[ODE] Joints and bodies

[Nate W]

On Sat, 11 Jan 2003, gl wrote:

> > Imagine joining two real-world blocks of wood with a real-world hinge,
> > like a door hinge... For any location of the hinge, there are an infinite
> > number of places you could ALSO put the hinge, which would create exactly
> > the same effect.  All that's necessary is that all of the hinge axes lie
> > along a common line in space.
> 
> I can't quite see that.  On those real blocks, the side the hinge is
> connected to very much affects where they end up in space (see attached
> picture).  Sure, the rotation is the same, but the resulting position isn't.
> So presumably the joints _are_ assumed to go through the body 'position',
> ie. currently the center of mass.  Or am I missing something?

Two things define the hinge axis: a point in space (what ODE calls the
"anchor") and a vector indicating the direction of the axis (ODE simply
calls it the axis).  In the two examples in your illustration, the
direction vectors are the same, but the anchor points are not the same.

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com