[ODE] point-on-plane constraint

[Achim Moller]

Hi,

has anyone some pointers how to create a 'point-on-plane' constraint?
I checked Russ' "How to make new joints" and the GameGems4 article,
but don't understand how I would build such a constraint:

I would like to have the socket of the ball constraint 'attached'
to a plane and the position of the ball socket should be constrained
to the plane. The socken then is allowed anywhere on the plane.

I don't know how this could work, because the standard ball constraint
already removes the 3 DOF for the xyz movement, but how could I
archive the required motion then?

Thanks a lot for some tips.

Achim