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

[Ben Sunshine-H. <sn...@gm...>]

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 <rf...@tb...> 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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