1. Virtual Light Field Group vlfproject@cs.ucl.ac.uk University College London Ray Tracing with the VLF (VLF-RT) Jesper Mortensen j.mortensen@cs.ucl.ac.uk GR/R13685/01 Research funded by:

2. VLF Project VLF-RT

3. VLF Project Aims of the Virtual Light Field Efficient data structure for storing directionally dependent radiance Fast generation (propagation of radiance) Fast rendering of novel views from the data structure

4. VLF Project Aims of VLF-RT Make use of the (existing) VLF data structure Provide fast ray queries for arbitrary rays Provide accurate ray queries for arbitrary rays (conservative)

5. VLF Project Data structure Use existing pre-computed tile visibility? Yes, but tile visibility must be accurate and inclusive, openGL/D3D is approximate GDI/D3D/OGL e.g. use integer top left rasterization convention If not done properly:- nasty aliasing….

6. VLF Project Rasterization to tiles

7. VLF Project Rasterization to tiles (cont’d)

8. VLF Project Rasterization to tiles (cont’d) Rasterization is done for each polygon in each direction Creates a list of polygon indices for each tile Many tiles will be empty Depth information is lost

9. VLF Project Tile BSP trees Tiles that intersect complex regions of a scene may have long tile lists Acceleration structure in effect only 2D Introduce 1D BSP tree for a tile along the direction of the PSF the tile belongs to We use spatial median subdivision and force polygons to BSP leaves at a fixed depth

10. VLF Project Tile BSP trees (cont’d) Example of a tile BSP tree

11. VLF Project Ray projection The algorithm for intersecting a ray with the acceleration structure is seemingly simple: 1) Find PSF that corresponds to ray 2) Project ray origin onto PSF to find tile 3) Intersect polygons whose id is in the tile 4) Return intersected id of closes intersection However, the ray may not correspond exactly to a PSF in the set

12. VLF Project Ray projection (cont’d) Project ray onto nearest PSF Compute a list of intersected tiles with relevant t-intersection ranges This can be done all at once, or incrementally Incremental approach saves work due to early ray termination

13. VLF Project Ray projection (cont’d) Correct image Directional resolution (.5K 2K 8K) Tile resolution (16 2 32 2 64 2 )

14. VLF Project Ray intersection overview To intersect a ray it is projected to the nearest PSF Tiles are visited in near-far order, incrementally BSP tree for a tile is searched in near-far order for the intersection range defined by the rays intersection with the tile Ray can terminate as soon as intersection is found in a BSP node Traversal must be fast BSP traversal is recursive, we avoid explicit recursion by unrolling and using a stack

15. VLF Project Intersection kernels The acceleration structure provides identifiers of polygons that may intersect the ray, to resolve these we use: Wald’s optimized intersection - Requires pre-computed information - But very fast Möller-Trumbore intersection - Only requires triangle vertices - But significantly slower than Wald Both implemented as macros to ensure inlining

16. VLF Project Intersection kernels (Wald) Barycentric projection method Pre-compute and store normals, edges, projection case, and simplify terms Triangle datastructure is 48bytes + vertices Use cache alignment, ordering matched to algorithm flow Worst case 10 muls, 1 div, 11 adds Best case (depth rejection) 4 muls, 1 div, 5 adds Wald, I, “Realtime Ray Tracing and Interactive Global Illumination”, PhD Thesis, 2004.

17. VLF Project Intersection kernels (Möller- Trumbore) Minimum storage, only triangle vertices No pre-computation But no early depth rejection Worst case 27 muls, 1 div, 23 adds Best case - barycentric u rejection 12 muls, 16 adds - barycentric v rejection 21 muls, 21 adds Good for unstructured motion, more on this later… Möller, T, Trumbore, B, “Fast, minimum storage ray-triangle intersection”. Journal of graphics tools, 2(1):21-28, 1997.

18. VLF Project Intersection kernels In the walkthrough scenario shown below the Wald intersection kernel was 20% faster than Möller-Trumbore

19. VLF Project Rendering Flexible ray tracing framework Shading - Simple diffuse - OpenGL like local illumination - Full Whitted ray tracing

20. VLF Project Rendering (cont’d) Spatial median 3D BSP acceleration structure Depth first and Breadth first ray tracing Single and multi-threaded Support triangles only – automatic triangulation 3D file format support: - Alias Wavefront (OBJ) - Benchmark for Animated Ray Tracing (BART) - Procedural random triangle clouds

21. VLF Project Results Early results on artificial scenes showed promising performance Recent results on realistic scenes confirm this Comparison with single ray 3D BSP ray tracing (CRT) favors VLT-RT

22. VLF Project Results (random scene) Walkthrough of 16K randomly distributed triangles Superior performance throughout Less inter frame variation

23. VLF Project Results (random scene) SHOW RANDOM SCENE VIDEO

24. VLF Project Results (classroom scene) Walkthrough of 46K realistic scene Superior performance on average Again, less inter frame variation Memory consumption high (>1GB)

25. VLF Project Results (classroom scene) SHOW CLASSROOM VIDEO

26. VLF Project Future work SIMD extensions for ray bundle intersection Hierarchical multi VLF approach Analytical objects - spheres - parametric surfaces Transparency (trivial, but missing)

27. VLF Project Publications All publications can be found here: http://www.cs.ucl.ac.uk/research/vr/Projects/VLF/Media/