Advanced Skin Rendering Techniques: Achieving Realistic Skin Appearance

1. SIGGRAPH 2007 GPU Gems 3:Advanced Skin Rendering Eugene d’Eon

2. Outline • Demo: Human Head • The Appearance of Skin • An overview of the system • Specular Surface Reflectance • Scattering Theory • Advanced Subsurface Scattering

3. Demo

4. The Appearance of Skin • Difficult to simulate:

5. The Appearance of Skin • Difficult to simulate: • Detailed

6. The Appearance of Skin • Difficult to simulate: • Detailed • Modern scanning

7. The Appearance of Skin • Difficult to simulate: • Detailed • Modern scanning • Translucent

8. The Appearance of Skin • Difficult to simulate: • Detailed • Modern scanning • Translucent • Subsurface scattering

9. Two Component Reflectance Model • Most materials use two components

10. Two Component Reflectance Model • Most materials use two components • Surface reflectance (specular)

11. Two Component Reflectance Model • Most materials use two components • Surface reflectance (specular) • Subsurface reflectance (diffuse)

12. Two Component Reflectance Model • Most materials use two components • Surface reflectance (specular) • Subsurface reflectance (diffuse) • Use Physically based models

13. Two Component Reflectance Model • Most materials use two components • Surface reflectance (specular) • Subsurface reflectance (diffuse) • Use Physically based models • Specular BRDF

14. Two Component Reflectance Model • Most materials use two components • Surface reflectance (specular) • Subsurface reflectance (diffuse) • Use Physically based models • Specular BRDF • Efficient subsurface scattering approximations

15. Skin Surface Reflectance • Start with a physically based model for skin [DJ2006]

16. Skin Surface Reflectance • Start with a physically based model for skin [DJ2006] • Consider top-most interaction

17. Skin Surface Reflectance • Start with a physically based model for skin [DJ2006] • Consider top-most interaction • How much light reflects?

18. Skin Surface Reflectance • Start with a physically based model for skin [DJ2006] • Consider top-most interaction • How much light reflects? • What direction(s)?

19. Skin Surface Reflectance • Only ~6% of the incident light (on average) reflects directly [Tuchin 2000]

20. Skin Surface Reflectance • Only ~6% of the incident light (on average) reflects directly [Tuchin 2000] • This is due to a Fresnel reflection • Not colored by the skin

21. Skin Surface Reflectance • Only ~6% of the incident light (on average) reflects directly [Tuchin 2000] • This is due to a Fresnel reflection • Not colored by the skin • The topmost skin surface is rough • Single incoming direction: many outgoing directions

22. Skin Surface Reflectance • Only ~6% of the incident light (on average) reflects directly [Tuchin 2000] • This is due to a Fresnel reflection • Not colored by the skin • The topmost skin surface is rough • Single incoming direction: many outgoing directions • Use a specular BRDF function

23. Skin Surface Reflectance • Which BRDF should we use?

24. Skin Surface Reflectance • Which BRDF should we use? • Phong and Blinn-Phong • Not physically based • We can do better

25. Skin Surface Reflectance • Which BRDF should we use? • Phong and Blinn-Phong • Not physically based • We can do better • Turn to more physically based models

26. Skin Subsurface Reflectance • What about the remaining 94%?

27. Skin Subsurface Reflectance • What about the remaining 94%? • We need to compute subsurface scattering

28. Skin Subsurface Reflectance • Subsurface scattering • Gives skin soft appearance and color

29. Skin Subsurface Reflectance • Subsurface scattering • Gives skin soft appearance and color • Expensive to compute

30. Skin Subsurface Reflectance • Subsurface scattering • Gives skin soft appearance and color • Expensive to compute • Essential for realistic appearance Subsurface scattering No Subsurface scattering *Scan data courtesy of XYZRGB Inc.

31. Overview • Two component reflectance • Specular BRDF • Subsurface scattering approximation

32. Overview • Two component reflectance • Specular BRDF • Subsurface scattering approximation • Specular • Kelemen Szirmay-Kalos 2001 BRDF • Schlick fast Fresnel • Weyrich et al. 2006 measured parameters

33. Overview • Two component reflectance • Specular BRDF • Subsurface scattering approximation • Specular • Kelemen Szirmay-Kalos 2001 BRDF • Schlick fast Fresnel • Weyrich et al. 2006 measured parameters • Subsurface • Advanced texture-space diffusion • Stretch map correction • Multiple Gaussian blurs mixed

34. Overview Diagram Start … blur blur blur Linear combination texture mapping Stretch maps Final pass: combine blurs + specular

35. Specular Surface Reflectance • Use a physically based BRDF

36. Specular Surface Reflectance • Use a physically based BRDF • Phong and Blinn-Phong can be improved upon

37. Specular Surface Reflectance • Use a physically based BRDF • Phong and Blinn-Phong can be improved upon • Torrance-Sparrow has proven realistic • Too lengthy to compute exactly

38. Specular Surface Reflectance • Use a physically based BRDF • Phong and Blinn-Phong can be improved upon • Torrance-Sparrow has proven realistic • Too lengthy to compute exactly • Kelemen Szirmay-Kalos 2001 • Faster than TS • Even faster with Schlick’s Fresnel term

39. Phong vs. physically-based BRDF * Phong KS BRDF Phong KS BRDF *[Kelemen and Szirmay-Kalos 2001]

40. Phong vs. physically-based BRDF • Increased specularity at grazing angles • Here we use Kelemen-Szirmay-Kalos 2001

41. Rendering with a BRDF • BRDFs • Several analytic terms • Fresnel

42. Rendering with a BRDF • BRDFs • Several analytic terms • Fresnel • Parameters • rho_s • Vectors N, V, L • Roughness (m) • Index of refraction (eta)

43. Rendering with a BRDF • BRDFs • Several analytic terms • Fresnel • Parameters • rho_s • Vectors N, V, L • Roughness (m) • Index of refraction (eta) specularLight += lightColor[i] * lightShadow[i] * rho_s * specBRDF( N, V, L[i], eta, m) * saturate( dot( N, L[i] ) );

44. Rendering with a BRDF • The dot( N, L ) is important • Only works for point/spot lights • Environment lights are expensive • Glossy reflections • See Kautz and McCool 2000 specularLight += lightColor[i] * lightShadow[i] * rho_s * specBRDF( N, V, L[i], eta, m ) * saturate( dot( N, L[i] ) );

45. Fresnel Reflectance for Rendering Skin • All physically based BRDFs have a Fresnel term, F

46. Fresnel Reflectance for Rendering Skin • All physically based BRDFs have a Fresnel term, F • Requires knowing index of refraction, eta • Look to skin research: use 1.4 [DJ2006]

47. Fresnel Reflectance for Rendering Skin • All physically based BRDFs have a Fresnel term, F • Requires knowing index of refraction, eta • Look to skin research: use 1.4 [DJ2006] • Dielectric, unpolarized Fresnel reflectance

48. Fresnel Reflectance for Rendering Skin • All physically based BRDFs have a Fresnel term, F • Requires knowing index of refraction, eta • Look to skin research: use 1.4 [DJ2006] • Dielectric, unpolarized Fresnel reflectance • Use Schlick’s fast Fresnel approximation: // H is the standard half-angle vector. F0 is reflectance at normal incidence (for skin use 0.028). float fresnelReflectance( float3 H, float3 V, float F0 ) { float base = 1.0 - dot( V, H ); float exponential = pow( base, 5.0 ); return exponential + F0 * ( 1.0 - exponential ); }

49. Fresnel Reflectance for Rendering Skin Using textbook Fresnel Formula Using Schlick’s Fresnel

50. Roughness parameter • How do we set roughness parameter m? *[Donner and Jensen 2005]