Separable Approximation Of Ambient Occlusion

Separable Approximation Of Ambient Occlusion

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

14 Separable Gaussian Filter 15 Separable Gaussian Filter 16 Bilateral Filter With gaussian filter

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

Stratified sampling 1. Regular 2. Jittered Low-discrepancy sampling 1. Halton sequence 2. Hammersley sequence Random Regular Jittered

Halton Hammersley 22 Interleaved Sampling [Keller 2001]

Regularly replicated basis pattern Between regular & irregular sampling Regular Pattern Interleaved Pattern 23 Interleaved Sampling [Keller

2001] Toasters 24 Interleaved Sampling [Keller 2001] Sampling

4 x 4 Boxfilter Results High Frequency Random AO in Toasters feet 4x4 Interleaved Advantage : easy to filter

save memory 25 Results 27 Performance Size 5 11 21

Separable Crytek Vol. Obs HBAO [Mit07] [LS10] [BSD08]

no 3.2ms 3.5ms 3.6ms yes 3.4ms

3.6ms 3.5ms no 13.9ms 14.8ms 15.0ms yes

5.8ms 5.9ms 6.0ms no 49.7ms 51.9ms

51.9ms yes 9.9ms 9.9ms 10.2ms Resolution 1024 x 768 GPU Nvidia GTX 480

28 Perceptual Error Reference HBAO Separable Perceptual Difference in Lab space. [Yee 2004]

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

Obrigado Gracias 35

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