text: #!/usr/bin/env python
  """
  This file demonstrates the creation of a tree using the
  Python interface to VTK.
  """
  
  from __future__ import print_function
  from vtk import *
  
  #------------------------------------------------------------------------------
  # Script Entry Point
  #------------------------------------------------------------------------------
  if __name__ == "__main__":
  
      print("vtkTree Example 1: Building a tree from scratch.")
  
      # Create an empty graph
      G = vtkMutableDirectedGraph()
  
      vertID = vtkIntArray()
      vertID.SetName("ID")
  
      G.GetVertexData().AddArray( vertID )
  
      # Add a root vertex
      root = G.AddVertex()
      vertID.InsertNextValue(root)
  
      # Add some vertices
      for i in range(3):
          v = G.AddChild(root)
          vertID.InsertNextValue(v)
          for j in range(2):
              u = G.AddChild(v)
              vertID.InsertNextValue(u)
  
      T = vtkTree()
      T.ShallowCopy(G)
  
  
      #----------------------------------------------------------
      # Draw the graph in a window
      view = vtkGraphLayoutView()
      view.AddRepresentationFromInput(G)
      view.SetVertexLabelArrayName("ID")
      view.SetVertexLabelVisibility(True)
      view.SetLayoutStrategyToTree()
      view.SetVertexLabelFontSize(20)
  
      theme = vtkViewTheme.CreateMellowTheme()
      theme.SetLineWidth(4)
      theme.SetPointSize(10)
      theme.SetCellOpacity(1)
      view.ApplyViewTheme(theme)
      theme.FastDelete()
  
      view.GetRenderWindow().SetSize(600, 600)
      view.ResetCamera()
      view.Render()
  
      view.GetInteractor().Start()
      print("vtkTree Example 1: Finished.")
  