In computer graphics, ambient occlusion is used to represent how exposed each point in a scene is to ambient lighting. So the enclosed inside of a tube is typically more occluded (and hence darker) than the exposed outer surfaces; and deeper inside the tube, the more occluded (and darker) it becomes. The result is diffuse, non-directional lighting throughout the scene, casting no clear shadows, but with enclosed and sheltered areas darkened. In this way, it attempts to approximate the way light radiates in real life, especially off what are normally considered non-reflective surfaces.
Unlike local methods like Phong shading, ambient occlusion is a global method, meaning the illumination at each point is a function of other geometry in the scene. However, it is a very crude approximation to full global illumination. The soft appearance achieved by ambient occlusion alone is similar to the way an object appears on an overcast day.
I'm toying with the screen-space approximation of this technique in my Isle of marooned project.
It suffers some limitations and artifacts, that I will discuss and provide more info about in further posts.
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