ok there is a major difference between an orthographic camera and a persp one. An orthographic igrnores the whole idea of perspective which means that things dont appear to get smaller as they dissapear into the distance. So that's why you really shouldnt render from the top camera.
I'm not great on displacements so somebody may be able to help you out with that part but you should always render the displacements from a persp camera.
Alan
Technical Director - Framestore
Currently working on: Your Highness
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