[Opensim-dev] Using ODE for vehicles and ragdolls

Wed Nov 19 13:50:40 UTC 2008

Heh.

For the record:

It was I who first demostrated the possibility of using
llSetForce/llApplyImpulse as a viable method of making a prim behave like a
vehicle, some 10 months or so ago. During a conversation with one Owen Oyen
(who is something of a physics guy), of SL and OSGrid, he became aware of my
proof-of-concept efforts along these lines and produced a working vehicle
script.

On the linden grid, Owen is a boat scripter and scripter of related
technologies; he has, for instance, devised a means of producing current
effects based on a model of the contour of the sea floor

While our current work is targeted directly at boating, it can be readily
adapted to encompass all of the typical vehicle types of SL, without any of
their vehicle functions.

This model is very primitive, as are those presented in the LSL Vehicle API
as found in LL's LSL.

The characteristics of the LSL vehicle API as implemented by LL are quite
simple:

1. Linear motors. These motors provide motive force parallel to a given axis
or axes.
2. Torque motors. These motors provide rotational force about an axis or
axes.

That's it. Period.

These two models are further modified by a series of contraints that
introduce the concept of friction or other load against the forces provided
by these motors.

Tidy all this up into a 32 bit bitfield for describing the motive forces and
their loads and wrap it up in a nice set of functions exposing this via LSL
commands that characterise a vehicle 'type' (e.g., boat/car/plane) with any
related constraints/dampers and you have Linden vehicle physics.

As I said, this is a highly simplified vehicle simulation model. There is no
distribution of mass involved, no geometry-based drag. There is a single
center of gravity at the geometric center. All motion produced by the calls
to the API is produced around this center. The complexity all rests witth
the description of the motive forces and their respective loading and
damping. The forces are applied either to a simple prim in the simplest
case, or to a linkset. If to a linkset, the linkset is treated as a single
object except for collision purposes, the extent of which is to mesh any
geometric complexities of the object.

The short story is, we have modelled LSL-style vehicles using little but
llApplyImpulse and llSetForce, and a first-order friction calculation.
Ironically, our experiences indicate that these vehicles are consistently
better in their handling characteristics than what is available on the
Linden grid. Indeed, about half of Owen's current work is in the interest of
porting this work *back* to the linden grid.

Enough of that. Lets talk ODE :D

ODE *does* present some rich opportunities for simulation beyond the
(arguably oversimplified) model presented by the LSL vehicle API. given the
type of things I've seen reading the ODE documentation, my suspicion is that
it's strong points for such simulations will be wheeled vehicles, tracked
vehicles, walkers, and by extension, ragdoll/skeletal physics. These would
certainly add an interesting element to the mix, and I am all for seeing
these ideas roll forward.

I do, however, wish to draw a strong distinction between this ODE work and
basic vehicle physics ala Linden Labs, as the vehicle paradigm in LSL is so
heavily abstracted that 'simulation', for any meaningful employment of the
word, is at best inaccurate, and at most hype. Worse, their method of
'simplifying' (read 'dumbing down'), adds significant processing cycles;
were I to guess, I'd say that their API represents greater processing
overhead than do the physics proper.

I'd say +1 for rolling forward with some OS functions for ODE; indeed, I
would suggest (as I have been for some 9 months now) that ODE's interface be
exposed directly in a set of OS functions, allowing the scripter to work in
a relationship that is only once removed from the physics engine, in the
interest of providing the fullness of the ODE featureset to the scripter. I
think some fabulously interesting things would come of this, not the least
of which might be a more interesting in-world experience for those of us who
primarily employ this as an entertainment platform.

Keep up the good work y'all, this is an excellent dialogue and I hope I have
contributed in some way to a clarification of the events surrounding our
recent in-world vehicle work (btw we should be ready to demo some of this
broadly this coming weekend), and to the issues that lie ahead as we work to
bring more physics features to the scripting language and our virtual
environment.

Cheers!
Hiro/daTwitch/Laz...etc.

On Wed, Nov 19, 2008 at 6:14 AM, Stefan Andersson <stefan at tribalmedia.se>wrote:

>  I for one would love to see it unfold something like this
>
> 1) one or several prototypical and specialized implementations coding
> directly to ODE but in a nice manner.
> 2) the implementations refactored into a nice OpenSim API for ODE joints
> 3) one or several implementations of vehicles utilizing this API directly
> 4) these implementations refactored into a nice OpenSim vehicle API
> 5) implementing the LSL vehicle model using this API
>
> so yeah, bottom up is the way to go, but with a clear goal in the back of
> our heads.
>
> I for one would want to see the monorail module before the monorail lsl
> script.
>
> I also suspect that along the lines, we might see that we want to implement
> vehicle types that don't fit into the lsl model... like the 'humanoid' or
> 'animal' - anybody up for building a stone giant or re-casting an agent in
> pure prims? :D
>
> Best regards,
> Stefan Andersson
> Tribal Media AB
>
> Join the 3d web revolution : http://tribalnet.se/
>
>
>
>
>
> > From: adam at deepthink.com.au
> > To: opensim-dev at lists.berlios.de
> > Date: Wed, 19 Nov 2008 02:01:27 -0500
>
> > Subject: Re: [Opensim-dev] Using ODE for vehicles and ragdolls
> >
> > Doesnt ODE have vehicle support internally? The concern I'd have about a
> vehicle layer over the top might be that performance is suboptimal since we
> need to run a second physics step which could introduce issues.
> >
> > Regards,
> >
> > Adam
> >
> > > -----Original Message-----
> > > From: opensim-dev-bounces at lists.berlios.de [mailto:opensim-dev-
> > > bounces at lists.berlios.de] On Behalf Of Gerhard Dünnebeil
> > > Sent: Tuesday, 18 November 2008 10:40 PM
> > > To: opensim-dev at lists.berlios.de
> > > Subject: Re: [Opensim-dev] Using ODE for vehicles and ragdolls
> > >
> > > Hi
> > >
> > > having looked into the ODE engine I don't think it's usable for the
> > > vehicle API.
> > >
> > > Instead i think we should put the vehicle engine on top of it.
> > >
> > > This is pretty easily done by adding another layer (could/should be in
> > > a
> > > plugin).
> > > It can be called in each "Simulate" step and calculate the forces
> > > needed
> > > to achieve the vehicles behaviour.
> > > These forces are then applied -- using the phys engine's API -- to the
> > > objects.
> > >
> > > This way we are
> > > a) ... independant (at least theoretically) from the characteristics of
> > > the underlying Phys engine
> > > b) ... able to implement different engines for different purposes
> > >
> > > I know someone (is it you, nlin?) already implemented a sailing boat in
> > > OpenSim. A modular approach here would allow us to implement
> > > alternative
> > > friction models beyond what LSL gives us.
> > >
> > >
> > > Just my two cents
> > >
> > > Gerhard
> > >
> > >
> > > nlin (message) wrote:
> > > > Hello,
> > > >
> > > > There's been some discussion about SL-compatible vehicles in OpenSim
> > > > recently, with its higher-level abstraction of how vehicles work.
> > > This
> > > > would have the long-term goals of compatibility and ability to swap
> > > > out physics engines while still using the same vehicle layer.
> > > >
> > > > SL-compatibility and abstracting vehicles from the physics engine are
> > > > good goals for the long term; sort of a top-down approach (start with
> > > > the vehicle abstraction, map it to many different physics engines).
> > > >
> > > > Complementary to this, I'd like to explore and start discussion of an
> > > > alternative way of handling vehicles by using a bottom-up approach.
> > > > Starting with the specific physics engine of ODE, can we use
> > > > ODE-specific features to make simple vehicles?
> > > >
> > > > I did some experiments with OpenSim and ODE and the answer seems to
> > > be
> > > > yes, we can make simple vehicles with ODE by using joints. We can
> > > also
> > > > make ragdolls. I posted some videos to YouTube of the preliminary
> > > results:
> > > >
> > > > http://www.youtube.com/watch?v=iYIh-eIwmjs
> > > > http://www.youtube.com/watch?v=z9bedzIuxdM
> > > >
> > > > This is only prototype work for now, put together sort of quickly as
> > > a
> > > > proof-of-concept with just enough "plumbing" (data flow) to work.
> > > > There are a number of issues that need to be solved for this to work
> > > > cleanly. As far as I know (please correct me if I'm wrong) there
> > > > hasn't been much work done to try to get ODE joints/vehicles/ragdolls
> > > > working in OpenSim, but I think it's an avenue worth exploring.
> > > >
> > > > Is there interest in pursuing this approach for OpenSim vehicles? I
> > > > would look forward to discussing some of the issues that would need
> > > to
> > > > be solved.
> > > >
> > > > Thanks,
> > > > N Lin (nlin)
> > > >
> > > > ---------------------------------------------------------------------
> > > ---
> > > >
> > > > _______________________________________________
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> > > > Opensim-dev at lists.berlios.de
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> > > >
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