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

[Michael De R. <mic...@gm...>]

Hi Richard,

I believe you will face a couple of problems with the approach you describe
in 1)

- when comparing local quaternions for the joints, the comparison will take
into account the twist around the limbs even though any amount of twist
doesn't change the position of the limb. Therefore you could get drastic
differences when visually there are barely any.

- even if the joint orientation are different the position of the limbs,
especially their endings like hands of feet, could still be in very close
positions, i.e. potentially closer than limbs with similar rotations but
with their root joint (shoulder for instance) off by a bit. You mention
weighting system, but that is going to be a pain to tune.

An other approach would be to find a comparison algorithm that compares the
overall position of the limbs.

For instance you could consider;
- modeling triangles based of significant body joints, for instance
    + hips, shoulder, hand
    + hips, hand, foot
    + hips, shoulder, shoulder

Then use the normal of those triangles for your pose comparison.

You would still need to make the normals relative to the hips and then use
a hips orientation comparison process.

I haven't actually implemented this, but that's how I would go about it. I
hope this gives you a different perspective or more ideas.

Michael





On Thu, Dec 13, 2012 at 11:39 AM, Jeff Russell <je...@gm...> wrote:

> 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
>
>
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