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motion editing - hand and leg ik constraint pinning.


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Hi all, 
I've looked  - but I can't figure out how to pin a hand in Houdini using Kinefx. I've got two situations:
-My input motion capture has a person holding a mobile phone with both hands - I need to pin one of the hands to the other hand as they walk around so it looks like both hands are properly holding the phone.
-Also some of my motion capture has a bad arm that twitches erratically at times. I'd like to pin the hand of that arm at those times and bypass the twitch.

Does anyone have a suggestion of how to go about this. Or point me to where there's good info on this. Coming from Nuance, Motionbuilder, and Maya,  what I'm looking for is referred to as pinning (or 'hand/foot constraint pinning') Can't find anything here in Houdini around those terms.



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In kinefx, joints are just points. You can do anything to them that you can to points. It's a bit different approach than other programs, but liberating once you can wrap your head around it.

So one way is simply to use the world transforms of one hand as the world transforms of the other. In the file below, I just take the world transforms (the position and rotation in world space) of one hand and literally set the other hand to use the same transforms (with an optional offset) in a VOP. Then just use that as the goal for a simple IK chain. In this example the hands are a bit far apart so the arm gets a little over-extended, but you should get the idea. Finally, for good measure, I run it through a full-body IK.

For the second, there's lots and lots of ways to do that. You could use a motion clip SOP to convert the mocap into a static point cloud and then just use various SOP tools or CHOPs to smooth it out. But if you want to try to clean it up by constraining, you could try something similar to the approach above. I just grabbed the pelvis joint (which was pretty stable) and constrained the hand to that with another offset. You could do all kinds of things like taking a few points and averaging them or using the points of a curve as targets or bring in a totally different clip that doesn't have the jitter and blend them together...

Anyway, hope this is a start... The mocap I used is the Thriller Part 2 dance from Mixamo if you want to pipe it in.


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Hey Geoff,
that is... absolutely awesome!!

Thankyou so much for that. It's helped me out going through your explanation and that hip file you provided. I've wrapped my head around it. Happy to say it makes sense. But I was a long long way from getting to that kind of solution myself. (I'm off now to get comfortable with Rig Attrib VOPS.)

I've learnt. Thanks.


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