Mental ray glossy reflections are notoriously expensive to render (expensive in this case meaning taking a long time). In order to optomise the render time it can be advantageous to use “interpolation”.
In this example the MIA (mental images architectural shader) is applied to the spheres, one with a colored metal look and the other with a glass ball appearance. Both have glossy reflections and refractions. In order to greatly shorten the render time, the “interpolation” rollout in the MIA shader is employed in conjunction with the glossy samples in the refraction and reflection rollouts.
I wont go into the technical aspects of how the interpolation is achieved (you can read an in depth account in Boaz Livny’s book “Mental Ray for Maya, 3ds Max, and XSI”), suffice to say that interpolation of glossy elements for rendering is a life saver for a busy lighting and rendering artist.
By Robert Appleton, New York Film Academy 3D Animation School Instructor