A question about converting unstructured grid to explicit structured grid

Development
1

I have a question about converting unstructured grid to explicit structured using vtkUnstructuredGridToExplicitStructuredGrid:

Two errors are shown when I run this code using SetWholeExtent:

>>> converter = vtk.vtkUnstructuredGridToExplicitStructuredGrid()
>>> converter.SetInputData(ugrid)
>>> converter.SetWholeExtent(0, 46, 0, 69, 0, 15)
>>> converter.Update()
ERROR:root:Attempt to get an input array for an index that has not been specified
ERROR:root:An ijk array has not be set using SetInputArrayToProcess, aborting.

The code works when I include SetInputArrayToProcess:

>>> converter = vtk.vtkUnstructuredGridToExplicitStructuredGrid()
>>> converter.SetInputData(ugrid)
>>> converter.SetWholeExtent(0, 46, 0, 69, 0, 15)
>>> converter.SetInputArrayToProcess(0, 0, 0, 1, 'BLOCK_I')
>>> converter.SetInputArrayToProcess(1, 0, 0, 1, 'BLOCK_J')
>>> converter.SetInputArrayToProcess(2, 0, 0, 1, 'BLOCK_K')
>>> converter.Update()

But the code also works when I remove SetWholeExtent:

>>> converter = vtk.vtkUnstructuredGridToExplicitStructuredGrid()
>>> converter.SetInputData(ugrid)
>>> converter.SetInputArrayToProcess(0, 0, 0, 1, 'BLOCK_I')
>>> converter.SetInputArrayToProcess(1, 0, 0, 1, 'BLOCK_J')
>>> converter.SetInputArrayToProcess(2, 0, 0, 1, 'BLOCK_K')
>>> converter.Update()

Therefore, what is the SetWholeExtent for? And how to use it correctly?

The parameters of SetWholeExtent are correct. I got them from the original explicit structured grid.