IK -v- FK: Which is Best?

IK - v - FK - which is best?

One question that student animators often ask is about the merits of Inverse Kinematics (IK) vs Forward Kinematics (FK).  The IK -v- FK debate sounds rather abstract - a bit of a techie question. After all, at first, it's not immediately obvious what these terms even mean, let alone how to explain which option you should choose.

The question is really about hierarchies. Usually, animators are trying to decide whether to set the hands on a character to IK or FK.  If the hand controls are set to IK, then you are controlling the hands from the bottom of the arm hierarchy, from the hand control.  If you set the controls to FK, then upper arm, shoulder and body is in control of what the hands do.

In a walk cycle, you usually set arms and hands to FK, because the arms are swinging, controlled from the shoulders. But if you were to animate a sword fight, where the position of the hands is vital, then you would almost certainly choose IK.



IK is best - Usually
Most animators choose IK controls for the hands, most of the time. Using IK helps to speed up the animation process, because you have greater control over the position of the character's hands in any given pose. It's also much easier to control and to pose the arms and hands this way. If you are using FK, you sometimes have  to compensate or counter-rotate body parts to maintain the hand pose. Ultimately however it's all about the shot - which you choose depends on the shot itself.

Who is in charge? The hand or the body?
Ask yourself who needs to be in charge - the hand or the body. Use FK if you want the hand and arm rig to be influenced by the movement of the body. Use IK if you want the hand to dominate.  However, both approaches have their positives and negatives for different situations; ultimately your choice depends entirely on the kind of animation you want to do.

Walk Cycle - FK
For example, if you are animating a walk cycle, you would almost always choose FK, because the arms are swinging below the shoulders, follow the main motion of the body.

Sword Fight - IK
Conversely, if you are animating a sword fight, you would almost certainly choose IK, because you need the maximum control over the placement of the hands and swords.

Check the rig
Finally, much depends on the rig you are using, as the IK controls on some rigs can be awkward and might not work as well as you might like them to.

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