[ODE] penalty method

[Sergio Valverde]

In fact, when you told us about penalty forces I was thinking in a
spring-damping system. That is, we have a penalty which depends both on
penetration distance and the relative velocity between the interpenetrating
bodies.

I think you can implement "simple" simulations of "passive" rigid bodies
(like those piles of balls and boxes) with both methods in an acceptable
amount of time. Maybe the penalty method allows faster and efficient
simulations for these kind of systems than the LCP-based does. 

But when you need to implement complex mechanics and/or behaviour over your
physic engine the choose is clear to me: LCP.  The stability provided by
this method is worth the associated computation costs. Penalty method is
tricky and doesn't assure stability for certain simulation situations.
Unfortunately, a game designer is allways able to find such problematic
cases, breaking your trick code! :-(

The topic is : What is the best way to implement the high-level,
physically-based, behavior which is required by current and future 
games? 

Sergi

-----Original Message-----
From: Pierre Terdiman [mailto:p.terdiman@wanadoo.fr]
Sent: miércoles, 23 de octubre de 2002 13:00
To: ode@q12.org
Subject: Re: [ODE] penalty method


> The stiffness of the differential equations and the fact that the penalty
> forces rely solely on the position of the objects (not the velocities)
> demand that you use an integrator that does not amplify its own errors.

Who said the penalty forces didn't depend on velocities ? Mine do.

> Wu
> uses implicit Euler as integrator, which he claims keeps the system from
> exploding. Also, care should be taken not to add energy to the system by
> placing exerted springs on interpenetrating contacts, so either you have
to
> determine the exact time of collision, or you'll have to allow that
objects
> are interpenetrating to some extent.

Indeed, some interpenetration is allowed. That's good or bad, depending on
the way you see it. On the bad side, well, it might be visually unpleasant
if it's too obvious. On the good side, the system doesn't break or explode
when it happens. And it does happen.

Anyway I'm not sure the magic physics bullet has been found yet, each method
has its share of painful problems..

Pierre

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