Math & Rigging · 40 min

Inverse Kinematics with FABRIK

IntermediateNeeds: VectorsNeeds: Skinning

You will be able to

Describe the forward and backward passes of FABRIK.
Explain why FABRIK converges quickly compared with Jacobian methods.
Handle unreachable targets and joint limits gracefully.

The concept

FABRIK solves IK by repeatedly projecting joints onto the line toward their neighbour at the correct bone length: once from the effector back to the root, then from the root forward to the effector. It is iterative, cheap, needs no matrix inversion, and produces natural poses — which is why it is common for tails, tentacles and foot placement.

In your engine

Control Rig has a native FABRIK node; Full Body IK covers the constrained case.

Practice (4)

Concept check

The backward pass of FABRIK starts by:

Predict the result

The target is further away than the total chain length. A correct FABRIK implementation:

Tune the value

Typical FABRIK iteration count needed for a 4-bone chain to converge visually:

Find the bug

An IK leg pops between frames when the target moves slightly. Most likely cause?

Knowledge check

01FABRIK requires:

02A pole vector controls:

03Bone lengths during FABRIK are:

04FABRIK is popular for tails and tentacles because:

Build challenge

Implement FABRIK for a 5-bone chain with a reachability check and a pole constraint. Show it tracking a moving target with no popping across a 10-second capture.

SELF-CHECK RUBRIC