Screen space ambient occlusion
Screen space ambient occlusion (SSAO) is a rendering technique for efficiently approximating the computer graphics ambient occlusion effect in real time. It was developed by Vladimir Kajalin while working at Crytek and was used for the first time in 2007 by the video game Crysis, developed by Crytek and published by Electronic Arts.
The algorithm is implemented as a pixel shader, analyzing the scene depth buffer which is stored in a texture. For every pixel on the screen, the pixel shader samples the depth values around the current pixel and tries to compute the amount of occlusion from each of the sampled points. In its simplest implementation, the occlusion factor depends only on the depth difference between sampled point and current point.
Without additional smart solutions, such a brute force method would require about 200 texture reads per pixel for good visual quality. This is not acceptable for real-time rendering on current graphics hardware. In order to get high quality results with far fewer reads, sampling is performed using a randomly rotated kernel. The kernel orientation is repeated every N screen pixels in order to have only high-frequency noise in the final picture. In the end this high frequency noise is greatly removed by a NxN post-process blurring step taking into account depth discontinuities (using methods such as comparing adjacent normals and depths). Such a solution allows a reduction in the number of depth samples per pixel to about 16 or fewer while maintaining a high quality result, and allows the use of SSAO in soft real-time applications like computer games.
Compared to other ambient occlusion solutions, SSAO has the following advantages:
- Independent from scene complexity.
- No data pre-processing needed, no loading time and no memory allocations in system memory.
- Works with dynamic scenes.
- Works in the same consistent way for every pixel on the screen.
- No CPU usage – it can be executed completely on the GPU.
- May be easily integrated into any modern graphics pipeline.
Of course it has its disadvantages as well:
- Rather local and in many cases view-dependent, as it is dependent on adjacent texel depths which may be generated by any geometry whatsoever.
- Hard to correctly smooth/blur out the noise without interfering with depth discontinuities, such as object edges (the occlusion should not "bleed" onto objects).
In video games
- "Alan Wake FAQ – Alan Wake Community Forums". Forum.alanwake.com. Retrieved 2011-08-26.
- "Ask Bohemia (about Operation Arrowhead... or anything else you want to ask)! – Bohemia Interactive Community". Community.bistudio.com. 2010-05-06. Retrieved 2011-08-26.
- "Face-Off: Assassin's Creed: Brotherhood – Page 2 | DigitalFoundry". Eurogamer.net. 2010-11-18. Retrieved 2011-08-26.
- "Assassins Creed: Brotherhood PC Performance Analysis". Dasreviews.com. 2011-02-14. Retrieved 2012-05-10.
- "Assassin's Creed IV Black Flag - GeForce". Nvidia.
- "A man dressed like a bat: Arkham Origins – PC Features Further Explained & Demonstrated". DSOGaming. September 8, 2013. Retrieved 2013-09-08.
- "AMD’s Radeon HD 5850: The Other Shoe Drops". AnandTech. Retrieved 2011-08-26.
- "Head2Head: Bionic Commando". Lens of Truth. 2009-05-29. Retrieved 2011-08-26.
- "Head2Head: Borderlands Analysis". Lens of Truth. 2009-10-29. Retrieved 2011-08-26.
- "Borderlands 2 Gameplay Performance and IQ Review". HardOCP. 2012-10-01.
- "Benchmarks: SSAO Enabled : Burnout Paradise: The Ultimate Box, Performance Analysis". Tomshardware.com. Retrieved 2011-08-26.
- "Issue 17: Dark Mirror Patch Notes | City of Heroes® : The Worlds Most Popular Superpowered MMO". Cityofheroes.com. Retrieved 2011-08-26.
- "CryENGINE® 2". Crytek. Retrieved 2011-08-26.
- "Daylight - GeForce". Nvidia.
- "Dead Space 3 PC Graphics Settings Rundown". GameFront. February 5, 2013.
- "Empire: Total War – No anti-aliasing in combination with SSAO on Radeon graphics cards – Empire Total War, anti-aliasing, SSAO, Radeon, Geforce" (in German). PC Games Hardware. 2009-03-11. Retrieved 2011-08-26.
- CCP. "EVE Insider | Patchnotes". EVE Online. Retrieved 2011-08-26.
- "Gears of War Series | Showcase | Unreal Technology". Unrealengine.com. 2008-11-07. Retrieved 2011-08-26.
- "Bungie Weekly Update: 04.16.10 : 4/16/2010 3:38 PM PDT". Bungie.net. Retrieved 2011-08-26.
- "Halo: Reach beta footage analysis – Page 1 | DigitalFoundry". Eurogamer.net. 2010-04-25. Retrieved 2011-08-26.
- "Face-Off: L.A. Noire – Page 1 | DigitalFoundry". Eurogamer.net. 2011-05-23. Retrieved 2011-08-26.
- "Risen Tuning Tips: Activate Anti Aliasing, improve graphics and start the game faster – Risen, Tipps, Anti Aliasing, Graphics Enhancements" (in German). PC Games Hardware. 2009-10-06. Retrieved 2011-08-26.
- "STALKER: Clear Sky Tweak Guide". TweakGuides.com. Retrieved 2011-08-26.
- "Slender: The Arrival". Parsec Productions. Retrieved 2013-05-03.
- "Sleeping Dogs Gameplay Performance and IQ Review". HardOCP. 2012-10-02.
- Entertainment, Blizzard (2011-08-19). "Patch 1.2.0 Now Live – StarCraft II". Us.battle.net. Retrieved 2011-08-26.
- "The Settlers 7: Paths to a Kingdom – Engine". YouTube. 2010-03-21. Retrieved 2011-08-26.
- "Splinter Cell: Blacklist – PC Features Detailed + New Footage". DSOGaming. June 18, 2013.
- "Tom Clancy’s Splinter Cell Blacklist: Digital Deluxe Edition Now Bundled With GeForce GTX Graphics Cards". Nvidia. July 9, 2013.
- "Among Friends: How Naughty Dog Built Uncharted 2 – Page 3 | DigitalFoundry". Eurogamer.net. 2010-03-20. Retrieved 2011-08-26.
- Finding Next Gen – CryEngine 2
- Video showing SSAO in action
- Image Enhancement by Unsharp Masking the Depth Buffer
- Hardware Accelerated Ambient Occlusion Techniques on GPUs
Overview on Screen Space Ambient Occlusion Techniques(as of March 1, 2012)
- Real-Time Depth Buffer Based Ambient Occlusion
- Source code of SSAO shader used in Crysis
- Approximating Dynamic Global Illumination in Image Space
- Accumulative Screen Space Ambient Occlusion
- NVIDIA has integrated SSAO into drivers
- Several methods of SSAO are described in ShaderX7 book
- SSAO Shader ( Russian )
- SSAO Tutorial, extension of the technique used in Crysis