[ODE] Repositioning a jointed body

[Graham Fyffe]

Hi!  I have had similar mis-luck with trying to reposition joints,
i.e. dynamically changing the anchors or the axes of joints.  I ended
up creating a whole set of functions like dJointSetHingeAnchorRel,
dJointSetHingeAxisRel, dJointSetHingeQRel and so on and adding it to
my copy of ODE.  This works as expected.  I had to do this because I
wanted to dynamically update the center of gravity of my bodies, and
dMassTranslate plain doesn't work.  So I change the position of the
body instead, and I update all the joints that anchor to it.

Suffice it to say, a nice addition to ODE would be some helper
functions for dynamically adjusting things.  ODE works great if you
set up once, and run, but if you want to be changing things like this,
it's a monster :)

- Graham Fyffe


On Sat, 9 Oct 2004 11:44:29 -0400, John Buehler <johnbue at msn.com> wrote:
> John Buehler writes:
> 
> > I've been having some trouble repositioning a jointed body.  I
> > have a human figure which I create at the origin, then move to
> > its proper starting position by using dBodyGetPosition/
> > dBodySetPosition.  Originally, my joints were kept simple and
> > were all hinges.
> >
> > To move the figure, I get/set each body's position, and do the
> > same with the joints.  I don't touch the geoms.  I found that
> > the figure would rocket off into the distance if I tried that
> > combination.
> 
> After continued experiments, it seems that the reason that moving a jointed
> figure is a bad idea is because there is no way to move the entire jointed
> structure in a single operation.  Hinges will tolerate a separate move of
> each body that it is attached to, while ball joints, and possibly other
> complex joints, apparently will not.
> 
> I've worked around the problem by creating the jointed figure at its start
> position, and ODE behaves as expected.  Unfortunately, it limits my ability
> to directly manipulate the positions of my jointed figures through the user
> interface other than by destroying them and recreating them at the desired
> position.
> 
> I would recommend an addition to the ODE documentation that warns of trying
> to programmatically move a jointed body through a series of dBodySetPosition
> calls.
> 
> 
> 
> JB
> 
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