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Separable Approximation of Ambient Occlusion Jing Huang 1 Tamy Boubekeur 1 Tobias Ritschel 1,2 Matthias Holländer 1 Elmar Eisemann 1 1 Télécom ParisTech - CNRS/LTCI 2 Intel Visual Computing Institute. Ambient Occlusion.

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slide1

Separable Approximation of

Ambient Occlusion

Jing Huang1Tamy Boubekeur1 Tobias Ritschel1,2

Matthias Holländer1 Elmar Eisemann1

1Télécom ParisTech - CNRS/LTCI

2Intel Visual Computing Institute

ambient occlusion
Ambient Occlusion

Fast approximation of a subset of indirect lightingeffects

ambient occlusion2
Ambient Occlusion
  • AO = shadowcreated by ambient illumination

Ref photo : Guangzhou - Cloudy Day

  • Was first introduced by M. S. Langer & S. W. Zucker («Shape fromshading on a cloudyday», 1994)
  • Capturing surface variation usingaccessibiltyshading by Gavin Miller («Efficient Algorithms for Local and Global Accessibility Shading», 1994)
ambient occlusion3
Ambient Occlusion

Integral of Visibility all over the hemisphere

ambient occlusion4
Ambient Occlusion
  • Improving the perception of volume, concavities & contact areas of 3D objects

AO in

Starcraft II.

evaluation
Evaluation

Estimated by ray tracing

Prohibitive for real time rendering

screen space ambient occlusion
Screen-SpaceAmbient Occlusion

For Real Time Applications

screen space ambient occlusion1
ScreenSpaceAmbient Occlusion

SSAO

  • Main idea:
    • using the depth-buffer as a cheap, randomly accessible substitution for the actualscene’sgeometry.
    • Casting AO computation as a local filter in screenspace
  • CryEngine [Mittring2007]
screen space ambient occlusion2
ScreenSpaceAmbient Occlusion
  • [Shanmugam and Okan 2007], [McGuire 2010]
screen space methods
ScreenSpaceMethods
  • Horizon-Based AO [BAVOIL 2008] depends on the elevation angle e
hybrid ambient occlusion
HybridAmbient Occlusion
  • Accounts for «outside the view» occluders [Reinbothe 2009]
problem
Problem
  • Can we go a stepfurther ?
    • SSAO alreadyreformulated a 3D raytracingprocessinto a 2D screenspacelayout
  • Can wework in 1D ?

Observation:

  • SSAO looks like a local image filter.

Idea:

  • Propose a separable (2x1D) AO approximation instead of a full (2D) evaluation
bilateral filter
BilateralFilter

Withgaussianfilter

Withbilateralfilter

Original signal

separable bilateral filter
SeparableBilateralFilter

Mathematicallynot separable

But convincingseparableapproximation [Pham 2005]

separable ao
Separable AO
  • Separate AO samplingalong 2 orthogonal axes in a 2-passprocess
  • Evaluation alongeach axis usinganykind of SSAO
    • Occlusion estimation (Crytek, HBAO, etc)
    • Sampling (uniform, random, jittered, etc)

Needspatiallyvarying directions for removingdirectionalbandingartifacts

Solution: Local Frame Randomization

local frame randomization
Local Frame Randomization
  • Randomize the orientation of the local frame (axis pair) ateach pixel
  • Avoidsbandingartifacts.
distribution pattern pharr 2004
Distribution Pattern [Pharr 2004]
  • Usingdifferent distribution pattern:
    • Random
  • Stratifiedsampling
    • Regular
    • Jittered
  • Low-discrepancysampling
    • Halton sequence
    • Hammersleysequence

Random

Regular

Jittered

Halton

Hammersley

interleaved sampling keller 2001
InterleavedSampling[Keller 2001]
  • Regularlyreplicated basis pattern
    • Betweenregular & irregularsampling

Regular Pattern

Interleaved Pattern

interleaved sampling keller 20012
InterleavedSampling[Keller 2001]

Sampling

4 x 4 BoxfilterResults

High FrequencyRandom

AO in

« Toaster’sfeet »

4 x 4 Interleaved

  • Advantage:
  • easy to filter
  • savememory
performance
Performance

Resolution 1024 x 768

GPU Nvidia GTX 480

perceptual error
PerceptualError

Measuredagainst the exact computation for each technique using[Mittring 2007] [BAVOIL 2008]

Reference

HBAO

Separable

PerceptualDifference in Labspace. [Yee 2004]

limitations
Limitations
  • Inheritedfrom the particular SSAO technique using to evaluatealong the 2 axis
    • Missedoccluders (outside the frustum)
      • Couldbesolved by voxelization [Reinbothe 2009]
    • Over occlusion
      • Couldbesolved by depth peeling [Ritschel 2009]
conclusion
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
acknowledgements
Acknowledgements
  • ANR Projects « Cecil » and « MEDIAGPU »
  • European Network of Excellence « 3DLife »
  • Help from Telecom ParisTech: Catherine Pelachaud, Bert Buchholz, Jean-Marc Thiery
thank you for your attention
Thankyou for your attention
  • Thanks
  • Merci
  • 谢谢
  • Danke
  • Obrigado
  • Gracias