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Generating Realistic Terrains with Higher-Order Delaunay Triangulations

Generating Realistic Terrains with Higher-Order Delaunay Triangulations. Thierry de Kok Marc van Kreveld Maarten Löffler. Center for Geometry, Imaging and Virtual Environments Utrecht University. Overview. Introduction Results on local minima NP-hard Two heuristics

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Generating Realistic Terrains with Higher-Order Delaunay Triangulations

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  1. Generating Realistic Terrains with Higher-Order Delaunay Triangulations Thierry de Kok Marc van Kreveld Maarten Löffler Center for Geometry, Imaging and Virtual Environments Utrecht University

  2. Overview • Introduction • Results on local minima • NP-hard • Two heuristics • Results on valley components • A new heuristic

  3. Motivation • Terrain modeling for geomorphological applications • TIN as terrain representation • Realism necessary • Choice of triangulation is important

  4. Few local minima • Connected valley components • Wrong triangulation can introduce undesirable artifacts

  5. Triangulations

  6. Higher-Order Delaunay Triangulations • At most k points in circle • Order 0 DT is normal DT • If k > 0, order k DT is not unique • Introduced by Gudmundsson et al. (2002)

  7. Using HODT to Solve the Problem • Well shaped triangles, plus room to optimize other criteria • We want to minimize local minima • For k > 1, optimal order k DT is no longer easy to compute • Heuristics are needed

  8. Local Minima Results • Computing optimal HODT for minimizing local minima is NP-hard • Two heuristics • Experimental results comparing the heuristics and analysing HODT

  9. NP-hardness • Minimizing local minima for degenerate pointsets is NP-hard • Minimizing local minima for non-degenerate pointsets is NP-hard too, when using order k DT • Reduction from maximum non-intersecting set of line segments

  10. Flip Heuristic • Start with Delaunay triangulation • Flip edges that might potentially remove a local minimum • Preserve order k property • O (n.k2 + n.k.logn)

  11. New edge must be “lower” than old edge • New triangles must be order k

  12. Hull Heuristic • Compute a list of all useful order k edges that remove a local minimum • Add as many as possible • Make sure they do not interfere • O (n.k2 + n.k.logn)

  13. When adding an edge, compute the hull • Retriangulate the hull • Do not add any other edges intersecting the hull

  14. Experiments on real Terrains

  15. Quinn Peak • Elevation data grid • 382 x 468 • 1 data point = 30 meter

  16. Random sample • 1800 vertices • Delaunay triangulation • 53 local minima

  17. Hull heuristic applied • Order 4 Delaunay triangulation • 25 local minima

  18. hull heuristic flip heuristic

  19. Drainage on TIN • Complex to model due to material properties • Water follows path of steepest descent • Over edge • Over triangle

  20. Definitions • Three kinds of edges:

  21. Valley component: maximal set of valley edges s.t. flow from these edges reaches lowest vertex of the component

  22. Drainage quality of terrain • Quality defined by: • Number of local minima • Number of valley components not ending a local minimum • Improve quality by: • Deleting single edge networks • Extending networks downwards to local minima

  23. Isolated valley edge • Try to remove it • No new valley edges should be created • New triangle order k • Otherwise try to extend it

  24. Extending component • Extend: • Single edge network that cannot be removed (at this order) • Multiple edge networks that do end in a local minimum • Multiple edge networks that do not end in a local minimum

  25. Extend if: • bqrp is convex • br is valley edge • brp and bqr are order k • br is steepest descent direction from b • r < b, r < q, r < p • No interrupted valley components in p or q

  26. Results valley heuristic • 25-40% decrease in number of valley components • +/- 30 % decrease in number of local minima (far less than flip and hull heuristic)

  27. Results on a terrain

  28. Results compared to flip and hull

  29. Delaunay triangulation

  30. Flip-8

  31. Hull-8

  32. Valley-8

  33. Flip-8 + valley heuristic

  34. Hull-8 + valley heuristic

  35. Conclusions Local Minima • Low orders already give good results • Hull is often better than flip • Hull performed almost optimal

  36. Conclusions Drainage • Low order already give good results • Significant reduction in number of valley components • Drainage quality is improved the most when hullheuristic is combined with valley heuristic

  37. Future Work • NP-hardness for small k • Other properties of terrains • Local maxima • More hydrological features (watersheds) • Different local operators for valleyheuristic

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