This time, I would like to talk about the SSAO implemented in my little demo program. I write this demo because I spent most of my time using openGL and know little about DirectX, so I decided to learn DX by writing this demo. So it is not well optimized.

SSAO, short for Screen Space Ambient Occlusion, is a technique for approximating the indirect shadow casted by surrounding scene geometry which is done in screen space by sampling from the depth buffer.

The SSAO is implemented using the line integrals from "Rendering techniques in Toy Story 3"[1]. Here is my results:

With SSAO |

without SSAO |

SSAO texture |

Their method calculates the volume occluded by other objects inside a sphere at each fragment by sampling from the depth buffer.

From Slide 22 of the paper |

From Slide 51 of the paper |

But in my implementation, I didn't use the Voronoi Diagram, in stead, I tried to calculate the volume occupied by the depth sample using that equation in the pixel shader. However , due to the perspective projection, that equation no longer holds as the ray will not form a right angle triangle which is not the same as above figure, and resulting the artifact as below(the wall on the right side):

The artifact on the wall on the right side |

For example, when calculate the occlusion volume for the black cross in the above diagram (take 2 depth samples for easy explanation), I need to compute the length L1 and L2 by solving ray-sphere intersection. Also the length O1 and O2 can be computed by sampling from the depth buffer. Therefore, the volume of the sphere can be approximated by L1+L2 and the occlusion volume can be approximated by O1+O2. (I also added a distance attenuation factor to O1 and O2 if the depth difference is too large so that the tank does not occlude the wall in my demo program). And the AO value will be (O1+O2)/(L1+L2)

This eliminate the artifacts:

Solving ray-sphere intersection to eliminate the artifacts |

Rotating the sample points |

After bilateral blur |

Applying SSAO to the scene |

References:

[1]: Rendering techniques in Toy Story 3, http://advances.realtimerendering.com/s2010/index.html

[2]: The brick texture is obtained from Crytek's Sponza Model: http://crytek.com/cryengine/cryengine3/downloads

[3]: The tank model is obtained from an XNA demo project:http://create.msdn.com/en-US/education/catalog/?contenttype=0&devarea=0&platform=21&sort=1