[Opensim-dev] Using ODE for vehicles and ragdolls

[Gerhard Dünnebeil]

Hi

as an add-on i think joints might be interesting, yes. Personally I 
dream for years about a nice train in SL :-)

But I think this is future stuff that should not fall under the 
"standards".
Reason: This is only possible with ODE, which -- while obviously being 
the most spohisticated engine in use here -- isn't the only engine.

Instead I could imagine a script add one (like the os... functions) that 
do "odeModifyLinkBehaviour(.........)" or something like that.

But nevertheless, an idea that is worth to follow up.

BTW, the mass calculation for joint geometries is not done in ODE but in 
the surrounding code, i.e. in "our" code.
Mass and geometry are two clearly distinguished concepts in ODE.
It's up to the pplugin to calculate masses and their distribution 
cleanly and I know this will be a challenge, esp. for the distributions. 
I need to dig deep into old books of mine to understand mass 
distributions and tensors again I fear :-)

But honestly, this is late on my agenda. First I want to show that my 
approach is working and gives some advantages.


Gerhard


nlin (message) wrote:
> Hallo Gerhard,
>
> 2008/11/19 Gerhard Dünnebeil <Gerhard.Duennebeil at chello.at 
> <mailto:Gerhard.Duennebeil at chello.at>>
>
>     the vehicle support I found in ODE is closely related to the
>     heritage of
>     ODE when it started as an engine to simulate robots. It's far from the
>     vehicle model that Linden Labs implemented and that I think should be
>     used here.
>
>
> I agree that for SL compatibility, ODE by itself doesn't map 1:1 to 
> the SL vehicle model. To get SL-style vehicles, as you say, I think we 
> need some sort of additional force-providing layer/module.
>
> In addition, I think jointed assemblies in ODE-fashion could make an 
> interesting orthogonal addition to OpenSim, independent of any vehicle 
> model. Adding clean support for user-specified joints would allow fun 
> things like ragdolls, rolling car-like things, ropes, and other sorts 
> of mechanical/robotic devices.
>
>     The current approach uses joints to model linked prims inside the ODE.
>     This a mis-use of joints in think.
>
>
> I agree that the current OpenSim use of fixed joints to model linked 
> physical prims is not optimal, primarily because the ODE manual 
> specifically says that usage of fixed joints can lead to simulator 
> instability. I didn't look into it deeply yet, but linked physical 
> prims (using the current OpenSim approach) don't work well at all for 
> me; the linked assembly jumps all over the place.
>
>     Currently I work on an approach where each linked object (not each
>     prim!) is represented by one(!) ODE object.
>     Prims are represented by an (ODE) geometry.
>     ODE allows to assign more than one geometry to one object, so linked
>     prims can be modelled into one rigid ODE object by this approach while
>     conserving the collision behaviour as known from SL.
>     This will reduce the number of joints drastically as joints are only
>     used to model contact points and are only temporary after that.
>     There is
>     no need anymore to permanent joints.
>
>
> +1 on this idea.
>
> Any reason this should not be included in the core? I seem to recall 
> there was some issue about ODE's mass calculation being incorrect for 
> ODE composite geometries (i.e. that the way of creating composites 
> illustrated in ODE's boxstack demo was actually incorrect).
>
> While fixed joints are generally inadvisable to use, I think the other 
> joints (hinge, slider, ball, etc) could add a whole range of new 
> possibilities to our sims, which is why I'm interested in how to 
> cleanly allow their specification, editing, and persistence. Your 
> thoughts?
>
> mfg,
> N Lin (nlin)
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Opensim-dev mailing list
> Opensim-dev at lists.berlios.de
> https://lists.berlios.de/mailman/listinfo/opensim-dev
>