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Hi,

I'm using the vtkMergeCells class with unstructured grids with double precision points. In my problem, the merging process is very sensitive to the tolerance used in the vtkMergeCells object. However, as vtkMergeCells uses vtkKdTree to find duplicated points (if the tolerance != 0.0), the tolerance used to merge these points is limited to single precision values.

To ensure that the points are merged using a double precision tolerance, I need to rely on the class vtkCleanUnstructuredGrid implemented in Paraview. However, in my opinion the current implementation of vtkMergeCells could be compatible with double precision tolerance by using vtkIncrementalPointLocator instead of vtkKdTree.

If you are interested in this enhancement, I could work on a MR to ensure that the merging process of non-coincident points in vtkMergeCells is compatible with double precision tolerance.