Using RenderTargetBitmap with Viewport3D

Question

I use Viewport3d and show my 3D elements with the inherited UIElement3D classes ... Now I want to make different images of my viewport (different views). This means that I have to create a new viewport3d at runtime and apply a specific view to the image for it.

see the following code:

' Start creating Image
Dim bitmap As New RenderTargetBitmap(800,
                                     800,
                                     96,
                                     96,
                                     PixelFormats.Default)

'--------------------
'Render xamlViewport works perfect
bitmap.Render(xamlViewport)

Using stream As FileStream = New FileStream("C:\\temp\\test_xamlviewport.png", FileMode.Create)
  Dim encoder As New PngBitmapEncoder
  encoder.Frames.Add(BitmapFrame.Create(bitmap))
  encoder.Save(stream)
End Using


'---------------------
'Render local Viewport don't work
Dim oViewport As New Viewport3D

oViewport.Height = 800
oViewport.Width = 800

oViewport.Children.Add(New C3DCuboid())
oViewport.Camera = New PerspectiveCamera(New Point3D(4, 7, 6), New Vector3D(-4, -7, -6), New Vector3D(0, 1, 0), 45)

Dim oVisual As New ModelVisual3D
oVisual.Content = New DirectionalLight(Colors.White, New Vector3D(-1, -2, -3))

oViewport.Children.Add(oVisual)
oViewport.Measure(New Size(800, 800))
oViewport.Arrange(New Rect(New Size(800, 800)))

bitmap = New RenderTargetBitmap(800,
                                 800,
                                 96,
                                 96,
                                 PixelFormats.Default)

bitmap.Render(oViewport)

Using stream As FileStream = New FileStream("C:\\temp\\test_localviewport.png", FileMode.Create)
  Dim encoder As New PngBitmapEncoder
  encoder.Frames.Add(BitmapFrame.Create(bitmap))
  encoder.Save(stream)
End Using

The first image (test_xamlviewport.png) is shown correctly, but the second image, where I create my own Viewport3D object and print it, is empty (test_localviewport.png).

How can I make Render my UIElement3D objects, or why doesn't it work if I create a new Viewport and print an existing XAML viewport instance?

, http://cid-df67ca3f85229bd1.office.live.com/self.aspx/Development/WpfApplication2.zip

Roland