Common Hierarchy: For a rig to function properly, bones and joints must follow a logical hierarchy. When setting up a character`s skeleton, the first joint that is placed is called the root joint. Each subsequent joint is connected to the root either directly or indirectly via another joint.
Forward Kinematics: Forward Kinematics is one of two basic methods to calculate the joint movement of a fully rigged character. When using forward kinematics bracings, a given connection can only affect parts of the skeleton below in the connection hierarchy.
For example, the rotation of a character`s shoulder affects the position of the elbow, wrist, and hand. In Forward Kinematics animation, the artist typically has to adjust the rotation and position of each joint individually to achieve a desired position. To do this, the animator must work through the joint hierarchy sequentially. The end position of an end joint is calculated as a function of the joint angles of each joint above it in the hierarchy.
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. In an Inverse Kinematic rig, the final joint is placed directly by the animator, while the upper joints in the hierarchy are automatically interpolated by the software.
Inverse Kinematics is best used when the animation requires an end connection that needs to be placed very accurately. 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 ladder rungs instead of the animator having to adjust his position in an articulated manner, an Inverse Kinematics Rigg would make the animation process much more efficient. A disadvantage is that the Inverse Kinematics uses software interpolation. This means that a lot of cleanup work has to be done to complete the capture.
Degrees of freedom and limitations.
When rigging, keep in mind that in the real world, joints like elbows are limited to a single degree of freedom, i. e. they can only bend along one axis. Likewise, a human neck cannot be rotated 360 degrees.
In order to avoid unrealistic animations, it is adviseable to set up common constraints when building the rig.
Squash and Stretch.
Other considerations to be considered are whether the rig supports squash and stretching or whether the character is limited to realistic movements.