HOW DO I STOP MY IK LEG FROM FLIPPING?
George Flack, Cheshire
Inverse Kinematics, or IK as it’s more commonly known, can be a tricky beast to tame. What seems like a simple way of being able to plant the character’s feet as they walk, can turn into a complex battle with pole vectors and joint chains.
I’ve had my fair share of issues with IK; be it a Single Chain, Rotate Plane, Spline or even a Spring Solver, each has its rules to abide by to get them to behave.
The problem with IK is it can behave erratically if not set up correctly, but it’s not the IK itself that can be the root of the problem. Most issues can come down to something as simple as joint placement and orientation.
When it comes to a basic two-joint configuration, adding a slight bend in your knee or elbow joints will help to dictate which way they will bend when the IK handle is applied. This is because the pole vector, which is a triangular plane created by the IK, uses this bend to help align itself correctly. If the joints are straight, then the vector will get confused about which way to bend, but you can also get around this by using the Set Preferred Angle method, (more on that below).
With quadruped limbs you have the added complexity of there being three joints to deal with, but the same applies. Making sure you have a bend in each joint will help to dictate which axis the joints will bend through when the IK handle is moved, but there is an added issue when using a pole vector constraint which can cause the limb to flip.
The good news is that this too can be solved by adjusting the joints’ positions. Having the joints aligned above each other can cause a similar confusion about which way the limb should be pointing.
So, all you need to do is make sure you space the joints out more to better define the flow of the limb, and this way the IK will have a better idea of the layout of the hierarchy.