[ODE] Detecting sliding during collision

[Joshua Bongard]

I'm evolving a walking robot in ODE, and want to penalize gaits that cause
the legs or body to slide over the ground. Anyone know of any intelligent
way to do that? I guess I could poll the displacement of objects that are
in contact with the ground plane on 2 consecutive time steps, and detect
whether displacement is above a certain threshold, but maybe there's a
better way?

Thanks,
Josh