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BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions

BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions. Based on: "A Practical Model for Subsurface Light Transport" Henrik Wann Jensen, Steve Marschner, Marc Levoy and Pat Hanrahan. Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002. What is BSSRDF?.

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BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions

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  1. BSSRDF: Bidirectional Surface Scattering ReflectanceDistribution Functions Based on: "A Practical Model for Subsurface Light Transport" Henrik Wann Jensen, Steve Marschner, Marc Levoy and Pat Hanrahan Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002

  2. What is BSSRDF? • Bidirectional surface scattering reflectance distribution function • In other words… An equation that determines how incident light reflects and scatters from a surface • Also, simulates how light reflects off of and throughout a surface

  3. Example: Skin

  4. BRDF • Bidirectional reflectance distribution function • Simplification of BSSRDF • Assumes that light entering a material leaves it at the same point • Usually tries to approximate surface scattering in some way

  5. BSSRDF v. BRDF • BSSRDF allows simulation of light transport • Light can enter a material at one point and be reflected at another point. • BSSRDF  Translucence, Color Bleeding • BRDF  Opague, Hard-looking… • But… BSSRDF is EXPENSIVE

  6. BSSRDF v. BRDF BSSRDF BRDF

  7. Example BRDF BSSRDF

  8. The Theory • Outgoing radiance at point x0 is computed by integrating incident radiance at points xi over the surrounding area and incoming directions wi.

  9. Hendrik Wann Jensen et al • Basic Idea: Break into two parts • Diffuse = the radiance at a given point due to light transport within the media, via an approximation of the scatting • Single Scattering = the first order scattering from a surface… BRDF reflection + light refracted through the material

  10. Diffuse vs. Single Scatter

  11. The Diffusion Approximation • In highly scattering media (like milk), light scatters many times (often several hundred) • Causes a lot more calculations, more expensive (if you use full BSSRDF)

  12. Diff. Approx. (cont.) = Sd • It is observed that in such materials the scattering becomes isotropic (in all directions) even with a highly focused light distribution. • So the following two-term approximation is used: radiant fluence vector irradiance

  13. Single Scattering Term = S(1) • The light which refracts directly through or reflects off of the material • Directional component of incident light • Jensen (et al) extend a BRDF model (Hanrahan and Krueger) to achieve this scatter

  14. Putting the two together… • So Jensen’s model of BSSRDF becomes: • Or: • BSSRDF = Diffuse Approximation + Single Scatter Term

  15. Light propagation • sa= Absorption Coefficient • ss = Scattering Coefficient • st = Extinction Coefficient = sa +ss • These are used in the volume rendering equation, which describes how light moves through a medium

  16. Getting the Coefficients

  17. Got Milk?

  18. Marble bust 1.3 Million Triangles BRDF: 2 minutes BSSRDF: <5 minutes Monte Carlo: 1250 minutes

  19. Stanford Bunny

  20. Future Plans of Hendrik Wann Jensen • In an interview of Jensen by CGarchitect.com:“I would like to combine the BSSRDF with photon mapping, and also work on making these algorithms generally available and easy to use for complex models.”

  21. FinalRender • First Render Engine to support BSSRDF • More Info: http://trinity3d.comhttp://trinity3d.com/productinfo/FinalRender/finalrender.shtml

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