code: def left_button_down(self, obj, event_type):
          
          # Get 2D click location on window
          click_pos = self.iren.GetEventPosition()
  
          # Get corresponding click location in the 3D plot
          picker = vtk.vtkWorldPointPicker()
          picker.Pick(click_pos[0], click_pos[1], 0, self.renderer)
          self.pickpoint = np.asarray(picker.GetPickPosition()).reshape((-1, 3))
          if np.any(np.isnan(self.pickpoint)):
              self.pickpoint[:] = 0
docs: Register the event for a left button down click