bssrdf: bidirectional surface scattering reflectance distribution functions jared m. dunne c95 adv....
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![Page 1: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/1.jpg)
BSSRDF: Bidirectional Surface Scattering ReflectanceDistribution Functions
Jared M. Dunne
C95 Adv. Graphics
Feb. 7, 2002
Based on:
"A Practical Model for Subsurface Light Transport"
Henrik Wann Jensen, Steve Marschner, Marc Levoy and Pat Hanrahan
![Page 2: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/2.jpg)
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
![Page 3: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/3.jpg)
Example: Skin
![Page 4: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/4.jpg)
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
![Page 5: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/5.jpg)
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
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BSSRDF v. BRDF
BRDF BSSRDF
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BRDF BSSRDF
Example
![Page 8: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/8.jpg)
The Theory
• Outgoing radiance at point x0 is computed by integrating incident radiance at points xi over the surrounding area and incoming directions wi.
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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
![Page 10: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/10.jpg)
Diffuse vs. Single Scatter
![Page 11: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/11.jpg)
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)
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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
![Page 13: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/13.jpg)
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
![Page 14: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/14.jpg)
Putting the two together…
• So Jensen’s model of BSSRDF becomes:
• Or:
• BSSRDF = Diffuse Approximation + Single Scatter Term
![Page 15: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/15.jpg)
Light propagation
• a = Absorption Coefficient
• s = Scattering Coefficient
• t = Extinction Coefficient = a + s
• These are used in the volume rendering equation, which describes how light moves through a medium
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Getting the Coefficients
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Got Milk?
![Page 18: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/18.jpg)
Marble bust
1.3 Million Triangles
BRDF: 2 minutes
BSSRDF: <5 minutes
Monte Carlo: 1250 minutes
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Stanford Bunny
![Page 20: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/20.jpg)
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.”
![Page 21: BSSRDF: Bidirectional Surface Scattering Reflectance Distribution Functions Jared M. Dunne C95 Adv. Graphics Feb. 7, 2002 Based on: "A Practical Model](https://reader036.vdocuments.site/reader036/viewer/2022062300/56649d605503460f94a40e62/html5/thumbnails/21.jpg)
FinalRender
• First Render Engine to support BSSRDF
• More Info: http://trinity3d.comhttp://trinity3d.com/productinfo/FinalRender/finalrender.shtml