[ODE] LCP solver

[Martin C. Martin]

Hey SJS,

Can you provide a really small program which reproduces this bug?  From 
your description, the program should be tiny, and that will help people 
track down what's really going on.

Cheers,
Martin


SJS wrote:

> I've been trying to use ODE's LCP solver.  I've had quite a lot of success
> producing articulated objects, etc., but I've been having a bit of trouble
> in some specific cases.  Problems seem to arise in some "perfect" or trivial
> cases:
> 
> My most reproducible test case starts with 3 stacked frictionless spheres of
> equal mass and radius at rest on an infinite-mass plane.  Spheres are spaced
> so they touch exactly (contact points are generated at y=0, y=2*r, y=4*r,
> where y is the vertical axis and r is the radius of all 3 spheres).
> 
> I'm using the regular 'dStepWorld' function (not StepFast) and I've fixed
> the time step to 60fps.  Every time step yields 0 force on all three spheres
> (not just in y, but x and z as well), so they just drop straight down
> through the plane.  Is this a behavior that other people have observed?
> 
> While this case may seem unlikely, I believe the same failure is causing
> jittering in my simulation during situations that are much more complicated
> to describe here (at least in 1 email).
> 
> I should note that the LCP solver does NOT hit the "s<=0" internal error--
> it simply produces inadequate (0) force on the spheres to hold them above
> the plane.  I can provide a more details to anyone that is interested.  I
> HAVE managed to find some "tweaks" to the LCP solver that solve (or at least
> mask) the perfect 3-sphere problem, but I'd be very interested in a more
> rigorous solution if anybody has any ideas.
> 
> Steve
> 
> 
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