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Smoother Subsurface Scattering

Smoother Subsurface Scattering. Ivan Neulander ivan@neulander.org. Subsurface Scattering Overview. Light transport is mediated by BSSRDF Light can enter and exit material at different positions, unlike with BRDF Shading is affected by light incident on other parts of surface

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Smoother Subsurface Scattering

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  1. Smoother Subsurface Scattering Ivan Neulander ivan@neulander.org

  2. Subsurface Scattering Overview • Light transport is mediated by BSSRDF • Light can enter and exit material at different positions, unlike with BRDF • Shading is affected by light incident on other parts of surface • Expensive to render (but very useful) BRDF BSSRDF

  3. Jensen et al, SIGGRAPH 01 • Single + Multiple scattering • Single scattering • Monte Carlo integration along refractedoutgoing ray • Similar to volumetric ray marching • Multiple scattering • Diffusion approximation using dipole model,based on work by Eason and Farrell • Avoids ray marching but needs numerousirradiance evaluations on the surface

  4. Jensen & Buhler, SIGGRAPH 02 • Faster diffusion approximation for Multiple Scattering: Two-pass approach • Sample irradiance over surface, store in octree • Retrieve irradiance from octree in final pass • Each node contains an estimate of its children Irradiance Cache Rendered Image

  5. Jensen & Buhler, SIGGRAPH 02 • fast  used in production renderers; but: • Some materials require very high irradiance sample counts to achieve smooth shading • low mean free path Irradiance point density High Medium Low

  6. Langlands & Mertens, SIGGRAPH 07: Smoothing Solution • Applies smoothing to reduce “splotchy noise” in Jensen & Buhler at low point densities • Noise is due to a rapid falloff in the BSSRDFas a function of

  7. Langlands & Mertens, SIGGRAPH 07: Smoothing Solution • becomes increasingly peaked as mean free path decreases. = 1 = 1/2 = 1/4 = 1/8 = 1/16

  8. Langlands & Mertens, SIGGRAPH 07: Smoothing Solution • Idea: where = + Local component Global component 1 – w(r) Local term: More noisy Global term: Less noisy replace Noise-free!

  9. Langlands & Mertens, SIGGRAPH 07: Results = + unsmoothed Global term Local term = + smoothed Global term Smoothed local term

  10. Langlands & Mertens, SIGGRAPH 07: Problem 1 • Incorrectly hard shadow lines smoothing Ground truth

  11. Our Solution to Problem 1 • Problem lies in local term • Replace in smooth local term withwhere = + Global term Smooth local term Contributions: Current shading sample All samples in irradiance cache

  12. Our Solution to Problem 1: Results • We have replaced the hard edge with a noisier but appropriately soft one = + Global term Our smooth local term

  13. Our Solution to Problem 1: Results • Comparison of teapot example Unsmoothed L&M smoothing Our smoothing Ground truth

  14. Our Solution to Problem 1: Results

  15. Our Solution to Problem 1: Results

  16. Langlands & Mertens, SIGGRAPH 07Problem 2: Varying MFP • Continuous variation of requires arbitrarily many precomputations of because varies nonlinearly with . • Cannot texture-map • Avoid many discrete

  17. Our Solution to Problem 2 • Evaluate at discrete and interpolate result • Since can span range , at which points do we evaluate? • We use • Provides high resolution for near zero, lower resolution for large

  18. Our Solution to Problem 2: Results

  19. Our Solution to Problem 2: Results No Smoothing Our Smoothing Ground truth

  20. Conclusions • We have made two enhancements to the SSS smoothing model by Langlands & Mertens: • Fixed hard shadow edges • Allowed for continuous mean free path variation • Our solution builds on work by Jensen, Buhler, and others.

  21. Thank You Thanks to Rhythm & Hues Slides: www.neulander.org/work#sketch2009

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