SV: SV: [ODE] Particle/cloth/skeleton (was: Re: Choleski factorization)

[david@csworkbench.com]

I'm not so worried about stuff passing through the cloth, since it will
probably be integrated into my iterated integrator, and stuff would have
to be going pretty dern fast to get through it at a .01 or smaller
timestep.  I am worried, however, about penetration.  ODE doesn't
attempt to resolve a penetration until after it's happened in its
current state, which won't look all that nice for cloth.  But I could
probably do a little work to the cloth collider and set up an inequality
distance constraint between the cloth and every body "nearby" to fix the
most of that problem.... It'll take a while to work out the details.

David

>>  > If you do it wrong the body would simply
>> > pass through the cloth when it has a high enough speed.
>> > And that would sort of break ode in a way. Then you would be
>> > forced to think about max speeds and such
>>
>> This is a danger that ODE already has, though.  Geoms can
>> cheerfully pass through each other if they're going fast
>> enough, are thin enough in one dimension, and your collision
>> detection steps are infrequent enough, requiring hacks with
>> rays and ccylinders for bullets, etc.
>>
>
> Yes but that can be fixed with time extruded collision. However
> when it comes to the cloth you wont even need to be moving fast
> to mess it up. It would be enough to just push an object through it.
>
> Take a square cloth and lock each corner to the world so it forms a
> sort of safty net. Now let a rigidbody fall into that net.
> Because the integration loop most likely will look like this
>
> - Find collisions
> - Solve LCP
> - Solve "verlet" distance constraints (Thats the cloth)
> - Integrate cloth and rigid bodies
>
> Then the collision is found between the body and the cloth. The
> cloth gets to react on the body (and apply force to the body).
> Then you need to run the constraints in the cloth. Now because
> the cloth doesnt know about the rigidbody he will happly adjust
> the cloth so it goes through the rigidbody. This happen when
> you simply streach the cloth enough. You dont even need any
> high velocity for it to happen.
>
> There are a few solutions to the problem. One would be to time
> extrude the cloth. But that would be very slow. The other is to
> create distance constraints at the collision points. But you would
> also have to adjust the mass of the cloth during the normal collision
> step otherwise it would still pass through. That also means you
> have to calculate how streached the cloth is at any given point.
> Well.. simply to say it would get messy.
>
> If anyone has a robust solution to this problem then I'm happy to hear
> about it.
>
> /Joakim E. - http://www.planestate.net
>
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