# FK, IK, and Constraints

Hi all,

I was wondering if anyone could help me with a question on constraints. I’ve got IK working just fine, but the joints bend in strange directions.

Has anyone found any good tutorials on IK and limiting angles / axes for rotation on particular bones or at particular joints? I’m sure it’s possible, but I don’t know how.

The second question is this: is it possible to mix IK and FK on the same areas of an armature? IE, move something using IK, and then add finishing touches with FK?

If your IK joints are bending the wrong way, remember to create them in a half-flexed manner (knees half-bent, arms up 45 degrees from sides, etc.). Also, in edit mode, make sure you Ctrl-N all the bones to fix their rotations.

And no, you can’t pose with IK and tweak with FK (well, you can still roll the bones within in IK chain, but the usefulness of that is limited.)

So aside from creating them in a half-flexed manner, there are no other ways to use constraints to point a joint in a certain direction? I’m picturing how, in other programs, you can add a knee null and then the knee will always point forwards.

I guess it looks like my char will be using IK for his arms and tentacles, but FK for everything else. ^-^

Yup, totally possible. You just have to rotate the bones like usual and the additional rotation will be used to compute the IK solution. It’s a bit hard to explain how it will affect your joint (it depends on the original orientation really) so play with it to get a good feel.

Martin

AFAIK you can do this in blender as well. just add a small bone in front of the knee/joint/whatever. Then create a locked track constraint on the thigh bone/upperarm bone (I think thats the right one, it might be the lower bone) that points to the “null” bone that you created. This extra bone can be used to control the IK solution.