[ODE] Re: dynamic add of geom and mass to a body

[Adam D. Moss]

Guillermo Andrade-Barroso wrote:
>>>> See the boxstack demo for details.  I'm thinking of the compound 
>>>> object.
>>
>> The last time I checked, I'm pretty sure that the boxstack demo
>> didn't actually do this right.
> 
> Exact!
> Take a compound object from boxstack demo, this object has a center of 
> mass that coincides with its reference position and all initial geoms 
> (and perhaps
> some joints) are attached to this body.
> Now, you need to add another geom.In this case, the new geom changes mass,
> inertia and center of mass of body. Because the implementation of ODE 
> needs to preserve coincidence between center of mass and position of 
> reference, you
> need to change position of body and relative position of all geoms (and 
> all joints) attached.

Yeah, I wish I could provide a simple answer.

I've licked this in my engine, but it was a real nightmare.  I'm
going to have to revisit this part soon to make it properly
scriptable, so if it all starts to look simpler than it originally
did I'll post a summary.  (Actually IIRC the ODE side isn't that
bad, conceptually, it's just kinda horrifying to get this integrated
with a scenegraph because of the tangle of relative transformations
(world-relative, modelview-relative, body-relative) needed to
adjust the geoms' offset relative to the body while tieing the
rendered mesh corresponding to the geoms in the right place,
but that's perhaps a peculiarity of the way my engine disentangles
mesh from body from geom from mass)...

--Adam
-- 
Adam D. Moss   . ,,^^   adam@gimp.org   http://www.foxbox.org/   co:3
"I am NOT a nut!  I am the keeper of the seven universal truths!"