Re[1]: [ODE] Negative penetration depths

[Thomas Harte]

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> Although I've never tried it, the notion of a negative or zero penetration
> depth is quite reasonable, and doesn't mean that "no penalty need be
> applied." In fact, in a perfect simulation, when one object rests on
> another, a constraint with zero penetration depth is what would keep it
> from falling through. I suggest you try it and see what happens.

When I said "no penalty needs be applied", I was talking about the difference between 
penalty methods and genuine constraints in physics simulations. It is my understanding 
that a genuine constraint is modelled via an ordinary differential equation, which is 
solved in order to definitely prevent the geometry being able to break the constraints, 
whereas a penalty is something like a spring, which uses a simple feedback system to 
make the geometry 'want' not to break the constraints.

It is further my understanding that in the case of ODE, adding a contact constraint adds 
both a hard constraints - preventing interpenetration becoming any worse, and a penalty 
method - pushing the objects apart and hence making the penetration better.

In the case of negative penetration depth, it is my fear that ODE may still try to impose 
the penalty method, but in using a negative distance, will pull the objects together rather 
than letting them move correctly. Naturally this effect will be barely noticable as long as I 
don't declare my constraints massively early (which I understand is a bad idea anyway 
due to the massive computational costs), and I know the objects were going to collide 
anyway (which I will before I even think of adding a constraint). However, the strictly 
correct behaviour would be not to apply any penalty unless penetration depth were 
greater than 0.

So I was curious about how ODE would be likely to handle the situation, and therefore 
how much effort I should put into avoiding it. Also, is the idea of a negative penetration 
even sensible? I don't consider myself to have negative penetration with the wall at the 
end of the room, I consider myself just not to be penetrating it, so perhaps I should set 
penetration depth to 0 myself in cases where it should superficially seem to be negative?

That was the thrust of my question I guess. I'll try it out and report on how I get on!

-Thomas
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