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Efficient Rendering of Local Subsurface Scattering

Efficient Rendering of Local Subsurface Scattering. Tom Mertens 1 , Jan Kautz 2 , Philippe Bekaert 1 , Frank Van Reeth 1 , Hans-Peter Seidel 2. 1. 2. Overview. Problem Related Work Local Subsurface Scattering Our Approach Implementation & Results Discussion Summary & Future Work.

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Efficient Rendering of Local Subsurface Scattering

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  1. Efficient Rendering of Local Subsurface Scattering Tom Mertens1, Jan Kautz2, Philippe Bekaert1, Frank Van Reeth1, Hans-Peter Seidel2 1 2

  2. Overview • Problem • Related Work • Local Subsurface Scattering • Our Approach • Implementation & Results • Discussion • Summary & Future Work

  3. Subsurface Scattering BRDF translucent opaque BSSRDF

  4. BSSRDF model • introduced by Jensen et al.(SIGGRAPH’01) • multiple scattering • materials with high albedo: marble, milk, wax, skin,… function of distance

  5. BSSRDF model • introduced by Jensen et al.(SIGGRAPH’01) • multiple scattering • materials with high albedo: marble, milk, wax, skin,… function of distance

  6. Related Work • Jensen et al. ’02 • General scattering effects • Offline rendering • Mertens et al. ’03 • Dynamic models • General scattering effects • Per vertex • Our paper • Dynamic models • Local scattering effects • Per pixel

  7. Local Subsurface Scattering • Certain cases no global response • Dense materials • Large scale • Distinct appearance! • Rough surface • Local sampling sufficient • But accuracy is important! • Rd decays exponentially • Per vertex too coarse • Apply to skin rendering Global response Only local response

  8. Local Subsurface Scattering Local subsurface scattering Diffuse

  9. Local Subsurface Scattering Local Full

  10. Our Approach • High level description • Employ importance sampling scheme for Rd • Rendering algorithm • Generate importance samples • Render irradiance image • Integrate irradiance image locally in tangent plane

  11. Importance Sampling of Rd • Need to solve integral • Idea: sample according to Rd • Result: set of distances ri • Issues: • Need samples on surface, not ri’s • Need irradiance at sample

  12. Importance sampling of Rd • Solution: • Pick a view e • Render irradiance to image T • Generate sample p’ in tangent plane • Project p’ on surface  p • Project p’ into T • to retrieve irradiance E(p’)

  13. Importance sampling of Rd • We take eye position for e • p’ p implies a jacobian J • ratio of solid angles • Integral becomes:

  14. Rendering Algorithm • Generate importance samples in 2D 2D Rd ri

  15. Rendering Algorithm • Render irradiance image

  16. Rendering Algorithm • Integrate image locally in tangent plane

  17. Rendering Algorithm • Store result in final image

  18. Implementation • Variance reduction • Stratified sampling • Deterministic, pseudo random • Interleaved sampling • Noise  dither pattern • Combined sampling • Importance + uniform • Irradiance discontinuties • Software implementation • Programmable Graphics Hardware Combined sampling Uniform importance

  19. Implementation • Programmable Graphics Hardware • Overview: • generate 2D samples • quick per-frame preprocess in software • Render irradiance image T • Bind E as texture • For each sample • Look up sample E in T (pixel shader) • Accumulate E in temporary texture • Output temporary texture

  20. Results • ATI Radeon 9700 Pro • 500x500 image, 4 to 5 frames/sec • Some pictures…

  21. Image Quality Color bleeding (forehead) Shadow smoothing

  22. Image Quality nVIDIA’s skin shader Our method

  23. Complexlighting

  24. Demo video

  25. Discussion • No global effects • E.g. backlit ears • Prone to noise • Irradiance discontinuities • Shadow borders • Geometric discontinuities • Kills effect of importance sampling • Ghosting artifacts • Accumulation  fill-rate limited ghosting

  26. Summary • Novel technique for local subsurface scattering • Amenable for hardware implementation • Interactive frame rates • Dynamic models • Application: skin rendering

  27. Future Work • Hybrid algorithm • Global response per vertex • Local response per pixel • Eliminate ghosting • Apply technique in texture space • Combine with skin BRDF • Take into account varying blood concentrations

  28. Acknowledgments • Head model courtesy of nVIDIA • Funding: European Regional Development Fund

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