1. Smoother Subsurface Scattering
Ivan Neulander

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 refracted outgoing 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 numerous irradiance 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
High
Medium
Low
Irradiance point density

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 BSSRDF as 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 with where = +
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 07 Problem 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

21. 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.

22. Thank You Thanks to Rhythm & Hues Slides: