[ODE] Re: Possible solution to unstable rotating bodies problem

Matt Hancher mdh at email.arc.nasa.gov
Fri Apr 22 12:46:02 MST 2005

> Again, most people would rarely need this many freely-spinning objects 
> at once.

I think this is exactly this issue.  The question for me is *not*
what the performance hit is for spinning bodies relative to the
current situation in ODE.  The current situation in ODE is unusable
in this situations, so this is not the interesting question.  The
important questions revolve around the relative performance impact
of various techniques for addressing the problem, and also the
impact those techniques have on the performance of all the *other*,
*non-spinning* bodies.

My concern with your solution is that it *only* works for freely-
spinning objects.  It is possible to get bodies spinning themselves
into oblivion while in contact with the ground, for instance.  Any
solution you use to solve *that* problem will probably also prevent
your freely-spinning bodies from exploding, at which point adding
additional code to handle that case becomes silly.

The only reason I can think of to handle freely-spinning bodies
separately is if you wanted their physics to be more accurate than
a forward-Euler velocity integrator can provide.  Your approach
does improve this behavior, but if that's really what you want
then you're probably better separating them from the ODE integrator
entirely and using a more stable simulation technique to handle
those objects.


Matthew D. Hancher
Intelligent Systems Division
NASA Ames Research Center
mdh at email.arc.nasa.gov

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