FK-IK rigging

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This article was registered on July 8, 2020 on the quality assurance website. Please help to improve it and please take part in the discussion !
The following still needs to be improved:  Formulate the introduction in a generally understandable manner, check the redundancies for direct kinematics and inverse kinematics - Olaf Studt ( discussion ) 12:01 pm, Jul 8, 2020 (CEST)

Forward- ( FK ) and Inverse Kinematics ( IK ) are principles from robotics that have been transferred to digital rigs for computer animation in order to calculate joint rotations of 3D characters and other animatable 3D objects.

Forward Kinematics

Forward kinematics, also known as direct kinematics , is one of two basic methods of calculating the joint motion of a fully rigged character. When using a Forward Kinematics rig, joint rotation can only affect parts of the skeleton that are lower in the connection hierarchy.

So influences z. For example, the rotation of a character's shoulder changes the position of the elbow, wrist, and hand. When animating with Forward Kinematics, the animator typically has to adjust the rotation of each joint individually in order to achieve a desired position. To do this, the animator has to work through the joint hierarchy from the first to the last bone.

The end position of an end joint of a joint chain (for example a wrist at the end of an arm) is therefore dependent on the rotation of the previous joints.

With this method, control over the respective joint rotations lies entirely with the animator , but with correspondingly long joint chains or precise movements, the workload is quite large.

Inverse Kinematics

Inverse kinematics is the reverse process of forward kinematics and is often used as an efficient solution for rigging a character's arms and legs. With an inverse kinematics rig, the terminating joint is placed directly by the animator, while the rotation of the joints above it in the hierarchy is automatically calculated by the software.

Inverse Kinematics is best when the animation requires an end connection that needs to be placed very precisely. A good example is a character who has to climb a ladder.

Since a character's hands and feet can be placed directly on the rungs of the ladder, it is more efficient to use an inverse kinematics rig instead of rotating the joints one by one until the end joint is on the rung.

One disadvantage is that Inverse Kinematics uses software interpolation. The animator has no great influence on this interpolation and has to correct errors that occur between the key poses afterwards.

Advantages and disadvantages

FK: Disadvantages: A lot of time and patience must be invested in an FK animation . For each pose, the joints of the complete joint chain must be rotated again. This makes it difficult to reach the position of an end joint.

Advantages: FK can be used more flexibly than IK, and the rotation of each individual joint in the joint chain can be individually controlled. Since this principle comes close to how a real skeleton works, there is often not much need to correct between the poses, as the movement mimics the natural arc-shaped movement. It also makes it easy to mimic the natural delay that takes place between the joints.

IK: Disadvantages: With IK you have no control over the rotation of the joints, which are under the influence of the IK Kinematic, and you have to rely on the computer. This can lead to a lot of cleanup work.

Advantages: With IK, you can reliably determine the position of an end joint. This often makes the animation process itself faster.

FK / IK switch or blending

In order to get the best of both worlds, a so-called FK / IK switch is used for many rigs, especially characters.

Imagine three skeletons: one that controls the mesh and two that can control the first. Quasi a marionette with a marionette cross hanging on two marionette crosses. One of the two is an FK, the other an IK rig. Which one is currently being controlled by the actual skeleton is determined by a switch or slide control. For example, you could animate a movement with the FK Rig until the character should, for example, hold his hand in a certain position (support himself on the wall or hold a door handle). Then you switch off the FK rig with the switch, and the IK rig on, and then continue animating in IK.

To enable a smooth transition when changing between FK and IK, you can use a controller that can slowly cross-fade the FK / IK. In this way, the influence of the FK or IK rig on the actual skeleton can be mixed.

In most common 3D programs, you don't have to build 3 skeletons, but one that can switch between FK and IK according to this principle.

Web links

Individual evidence

  1. ↑ What are IK / FK? Retrieved July 9, 2020 .
  2. ↑ What are IK / FK? Retrieved July 9, 2020 .