[ODE] Re: Inside ODE

[François Beaune]

Thanks to your reply and the document about making new joints
in ODE, I finally understand how it actually works. But I wonder,
doesn't such a relaxation process (or error reduction as it is called
in the paper, I guess that error reduction is in the general case and
relaxation is in case of a contact) make assemblies more unstable ?

Hmmm, on second thoughts, I suppose it should not be a problem
since in most cases relaxation (error reduction) is involved because
of penetrating (thus moving) bodies. However, if you create a stable
assembly of bodies (a wall made of bricks) which interpenetrate
slightly each other and then run the simulation, the wall may crash,
am I wrong ?

Thanks again,
François

----- Original Message -----
From: <nlin@nlin.net>
To: <ode@q12.org>
Sent: Tuesday, March 12, 2002 9:03 AM
Subject: [ODE] Re: Inside ODE


> > I was wondering how ODE could achieve that level of robustness
> > and stability (not to mention the pretty good accuracy of produced
> > simulations !) without doing any time subdivision or performing any
> > root-finding process in order to find the approximative time of impact
> > of two interfering bodies.
>
> See the example "A contact joint" in the recently posted document on
> creating your own joints. http://q12.org/ode/joints.pdf
>
> The rigid bodies in ODE are subject to "constraints", which are
> restrictions on the positions and velocities (the "configuration") of
> the bodies in the system. Collision is treated as a constraint: a
"contact"
> constraint. When a collision is detected, the violation of the constraint
> is described in terms of one or more penetrating points. The constraint
> is enforced by requiring these penetrating points to have
non-inward-directed
> velocities.
>
> It would be more accurate to enforce the constraint at the exact time of
> impact by doing time subdivision as you mention. But with a
constraint-based
> approach, this isn't necessarily vital because the abstracted view of the
> system deals with constraint enforcement at each timestep. Indeed, even
starting
> from an illegal configuration (interpenetrating bodies, separated joints),
> ODE can (with the help of the stabilizing ERP) move the system back into
> a legal state (this is sometimes called self-assembly).
>
> With an impulse-based simulator (a la Mirtich), the collisions are the
> defining factor in system evolution: evolve until the next collision,
apply
> impulse at exact collision time, repeat. With a constraint-based simulator
like
> ODE, a collision is just another constraint (just like sliders,
ball/socket
> joints, etc) which has certain velocity invariants which need to be
maintained.
> It is most accurate if the constraint is enforced at the exact instant it
is
> violated (i.e. at the exact instant of collision), but enforcing the
> constraint a bit too late (even after some interpenetration) is not fatal.
> This is also why, for instance, test_boxstack behaves gracefully even if
> boxes are created in an initially interpenetrating state.
>
> At least, that's how I understand it :-)
>
> -Norman
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