[Algorithms] Finding the best pose to re-enter animation graph from ragdoll

[Richard F. <rf...@tb...>]

Hi all,

I've got a ragdolled character that I want to begin animating again. 
I've got a number of states in my animation graph marked as 'recovery 
points', i.e. animations that a ragdoll can reasonably be blended back 
to before having the animation graph take over fully. The problem is, 
I'm not sure how to identify which animation's first frame (the 
'recovery pose') is closest to the ragdoll's current pose.

As I see it there are two components to computing a score for each 
potential recovery point:

1) For each non-root bone, sum the differences in parent-space rotation 
between current and recovery poses. This is simple enough to do; in 
addition I think I need to weight the values (e.g. by the physics mass 
of the bone), as a pose that is off by 30 degrees in the upper arm 
stands to look a lot less similar to the ragdoll's pose than one that is 
only off by 30 degrees in the wrist. The result of this step is some 
kind of score representing the object-space similarity of the poses.

2) Add to (1) some value representing how similar the root bones are. 
The problem I've got here is that I need to ignore rotation around the 
global Y axis, while still accounting for other rotations. (I can ignore 
position as well, as I can move the character's reference frame to 
account for it).

Suppose I have a recovery pose animation that has been authored such 
that the character is lying stretched out prone, on his stomach, facing 
along +Z. If the ragdoll is also lying stretched out prone on his 
stomach, facing -X, then the recovery pose is still fine to use - I just 
need to rotate the character's reference frame around the Y axis to 
match, so the animation plays back facing the right direction. But, if 
the ragdoll is lying on his back, or sitting up, then it's not usable, 
regardless of which direction the character's facing in. So, I've got 
the world-space rotation of the ragdoll's root bone as a quaternion, and 
a quaternion representing the rotation of the corresponding root bone in 
the recovery pose in *some* space (I think object-space, but I'm not 
sure?) as starting points. What can I compute from them that has this 
ignoring-rotation-around-global-Y property?

It's been suggested there there's some canonicalization step I can 
perform that would just eliminate any Y-rotation, but I don't know how 
to do that other than by decomposing to Euler angles, and I suspect that 
would have gimbal lock problems.

This is probably some pretty simple linear algebra at the end of the 
day, but between vague memories of eigenvectors, and a general 
uncertainty as to whether I'm just overcomplicating this entire thing, I 
could use a pointer in the right direction. Any thoughts or references 
you could give me would be much appreciated.

Cheers!

- Richard