1. Precomputed Local Radiance Transfer for Real-time Lighting Design Anders Wang Kristensen Tomas Akenine-Moller Henrik Wann Jensen SIGGRAPH ‘05 Presented by Shao-Ti Lee 2010/04/08 1

2. Outline Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 2

3. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 3

4. Introduction MAIN IDEA ◦ Unstructured light cloud. 4

5. Introduction MAIN IDEA ◦ Unstructured light cloud. 5

6. Introduction FEATURES ◦ Local, not distant illumination.  Accurately represent incident radiance on different parts of the model. ◦ Light positions and intensities are changeable. ◦ Lights can be added or removed. ◦ Materials can be glossy. ◦ Camera is fully dynamic. ◦ Scene is assumed to be static. 6

7. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 7

8. Related Work Precompute the exitant radiance [Wood et al. 2000; Chen et al. 2002] ◦ Basis: sphere harmonics. 8

9. Related Work Sphere harmonics. 9 Approximated Exitant Radiance Approximated Exitant Radiance in the SH basis SH Basis

10. Related Work Clustered PCA(Principle Component Analysis) [Sloan et al. 2003] ◦ For geometry data representation compression. 10 Clustering Ex. K-Means PCA

11. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 11

12. Constructing the Light Cloud Before doing this, first divide the geometry into a set of discrete zones using a simple top-down partitioning algorithm. 12

13. Constructing the Light Cloud Use a two-stage approach to construct the light cloud. ◦ Stage 1 Make a sufficiently dense uniform sampling of the region of interest. ◦ Stage 2 The light cloud is then simplified by clustering similar lights using a bottom-up clustering algorithm. 13

14. Constructing the Light Cloud 14 This is for point lights.

15. Constructing the Light Cloud 15 This is for diffuse surface.

16. Constructing the Light Cloud 16 AC DEF GHI ABCDEFGHI Cluster List Priority queue sorted by △ jk △ AB △ AD △ BD △ CE ….. S S Compute △ SC, △ SD, △ SE, △ SF, △ SG, △ SH, △ SI and re-sort the priority queue to end a loop and stop until △ jk ≧ 1 for all j, k B

17. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 17

18. Compressing Surface Radiance Before compression ◦ Per vertex matrix X, with n p = n b x n l elements per color channel.  n b : The number of SH bases.  n l : The number of lights. After compression using CPCA 18 Cluster Mean PCA vectors (bases) weights

19. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 19

20. Real-time Relighting Organize the local lights in a kd-tree and locate the m nearest lights. 20

21. Real-time Relighting To avoid undesirable popping effects, set the weight for each pre-computed local light to After computing all weights, normalization is used. And all weights are multiplied by the power of the light at l. 21

22. Real-time Relighting Visibility problem 22 Solution: Use ray tracing with a few rays. Cannot assign weights!

23. Real-time Relighting Discontinuity Problem 23 Suddenly disappear/appear due to occlusion! Solution: Smoothly fade out/in lights over time, but that now have become obscured by geometry.

24. Real-time Relighting Computing Exitant Radiance For each cluster, x m and b i are constant. 24

25. Real-time Relighting Reconstruct the vector of SH coefficients representing exitant radiance in a vertex program by evaluating ◦ Where is the variable to ensure that we get correct blending at the borders between clusters and between zones. ◦ Recall that 25

26. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 26

27. Results 27

28. Results 28

29. Results 29 Left: Method of authors. Right: Ray tracing with per pixel lighting.

30. Results 30

31. Introduction Related Work Constructing the Light Cloud Compressing Surface Radiance Real-time Relighting Results Conclusion 31

32. Conclusion The system handles indirect illumination efficiently for models with more than 100,000 triangles. Future Work ◦ Soft shadow ◦ Spotlight 32