vtkAppendPolyData

[lassoan (Andras Lasso)]

@at123

I would like to form a convex hull that looks like a hourglass.

This is impossible because convex hull of an hourglass shape is a cylinder. You might be able to reconstruct hourglass shape from points using vrkDelaunay3D using non-zero alpha parameter or using other methods described in this post:

Problems using vtkContourWidget, vtkAppendPolyData, and vtkDelaunay3D

It seems that a convex hull is generated, which is what vtkDelaunay3D is expected to do. You can adjust alpha parameters on the vtkDelaunay3D filter, to make it generate concave objects, but it may make mesh generation fail. Alternatively, if the shape is just slightly concave then you may get away without using alpha shapes and applying vtkButterflySubdivisionFilter on the output. You can try both these options in 3D Slicer’s Markups To Model module (install 3D Slicer and then in the Slicer …