18 March, 2009

IK vs FK part 1

I have been studying the nature of IK and FK chains to study which one does better at what kind of movement in animation. Here are few simple observations that I wanted to note down.

FK Advantages:
  1. The greatest advantage I see with FK is ease in achieving overlapping action, e.g. between upper arm and forearm.
  2. Natural Arcs while interpolating between keys.
  3. When elbow is planted or when we just need to move forearm then FK is easier.

IK Advantages:
  1. The best thing about IK is the simplicity it offers in creating a pose. For example, positioning a hand with IK is very intuitive and predictable.
    (With FK, I believe the problem is that you have to think in terms of two rotation transforms to make hand reach a goal position. Also, Euler rotation introduces a certain level of difficulty if axes are not in proper alignment, i.e. two axes are closer to each other.)
  2. Animators have to deal less with Euler rotations, hence less Gimbal lock problems.

I understand that many times FK or IK is a personal preference. So a particular behavior of FK/IK may be difficult for one person, but easier for another. But my goal here is to list mainly logical advantages of each system.

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