Joint angles suck for pose comparisons -- they just aren't the basis of our intuitive notion of similarity. IMHO, point clouds work much better. Transform a few bone-attached points -- say, pelvis, left shoulder, right shoulder, left elbow, right elbow, left knee, right knee -- canonicalize by putting the pelvis at zero and the shoulder midpoint at +X, and find the minimum squared distance to a recovery pose (with some per-point weighting, if you like).

For references, the one that immediately comes to mind is "Dynamic Response for Motion Capture Animation". They're blending into a response animation while the character's still fully ragdoll, so they have to look at multiple frames to get velocity effects in there -- if you're waiting until the guy's all fallen over, your task will be simpler.

Ben

On Thu, Dec 13, 2012 at 6:22 PM, Richard Fine wrote:
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

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