning this option off causes the object to absorb final gathering rays.
The options give you fine control over a scene's final gathering computations. For example, you might want to limit the final gathering to specific objects, or remove only a few objects from the final gathering computations. By default, all of these options are turned on.
Select the object whose final gathering visibility options you wish to adjust, and open an explorer (press 8).
Expand the object's hierarchy and click its Visibility node to open the object's Visibility property editor.
On the Rendering tab, toggle Caster, Visible to Sampling, and/or Sampled options as needed.
If your scene contains a lot of objects, final gathering visibility options can be difficult to manage on a per-object basis. In complex scenes, it's easier to partition objects and use overrides to control final gathering visibility options for all of the objects in a partition. For example, you might use this technique to prevent all of a scene's background objects from casting, or being sampled by final gathering rays. This process is described in Overriding Rendering Visibility for Objects in Partitions.
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