[Opensim-dev] Using ODE for vehicles and ragdolls

[nlin (message)]

Hallo Gerhard,

2008/11/19 Gerhard Dünnebeil <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)
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