View Full Version : Flacky Knee joint

03-01-2005, 04:11 PM
Hi All:

I have a problem with a charactors knee. It is fully IK'd. When I move the goal up and forward (infront of the charactor) the knee binds OK to a point. As I start to move the goal forward all of a sudden the leg jumps straight see the second image. I have set limits on the knee rotation so it will not bend backwards. I what to have the charactor standup from a seated position, turn and walk toward the camera. Trouble is controling the flacky knee. This happons on both legs. Any ideas?

03-01-2005, 05:13 PM
My experience has been that if you have to set limits to keep the knee from popping then you should probably redo it.The character should be modeled so that you can have some prebend to the bones at those joints. When I do that I never have to set limits and I never get popping. :)

03-01-2005, 05:42 PM
Thanks Larry. Here is a side veiw of the model & skelegons in modeler. Is this prebend enough? When I said limits I was referring to the rotation limits on the 'controls and limits' panel of the Motion Options window. Do you use these limits when you rig a charactor?

03-01-2005, 05:58 PM
It doesnt look like it does to me but its whatever amount makes it work :)
I only use rotational limits in rare circumstances but never to correct popping joints.
What you might do is this:
Turn all the bones off by selecting all of them and hit ctrl + r
You can use joint move or just rotate the thigh and shin bones to get the knee bent enough.
Use the load bones plugin to get you bones into modeler as skelegons so you have a reference to readjust your model from. :)

03-01-2005, 06:49 PM
I would have to dissagree with Larry here. What works for me is to model the legs straight and then in Layout to bend the knees a bit before setting up the IK. I usually only need to bend them 1 degree or so and then I set the limits to not allow the knee to go past that bend. After that I can IK the leg and it doesn't POP. Hope this helps!

03-01-2005, 07:05 PM
Theres several reasons I do it that way. One is because Ive found that if you have enough prebend you never have to use limits to prevent flipping or popping.
Setting limits can also sometimes cause other issues. For example if you try and pose a character in a way that runs into those limits the character can be pushed away from its goals.
I also like to have a clean rest pose with everything already prebent that I can easily go back to if something gets messed up later.
Regardless, the key thing is the prebending. :)

03-01-2005, 07:13 PM
For example if you try and pose a character in a way that runs into those limits the character can be pushed away from its goals.I still would have to dissagree because I only set limits to the knee bend. I can't imagine a situation where I would need to pose a character in such a way that I would need to bend his knee backwards... That would just be painful to watch :eek:. Limits on the bone rotation can keep you from posing your character in a way that wouldn't be possible or that would bend the appendage in the wrong direction. Limits can definately be helpful!

03-01-2005, 08:31 PM
Some types of animation rely to a degree on breaking the joints to make it look right. It also depends on whether Im rigging for myself or someone else.
Like I said, if a rig needs limits to keep joints from breaking the wrong way, popping or flipping theres usually something wrong that needs to be fixed :)

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