As discussed earlier, the ambient term in the lighting equation models light that reaches surfaces indirectly, for example light bouncing from nearby surfaces.  This ambient term was the same for every point in the scene.

However, in real life, due to creases or gaps between objects, less light will impinge on a surface, essentially occluding them from the light source.

Ambient occlusion models this, by including an accessibility factor as a measure of how much ambient light reaches a point on a surface.


Source: Coderwall

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