A few months back I created a game asset for Roundabout and was faced with the issue of baking an occlusion map using Mental Ray. It was surprisingly hard to find a decent reference for what I ended up doing, so I hope this helps somebody else trying to do the same thing.
mib_amb_occlusion or Render Settings Ambient Occlusion?
I generally avoid using the render settings controls for ambient occlusion as I really like the control I get through the mib_amb_occlusion node. Unfortunately, the "Transfer Maps..." does not support this node and the only option you are left with is the Render Setting Ambient Occlusion option.
So using this technique for baking Mental Ray textures and shaders, we are able to bake the mib_amb_occlusion.
- Plug the occlusion outValue into "Additional Color" and make the diffuse black so that it acts like an incandescence shader. Otherwise scene lighting will affect your occlusion map.
- Use an mia_material_x shader and not a maya shader like a lambert, as it will cause tessellation (shown in the image below).