Ambient occlusion is a fast and computationally inexpensive way to simulate indirect illumination. It works by firing sample rays into a predefined hemispherical region above a given point on an object's surface in order to determine the extent to which the point is blocked - or occluded - by other geometry.
Once the amount of occlusion has been determined, a bright and a dark color are returned for points that are unoccluded and occluded respectively. Where the object is partially occluded the bright and dark colors are mixed in accordance with the amount of occlusion.
The image above shows a scene rendered using only the Ambient Occlusion shader.
The bright color is set to white and the dark color to black. This type of rendering can be composited with other passes to add the occlusion effect to the scene's color and illumination.
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