1. Zhao Dong 1, Jan Kautz 2, Christian Theobalt 3 Hans-Peter Seidel 1 Interactive Global Illumination Using Implicit Visibility 1 MPI Informatik Germany 2 University College London UK 3 Stanford University US

2. Zhao Dong 2 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Motivation Global Illumination (GI) Effects Hard Shadow Soft Shadow [HLHS03] Direct Lighting Direct +Indirect Lighting [Fantasylab] Arbitrary BRDF [KSS02]

3. Zhao Dong 3 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Motivation GI effects is important for realistic image synthesis. Real-time GI rendering is an open problems, especially for dynamic scene. Visibility update is always the key bottleneck.  Implicitly evaluate visibility info.

4. Zhao Dong 4 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Related Works Hierarchical Radiosity Explicit visibility evaluation is expensive. Dynamic scene  global update “links” per frame. Interactive Ray Tracing Difficult to interactively handle indirect lighting or environmental (area) light sources. Dynamic Ambient Occlusion Multiplying incident lighting with a precomputed factor that represents the visible hemispherical area at a point. Just a ratio number, directional info is incorrect. [Hanrahan91] [Wald07] [Bunnell05]

5. Zhao Dong 5 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Related Works Precomputed Radiance Transfer Dynamic Scene make the precomputation difficult SH Exponentiation  Direct lighting only. Implicit Visibility and antiradiance Many similarities with our method Our implicit visibility handling is slightly more flexible, as we impose no restrictions on the dynamics of the scene, but is also slightly more expensive. [Ren06] [Dachsbacher07]

6. Zhao Dong 6 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Light Transport Rendering Equation [Kajiya86]

7. Zhao Dong 7 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Light Transport Approximate Render Equation

8. Zhao Dong 8 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Basic Implicit Visibility Concept ShadowMap concept: Keep the link with the shortest distance in each bin. Assumptions: Each element is small. Constant energy across each element’s extent. A surface element covers the extent of the spherical bin. Nearest Element

9. Zhao Dong 9 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Basic Implicit Visibility Concept Naive Solution O(N 2 ), N is the Number of scene Element. Hierarchical Solution Similar with hierarchical Radiosity[Hanrahan91] O(NlogN)

10. Zhao Dong 10 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Algorithm Overview Preprocess Geometry: Create surface elements (surfels) based on input geometry Create Hierarchical geometric structures (QuadTree) For each frame Do: Update the geometry information for initial geometric hierarchy. Create hierarchical links between elements. Refine hierarchical links, Top  Down, Remove unnecessary links. Push down all the links to leaf node. For each bounce Do: Gathering incident energy from links and compute illumination in leaf nodes. Pull up the indirect lighting energy from leaf to root. End For

11. Zhao Dong 11 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Preprocess Geometry Create Surfels: One vertex  One Surfel Surfel position = vertex position Surfel normal = vertex normal Surfel texture coords = vertex texture coords Texture coords is for Parameterization. Surfel area [Bunnell05]: Normal Position Area Surface Elements (Surfels) Texture Coords

12. Zhao Dong 12 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Preprocess Geometry Create hierarchical geometric structures Based on UV texture coords (One texture atlas  One QuadTree) For deformable model, geometric hierarchy created once. For parent node in hierarchy:

13. Zhao Dong 13 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Create Hierarchical Links Similar with Hierarchical Radiosity [Hanrahan91] Plus implicit visibility check: Shortest distance link Atlas0 Atlas1 Atlas2 Atlas3 BinArray of Stored Link Shortest Link

14. Zhao Dong 14 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Refine Hierarchical Links Why? Unnecessary links for the same bin appear in different tree levels A B C A B C A BinArray of Shortest Link BinArray of Shortest Link Same bin

15. Zhao Dong 15 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Refine Hierarchical Links For the arrow in the figure: Different Color  Different Bins The length of arrow  distance of link Level2 Level1 Level0

16. Zhao Dong 16 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Illumination Computation Push down the links to leaf node Leaf node is vertex, so we evaluate illumination for each vertex (Leaf node level). Illumination Computation For each vertex, Gathering incident energy from hierarchical links. For deformable model, each frame we can reuse the hierarchical links to compute N-bounces lighting. Similar with Hierarchical Radiosity [Hanranhan91], each leaf node should Pull up its out shooting energy to it’s parent node! (multiply with its area ratio)

17. Zhao Dong 17 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Algorithm Implementation Preprocess Geometry: Create surface elements (surfels) based on input geometry Create Hierarchical geometric structures (QuadTree) For each frame Do: Update the geometry information for initial geometric hierarchy. Create hierarchical links between elements. Refine hierarchical links, Top  Down, Remove unnecessary links. Push down all the links to leaf node. For each bounce Do: Gathering incident energy from links and compute illumination in leaf nodes. Pull up the indirect lighting energy from leaf to root. End For Preprocess once, which depends on the model’s Complexity, and it is very fast on CPU side! Execute for each frame. Create And Refine hierarchical links is the main Computation cost! Currently implemented on CPU! Fully run on GPU side and it is Quite fast!

18. Zhao Dong 18 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Results: Various Bin Numbers Path Tracing 6x12x12, 6.23FPS 6x16x16, 4.42FPS 6x8x8, 7.89FPS

19. Zhao Dong 19 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Results: Efficiency Analysis 6x8x86x12x126x16x16

20. Zhao Dong 20 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Results: Indirect Lighting Direct, 6x16x16, 5.73FPSOne-bounce, 6x16x16, 4.95FPSTwo-bounces, 6x16x16, 4.43FPS

21. Zhao Dong 21 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Results: Non-hierarchical VS Hierarchical Path Tracing Non-hierarchy, 6x16x16, 8.5s6x16x16, 4.83FPS 6x16x16, coarse mesh

22. Zhao Dong 22 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Results: Area Light (One-bounce) Path Tracing6x16x16, 5.12FPS

23. Zhao Dong 23 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Results: Glossy && Between objects Path Tracing6x12x12, 8.02FPS Direct Lighting,6x12x12,3.86FPS

24. Zhao Dong 24 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Result: Video

25. Zhao Dong 25 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Limitation && Problems Limitations: Parameterization based on the texture coords, so input model must be textured. Our method is related with space distribution of vertex, so for too coarse input mesh, the light leaking will happen. Create && Refine hierarchical links currently only implemented on CPU, and occupy most of the time  Moderately complex scenes. Currently no explicit strategy for maintaining the temporal coherence of hierarchical links structure, so some flickering effects appear in animation.

26. Zhao Dong 26 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Conclusion && Future Works Conclusion A novel global illumination method that builds on and extends the traditional hierarchical radiosity approach by implicitly evaluating visibility. Full global illumination solutions for moderately complex and arbitrarily deforming dynamic scenes at near-real-time frame rates on a single PC. Future Works Investigate explicit temporal coherence strategies  improve animation quality Decoupling the tessellation of the mesh from shading computation. Integration with NormalMap/DisplacementMap

27. Zhao Dong 27 Interactive Global Illumination Using Implicit Visibility Pacific Graphics 2007 Questions and Thanks!