[Opensim-dev] Prospective ODE physics changes

[Justin Clark-Casey]

On 03/01/12 21:58, Adams, Robert 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?

This is a good point.  I need to go back and poke at ODE some more to see how many collisions are coming back.  But from 
my avatar stress testing, the reducing in physics time from reducing the collision number is very real.

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Justin Clark-Casey (justincc)
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