[Algorithms] Trouble with translating ragdoll joint rotation to animation joint rotation

[Megan F. <sha...@gm...>]

I'm having trouble with my ragdoll code, but it isn't the physics side
- ODE has that handled famously - the trouble I'm having is with
translating the rotation of the physical joint to the rotation of the
animation joint.  Specifically, getting the coordinate space of the
physical side of things to match the coordinate space used by the
joint in question.

In the physical world, I have my "root" body for a creature, which is
roughly equivalent to the root node in the animation system.  To
create, say, the waist joint, I make the "root" physical
representation resemble the hips, create another physical body to
resemble the chest, and joint the two.  To get the rotation between
the two, I would do chestMat * (hipMat ^ -1), and take the quaternion
of the result matrix.

This works fine for exactly one test matrix - an arm - because it just
so happens that the single joint of the arm has axis which align with
the object-space axis.


The trouble arrives when you look at the coordinate systems for the
hips and chest.  The torso ends up having axis which go xvec =
-objecty, yvec = -objectx, and zvec = -objectz - this is because in
the animation skeleton, there's a good 3 bones between the root and
hips (all of which liking to tweak the coordinate space - it ends up
that positive X always points "up" the bone towards the parent joint),
while in the physical skeleton it's just a straight connection.

I've tried canning it, by working through the pose (default joint
position) quaternions, quat1 * quat2 * ... etc up to the hips,
hard-setting that as an assumed hips rotation, setting the initial
orientation of the chest body to match the pose joint rotation of that
bone, then doing the rest of the joint rotation calc normally from
there... but even that didn't give me the results I was expecting - it
aligned the torso's axis with object-space axis, though the joint
movement past that was seemingly correct.

So, I assume my understanding of all of this is flawed on a basic
level, probably relating to my fairly weak understanding (self-taught
as I've needed it) of the application of matrices.  I could very much
use some help in figuring out where I've gone wrong.


Thanks,
-Megan