[ODE] Fixed Joints only for debugging?

Tim Sorrells tsorrells at breakawayltd.com
Fri Oct 27 08:22:44 MST 2006

I've been using fixed joints extensively in my simulation to glue ODE
driven objects to non-ODE game objects.  For the most part this works
reasonably well, although not perfectly.  Ideally a fixed joint would
prevent the objects from moving in relation to each other, but I've
found that ODE tolerates a small but noticeable separation even when a
small constant force is applied.  I suppose this is consistent with how
other joints work, although I'd prefer it to short-circuit the CFM and
ERP parameters and make the two objects joined this way hard and
inseparable.  I would think this would make calculations easier too, as
there is no math involved between the two objects, only forces that act
on the two as a whole.


I'm having a problem where I have two Bodies and the world.  One Body is
attached to the other via a fixed joint, the other is attach to the
world with another fixed joint.  The first body is correctly "glued" to
the position of the second body, but the second body jitters noticeably.
There is no substantial force being exerted on the second body, as the
first body is able to comfortably reach its position.  So I'm not sure
what's causing the jitter.  I was also a bit worried about the statement
on the Wiki page about fixed joints: "Using this joint is almost never a
good idea in practice, except when debugging."


I wonder if this means that fixed joints aren't really intended to be
used because better methods are available, or if it's more an issue of
this feature isn't done yet, but eventually will be.  The proposed
solution is "If you need two bodies to be glued together it is better to
represent that as a single body."  Is the suggestion here that when I
want to attach two bodies I should delete the Body associated with one
and associate its relevant Geoms with the Body of the other?  Then when
the two objects are disconnected again I have to recreate the Body and
change the Geoms back over to their original owner?  That seems rather
tedious to me when the idea of having a fixed joint between the two
Bodies really seems to encapsulate my intention entirely.

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