I think it's this one? http://advances.realtimerendering.com/s2010/Hable-Uncharted2(SIGGRAPH%202010%20Advanced%20RealTime%20Rendering%20Course).pptx Basically, what Cyrid said, the AO map should 'appear' in the shadow-ed part of mesh, while anything hitting with direct light should be washed out, true values, bleached, etc…