The Full Wiki

Ambient occlusion: Wikis

Advertisements
  

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

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. Ambient occlusion 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.

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 the industry, ambient occlusion is often referred to as "sky light."[citation needed]

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 Ap 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:

 A_p = \frac{1}{\pi} \int_{\Omega} V_{p,\omega} ( N \cdot \omega ) \, d\omega

where Vp 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 Nb 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.,

Aocclude bentnormal.png

In this example, light may reach p only from the left or right sides, but the bent normal points to the average of those two sources, which unfortunately, is directly toward the obstruction.

Contents

Scientific and Technical Academy Award

In 2010, Hayden Landis, Ken McGaugh and Hilmar Koch were awarded a Scientific and Technical Academy Award for their work on ambient occlusion rendering.[2]

See also

References

  1. ^ "Depth discrimination from shading under diffuse lighting", M.S. Langer and H. H. Buelthoff, Perception. 29 (6) 649-660, 2000.
  2. ^ http://www.altfg.com/blog/awards/oscar-2010-scientific-and-technical-awards-489/

External links

Advertisements

Simple English

Ambient Occlusion is a technique used in 3D computer graphics. In real life, if something is surrounded by other things, it is more likely to be blocked out by a shadow cast upon it, or "occluded". Ambient Occlusion calculates how much of some object, such as a surface, is blocked out by something else as compared to how much of that object is open to the rest of the environment. Things surrounded by other things that can occlude it or block it out are drawn darker than things more open to the environment, which are drawn lighter.


Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message