topic Re: Getting rotations of bones animated with IK in 3ds Max Programming Forum

Getting rotations of bones animated with IK

Anonymous — Fri, 23 May 2008 15:53:24 GMT

I have a hierarchy made like this (parent->child):
arm->forearm->hand
and an IK chain going from arm to hand.
I need to know at every key how much the rotation of the arm and the forearm is.
How can I do this in MaxScript?

Re: Getting rotations of bones animated with IK

Anonymous — Sat, 24 May 2008 01:25:22 GMT

Depends on what you want back but here are a few options.

Quaternion:
coordSys parent forearm.rotation
or
coordSys arm forearm.rotation

Euler:
(coordSys parent hand.rotation) as eulerAngles

Degrees via dot product:
(1-(dot forearm.dir hand.dir))*180

Re: Getting rotations of bones animated with IK

Anonymous — Sat, 24 May 2008 15:12:15 GMT

Thanks for the answer but there is one problem. What seemed to me is that, if the hierarchy is animated with inverse kinematics, the rotation of the arm, for example, always remains the same, because it is computed by IK.
I tried to solve this using an expose transform helper. Am I wrong?

Re: Getting rotations of bones animated with IK

Anonymous — Sat, 24 May 2008 15:32:13 GMT

post a file and I'll post a fix:)

Re: Getting rotations of bones animated with IK

Anonymous — Sat, 24 May 2008 17:05:45 GMT

post a file and I'll post a fix:)

I don't have a file, I was just doing some experiment. Let me explain the complete problem.
I have a skeleton made using 3ds bones. I have recorded hands' rotation and position data using 2 6DOF sensors and then inserted these data in a HTR file with the skeleton definition. I only have these two sensors for MoCap.
Because of the way HTR data is imported, the hands detached from the other bones while moving, so I decided to apply position data to two IK chains going from the arm to the hand bone.
This way, arms and hands movements work, but there's a something to correct about hands rotation.
In fact, since arms rotation is not recorded and rotations are absolute relatively to the magnetic field emitter hands beahvior is not correct.
For example, if the right arm rests along the right side of the body, hand recorded rotation is -90 on the Y axis. After importing these data in 3DS the hand is rotated of -90 degrees in its local coordinate system, so it is rotated relative to the forearm, while it must have no rotation relative to the forearm.
To correct this problem I have to set hand rotation this way:
HandRotation -= ArmRotation+ForeArmRotation (for each key)

Which is the best way to do this?

Re: Getting rotations of bones animated with IK

Anonymous — Sun, 25 May 2008 08:16:03 GMT

I'm not sure I entirely understand whether you have made the adjustment to the hands or not, but...

An easer way to make an adjustment to an IK rig than altering a targets keys would be to place a null or dummy between the object with the motion on it, and a new object, in this case a new hand. So you would have a duplicate hand follow the motion of the Target and not have to modify the keys at all. You can turn and twist the duplicate hand any way you want.

Re: Getting rotations of bones animated with IK

Anonymous — Sun, 25 May 2008 21:01:10 GMT

I'm not sure I entirely understand whether you have made the adjustment to the hands or not, but...

An easer way to make an adjustment to an IK rig than altering a targets keys would be to place a null or dummy between the object with the motion on it, and a new object, in this case a new hand. So you would have a duplicate hand follow the motion of the Target and not have to modify the keys at all. You can turn and twist the duplicate hand any way you want.

In my case, with the chain described in the first post, which is the object with the motion on it?
I can't understand how to apply your solution to my problem and in what way it is useful.