“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” [Wikipedia.org]
The proper way to do ambient occlusion is very expensive for today’s hardware and especially without the use of a ray-tracing renderer. For that reason a few new techniques developed that tried to produce the same result using simpler and faster algorithms. One of these approaches is the Screen Space Ambient Occlusion (aka SSAO) that makes the calculations of ambient occlusion in 2D space just like a normal 2D filter. This article will not go deep into what SSAO is, there are many great readings around the web that cover the theory behind SSAO, in this article we will jump into the implementation that AnKi uses.
There are quite a few techniques to produce SSAO, a top level way to group them is from the resources they need to produce the SSAO:
- Some use just the depth buffer (Amnesia, Aliens VS Predator)
- others use the depth buffer and the normal buffer (Crysis, gamerendering article)
- and others use the pixel position and the normal buffer (gamedev article)
AnKi used to implement the second technique for quite some time. The results were good but apparently not that good, so for the past couple of weeks I’ve tried to implement the third technique by reading this great article from gamedev. The present article extends the gamedev one by adding a few optimization tips and by presenting the code in GLSL.
Bellow you can see the old and the new SSAO, rendered at the half of the original rendering size with two blurring passes.