Image Based Occlusion
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Image-based Occlusion is a real-time version of Ambient Occlusion via deferred shading.
Ambient occlusion estimates the indirect light received at a particular point of the surface by the largest unoccluded opening angle visible from the point on the surface.
Although faster than indirect lighting it is still a costly operation that either needs to be precomputed or approximated further using image-based techniques (SSAO = Screen-Space Ambient-Occlusion).
SSAO estimates the opening angle from the geometry of adjacent Z-buffer values. This produces visibly appealing results, but it does not produce results that qualify for photorealistic rendering, though.
For further reading see this article about SSAO published by Crytek in 2007.
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