Ray tracing via GPU Rasterization

1. Ray tracing via GPU Rasterization Wei Hu1 Yangyu Huang1 Fan Zhang1 Guodong Yuan2 1Beijing University of Chemical Technology, Beijing 2Tsinghua University, Beijing

2. The Goal Input: A ray and scene objects Output: The intersection position(if exists)

3. GPU Rasterization Method • Sampling-based • Environment Map and Shadow Map approximate effects • Layered Depth Images(LDI) too slow for accurate ray-scene intersections GPU Rendering of Secondary Effects[K. Burger et al. 2007] • Voxel-based • Uniform Grid cost too much memory for accurate ray-scene intersections • Sparse Voxel Octree(SVO) use cone tracing to generate glossy reflection, not applicable for rendering fully dynamic scenes Gigavoxels: ray-guided streaming for efficient and detailed voxel rendering[C. Crassin et al. 2009]

4. Our Method Introduction • Our idea is simple but very effective • Combine low resolution uniform grid and high resolution A-buffer(LDI) Achieve balance between accuracy and performance • Support various GI effects • Ray casting • Whitted ray tracing(reflections/refractions, shadows) • Ambient occlusion • Soft shadow • Path tracing

5. Our Ray-tracing Algorithm Scene Representation • Two-level representation of scene(All generated in one pass) • One uniform voxel grid(low resolution, one 3D texture) • Three A-buffers(high resolution, three texture buffers) • A-buffer sorting(One pass) Sort each A-buffer by it’s main axis from near to far(x, y, z axis) • Deferred shading(One pass) Compute first ray-tracing scene informations(vertex, normal, texture, etc.)

6. Our Ray-tracing Algorithm

7. Our Ray-tracing Algorithm

8. Our Ray-tracing Algorithm Ray-scene Intersection • Traverse the ray on the coarse-level uniform grid in the scene(use 3D-DDA) • Select a A-buffer from three A-buffers • Traverse the ray on the selected precise A-buffer in the non-empty grid(2D-DDA) • determine whether the point is inside scene objects by comparing depth values in sorted link-listed values

9. Our Ray-tracing Algorithm Improve the performance of ray-tracing in A-buffer • Linear search accurate but slow reflection, refration • 1-step binary search fast but Inaccurate ambient occlusion, path tracing • 3-step binary search balance the accuracy and speed shadow A non-empty voxel search in A-buffer

10. Implementation Challenges Ray-tracing in multiple layers Reasons: • Shader don’t support recursion Solutions: • Replace recursion by loops • Forward compute results • Backward combine results Scene P1 P2 P3 Screen P0 P4

11. Implementation Challenges Low frequency effect need large sampling rays Reasons: • Shader limit sampling rays number Solutions: • Implement multiple-pass rendering pipeline • Two buffer(FBcurr and FBcomp) maintain result • One for current result • One for final result

12. Implementation Challenges Multiple-pass rendering methods • General rendering For i = 0:passNum-1 Compute Fbcurr Fbcomp += Fbcurr / passNum End updateScene() • Progressive rendering For i = 0:inf Compute Fbcurr Fbcomp = (i * Fbcomp + Fbcurr) / (i + 1) updateScene() End

13. Experimental Results & Discussion • Ray Casting • Ray Tracing • Ambient Occlusion • Path Tracing Result Size: 800 600 All generated with full dynamic scenes

14. Experimental Results & Discussion • Language: OpenGL/GLSL • Equipment: Intel Xeon R CPU E5-1620 3.7GHz NVidia GeForce R780 GTX graphics card • A-buffer: 1280 1280 • Uniform grid: 64 64 64 sqrt(3n)^3 ugrid is a better choice for n^2 A-buffer • Screen buffer: 800 600 • Trace depth: 3 • Fully dynamic scene: uniform voxel grids and A-buffers are reconstructed for each frame

15. Experimental Results & Discussion A display of video All effects are generated by our method Fully dynamic scene

16. Experimental Results & Discussion Whitted ray tracing effect Accuracy of ray-scene intersection Result: Our results closely match the references(Optix) • Compare with optix Ours Optix

17. Experimental Results & Discussion • Compare with uniform grid Ambient occlusion effect • Ours: 64^3 / 256 / 64 / 16.8fps • Ugrid: 64^3 / 64 / 46.3fps • Ugrid: 256^3 / 96 / 7.7fps Result: Reach the approximate same effect, our method is faster than uniform grid

18. Experimental Results & Discussion • Compare with sparse voxel octree Result: Better for full dynamic scenes, for which high-precision results are usually required.

19. Comparison bad general good Time slow: -1 medium: 0 fast: +1 Accuracy low: -1 medium: 0 high: +1 Memory large: -1 medium: 0 small: +1 Scene small: -1 medium: 0 large: +1

20. Applications • Ward BRDF

21. Applications • Soft shadow

22. Applications • Slightly larger scene

23. Limitations • Gracile objects might be missed in A-buffer • Large and complex scenes may beyond the capability of current GPU • When tracing, a ray can‘t branch into two rays in multiple layers

24. Conclusion & Future Works • Conclusion • A novel ray-scene intersection technique • Balance the accuracy and performance • A new ray-tracing framework • Wide range use in GI effects • Future works • Improve the intersection accuracy • Accelerate representation construction • Apply to more large and complex scenes

25. Thanks & Questions Contact me: memoryemon@gmail.com Yang-yu Huang the second year of master's degree