On 05/01/12 12:19, AJLDuarte wrote:
Hi,
Reading back my previus posts, They may look a bit unfriendly.
If so please consider it just resulting from of my non-native english nature.
Hi Ubit. No problem. If my own tone is occasionally terse and robust that's because I have to read/write lots of
e-mails quickly :). I do value your input and opinion.
Regards,
Ubit
----- Original Message -----
*From:* AJLDuarte <mailto:[email protected]>
*To:* [email protected] <mailto:[email protected]>
*Sent:* Thursday, January 05, 2012 12:04 PM
*Subject:* Re: [Opensim-dev] Prospective ODE physics changes
Hi again,
Forgot to mention that you can find working code at
https://github.com/UbitUmarov/Ubit-opensim where i did tried to
fix those and other issues
You may see, use, adapt and of course improve.
Thanks, I'll certainly bear that in mind.
Regards,
Ubit
----- Original Message -----
*From:* AJLDuarte <mailto:[email protected]>
*To:* [email protected] <mailto:[email protected]>
*Sent:* Thursday, January 05, 2012 11:39 AM
*Subject:* Re: [Opensim-dev] Prospective ODE physics changes
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.
I did review those changes but I'm not convinced that we aren't already using
pinned memory. For instance
public static extern int Collide(IntPtr o1, IntPtr o2, int flags, [In, Out]
ContactGeom[] contact, int skip);
has the contact parameter with both [In and Out] attributes. According to the ms docs that I've read, this means that
pinned memory will be used - though I find the available docs really quite hard to read. Is this incorrect? A similar
thing is true for
public static extern IntPtr JointCreateContact(IntPtr world, IntPtr group, ref
Contact contact);
where the Contact is a ref and so occupies pinned memory, according to my
interpretation of ms docs.
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.
Isn't 'useless state' a bit strong? Reduce the collisions all the way down to 1 had no noticeable effect in my tests,
even with physical objects. I'm not advocating this number - it's just an illustration.
I think there has to be a balance between physical fidelity and the need for a given sim to be able to host more
avatars. On balance, I think people would prefer avatars. In any case, one can always adjust those parameters in config.
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 ? ....
I have already done actual stress testing. The reason for my conclusions is that ODE does not fail on a single sim, as
shown by the thousands of hours that it now doesn't crash on osgrid, for instance. If there was a problem with freeing
memory due to misuse of p/Invoke. I would except this scenario to crash as well.
However, two regions does crash and different OdeScene classes have no common data (e.g. static variables) at the C#
level, whilst the mailing list messages I've read do suggest that they share a global cache on the ODE level.
Moreover, GIMPACT does not crash with two regions. If there was a p/Invoke issue wouldn't we expect this collider to
probably have the same problem?
Having said that, it's certainly not impossible that there could be a complicated interaction with p/Invoke and ODE with
multiple regions. But I simply lack the time to test every scenario when there's a solution that appears to work and
can be reversed if it turns out to be bad. Development is a learning process.
Regards,
Justin
Best Regards,
Ubit Umarov
----- Original Message -----
*From:* Teravus Ovares <mailto:[email protected]>
*To:* [email protected]
<mailto:[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]
<mailto:[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]> <mailto:[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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http://justincc.org/blog
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