Joints and bones

Joints are the building blocks of skeletons and their points of articulation. Also, joints have no shape and therefore can not be rendered. Each joint can have one or more bone attached to it, and more than one child joint. Joints let you transform a skeleton when posing and animating a bound model.

Bones do not have nodes, and they do not have a physical or calculable presence in your scene. Bones are only visual cues that illustrate the relationships between joints.

Setting Limits on Joint Rotation:

It is only useful to set joint limits when you are planning to use FK to pose your joints.

Joint limit information specifies the minimum and maximum translation, rotation, and scaling values for a joint. You can set a joint's limits if you want the joint to behave in a specific manner. For example, if you want to create an elbow joint for a human skeleton, you need to set its rotation limits so that it cannot rotate in X and Z, but can rotate only a specified amount in Y. Alternately, you can turn off transformation channels or use Set Driven keys to restrict the transformations of an object. See Set Driven Keys in the Animation guide.

Damping and Stiffness

It is only useful when used with IK handles

You can set the damping and stiffness for joints that are influenced by IK. The damping and stiffness values let you specify how joints behave when posed by an IK handle.

Joint damping applies resistance to a joint as it approaches its joint limits. Instead of the joint abruptly stopping when it reaches its limits, you can use damping to slow it down smoothly. Depending on the strength and range you set, a joint with dampening will not reach its limit boundary unless forced. For most living creatures, when a joint rotates as far as it can, it tends to slow down or dampen before reaching its limit. For example, an elbow does not snap straight, but gradually slows down as the lower arm aligns with the upper arm. In animation terminology, this type of effect is called an ease-in.

Joint stiffness specifies a joint's resistance to rotation during inverse kinematics (IK) posing. Set the stiffness value for a joint only if you want certain joints in a joint chain controlled by an IK handle to rotate less freely than others. For example, you can set the stiffness of joints in the mid-back of a human character so that they rotate less freely than those in the lower back. The higher the stiffness value, the less the joint will bend.

You have to set the stiffness value for all the joints in a joint chain in order for stiffness to work properly because stiffness calculations are relative between joints.

EULER rotation (pronounced "oiler")

In animation, a method of rotation that uses three angle values (X, Y and Z) plus a rotation order to specify three-dimensional orientation.

Euler rotation has several drawbacks, including gimbal lock and choppy interpolation between keyframes.

gimbal lock!!!!!

...is an annoying situation where one rotational axis is placed over another, causing the two axes to lead to the same result when animated.

To avoid gimbal lock, either quaternion rotation must be used instead of Euler rotation, or a node must be provided for each rotational axis.

quaternion rotation

A method of rotation that uses a unit quaternion (a quaternion of length one) to represent an orientation in three dimensions. A quaternion is a four dimensional extension to complex numbers. Maya uses XYZ curves and computes a fourth curve (W) to determine the quaternion rotation.

See also rotate order.