[vtkusers] Rendering a scene in which actors move

[Gib Bogle]

I wish to repeatedly render a scene with many actors (cones for now) that may 
change their positions as time progresses (i.e. I am creating a movie).  I can 
render the scene OK, but I see that the way I'm doing it is just adding actors 
each time step, whereas I want to redraw all the actors each time (or possibly, 
if I get clever, only those that have moved).  This what I'm doing in Python:

     def QVTKRenderWidgetCones(self,page_VTK):
         """A simple example that uses the QVTKRenderWindowInteractor class."""
         if self.first_VTK:
             self.first_VTK = False
             layout = QtGui.QVBoxLayout(page_VTK)

             # create the widget
             self.VTKscene = QVTKRenderWindowInteractor(page_VTK)
             self.VTKscene.Initialize()
             self.VTKscene.Start()
             renWin = self.VTKscene.GetRenderWindow()

             layout.addWidget(self.VTKscene)
             self.ren = vtk.vtkRenderer()
             renWin.AddRenderer(self.ren)

             cone = vtk.vtkConeSource()
             cone.SetResolution(8)
             self.coneMapper = vtk.vtkPolyDataMapper()
             self.coneMapper.SetInput(cone.GetOutput())

#        self.ren.Clear()
#        for actor in self.ren.GetActors():
#            self.ren.RemoveActor(actor)

         print self.ren.GetActors()

         n = self.ncells
         coneActor = [0]*n
         for i in range(0,n):
             coneActor[i] = vtk.vtkActor()
             coneActor[i].SetMapper(self.coneMapper)
             self.ren.AddActor(coneActor[i])
             cp = self.cellpos[i]
             x = int(cp[2])
             y = int(cp[3])
             z = int(cp[4])
             coneActor[i].SetPosition(x, y, z)

         # show the widget
         self.VTKscene.show()
         self.VTKscene.Render()

I tried to use ren.Clear(), but that obviously clears everything, and I 
discovered that a collection is not a list and therefore my attempt to use 
ren.RemoveActor() failed.  What is the right (i.e. fast) way to do this?

Thanks
Gib