If Rambo ever got the idea to do some global illumination he would do it the most brute force way there ever was. In mental ray this method is called path tracing.No mumbojumbo, no cryptic controls or shaders.
It is all super mega ray tracing, which means two things:
1. It's super accurate.
2. It's super silly slow.
There are a few things to keep in mind when path tracing. Most important is to use only the path_material, and since this is brute force there are no photon shaders or such needed. In your render settings be sure to disable all photon techniques ( GI and caustic ) as well as final gather.
As we are about to use pure ray tracing as GI we need to make sure that our standard ray tracing depths are high enough to distribute the light through out the scene.
Last but not least, we need a ton of samples to get the render smooth.
( You probably need to look into the oversampling_lens )
Here is an example of the difference between samples, these images were rendered
with min/max 2/2, then oversampled with the oversampling_lens.
Those over samples are the image annotations.
Note. render times are per full frame (415x415)
the images were cropped for easier compare.
This special technique might not be something you would want to use in your pipeline. But it's always a plus to know as much as possibly about your tools of trade.