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Enhancing Subsurface Scattering Smoothing Techniques

Learn about improving subsurface scattering smoothing models, addressing hard shadow edges and continuous mean free path variations for realistic renderings.

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Enhancing Subsurface Scattering Smoothing Techniques

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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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