Understanding the effects of IK and FK in animation

While you may not be an aspiring animator, it’s somewhat important that you understand some key theories within animation. This knowledge will allow you to decide where to use FK and IK on rigs. If you can preemptively decide what system belongs where, you can save a lot of time and people will appreciate your work just that little bit more. In general, IK goes on arms and legs while FK fills in the gaps such as the spine, neck, fingers, and toes.

In the real world nothing organic moves in straight lines; all of our motions occur in arcs. From humans walking to birds flying, there is never a straight line. In animation, straight lines are a result of linear interpolation between two points. Take a look at Figure 6.1 and Figure 6.2, which show how FK and IK systems move from keyframe to keyframe when animating.

Figure 6.1 – FK controls form a clear arc

Inverse kinematics form a linear...