[Opensim-dev] PhysX Development

Glenn Martin gamucf at gmail.com
Fri Aug 14 21:27:13 UTC 2015 

That's a very good point and the model we are using. We originally were looking at PhysX.Net but it is difficult to make it work with OpenSim and mono (at least without some OpenSim compilation changes and we didn't feel that would be accepted), and it likely would have a lot more overhead. 

Glenn

Note: Sent from my cell phone. The opinions and thoughts in this email are my own and do not reflect those of any other person or organization. 

> On Aug 14, 2015, at 4:18 PM, Dahlia Trimble <dahliatrimble at gmail.com> wrote:
> 
> It might be worth noting that making a call from managed to unmanaged code typically has a lot of overhead. I've found that making fewer calls while passing many data in arrays of native types is often a lot faster than making many calls with fewer data. I suspect you've probably experienced similar but I thought I'd mention it on the odd chance you haven't. Here's an interesting article about it: http://www.codeproject.com/Articles/253444/PInvoke-Performance
> 
>> On Fri, Aug 14, 2015 at 12:26 PM, <gamucf at gmail.com> wrote:
>> 
>> Hi!  That is very useful information, many thanks!  As we go forward, I'd like to reach out to you more if that would be okay.  We are making the C#/C++ interface be as simple as possible and trying to keep as much native stuff on the native side as possible.  Just a whole lot easier especially with differences in compiler/linker flags between OpenSim and PhysX (Robert can go into a ton more on that).
>> 
>> Glenn
>> 
>> 
>> 
>> 
>> 
>> On Thu, 13 Aug 2015, Mister Blue wrote:
>> 
>>> Hello Robert and Glenn,
>>> I'm the main person behind BulletSim and feel free to send me email (
>>> misterblue at misterblue.com) if you have any questions about integrating a
>>> physics engine with OpenSimulator.
>>> 
>>> Some general ramblings about BulletSim:
>>> 
>>> The C# part of BulletSim can be in addin-modules -- it doesn't need to be
>>> 'in core' but needs to be built with
>>> core so it can be an addin module. Your new physics engine can start out
>>> as an addin module and, like BulletSim, prove itself before graduating into
>>> core.
>>> 
>>> There is a separate OpenSimulator source tree... opensim-libs at "git://
>>> opensimulator.org/git/opensim-libs" that has a bunch of the non-core parts
>>> of OpenSimulator (http server, old and ancient other tries at he physics
>>> engine, ...). The C++ portion of BulletSim is in
>>> 'opensim-libs/trunk/unmanaged/BulletSim' and there are the instructions for
>>> fetching the Bullet sources, patching same, and then building with the
>>> interface to the C# code). The C++ wrapper mostly deals with passing the
>>> structures back and forth between the C# and C++ code (pinned memory for
>>> the position updates and collisions, copying meshes in arrays of floats,
>>> ...)
>>> 
>>> The BulletSim design is around making a simulation step be only one
>>> transition between C# and C++. So, under normal running conditions, there
>>> is only one transition per simulation step and the data (position updates
>>> and collisions) are passed in pinned memory so there is no copy. 98% of the
>>> C# code deals with doing and adapting Bullet to what OpenSimulator required
>>> (link sets (ugh!), ...). The C# -> C++ interface for BulletSim is rather
>>> large... physics engines seem to have lots of calls for all their features.
>>> Bullet, for instance, has what seems like zillions of methods for changing
>>> constraint parameters. I made those appear in the interface to C#. If I had
>>> it to do over again, I'd probably go more with a functional design where
>>> there is a "call a named function with parameter blob" design so the C#/C++
>>> interface was smaller and new function could be added without changing the
>>> binding of the DLL then use some fancy reflection to build the binding on
>>> both sides
>>> 
>>> The .NET C#/C++ binding is pretty good except that int's and boolean's
>>> change size between 32 and 64bits... if you look at the BulletSim interface
>>> you'll see I use floats and arrays of floats everywhere because they are
>>> always 32 bit.
>>> 
>>> I recently played with building "BulletThrift"... a version of BulletSim
>>> that used Thrift to call a remote process physics engine (experiment in
>>> distributed physics). It didn't get finished mainly because the existing
>>> interface to the C++ module is so large. BulletSim actually has a HAL layer
>>> to access the physics engine and there are two physics engines: the C++
>>> Bullet and a C# port of Bullet. The latter was last used by Nebadon to run
>>> OpenSimulator on a Raspberry PI. But this also means it is easy to add a
>>> link to a remote Bullet. That's where I was going to add BulletThrift that
>>> would call across the network to a remote Bullet server. My main reason for
>>> doing this was to be able to run Bullet in a pure C++ environment where
>>> debugging wouldn't be complicated by the managed/unmanaged environment.
>>> 
>>> If you distributed the physics engine, operationally, I'd expect you'd see
>>> some of the things that happen when
>>> running BulletSim on its own thread like jitter caused when there is a
>>> 'beat' between the physics simulation
>>> time and the simulator heartbeat.  BulletSim running on its own thread
>>> means that the physics engine is called
>>> on its own thread and  the passing back of collisions and position updates
>>> happens when the simulator heartbe
>>> at thread calls into the physics engine.
>>> 
>>> Anyway, feel free to ask about my learnings.
>>> 
>>> -- mb
>>> 
>>> 
>>> 
>>> 
>>> On Tue, Aug 11, 2015 at 8:45 AM, Myron Curtis <myronjc at virtualworldsgrid.com
>>>> wrote:
>>> 
>>>> Thanks,
>>>> It would add the extra overhead, but it might also enable a central server
>>>> to manage the physics for several grid instances, and it might be more
>>>> compatible with web based access to a virtual world if we can ever get that
>>>> built.
>>>> Myron
>>>> 
>>>> -----Original Message-----
>>>> From: opensim-dev-bounces at opensimulator.org
>>>> [mailto:opensim-dev-bounces at opensimulator.org] On Behalf Of Kevin Cozens
>>>> Sent: Monday, August 10, 2015 7:00 PM
>>>> To: opensim-dev at opensimulator.org
>>>> Subject: Re: [Opensim-dev] PhysX Development
>>>> 
>>>>> On 15-08-10 06:39 PM, Myron Curtis wrote:
>>>>> Could you use PHP as the wrapper?
>>>> 
>>>> Interesting idea but I could see it adding (a lot of?) extra overhead
>>>> compared to coding the wrapper in the same language as Open Simulator or
>>>> PhysX.
>>>> 
>>>> --
>>>> Cheers!
>>>> 
>>>> Kevin.
>>>> 
>>>> http://www.ve3syb.ca/           |"Nerds make the shiny things that
>>>> distract
>>>> Owner of Elecraft K2 #2172      | the mouth-breathers, and that's why we're
>>>>                                  | powerful!"
>>>> #include <disclaimer/favourite> |             --Chris Hardwick
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