[vtkusers] 3D Visualization of Map Data

[Malcolm Drummond]

The naive approach will work but you can drastically reduce the number of intersection tests by running a sweep algorithm (many computational geometry textbooks cover this). Basically you sweep a vertical line through a horizontally sorted set of line segments (or vice versa). You will only have to check for intersections between those line segments that simultaneously intersect your sweep line - i.e. you maintain a subset of candidates, dynamically discarding some and appending others as the sweep line proceeds through the sorted events (events = beginnings and ends of line segments). It's a simple concept but can be difficult to implement - especially in cases like yours where considerable book-keeping would be required to insert new points correctly into existing polylines. Is this something you'll have to do often or just a one-off?

HTH
Malcolm Drummond

  ----- Original Message ----- 
  From: Ron Chapman 
  To: vtkusers at vtk.org 
  Sent: Thursday, February 16, 2006 1:38 AM
  Subject: [vtkusers] 3D Visualization of Map Data


  Malcolm,

  Thanks for your help, once I figured out some basic vtk'isms your suggestion worked well, albeit roads are sometimes elevated, and/or disappear through my surface (see the below pipeline).  Now I need to figure out how to split my road (line segments) into polylines that intersect with my 2D mesh so that the probing produces better results.  Does anyone have some hints on how to do this?  I was naively thinking of the following:

  create an empty 2d polydata set -> call this NEW
  for each 2d line segment in my  2d polydata
      for each line in 3d mesh
          if intersection
              create new line segment
              add new line segment to NEW

  Then use NEW with the pipeline below insead of Shape2D:

  // get the evalation scalars

   float bounds[6];
   Mesh3D->GetBounds(bounds);
   vtkElevationFilter *Elevation = vtkElevationFilter::New();
   Elevation->SetInput(Mesh3D);
   Elevation->SetLowPoint(0,0,bounds[4]);
   Elevation->SetHighPoint(0,0,bounds[5]);
   Elevation->SetScalarRange(bounds[4], bounds[5]);
   Elevation->Update();

    // convert our 3d mesh to 2d

   vtkTransform *Transform = vtkTransform::New();
   Transform->Scale(1.0,1.0,0.0);

   vtkTransformPolyDataFilter *transFilter = vtkTransformPolyDataFilter::New();
   transFilter->SetInput((vtkPolyData *) Elevation->GetOutput());
   transFilter->SetTransform(Transform);
   transFilter->Update();
   
   // probe 2d mesh with shape data

   vtkProbeFilter *Probe = vtkProbeFilter::New();
   Probe->SetInput(Shape2D);
   Probe->SetSource(transFilter->GetOutput());
   Probe->Update();

   // apply the sampled elevation to the overlay

   vtkWarpScalar *Warp = vtkWarpScalar::New();
   Warp->SetInput(Probe->GetPolyDataOutput());
   Warp->SetScaleFactor(1.0);
   Warp->Update();

  Ron



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