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Ambient occlusion is a shading method used in 3D computer graphics which helps add realism to local reflection models by taking into account attenuation of light due to occlusion. 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.

Ambient occlusion is most often calculated by casting rays in every direction from the surface. Rays which reach the background or “sky” increase the brightness of the surface, whereas a ray which hits any other object contributes no illumination. As a result, points surrounded by a large amount of geometry are rendered dark, whereas points with little geometry on the visible hemisphere appear light.

Ambient occlusion is related to accessibility shading, which determines appearance based on how easy it is for a surface to be touched by various elements (e.g., dirt, light, etc.). It has been popularized in production animation due to its relative simplicity and efficiency. In industry, ambient occlusion is often referred to as "sky light."

The ambient occlusion shading model has the nice property of offering a better perception of the 3d shape of the displayed objects. This was shown in a paper ^[1] where the authors report the results of perceptual experiments showing that depth discrimination under diffuse uniform sky lighting is superior to that predicted by a direct lighting model.

ambient occlusion

diffuse only

combined ambient and diffuse

The occlusion A_p at a point p on a surface with normal N can be computed by integrating the visibility function over the hemisphere Ω with respect to projected solid angle:

where V_p,ω is the visibility function at p, defined to be zero if p is occluded in the direction ω and one otherwise. A variety of techniques are used to approximate this integral in practice: perhaps the most straightforward way is to use the Monte Carlo method by casting rays from the point p and testing for intersection with other scene geometry (i.e., ray casting). Another approach (more suited to hardware acceleration) is to render the view from p by rasterizing black geometry against a white background and taking the (cosine-weighted) average of rasterized fragments. This approach is an example of a "gathering" or "inside-out" approach, whereas other algorithms (such as depth-map ambient occlusion) employ "scattering" or "outside-in" techniques.

In addition to the ambient occlusion value, a "bent normal" vector N_b is often generated, which points in the average direction of unoccluded samples. The bent normal can be used to look up incident radiance from an environment map to approximate image-based lighting. However, there are some situations in which the direction of the bent normal is a misrepresentation of the dominant direction of illumination, e.g.,

• Global illumination
• Photon mapping
• Radiosity
• Ray tracing

1. ^ Michael S. Langer and Heinrich H. Bülthoff. Perception of shape from shading on a cloudy day. Technical Report No. 73, Max-Planck-Institut für biologische Kybernetik, October 1999. (pdf)

• Depth Map based Ambient Occlusion
• Ambient Occlusion
• Assorted notes about ambient occlusion
• Ambient Occlusion Fields — real-time ambient occlusion using cube maps
• Fast Precomputed Ambient Occlusion for Proximity Shadows real-time ambient occlusion using volume textures
• Dynamic Ambient Occlusion and Indirect Lighting a real time self ambient occlusion method from Nvidia's GPU Gems 2 book
• ShadeVis an open source tool for computing ambient occlusion
• xNormal A free normal mapper/ambient occlusion baking application