code: def normalLines(actor, ratio=1, c=(0.6, 0.6, 0.6), alpha=0.8):
      
      maskPts = vtk.vtkMaskPoints()
      maskPts.SetOnRatio(ratio)
      maskPts.RandomModeOff()
      actor = actor.computeNormals()
      src = actor.polydata()
      maskPts.SetInputData(src)
      arrow = vtk.vtkLineSource()
      arrow.SetPoint1(0, 0, 0)
      arrow.SetPoint2(0.75, 0, 0)
      glyph = vtk.vtkGlyph3D()
      glyph.SetSourceConnection(arrow.GetOutputPort())
      glyph.SetInputConnection(maskPts.GetOutputPort())
      glyph.SetVectorModeToUseNormal()
      b = src.GetBounds()
      sc = max([b[1] - b[0], b[3] - b[2], b[5] - b[4]]) / 20.0
      glyph.SetScaleFactor(sc)
      glyph.OrientOn()
      glyph.Update()
      glyphActor = Actor(glyph.GetOutput(), c=vc.getColor(c), alpha=alpha)
      glyphActor.mapper.SetScalarModeToUsePointFieldData()
      glyphActor.PickableOff()
      prop = vtk.vtkProperty()
      prop.DeepCopy(actor.GetProperty())
      glyphActor.SetProperty(prop)
      return glyphActor
docs: Build an ``vtkActor`` made of the normals at vertices shown as lines.