[vtkusers] vtkCellLocator

John Biddiscombe jbiddiscombe at skippingmouse.co.uk
Mon Sep 10 10:17:24 EDT 2001

CellLocator should work fine. (The tolerance is used, but the details are 
hidden away deep inside the vtkCell subclasses (polygon, triangle etc etc) 
where it controls line intersection tests - although you didn't specify how 
you are using the locator).

If you use an implicit function to describe a polyhedral radar beam (or 
even a cone) and intersect that with the Dataset using a celllocator you 
can do what you want. [You will get the front and back of a hill though. If 
you are implementing a full backscatter type problem then you can take the 
clipped dataset, run it through a normal generator, and using the antenna 
as a reference then reject the rear facing polygons and compute field 
strength at the remaining points. use these asscalars on the original 
dataset and you get some nice piccies]


At 13:41 10/09/2001, James A Galbraith/GALBJA/CC01/INEEL/US wrote:
>Good morning everyone.
>We are modelling radar beams over terrain using vtk with java.  In order to
>mask the radar beams where they intersect the terrain we need to determine
>where this intersection occurs.  We are currently using the vtkCellLocator
>class with mixed results.  For the most part, we are receiving what we
>consider valid intersections, but occasionally we receive intersections
>that are not correct.  In some instances the intersection comes at the back
>side of a hill the radar beam is "passing through" rather than the front
>edge.  In other instances, the beam is allowed to pass through several
>hills, intersecting on , say, the third hill.  Instances where the beam is
>allowed to pass through several intersections before arriving at the "best"
>intersection are very consistent and are always returned this way.  When we
>overlay radar beams on the terrain, the terrain then masks the radar beam
>wherever it is above the beam - which supports our view that we are getting
>invalid intersections from vtkCellLocator.
>Can anyone shed some light on how the vtkCellLocator works and why it
>possibly is obtaining a "best" cell which is incorrect as far as we are
>concerned?  Are we using the correct class for detecting these locations,
>or is there another class we should look at?
>I tried tweeking the tolerance value, but after looking at the source it
>doesn't appear to be used.
>Thanks again for all your help
>James Galbraith
>Idaho national Engineering & Environmental Laboratory (INEEL)
>Bechtel BWXT Idaho, LLC
>P.O. Box 1625
>Idaho Falls, ID  83415-3779
>Email: galbja at inel.gov
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