def image_depth(self):
        """ Returns an image array of current render window """
        ifilter = vtk.vtkWindowToImageFilter()
        ifilter.SetInput(self.ren_win)
        ifilter.ReadFrontBufferOff()
        ifilter.SetInputBufferTypeToZBuffer()
        return self._run_image_filter(ifilter)
Returns an image array of current render window