Reference

http://www.neilblevins.com/cg_education/ambient_occlusion_rust/ambient_occlusion_rust.htm

http://www.artbycrunk.com/blog/2009/03/ambient-occlusion-using-mib_amb_occlusion_node/

http://cg.tutsplus.com/articles/web-roundups/72-magnificent-maya-tutorials/

http://wiki.bk.tudelft.nl/toi-pedia/Rendering_Mental_Ray:_Final_Gather