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

[Jeff R. <je...@gm...>]

There are probably a number of ways to do it. My first guess would be to
compute the difference in rotation for the root bone (that is, what
rotation takes you from your starting frame to the current ragdoll
orientation), and then examine the "up" vector of the resulting transform.
If it's too far from vertical, you don't have a very good match. You can
compute a score perhaps based on the dot product between the "up" basis of
this transform and the global up direction.

On Thu, Dec 13, 2012 at 1:22 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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-- 
Jeff Russell
Engineer, Marmoset
www.marmoset.co