Ambient Occlusion and Ambient Obscurance are coarse approximations to global illumination from ambient lighting, commonly used in film and games. This paper describes a system that computes Ambient Obscurance over the vertices of complex polygon meshes. Novel contributions include pre-processing necessary for “triangle soup” scene representations to minimize artifacts, a compact model for different classes of instanced decorator objects such as trees and shrubs, a compact model for pre-computed visibility to be used on dynamically placed objects, and an approximation to model the occlusion of small decorator objects when ray tracing.