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Jun 18th, 2019
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  1. paths = {{{348.488, 132.622}, {336.333, 63.6857}, {394.365, 24.5422},
  2. {39.3603, 78.1653}, {109.094, 84.2662}, {170.317, 50.3295},
  3. {195.403, 115.68}, {263.324, 132.615}, {316.947, 177.61},
  4. {381.382, 150.259}, {49.8526, 164.812}, {41.3217, 95.3342},
  5. {11.7384, 158.776}, {65.3616, 113.781}, {5.35985, 77.728},
  6. {18.7165, 9.01408}, {358.715, 372.961}, {394.767, 312.96},
  7. {340.367, 268.907}, {313.016, 333.343}, {269.92, 388.503}}};
  8.  
  9. r = 3;
  10. torus[{u_, v_}] := {Cos[u]*(Sin[v] + r), Sin[u]*(Sin[v] + r), Cos[v]}
  11.  
  12. Needs["VariationalMethods`"]
  13. eq = EulerEquations[Sqrt[Total[D[torus[{u, v[u]}], u]^2]], v[u], u];
  14.  
  15. geodesic[{{u1_, v1_}, {u2_, v2_}}] := Module[{start, g, sol},
  16. If[u2 < u1, Return[geodesic[{{u2, v2}, {u1, v1}}]]];
  17. sol = ParametricNDSolve[Flatten[{
  18. eq, v[0] == v1, v'[0] == a
  19. }], v, {u, 0, u2 - u1}, {a}];
  20. start = a /. FindRoot[Evaluate[(v[a][u2 - u1] - v2 /. sol)], {a, 0}];
  21. g = v[start] /. sol;
  22. Function[t, {u1 + t*(u2 - u1), g[t*(u2 - u1)]}]
  23. ]
  24.  
  25. LocatorPane[
  26. Dynamic[pts],
  27. Dynamic[ParametricPlot[Evaluate[geodesic[pts][t]], {t, 0, 1},
  28. PlotRange -> {{-π, π}, {-π, π}}, Axes -> True,
  29. AspectRatio -> 1/r]]]
  30.  
  31. Show[
  32. ParametricPlot3D[
  33. torus[{u, v}], {u, -π, π}, {v, -π, π},
  34. PlotStyle -> White, ImageSize -> 500],
  35. ParametricPlot3D[Evaluate[torus[geodesic[pts][t]]], {t, 0, 1},
  36. PlotStyle -> Red]
  37. ]
  38.  
  39. Clear[geodesicFindMin]
  40. geodesicFindMin[{p1_, p2_}, nPts_: 25] :=
  41. Module[{approximatePts, optimizeOffset, optimizeOffsets, direction,
  42. normal, pathLength, optimalPath, interpolations, len, solution},
  43. direction = p2 - p1;
  44. normal = {{0, 1}, {-1, 0}}.direction;
  45.  
  46. approximatePts = Join[
  47. {p1},
  48. Table[
  49. p1 + i*direction/(nPts + 1) + optimizeOffset[i]*normal, {i,
  50. nPts}],
  51. {p2}];
  52.  
  53. pathLength = Total[Norm /@ Differences[torus /@ approximatePts]];
  54.  
  55. {len, solution} =
  56. Quiet[FindMinimum[pathLength,
  57. Table[{optimizeOffset[i], 0}, {i, nPts}]]];
  58. optimalPath = approximatePts /. solution;
  59.  
  60. interpolations =
  61. ListInterpolation[#, {{0, 1}}] & /@ Transpose[optimalPath];
  62.  
  63. Function[t, #[t] & /@ interpolations]
  64. ]
  65.  
  66. LocatorPane[
  67. Dynamic[pts],
  68. Dynamic[ParametricPlot[Evaluate[geodesicFindMin[pts][t]], {t, 0, 1},
  69. PlotRange -> {{-π, π}, {-2 π, 2 π}}, Axes -> True,
  70. AspectRatio -> 2/r]]]
  71.  
  72. Show[
  73. ParametricPlot3D[
  74. torus[{u, v}], {u, -π, π}, {v, -π, π},
  75. PlotStyle -> Directive[White], ImageSize -> 500],
  76. ParametricPlot3D[Evaluate[torus[geodesicFindMin[pts][t]]], {t, 0, 1},
  77. PlotStyle -> Red]
  78. ]
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