Pivoting a character around his knee?



here’s a quick mockup I threw together in 5 minutes to show what I mean:

Animation: https://i.imgur.com/eYOqIuW.gifv
The 4 keyframes I used:
Frame 0: https://i.imgur.com/ZKkd1nJ.png
Frame 20: https://i.imgur.com/nmHwTCu.png
Frame 40: https://i.imgur.com/KZpPAia.png
Frame 60: https://i.imgur.com/jiItpqu.png

Now, the character puts his weight on his knees to lift himself up a bit, then shifts the weight only on the right knee to move the left leg forwards. As you can tell in the render, somewhere between Frame 20 and 40 the knee sinks into the ground a bit.

Is there a specific trick or workflow to make sure that the Hip stays the same distance from the knee’s point of contact on the ground for a certain duration? Do you just eyeball it frame-by-frame?

I tried setting the IK target for the leg to the knee (which is easy to do with CAT Num of Bones), but that didn’t really help, because the Hip is still the deciding factor of whether or not the knee makes contact with the ground.
Another thing I thought might work is setting up a helper at the contact point and link contraining the Hip to that point while there is weight on the knee and then use the Helper to animate the position of the Hip by rotating the helper and thus locking the distance between the hip and the point of contact.

Thanks in advance.



You can try just to add an extra Helper which will have its pivot on the knee and, depending on your rig, constraint or parent your hips controlller under this helper.

Hope this will help