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Concept
Ambient occlusion
Summary
In 3D computer graphics, modeling, and animation, ambient occlusion is a shading and rendering technique used to calculate how exposed each point in a scene is to ambient lighting. For example, the interior of a tube is typically more occluded (and hence darker) than the exposed outer surfaces, and becomes darker the deeper inside the tube one goes.
Ambient occlusion can be seen as an accessibility value that is calculated for each surface point. In scenes with open sky, this is done by estimating the amount of visible sky for each point, while in indoor environments, only objects within a certain radius are taken into account and the walls are assumed to be the origin of the ambient light. The result is a diffuse, non-directional shading effect that casts no clear shadows, but that darkens enclosed and sheltered areas and can affect the rendered image's overall tone. It is often used as a effect.
Unlike local methods such as Phong shading, ambient occlusion is a global method, meaning that 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 appearance achieved by ambient occlusion alone is similar to the way an object might appear on an overcast day.
The first method that allowed simulating ambient occlusion in real time was developed by the research and development department of Crytek (CryEngine 2). With the release of hardware capable of real time ray tracing (GeForce 20 series) by Nvidia in 2018, ray traced ambient occlusion (RTAO) became possible in games and other real time applications. This feature was added to the Unreal Engine with version 4.22.
In the absence of hardware-assisted ray traced ambient occlusion, real-time applications such as computer games can use screen space ambient occlusion (SSAO) or horizon-based ambient occlusion (HBAO) as a faster approximation of true ambient occlusion, using per-pixel depth, rather than scene geometry, to form an ambient occlusion map.
Official source
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