Ambient occlusion

Posted: 1264200022|%e %B %Y, %H:%M|agohover
Tags: ambient occlusion


Ambient occlusion measures how much of the hemisphere above each surface point is occluded by other objects in the scene. It is computed by tracing rays randomly distributed around the hemisphere above each shaded point. In the example picture, the shader colors pixels white if they have a completely unoccluded view of the hemisphere, and black if the view of the hemisphere is completely occluded: the result looks a bit washed out similar to the way an object would appear on an overcast day.
Because it takes into account other objects for lighting computations, it pretends to be a global illumination solution but is not physically based. Anyway, this is a much better solution for computing the ambient term compared to adding the usual constant to the rendering equation. The only drawback is that you need to fire lots of ray per pixel to get rid of the noise (random distribution of rays = noise).
Implementing ambient occlusion is as simple as its definition: if you already have shadow rays in your renderer, it takes a mere 20 lines of code.
Despite its simplicity, it gives surprisingly convincing results. Therefore, it is part of the toolbox of all major CGI studios: Pixar, DreamWorks, Sony Pictures and the likes. You will find more examples of this feature in the gallery (look here)

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