Hi guys, thanks for the replies.
The detection in the z direction does seem to work. After trying it out we found a problem:
The robot doesn't collide with a model if the model is above the ground (but doesn't clear the robot's height), it seems like the robot only signals a collision with a model that is sitting on the ground. We are currently working on the problem.
Also, we are looking to change the way objects are treated in Stage so that they cannot overlap each other, another idea is to introduce the concept of gravity.
Our final year project is to work on and improve the 2.5D support for Stage, we are open for any suggestions as to what features to work on. Our time available to work on Stage is between now and the 21st of July.

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Warm regards,
Nick, Jimmy