Create an approximation node

Select the geometry to which you want to assign an approximation node.
For compatible geometry types, see Approximation nodes.
Click Window > Rendering Editors > mental ray > Approximation Editor.
The Approximation Editor opens. For more information on the Approximation Editor, see mental ray Approximation Editor.
Click the Create button for the type of approximation node you want to create.
For descriptions, see Approximation nodes.

The approximation node is created and assigned to the selected geometry.