[ODE] direction of contact normal

[Guru2012]

I'm using ODE for a 2d engine, and I've written my own collision detection.
The problem I'm having is setting the normal so that the objects separate
instead of stick.
What I'm finding is that if a certain normal for the contact joint causes
the two objects to separate, the negative of that normal (scaled by -1)
causes them to go flying into each other.

For example, when I drop a square on top of another square I find that the
normal needs to point downwards, but looking at the sample code, a collision
between a box and a plane gives a normal pointing upwards.

My question is how to choose the normal so that the objects separate and
what criteria determine separation.

Thanks,
Andrew
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