[Opensim-dev] PhysX Development

[Mister Blue]

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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