[ODE] Negative penetration depths

[Russ Smith]

> contact constraints will add a force to try and reduce the penetration
> depth. this is not really a 'penalty force' however. this is a terminology
> thing - penatly forces are usually produced by some kind of spring and
> damper equation, and can result in hard-to-tune semi-stable systems. the
> ODE forces are exactly calculated to reduce the penetration depth along
> an exponential profile. this force can only be positive, not negative,
> which means that a contact will push bodies apart but never suck them
> together. hope that helps.

oh ... to clarify this, ODE's notion of 'push apart' and 'suck together'
depend on the contact normal and the penetration depth. so a negative
penetration depth will suck bodies together, as will a flipped normal
vector. many instabilities can be traced to bad contact generation that
gets these signs wrong.

russ.

--
Russ Smith
http://www.q12.org