[ODE] fixed joints

[Martin C. Martin]

> For example, lets say I have a car, and I want to represent car engine as a
> separate object. Why would I want to do that? because I want to be able to
> customize a car with different engines. Engines could have different mass
> properties.

Then have a single body, and change it's mass.  Play games with dMassAdd,
dMassTranslate, etc.

> Now what if I want to add some external attachments? customize
> car exterior. Then it's important to have Geom class attached properly for
> collision detection.

So change the geom attached to the body.  Have more than one attached.  If
you want it/one of them off the center of mass, use a transform geom.

> There needs to be a simple way to do this using ODE.

There is.  Change the geom attached to the body.

If two bodies, A and B, are always fixed to each other, why do you want to
represent them as separate entities, and use forces to keep them mostly
fixed to each other?  Especially when this will slow down ODE?

The best use I can see for fixed joints is when two objects that are fixed
to each other become separated, like explosive bolts on spacecraft.  It
might be a lot easier to use a fixed joint for the bolt in that case,
although you could always get the same effect by destroying the single
body and creating two others.  It's more coding work, but better run time
performance before the split.

- Martin