[ODE] A slider joint to bond an axis-aligned body with a rigid body group?

[Megan Fox]

Let me try to explain exactly what I have: I have a player body which
is forced to remain upright via direct control of torque and
orientation (I know, I know, it isn't "fancy", but it works perfectly
and never requires tweaking).  I need to join this to the root of a
collection of jointed rigid bodies via a slider joint.

This slider joint must remain at the same alignment as the player
body, but the root body of the collection attached at the other end
can rotate about the point of the attachment (and of course the joints
within the collection can bend and flex and whatever else they need to
do).

How would I go about implementing this?  I guess what I need is sort
of like a suspension + axle, but not quite - replace the axle with a
balljoint and you're getting closer, but even then setting the
orientation of the "top" body would seemingly break the construct?


Now, why I need it:

I'm trying to implement resumable ragdoll - that is, a character can
get back up out of ragdoll state.  Now for this to work, I need a
standard center of character, as well as a region of set-aside space
so that the player can get back up out of ragdoll, resume their old
single-body collision shape,and not explode the surrounding
environment in the process.

For this, I need to keep the original hovering body active throughout
the entire process, situated as if the entity were still alive (and
the ragdoll animation was just that - animation, pre-canned, relative
to the central point).  The root node must be capable of going from
where it is during normal animation (resuming animation before a
stunned character has actually fallen completely) to where the feet
normally are (resuming animation when the player is laying on the
ground looking dead-ish).

Resuming animation when the player is in any position other than
laying on the ground isn't really important, it's just a side-effect
of the system of sliding joint I've spec'ed out.  The "get up from
fall" animation is typically hip-centric, but takes place when the
hips are at the level of the feet, and a sliding joint connecting the
player body to the rigid ragdoll is the only way I can come up with
that makes this possible.



Any ideas on this?  The "simpler" alternative would be to fix the
hovering player body to be relative to the position of the feet, and
make the get-up animations foot relative, but I'm not sure that would
look good at all (how would you stand up from laying down such that
you end up standing over the point where your feet used to be?  You
usually get up such that your body is above the point the torso was
at).

Thanks,
-Megan