Mental ray's
ambient occlusion material doesn't respect transparency and this can create
undesired occlusion for transparent materials like windows.
You can
easily turn on and off AO for a certain object using miLabel attribute.
Let's look at
the simple scene here.
All three
objects in this scene are assigned with the same ambient occlusion material.
What if
object#2 is a transparent object like window. If the window is transparent enough,
it should not cast that much occlusion
on the floor.
