Raytracing with terrain

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Picking at terrain for NASA World Wind 1.4

by Patrick Murris <patmurris@videotron.ca> March 16, 2007

These sources are modifications to some WW classes to add a PickingRayIntersectionWithTerrain() method to the CameraBase class.

Included is PickAtTerrain.cs a test plugin that displays the lat/lon/alt under the mouse pointer.



  • Added PickingRayIntersectionWithTerrain(): Same arguments as PickingRayIntersection() with the addition of the current world (needed to use the TerrainAccessor).
    • Use ray casting with a sampling length of 100m.
    • Use a new memory only method to get elevation from terrain (see below)
    • More suited for close range picking (under 60km altitude)
    • Needs some optimization and tuning but works well.
  • Changed ComputeAbsoluteMatrices() - View matrix logic
    • Use currentCameraElevation in the same way as the 'real' view matrix in ComputeViewMatrix(): WorldRadius + curCameraElevation instead of WorldRadius
      • this._distance + curCameraElevation instead of
      • this._distance - curCameraElevation
    • Affects picking and view frustrum when ElevateCameraLookatPoint is true.
    • Tests ok so far.


  • Added GetCachedElevationAt() method to retreive elevations without triggering downloads in foreground. Uses memory data only.
    • Will get the best available data from the terrain accessor cache.
    • Will return float.NaN is no data found.


  • Added virtual GetCachedElevationAt()


  • Made NumberLevels public

Known issues

  • Terrain picking doesnt work well and can become very slow if used from far away - especially at startup when no elevation data is in the cache yet. Use the regular PickingRayIntersection() above 60km altitude.
  • Ray casting sampling logic needs some optimization.
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