[Opensim-dev] Prospective ODE physics changes

[Justin Clark-Casey]

Hi Teravus, nice to hear from you again!

Yes, more testing is needed, hopefully on OSGrid.  But it seems there may be a tradeoff between having super smooth 
physics objects and being able to get more avatars in a scene without encountering cpu limits.  My perception is having 
more avatars is a more common use case then lots of physics objects, particularly as OpenSim's current ODE use does not 
seem to provide a good physics simulation).  Anybody who does want to try for better physics could always turn the 
collision number back up.

In any case, what was the rationale for choosing 80 as the default?

On 03/01/12 22:30, Teravus Ovares wrote:
> 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 <mailto: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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Justin Clark-Casey (justincc)
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