"Boundary" of unstructured point data

[Mark Beall]

>I would like some advice on how to compute the "boundary mesh" of a set
>of points in an UnstructuredGrid dataset. In other words, construct a
>mesh from only the boundary of the set of points.
>
>The dataset is quite large: ~50,000 points, so Delaunay3D for example is
>prohibitively expensive.
>
>I thought perhaps some combination of parameters to vtkDelaunay3D might
>do the trick? Apparently vtkExtractEdges and vtkFeatureEdges require
>polygons and dont work on point data alone.  ContourFilter wont do the
>trick either.

Do you have just the points or also the connectivity of the points.
>From the way you asked the question, I'm assuming you just have
the points. In that case you'll need to define what "boundary"
means before you can answer the question.

for example what's the boundary of this point set:
  +      +

       +
         
  +      +

this?
  +------+
  |     /
  |    +
  |     \
  +------+
or this:
  +------+
  |      |
  |    + |
  |      |
  +------+

What you likely want is the convex hull of the points (although
you may acutally want something different), there are well
known algorithms to determine this. I don't know if there is
an implementation of one in vtk or not.

mark


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