1. computer graphics & visualization Global Illumination Effects

2. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Realistic illumination of the scene

3. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Soft shadows

4. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Subsurface scattering

5. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Motivation Many algorithms exist – Photon mapping – Ambient Occlusion – … Common goal: Solving parts of the Rendering Equation

6. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Problems Scene changes -> New computation Still not possible in real-time Uses Raytracing or Radiosity Already explained Going to be explained now

7. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Photons Have energy h: Planck constant v: Frequency of light

8. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiometric Quantities Radiant energyJ Radiant powerW IrradianceW/m² RadiosityW/m² Radiant intensityW/sr

9. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiance θ: angle between surface‘s normal and ω cosθ: Lambertian law Constant along a ray

10. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Irradiance

11. computer graphics & visualization Global Illumination Effects Christian A. Wiesner BRDF Bidirectional reflectance distribution function How much light is reflected

12. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Reflection Equation Integrate over the hemisphere

13. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Rendering Equation Radiance Emitted light Surfaces BRDF Visibility Geometry factor

14. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Ideal diffuse reflection can be simulated with Radiosity Uses finite elements Introduced by Goral et al.

15. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Origin: Thermal heat transfer Developed in 1984, still in use Modelling of diffuse lighting – Doesn‘t account for specular lighting – Independent of viewer – Therefore: Stays constant in constant scene

16. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Equation RadiosityEmissivity Reflectivity Form factor constant

17. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Form Factors

18. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Form Factors

19. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Nusselt Analog Simple geometric analog for calculating form factors B A

20. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Algorithm Hemicube instead of hemisphere

21. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Algorithm Idea: – Precompute delta form factors analytically – Count covered pixels – Sum up covered delta form factors to the true form factor

22. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Example

23. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Hemicube Algorithm on GPU Use projection center as viewport Use current face as viewing plane Do the rendering Grab the colour buffer (IDs of patches) Count coloured pixels Visibility test performed by depth test

24. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Algorithm Compute form factors Solve linear equation system for i = 1, …, n

25. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Von Neumann Series 0 Bounces 1 Bounce 2 Bounces 3 Bounces

26. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Jacobi Iteration

27. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Shooting / Gathering

28. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Result

29. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity vs. Ray Tracing

30. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Radiosity Conclusion Old, but still in use Used for simulating diffuse lighting Result can be used in combination with other GI algorithms

31. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Motivation Ambient term constant in Phong model Not very realistic Idea: Compute occlusion of each face

32. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Result: Occluded areas appear darker than brigther ones Multiply with usual Phong model 2 possibilities: – Screen space – Object space

33. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Screen Space Ambient Occlusion Can be completely done on GPU No preprocessing Independent of scene complexity Idea: Instead of performing full raytracing use occlusion information from z-buffer

34. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Screen Space Ambient Occlusion Take 3D samples around each point Determine occlusion of each point by testing against the depth buffer Far samples with less influence Use blurring for smooth results

35. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Screen Space Ambient Occlusion

36. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Object Space Ambient Occlusion Define surface element as an oriented disk Use Heron‘s formula, Store position, normal and area in texture for pixel shader

37. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Object Space Ambient Occlusion Compute accessibility value at each element (% of hemisphere) Approximation based on the solid angle of an oriented disk Strongly dependent on scene complexity

38. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Object Space Ambient Occlusion

39. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Results

40. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Results

41. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Ambient Occlusion Conclusion Can be preprocessed for each object Used in the current version of PIXAR‘s RenderMan

42. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Outlook Faster computation – Cheaper – Artists can see results faster More realistic lighting

43. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Conclusion Very important for any animated movie Computation time not too important

44. computer graphics & visualization Global Illumination Effects Christian A. Wiesner Thanks for your attention!