# File vtk/tk/vtkTkImageViewerWidget.rb, line 190
  def StartWindowLevelInteraction(x,y)
    if ! @FocusOnEnter
      _GrabFocus
    end
    viewer = @ImageViewer
    @LastX = x
    @LastY = y
    @Window = viewer.GetColorWindow.to_f
    @Level = viewer.GetColorLevel.to_f

    # make the window level text visible
    @LevelActor.SetVisibility(1)
    @WindowActor.SetVisibility(1)

    self.UpdateWindowLevelInteraction(x,y)
    return nil
  end