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Efficient Ray Tracing of Compressed Point Clouds

Efficient Ray Tracing of Compressed Point Clouds. The Quantized kd-Tree. Erik Hubo Tom Mertens Tom Haber Philippe Bekaert. Expertise Centre for Digital Media Hasselt University. Goal: Display Large Models. 3D scanners Industrial inspection Preservation of cultural heritage ….

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Efficient Ray Tracing of Compressed Point Clouds

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  1. Efficient Ray Tracing of Compressed Point Clouds The Quantizedkd-Tree Erik HuboTom MertensTom HaberPhilippe Bekaert Expertise Centre for Digital Media Hasselt University

  2. Goal: Display Large Models • 3D scanners • Industrial inspection • Preservation of cultural heritage • …

  3. Goal: Display Large Models • How? • Use points to represent model • Compress model • Ray trace compressed model

  4. Why Compression? • Increasing resolution of 3D scanners • Ray tracing efficient if model in main memory • Models: +100 M points > 1GB (only positions) • Model size > memory size  stalls due to slow disk access • Solution? out-of-core (disk access needed) • Compression reduces model size

  5. Our approach • Ray tracing requires local neighborhood • Caching precomputed neighborhoods? • Additional memory cost • Querying on the fly • Challenge: compression + spatial querying Moving Least Squares [Adamson and Alexa 2003,2004]

  6. Related Work • Point-based rendering: • QSplat [Rusinkiewicz and Levoy 2000] • Sequential Point Trees [Dachsbacher 2003] • Ray-tracing point clouds • [Schaufler and Jensen 2000] • [Adamson and Alexa 2003] • [Bart Adams et al. 2005] • [Ingo Wald et al. 2005]

  7. Related Work • Compression • 1.QSplat _ • 2.[Botsch 02] _ _ _ _ _ _ _ _ _ _ _ _ • 3.[Gandoin 02] _ • 4.[Krüger 05] _ _ _ _ _ _ _ _ _ _ _ _ • 5.[Waschbüsch 04] _ • 4.[Ochotta 04] _ _ _ _ _ _ _ _ _ _ _ _ • 7.[Gumhold 05] _ • No efficient spatial querying: • Pointers are discarded: 1,2,3,4 • Sequential  full decompression: 4,5,6,7 The Quantized kd-tree: Local Decoding

  8. 1 2 3 2 3 8 1 9 4 5 6 7 4 5 6 7 8 9 Quantized kd-Tree • Left-Balanced kd-Tree [Jensen01]: • Subdivides space in 2 axis aligned box shaped regions (binary tree) • Splitting plane at median point • Splitting direction X, Y, Z

  9. Quantized kd-Tree • Motivation for kd-Tree: + Efficient spatial querying + Efficient ray tracing [Wald 05] • Motivation for Left-Balanced kd-Tree + Stored as heap  no pointers - Balancing may harm performance [Wald 04]

  10. Quantized kd-Tree • Derived from Left-Balanced kd-Tree: • Normalized splitting plane offset d, w.r.t. current bounding box, d  [0,1) and quantize d  D • Leaf nodes represent input points Normal Median Split Quantized kd-Tree Median Split Quantization

  11. 0 1 0 .6 Internalnode .4 .6 .5 .6 .5 + (.6 - .5)*(7/8) = .59 Local Decoding Root leaf 3 bit per split plane – 8 quantization steps

  12. Precision: 1D q = 8 = 23 1 0 d [0,1) • Precision depends on tree depth: L • Error decreases: • Worst: 1 • Best: 1/q (= 1 quantization step) • Avg: 0.5 < α = q+1 < 1 2q •  Precision ~ αL Quant. split = q*d 1/q

  13. Ray Ray Ray Unquantized Splitting Position Local Decoding:Quantization uncertainty • Let child bounding boxesoverlap by one quantization step • Extend tree traversaland spatial query algorithms Quantization Left Region Right Region Overlap Region This point is missed 

  14. Ray Overlap Local Decoding: The Overlap Region • Low #quantization steps Large overlap region • Top of tree: examine both subtrees! Large overhead! • E.g. q = 4  56 times slower

  15. Fixed vs. Variable Quantization Fixed Variable 2 16 16 2 2 16 2 8 2 3 2 2 2 1 q = 2 q = 1..16 • Traversal overhead • Poor quality + Less overhead, better quality + Only small additional cost

  16. Fixed vs. Variable Bitstring Length

  17. Fixed vs. Variable Bitstring Length

  18. Fixed vs. Variable Bitstring Length

  19. Fixed vs. Variable Bitstring Length

  20. Fixed vs. Variable Bitstring Length

  21. Fixed vs. Variable Bitstring Length

  22. Results Bpp: 15.5 PSNR: 79.1dB Bpp: 5.6 PSNR: 62.8 dB Bpp: 8.6 PSNR: 69.4 dB Uncompressed

  23. Results

  24. Multiresolution Representation • Inner nodes represent LOD points • Median position = LOD point • Detail level based on projected screen size Level 13 Level 15 Level 18 Full

  25. Results • Multiresolution • Rendering times are 9 times faster with LOD • Reduces aliasing artifacts: Without LOD With LOD

  26. Camera position changed Results • Comparison of rendering times on 1 PC, 2GBRAM: • Compressed St Matthews (890 MB) : 9s • Uncompressed St Matthews (3.6 GB) : 36 …150s • In both cases normals were compressed to 12 bit Compressed Uncompressed

  27. Results • Rendering large scenes : • 10 sec 1 PC: 512x512 374M points(without instancing) 229M points

  28. Conclusion • Compress and display huge models • Spatial querying on compressed data • Suited for ray tracing • Multiresolution representation • Future work • Compress other attributes, e.g. normals, colors

  29. Questions? • Acknowledgments • Stanford Scanning Repository • BOF: Bijzonder Onderzoeks Fonds • Belgian American Educational Foundation • Contact: erik.hubo@uhasselt.be http://research.edm.uhasselt.be/ehubo

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