# [ODE] Silly theoretical question

Erin Catto erincatto at sbcglobal.net
Wed Feb 23 23:46:07 MST 2005

```I suspect that Novodex uses a penalty method. Also Pseudo Interactive used a
penalty method for their game Cel Damage.

If you can make a fast Implicit Euler integrator then penalty methods become
more interesting. The advantage is that you can handle just about any model
and produce a stable result. The disadvantage is that you have to tune the
penalty parameters and your solver may damp out motion too quickly. Also,
you still my have to deal with constraints to model joints well.

ODE uses rigid bodies and constraints, and is inspired by David Baraff's
work, but it does not use his algorithms. ODE has more in common with the
time stepping methods of Mihai Anitescu and David Stewart. Basically, take
Anitescu's formulation, make the friction force bound constant, add
Baumgarte position stabilization, and you have ODE.

Erin

-----Original Message-----
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org] On Behalf Of Serguey
Zefirov
Sent: Wednesday, February 23, 2005 10:19 PM
To: ode at q12.org
Subject: [ODE] Silly theoretical question

ODE was done on the basis of David Baraff works. As far as I can remember,
(and confirmed right now by reading his thesis, part 1, 2.4 The penalty
method)
he considered not to use penalty-based (where constraints are substituted
by penalty forces) equation of motion because of stiffness of resulting
equations of motion.

So, supposing we get a solver that could relatively inexpensively solve
set of such equations what kind of benefit we could obtain? Should we
search for such a solver? And, to triple confusion of the reader and
possibler answerer, why ODE mentions in documentation that stiff systems

PS
I read overview of Havoc physics library and they mention that using stiff
springs for ragdolls might be beneficial comparing to constrained ragdolls
(cannot find that overview right now, though).
PPS
It seems that not only ODE is based on Baraff's work. As far as I can see,
everyone and his dog done their physics library that way.

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