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This paper presents a new GPU-based algorithm to compute ambient occlusion. We first examine how ambient occlusion is related to the physically founded rendering equation. The correspondence is made by introducing a fuzzy membership function that defines what ``near occlusions'' mean. Then we develop a method to calculate ambient occlusion in real-time without any pre-computation. The proposed algorithm is based on a novel interpretation of ambient occlusion that measures how big portion of the tangent sphere of the surface belongs to the set of occluded points. The integrand of the new formula has low variation, thus can be estimated accurately with a few samples. Thus, the algorithm can effectively be used in real-time systems and games to cheaply approximate global illumination effects.