1. Surface Material Physicsfor Shader Writers Chas. Boyd DirectX® Graphics Architect Windows® Gaming & Graphics Technology Microsoft® Corporation

2. Motivation • Lighting and surface effects are clearly discernable to end-users • Enables novelty for them, and differentiation for you • Enable a title to have a distinct “look” • See other talks on lighting models, this one focuses on surfaces

3. Overview • Surfaces vs. Lighting • Surface Types • Metal • Rough • Specular • Subsurface • Layered Materials • Maya, SOFTIMAGE, Renderman

4. Assumptions • For all surfaces: • Other effects such as shadows will be applied in the lighting model • These are independent of the surface effects described here

5. Surfaces and Lighting • Key participant in visual “Look” • Interacts with lighting environment in various ways • Need to take both into account in any given situation • Design content to have surfaces that look best in the lighting environment they will be used in and vice versa

6. Surface-Light Interaction • Light comes in • Surface modifies it • Light leaves towards camera/eye • Surface interactions include • Absorption, reflection, self-shadowing, self-occlusion, scattering • Subsurface effects • Absorption, scattering, re-emission

7. Fundamentals of Surfaces • Surfaces often contain multiple constituents • Absorptive dies, reflective layers, scattering particles of various sizes • This talk proceeds from simplest to more complex surface structures

8. Polished Metal Surface • Process • Light comes in • Some colors are absorbed • Remaining light reflects • Atomically smooth—mirror finish • Structured highlights • image of environment are visible in them

9. STM Micrograph

10. Polished Metal

11. Map-Based Technique • Apply environment map for reflection • Scale it by metal color • Accounts for some colors absorbed • That is the final color for the screen • C = envmap*base • Gamma Note: no effect

12. Modulate Environment Map

13. Procedural Technique • Use very high specular exponent • ~100 or higher • Accurate model for single constant color round light source • C = light*dot(N,H)^100 * base • Gamma Note: picking exponent

14. Use in Games • Works for gold, copper, bronze • some anodized effects • Ideal for weapons • “Gun metal blue” • Not appropriate for enameled metal • More metallic than any “metallic” auto-body enamel • Nor for oxidized e.g. verdigris • These are too rough for this model

15. Rougher Surfaces • Smudges, scuffs, dust, oxides • At high magnification we see:

16. Rough Surface Diffuse Scattering Reduced Specular

17. Map-based Technique • Roughness varies per-pixel in “gloss” map e.g. alpha of base tex • Scale down environment map • c = gloss*envmap*base • Blur environment map in areas of low gloss • MIP LOD or post-process effect • MIP LOD is per-pixel in ps.2.0 • Blur is partial integral of map

18. Procedural Technique • Use procedural specular • spec = light*dot(N,H)**25 • but with same gloss term • c = gloss*spec*base • Decrease exponent to achieve blur

19. Both Techniques • Add diffuse scattering term

20. Increased Roughness

21. Increased Roughness Surface

22. Fully Rough Surface

23. Fully Rough Surface Diffuse Scattering No Specular

24. Rough Surfaces • Concrete • Sand • Cloth • Plaster • Marble, non-polished • Pavement • Stone

25. Moon Photograph

26. Moon Rendering

27. Rough Surfaces • Diffuse reflector • Lambertian or Oren-Nayar effects

28. Map-based Technique • Blur map across very large scale • Integrate entire hemisphere

29. Cube Map And Its Integral

30. Diffuse only

31. Procedural Technique • Model only Diffuse term • For directional light: • c = light * dot(N,L) * base • Or use hemisphere light • Gamma Note: • Correct using sqrt or 1-(1-x)**2 • for non-CAD applications

32. Layered Surface Varnish Dye Layer

33. Layered Surface Larger Varnish Dye Particles

34. Layered Surface • Thin layer of smooth clear material • Over rough dye particles • Underlayer is rough because particles may be suspended • Plastics • Linoleum • Enameled metal –car finish • Varnished surface –wood

35. Map-Based Technique • Envt. map scaled by gloss term • spec = gloss*envmap • Added to diffuse term: • c = dot(N,L)*base + spec • Base color does not tint highlight • Gamma Note: add should be linear • Correct to linear, add, convert back • if have enough precision and time

36. Add Environment + Diffuse

37. Specular Only

38. Procedural Technique • Base color does not modulate specular, only light color does • spec = light*dot(N,H)**100 • c = dot(N,L)*base + spec*gloss • Top layer + bottom layer

39. Summary Polished specular*gloss*base Rough diffuse*base Varnished spec*gloss + diffuse*base

40. Fresnel Effect • At top layer interface • Some light is reflected, • Remainder is transmitted through • Depends on surface material • And on incident angle! • Fresnel term f is proportion reflection • grazing angles: 100% reflected • normal angles: 5% reflected

41. Fresnel Mid 60% reflected Air Material 40% transmitted

42. Fresnel Normal 10% reflected Air Material 90% transmitted

43. Fresnel Grazing 90% reflected Air Material 10% transmitted

44. Fresnel Profile Measurement 0o 45o 90o

45. Fresnel Term Approximation • Use f = 1 - cosq = (1 - E dot N) • Raise to power for better approximation • f = (1 – cosq)4 • [Schlick] used 5th power • Hack curve fit to Glassner

46. Fresnel Vertex Shader m4x4 oPos,v0,c8; // xform pos. to output space m4x4 r0,v0,cW // xform pos. to world space, m3x3 r1,v3,cW // xform normal to world space add r0,-r0,c14 // compute eye ray dp3 r3.x,r0.xyz,r0.xyz rsq r3.xyz,r3.x mul r0.xyz,r0.xyz,r3.xyz// normalize it dp3 r0,r0,r1; // dot eye ray with normal add r0,c1111,-r0 // complement color mul r1,r0,r0 // **2 mul r0,r1,r1 // **4 mul r0,r0,c1 // scale for fresnel add oD0,r0,c0 // bias and emit as diffuse

47. Fresnel Pixel Shader ps.1.1 ; def v0 // eye ray def c0 // 5% bias def c1 // 95% scale tex t0 // normal map dp3 r0,v0_bx2,t0_bx2; // dot eye ray with normal mul r1,1-r0,1-r0 // complement and square mul r0,r1,r1 // **4 mad r0,r0,c1,c0 // scale & bias

48. Fresnel Comparison 1- cosq (1- cosq)4 Measurement 0o 45o 90o

49. Fresnel Term

50. Fresnel Affects Diffuse Layer • Only light that was not reflected illuminates diffuse “dye” layer • In single light case, incident angle can be used to compute f • Then f is used to blend (lerp) between specular and diffuse • Not as useful for multi-light case • diffuse layer is illuminated by light from all directions in that case