Separable Approximation of Ambient Occlusion Jing Huang1 Tamy Boubekeur1 Tobias Ritschel1,2 Matthias Hollnder1 Elmar Eisemann1 Tlcom ParisTech - CNRS/LTCI 2 Intel Visual Computing Institute 1 Ambient Occlusion Fast approximation of a subset of indirect lighting
effects 2 Ambient Occlusion 3 Ambient Occlusion AO = shadow created by ambient illumination
Ref photo : Guangzhou - Cloudy Day Was first introduced by M. S. Langer & S. W. Zucker (Shape from shading on a cloudy day, 1994) Capturing surface variation using accessibilty shading by Gavin Miller (Efficient Algorithms for Local and Global Accessibility Shading, 1994)
4 Ambient Occlusion Integral of Visibility all over the hemisphere 5 Ambient Occlusion Improving the perception of volume, concavities & contact areas of 3D objects
AO in Starcraft II. 6 Evaluation Estimated by ray tracing Prohibitive for real time rendering 7 Screen-Space
Ambient Occlusion For Real Time Applications 8 Screen Space Ambient Occlusion SSAO Main idea: using the depth-buffer as a cheap, randomly accessible substitution for the actual scenes geometry.
Casting AO computation as a local filter in screen space CryEngine [Mittring 2007] 9 Screen Space Ambient Occlusion [Shanmugam and Okan 2007], [McGuire 2010]
10 Screen Space Methods Horizon-Based AO [BAVOIL 2008] depends on the elevation angle e 11 Hybrid Ambient Occlusion Accounts for outside the view occluders
[Reinbothe 2009] 12 Problem Can we go a step further ? SSAO already reformulated a 3D raytracing process into a 2D screen space layout Can we work in 1D ? Observation: SSAO looks like a local image filter.
Idea: Propose a separable (2x1D) AO approximation instead of a full (2D) evaluation 13 Separable Approximation of Ambient Occlusion
Original signal With bilateral filter 18 Separable Bilateral Filter Mathematically not separable But convincing separable approximation [Pham 2005]
19 Separable AO Separate AO sampling along 2 orthogonal axes in a 2-pass process Evaluation along each axis using any kind of SSAO Occlusion estimation (Crytek, HBAO, etc) Sampling (uniform, random, jittered, etc)
Need spatially varying directions for removing directional banding artifacts Solution: Local Frame Randomization 20 Local Frame Randomization
Randomize the orientation of the local frame (axis pair) at each pixel Avoids banding artifacts. 21 Distribution Pattern [Pharr 2004] Using different distribution pattern: Random
Measured against the exact computation for each technique using [Mittring 2007] [BAVOIL 2008] 29 Video 31 Limitations Inherited from the particular SSAO technique using to evaluate along the 2 axis
Missed occluders (outside the frustum) Could be solved by voxelization [Reinbothe 2009] Over occlusion Could be solved by depth peeling [Ritschel 2009] 32 Conclusion Separable Approximation of Ambient Occlusion
Faster than complete evaluation Compatible with all SSAO methods No visible error Same principle can be applied to other screen-space rendering techniques 33 Acknowledgements ANR Projects Cecil and MEDIAGPU
European Network of Excellence 3DLife Help from Telecom ParisTech: Catherine Pelachaud, Bert Buchholz, Jean-Marc Thiery 34 Thank you for your attention Thanks Merci Danke
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