[vtkusers] Convex hull of spherical point cloud has holes

Amit amit112amit at yahoo.co.in
Wed Aug 9 23:34:19 EDT 2017

Thank you Bill and David for your replies.

David: I did not use vtkCleanPolyData. I had read the doc multiple times 
but I did not have the patience to understand the significance of alpha.

Bill: You are right. I was using the default value of alpha=0.0. Just 
out of trial and error I set alpha=1.0 and the issue got resolved. I 
think that as I am working with unit spheres, alpha_radius=1.0 is 
sufficiently large value so that all faces of all tetrahedron get 
included in the output convex hull.

Thanks once again for the pointers.


On 08/09/2017 04:03 PM, Bill Lorensen wrote:
> If you save your polydata in a .vtp file you can try this example:
> https://lorensen.github.io/VTKExamples/site/Cxx/Modelling/Delaunay3D/
> It reports the # of verts, lines, triangles and tets.
> On Wed, Aug 9, 2017 at 4:56 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
>> I assume Alpha=0.0 for the Delaunay filter?
>> Can you post a small, standalone compilable program that produces this output?
>> Bill
>> On Wed, Aug 9, 2017 at 2:18 PM, Amit via vtkusers <vtkusers at vtk.org> wrote:
>>> Dear all.
>>> I have a point cloud made up of points lying on surface of a unit sphere
>>> stored in a vtkPolyData. I want to get the convex hull of this point cloud
>>> using the vtkDelaunay3D filter. Then using vtkDataSetSurfaceFilter I want to
>>> extract the surface of the convex hull. This way, I can get a triangulation
>>> of the point cloud as a hollow spherical shell. Kindly open the attached
>>> UnstructuredGrid file in Paraview. It represents the output of vtkDelaunay3D
>>> filter made up of tetrahedrons. But the surface of the convex-hull has some
>>> missing triangles. Using the SpreadSheet View, I can see that cells with Ids
>>> 60, 61, 64 have triangular faces in the missing region. Hence, I would not
>>> have expected these "hole" in the surface. Can somebody explain why the
>>> triangles are not being rendered? When I extract surface from this
>>> unstructured grid I get a spherical shell with missing triangles as holes.
>>> Regards,
>>> Amit
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