Raytracing with terrain

Picking at terrain for NASA World Wind 1.4

by Patrick Murris <patmurris (at) 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.

[edit] Camera.cs

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.

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.