[Opensim-dev] Prospective ODE physics changes

[Teravus Ovares]

With ODE, it depends on the physics situation.

With Tri-Mesh and the heightfield collider specifically, ODE generates lots
of small effect contacts and then the stepper integrates them all into a
contact resolution force.  With tri-mesh and the heightfield, depending on
how an object collides with another, there could be 20 or 30 contacts that
all factor into getting the object to react normally.   So, to test, you're
going to want to use a stack of 'active'(physical in the client) tri-mesh
objects.   You may also want two or more trimesh LINKSETS to see how they
react.

My guess, is the first thing that you're going to notice is that a tri-mesh
object sitting on another object will become more unstable (vibrate more).
Each mini-contact represents a part of the force to keep the object from
rotating from the other parts of the contact resolution force.   As the
effect gets worse, you're going to notice 'rotation anomolies' that occur
when objects collide.

Think of it like...    you have a cube shaped trimesh...   and the cube's
corners are touching a flat ground.   In theory, that would generate 4
contact points for each of the vertices touching the flat ground.   If you
cut one off, then only three of the corners are being held above
ground.   On a larger scale,   If you do that enough, then the object will
partially fall through the ground and then bounce back up from an excessive
contact resolution force creating instability and vibrating.

Those are the indicators that I would use to determine if it's OK to make
that change.   Are 8 contacts enough for ODE to react properly in our
usage?   That remains to be seen :).

Regards

Teravus

On Tue, Jan 3, 2012 at 4:58 PM, Adams, Robert <robert.adams at intel.com>wrote:

> > ...
> > According to [2], the maximum reported scripting collision contacts is 8.
> >
> > Testing with 8 on Wright Plaza today in the Tuesday meeting seemed to
> greatly reduce physics scene time compared to
> > previously without any apparent loss of required fidelity (50ms with 18
> avatars, albeit mostly sitting down -
> > unfortunately I didn't record previous week's numbers but they were
> higher.  Nebadon tested one of his vehicles).
>
> Looking at the code, contacts_per_collision is the number of collision
> points reported by ODE for each collision -- like a prim sitting on rough
> terrain and touching multiple places on the ground. Reducing the count to 8
> means that no more than 8 contact points will be reported by ODE and, if
> there are more, you can't be sure you get the 'best' ones.
>
> I suspect that most of the time there are only a few contact points so it
> doesn't make sense that reducing the number from 80 to 8 would
> significantly reduce the compute time. If it is the number of contact
> points causing the computation overhead then ODE must be normally returning
> more than 8 contact points. Is this really the case? Or is something else
> going on?
>
> -- ra
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