Trailing/Smeared WPF 3D Drawings on Mouse Move event using ScreenSpaceLines3D (C#)

Question

Hi,

I'm having a problem when drawing a 3D line on MouseMove event in WPF. The drawing seemed to be trailing (like mouse cursor-trailing effect)/smeared in Viewport3D, and it doesnt 'clean' the previous drawing (from previous mouse move)

I'm using ScreenSpaceLines3D from CodePlex to draw the 3D line on top of a 3D cube. I want it to be cleaned in every mouse move so that viewport3d will display the current updated drawings.

It should behave like: (1) onLeftButtondown, get 3D Point from HitTest on existing 3D cube == first point for 3d line (2) onMouseMove, always get current mouse move == second point for 3d line

private void onMouseMoveViewPort3D(object sender, System.Windows.Input.MouseEventArgs e)
  {
  if (e.LeftButton == MouseButtonState.Pressed)
    {
    if (mainViewport.IsMouseCaptured)
      {
       Draw3DLineOn3DObject(e);
      }
    }...
  private void Draw3DLineOn3DObject(System.Windows.Input.MouseEventArgs e)
  {
  RayMeshGeometry3DHitTestResult rayMeshResult = (RayMeshGeometry3DHitTestResult)
                                                 VisualTreeHelper.HitTest
                                                 (mainViewport,e.GetPosition(mainViewport));

  if (rayMeshResult == null) 
    return;

  _point3DBtnCurrent = rayMeshResult.PointHit;

  if (_thisLine== null)
    {
    _thisLine = new ScreenSpaceLines3D();
    _thisLine.Color = Colors.Red;
    _thisLine.Thickness = 1;

    this.mainViewport.Children.Add(_thisLine);
    }

  _thisLine.Points.Add(_point3DBtnDown);
  _thisLine.Points.Add(_point3DBtnCurrent);
  }

I tried to invalidate visual the mainViewport, but it still nave no effect. Result of the above code:

http://www.imagechicken.com/viewpic.php?p=1258950572004579200&x=jpg

Thanks & Best Regards,

F.Notts