Hi,
Justin, before thicking about reducing the size of the array passed to ode
collide function to receive the colision contacts information, maybe you should
remember what i told you about managed versus unmanaged memory use in the ode
plugin.
Maybe you should think about what is being done by framework to convert the
array from managed memory space to unmanaged and then back again on each call.
If you do that (or just remember the details i told you) maybe you will
see how to save some cpu without reducing the stability of the simulation to a
useless state.
Also before doing hard testing with diferent ode supporting libs, maybe you
should also review managed/unmanaged issues on other parts of the plugin.
JointCreateContact ? GeomHeightfieldDataBuildSingle ? ....
Best Regards,
Ubit Umarov
----- Original Message -----
From: Teravus Ovares
To: [email protected]
Sent: Wednesday, January 04, 2012 7:53 PM
Subject: Re: [Opensim-dev] Prospective ODE physics changes
ODE Documentation and examples :)
Regards
Dan
On Wed, Jan 4, 2012 at 2:46 PM, Justin Clark-Casey <[email protected]>
wrote:
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 <[email protected]
<mailto:[email protected]>> 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)
http://justincc.org/blog
http://twitter.com/justincc
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