不能在vtkImagevi中画点

Question

我想在灰色图像上画一个点，我使用vtkImageViewer来显示它，但是我发现在我画了一个点并渲染它之后，这个点不会被显示，怎么做？ 我使用vtk 8.1.0、python 3.6.4，我的代码如下：

self.points = vtk.vtkPoints()
self.points.SetNumberOfPoints(1)
self.cells = vtk.vtkCellArray()
self.cells.SetNumberOfCells(1)
self.mapper = vtk.vtkPolyDataMapper2D()
self.pd = vtk.vtkPolyData()
self.actor = vtk.vtkActor2D()
clickpos = self.iren.GetEventPosition()
self.points.SetPoint(0, [clickpos[0], clickpos[1], 0])
self.cells.InsertCellPoint(0)
self.pd.SetPoints(self.points)
self.pd.SetPolys(self.cells)
elf.mapper.SetInputData(self.pd)
self.actor.SetMapper(self.mapper)
self.actor.GetProperty().SetColor(vtk.vtkNamedColors().GetColor3d('Black'))
self.actor.GetProperty().SetPointSize(1)
self.imageviewer.GetRenderer().AddViewProp(self.actor)
self.iren.Initialize()
self.imageviewer.Render()
self.iren.Start()