Scalable many-light methods

1. Scalable many-light methods Jaroslav Křivánek Charles University in Prague

2. Instant radiosity • Approximate indirect illumination by Virtual Point Lights (VPLs) • Render with VPLs Generate VPLs

3. Instant radiosity with glossy surfaces • LargenumberofVPLsrequired • Trueevenfordiffusescenes • Scalabilityissues Ground truth 1,000 VPLs 100,000 VPLs

4. Scalable many-light methods Generate many, many VPLs Pick only the most relevant VPLs for rendering

5. Scalable many-light methods • Choosing the right VPLs • Per-pixel basis • Lightcuts [Walter et al 05/06] • Per-image basis • Matrix Row Column Sampling [Hašan et al. 07] • Somewhere in-between • LightSlice[Ou & Pellacini 2011] • Importance caching [Georgiev et al. 2012]

6. Scalable many-light renderingLightcutsMultidimensional Lightcuts Walter et al., SIGGRAPH 2005/2006 Slides courtesy Bruce Walter: • http://www.graphics.cornell.edu/~bjw/papers.html

7. Lightcuts • http://www.graphics.cornell.edu/~bjw/papers.html

8. Complex Lighting • Simulate complex illumination using VPLs • Area lights • HDR environment maps • Sun & sky light • Indirect illumination • Unifies illumination Area lights + Sun/sky + Indirect

9. Scalable • Scalable solution for many point lights • Thousands to millions • Sub-linear cost Tableau Scene

10. Lightcuts Problem Visible surface

11. Lightcuts Problem

12. Lightcuts Problem Camera

13. Key Concepts • Light Cluster • Approximate many lights by a single brighter light (the representative light)

14. Key Concepts • Light Cluster • Light Tree • Binary tree of lights and clusters Clusters Individual Lights

15. Key Concepts • Light Cluster • Light Tree • A Cut • A set of nodes that partitions the lights into clusters

16. Simple Example Light Tree Representative 4 #1 #4 #2 #3 Light Clusters 1 4 Individual 1 2 3 4 Lights

17. Three Example Cuts Three Cuts #1 #1 #1 #4 #4 #2 #3 #4 4 4 4 1 4 1 4 1 4 1 2 3 4 1 2 3 4 1 2 3 4

18. Three Example Cuts Three Cuts #1 #1 #1 #4 #4 #2 #3 #4 4 4 4 1 4 1 4 1 4 1 2 3 4 1 2 3 4 1 2 3 4 Good Bad Bad

19. Three Example Cuts Three Cuts #1 #1 #1 #4 #4 #2 #3 #4 4 4 4 1 4 1 4 1 4 1 2 3 4 1 2 3 4 1 2 3 4 Bad Good Bad

20. Three Example Cuts Three Cuts #1 #1 #1 #4 #4 #2 #3 #4 4 4 4 1 4 1 4 1 4 1 2 3 4 1 2 3 4 1 2 3 4 Good Good Good

21. Algorithm Overview • Pre-process • Convert illumination to point lights • Build light tree • For each eye ray • Choose a cut to approximate the illumination

22. Convert Illumination • HDR environment map • Importance sampling • Indirect Illumination • Convert indirect to direct illumination using Instant Radiosity [Keller 97] • Caveats: no caustics, clamping, etc. • More lights = more indirect detail

23. Build light tree • Cluster spatially close lights with similar orientation Light Tree Representative 4 Light Clusters 1 4 Individual 1 2 3 4 Lights

24. Choose a cut • Approximate illumination with a bounded error • Different cut for each pixel

25. Illumination Equation S result=MiGiViIi lights Material term Visibility term Light intensity Geometric term

26. S lights Cluster Approximation Sum pre-computed during light tree construction result≈ MjGjVjIi j is the representative light Cluster

27. Cluster Error Bound • Bound each term • Visibility <= 1 (trivial) • Intensity is known • Bound material and geometric terms using cluster bounding volume error<MubGubVubIi S - lights Cluster ub= upper bound

28. Perceptual Metric • Weber’s Law • Contrast visibility threshold is fixed percentage of signal • Used 2% in our results • Ensure each cluster’s error < visibility threshold • Transitions will not be visible • Used to select cut

29. Perceptual Metric • Problem: • We don’t know the illumination so we don’t know the threshold either • (because threshold = 2% illumination) • Solution: • As we traverse the tree, gradually improve the illumination estimate. • Stop the traversal if the error bound for all cut nodes is below threshold.

30. Cut Selection Algorithm • Start with coarse cut (eg, root node) Cut

31. Cut Selection Algorithm • Select cluster with largest error bound Cut

32. Cut Selection Algorithm • Refine if error bound > 2% of total Cut

33. Cut Selection Algorithm Cut

34. Cut Selection Algorithm Cut

35. Cut Selection Algorithm Cut

36. Cut Selection Algorithm • Repeat until cut obeys 2% threshold Cut

37. Lightcuts (128s) Reference (1096s) Error Error x16 Kitchen, 388K polygons, 4608 lights (72 area sources)

38. Combined Illumination Lightcuts 128s 4 608 Lights (Area lights only) Avg. 259 shadow rays / pixel Lightcuts 290s 59 672 Lights (Area + Sun/sky + Indirect) Avg. 478 shadow rays / pixel (only 54 to area lights)

39. Reference Lightcuts Error x 16 Cut size

40. Scalable • Scalable solution for many point lights • Thousands to millions • Sub-linear cost Tableau Scene Kitchen Scene

41. Why does it work so well? • Data-driven stratification & importance sampling • Stratification • Clustering of similar lights in the light tree • Importance sampling • Subdividing clusters with high contribution

42. Main issue Problem: Large cuts in dark areas

43. Lightcuts Recap • Key ingredients • Upper bound on error • Refinement of the highest-error nodes first

44. Multidimensional Lightcuts • http://www.graphics.cornell.edu/~bjw/papers.html

45. Problem • Simulate complex, expensive phenomena • Complex illumination • Anti-aliasing • Motion blur • Participating media • Depth of field

46. Problem • Simulate complex, expensive phenomena • Complex illumination • Anti-aliasing • Motion blur • Participating media • Depth of field

47. Problem • Simulate complex, expensive phenomena • Complex illumination • Anti-aliasing • Motion blur • Participating media • Depth of field

48. Problem • Complex integrals over multiple dimensions • Requires many samples camera

49. Multidimensional Lightcuts • Solves all integrals simultaneously • Accurate • Scalable

50. Direct only (relative cost 1x) Direct+Indirect (1.3x) Direct+Indirect+Volume (1.8x) Direct+Indirect+Volume+Motion (2.2x)