Why would `vtkPolyDataNormals` change the number of nodes?

Support
1

I have a *.vtp file that does not have normals (but needs them for what I am doing next). I wanted to use vtkPolyDataNormals to add them … but it changes the number of nodes??

rd = vtk.vtkXMLPolyDataReader()
rd.SetFileName("input.vtp")
rd.Update()
pd = rd.GetOutput()

print(pd.GetNumberOfPoints())
104923

nf = vtk.vtkPolyDataNormals()
nf.SetInputData(pd)
nf.Update()
print(nf.GetOutput().GetPointData().GetNormals().GetNumberOfTuples())
104670
2

From vtkPolyDataNormals documentation

The filter can reorder polygons to ensure consistent orientation across polygon neighbors. Sharp edges can be split and points duplicated with separate normals to give crisp (rendered) surface definition. It is also possible to globally flip the normal orientation.

Experiment with Splitting property as well as the FeatureAngle

3

Ah, forgot that, thanks