Ambient Occlusion

The measure of how much a surface point is occluded by the environment. Analytically:

Where:

• the ambient coefficient; how much ambient light is received
• the normal oriented hemisphere
• Visibility Function for direction , outputs 0 or 1
• angle between normal vector and

Original paper #

@inproceedings{miller1994efficient,
  title={Efficient algorithms for local and global accessibility shading},
  author={Miller, Gavin},
  booktitle={Proceedings of the 21st annual conference on Computer graphics and interactive techniques},
  pages={319--326},
  year={1994}
}

https://dl.acm.org/doi/pdf/10.1145/192161.192244

Note about why is used for normalization: #

Accounting for Area Foreshortening , the resulting irradiance at a completely unoccluded surface point equals to :

Possible implementations #

• Prebaked an AO map by Raytracing (PBAO)
• Object Space Ambient Occlusion (OSAO)
• Screen Space Ambient Occlusion (SSAO)
• Horizon Basesd Ambient Occlusion (HBAO)
• Alchemy Ambient Obscurance (AOO)
• Ground Truth based Ambient Occlusion (GTAO)

AO Pipeline #

Bluring in X and Y separate will take two render passes but is in . It a better complexity than a normal gaussian blur which is .

Problems #

AO leaving image problem #

Depth image based AO methods like SSAO or HBAO suffer AO that is leaving the view:

Solution: Render a bigger depth image

Discontinuity problem #

The computed AO from SSAO or HBAO both work with a radius. So the AO is not continuous.

Noise #

Banding happens when samples are not chosen randomly but same random directions are used throughout the rendering.

To help with the noise, you can blur it.