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-Occlusin).
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 gamedev article about SSAO.