VLF Rendering & Implementation Details

1. Research funded by: Virtual Light Field Group vlfproject@cs.ucl.ac.uk University College London GR/R13685/01 VLF Rendering & Implementation Details Jesper Mortensen j.mortensen@cs.ucl.ac.uk

2. Overview • Rendering from the VLF • Implementation details • Walkthrough results • Limitations • Future work • Questions

3. Rendering: VLF • The VLF can be used to render any ray • Determine intersected polygons with false colour rendering • Lookup hemisphere triangle in which direction falls, with barycentric coordinates • In each direction lookup the TRM for the intersected polygon • Interpolate radiance in TRM from 8-neighbourhood • Apply barycentric weights to interpolate the 3 radiances

4. Rendering: VLF • However, due to limited resolution blurring may occur • Especially for specular surfaces • Expensive:- 3 dirs * 9 TRM cells * rays • Example has 2K directions, 128x128 resolution

5. Rendering: Diffuse textures • The VLF also stores diffuse maps for any diffuse surface • These can be efficiently rendered using D3D/OpenGL • Limiting blurring to non-diffuse surfaces

6. Rendering: Backwards ray tracing • Backwards ray tracing can be used for specular parts • Can reconstruct specularly reflected geometry well • Bounces until diffuse surface hit • Can be slow if a large part of the scene is specular

7. Rendering: Progressive method • Hybrid method • Uses OpenGL texturing for diffuse surfaces • Uses direct VLF lookup for specular surfaces during motion • If viewpoint is stationary renders specular reflection using backwards ray tracing

8. Implementation: Language • Class based C++ • Heavy use of class templates- flexibility- efficiency, can tailor implementation- con: must recompile

9. Implementation: Platform • PC based • Windows 2000/XP • Visual Studio .NET

10. Implementation: Graphics API • Graphics API: OpenGL • Standard pbuffers • Used for ‘false colour’ rendering [visibility]- exchange buffers- rendering phase • Main issue is slow framebuffer readback

11. Implementation: Libraries • Hierarchical scene graph library- facesets, spheres, blobs etc. • Graphics library- matrices, vectors, materials, cameras etc. • VLF library- tiles, radiance maps, diffuse maps etc.

12. Implementation: Dependencies • Glut- http://www.opengl.org/developers/documentation/glut/ • Zlib- http://www.gzip.org/zlib/ • Jpeglib- http://www.ijg.org/

13. Implementation: GI framework • General framework for Global Illumination • Supports many approximations…

14. Implementation: OpenGL real-time • OpenGL rendering for real-time local illumination

15. Implementation: Radiosity • Progressive radiosity- Cohen et .al 1988

16. Implementation: Classic ray tracing • Whitted ray tracing- Turner Whitted 1980

17. Implementation: Distributed ray tracing • Distributed ray tracing- Robert L. Cook et. al. 1984

18. Implementation: Path tracing • Coming soon … Monte Carlo path tracing- James T. Kajiya 1986

19. Implementation: VLFs • And of course – Virtual Light Fields… • Diffuse • Specular • Caustics

20. Implementation: VLF applications • There are three main applications currently: • VLF analyser- visualises elements of the VLF, PSFs, tiles, maps, visibility etc. good debugging tool • VLF propagator- solves GI for a VLF, outputs binary VLF files, can reload and continue from a binary file • VLF walkthrough- loads a binary VLF file and renders it in real-time

21. Implementation: HDR viewer • GI results are natively HDR but display devices are inherently LDR [24 bit RGB]! • We have our own file format- similar to Wards radiance format • 32 bit floats RGB interleaved- optionally zip compressed • … and an associated viewer

22. Implementation: HDR viewer (contd.) • Uses simple linear scaling approaches • Or tone mapping a la Reinhard et. al. 2002

23. Walkthrough results • The following shows results from walkthroughs of VLFs • The illustrate real-time performance for scenes with global illumination effects • They were all rendered on a dual 2.8 GHZ Pentium 4 Xeon with 3GB RAM, and a NVIDIA GeForce FX5800

24. Walkthrough results: Cornell scene • This illustrates diffuse GI effects such as colour bleeding and soft shadows • The VLF uses 2K directions and 8x8 tiles each having 16x16 cells • The progressive method is used for rendering

25. Walkthrough results: Cornell scene SHOW CORNELL VIDEO

26. Walkthrough results: Office scene • This illustrates GI effects such as specular reflections, soft shadows and colour bleeding • The VLF uses 2K directions and 8x8 tiles each having 16x16 cells • Memory usage is 980MB, propagation time was 36 hrs. • The progressive method and CRT method used for rendering

27. Walkthrough results: Office scene SHOW PROGRESSIVE OFFICE VIDEO SHOW CRT OFFICE VIDEO

28. Walkthrough results: GI benchmark • Global illumination test scene from Smits & Jensens repository:- http://www.cs.utah.edu/~bes/papers/scenes/ • Illustrates several interesting light paths: LDE, LDSE, LDSDE (of which the last is a caustic) • The VLF uses 2K directions and 8x8 tiles each having 16x16 cells • Propagation time was 33 hrs. • The progressive method used for rendering

29. Walkthrough results: GI benchmark SHOW GI BENCHMARK VIDEO

30. Limitations • Limited to planar geometry • Support for basic materials • No participating media • Closed polyhedra

31. Whats next? • Investigate alternative rendering methods- Lens approach • Optimise, graphics hardware

32. Questions ?