Although the mental ray renderer is relatively easy to use once you’ve set it up correctly, there are several "gotchas" that you might encounter immediately, especially if you’re primarily accustomed to the 3ds Max Design scanline renderer and its workflow. For example, see . Following are some basic rules of thumb for using mental ray in 3ds Max Design:
The mental ray renderer does not fully support cubic maps for . It uses them if they have already been generated by the default scanline renderer, but it does not generate them. If Source From File is active and the mental ray renderer can find the six cubic maps, it uses them. If Source Automatic is active, or if the cubic maps cannot be found, the mental ray renderer generates ray-traced reflections or refractions instead.
On the rendering menu, Ray Tracer Settings and Raytrace Global Include/Exclude are disabled while the mental ray renderer is active. These controls adjust ray-trace settings for the scanline renderer only. The settings of these controls have no impact on the mental ray renderer. The ray-tracing controls for mental ray appear on the Renderer panel .
Say you’re rendering a (lathed) wineglass, with an inner and outer surface and a piece of geometry representing the wine. The wine geometry is just slightly smaller than the inner surfaces of the wineglass, and capped with a flat top. Now, you go to render the glass. After rendering the scene, however, there’s something wrong: the inner surfaces of the glass don’t seem reflective enough, and the wine isn’t refracting properly. What’s wrong?
It’s possible that you have the number of reflections and refractions set too low for the number of surfaces you have. To check this, go to the Renderer panel and look at the Maximum Trace Depth settings. If you havent changed the parameters, then you should see Max. Reflections and Max. Refractions set to the default of 6, and Max. Depth set to 6.
There’s the problem: you actually have six surfaces that need to be traced by the light rays for both reflections and refractions. The way to always calculate the number of rays needed for a scene is to take the ray-traced objects in your scene and draw an imaginary line through them, originating at the point of view. Then, count the number of surfaces the line intersects.
When it encounters coincident faces, the mental ray renderer can produce artifacts, because it can't decide which face is nearer the camera (neither is). To fix this, move or scale one of the objects so faces are no longer coincident.