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With the linked system created, position the child object's pivot point at the center of the joint between it and its parent. For example, the joint between an upper and lower arm would be at the elbow, so this is where the pivot point for the lower arm should be located.

Cross-Reference

Chapter 9, “Grouping, Linking, and Parenting Objects,” covers creating linked systems, and Chapter 7, “Transforming Objects, Pivoting, Aligning, and Snapping,” covers moving pivot points. •

While creating the linked system, you can select the IK Solver in the Create panel, and later after correctly positioning your pivot points, you can access the IK parameters in the Hierarchy panel by clicking the IK button. Several rollouts open that let you control the IK system, including the Object Parameters rollout shown in Figure 38.6.

FIGURE 38.6

The IK rollouts let you control the binding of an IK system.

Selecting a terminator

Because child objects in an IK system can cause their parents to move, moving a child could cause unwanted movements all the way up the system to the root object. For example, pulling on the little finger of a human model could actually move the head. To prevent this, you can select an object in the system to be a terminator.

A terminator is the last object in the IK system that is affected by the child's movement. Making the upper arm a terminator prevents the finger's movement from affecting any objects above the arm.

To set a terminator, select an object and enable the Terminator option in the Object Parameters rollout.

For Interactive IK mode, you also can enable the Auto Termination option included in the Auto Termination rollout. The Number (#) of Links Up value sets the terminator a specified number of links above the current selection.

Defining joint constraints

The next step is to define the joint constraints, which you specify in the Sliding Joints and Rotational Joints rollouts. The availability of these limits depends on the selected IK Solver and its options. By default, each joint has six degrees of freedom, meaning that the two objects that make up the joint can each move or rotate along the X-, Y-, or Z-axis. The axis settings for all other sliding and rotational joints are identical. Defining joint constraints enables you to constrain these motions to prevent unnatural motions, such as an elbow bending backward. To constrain an axis, select the object that includes the pivot point for the joint, locate in the appropriate rollout the section for the axis that you want to restrict, and deselect the Active option. If an axis's Active option is deselected, the axis is constrained. You also can limit the motion of joints by selecting the Limited option.

When the Limited option is selected, the object can move only within the bounds set by the From and To values. The Ease option causes the motion of the object to slow as it approaches either limit. The Spring Back option lets you set a rest position for the object; the object returns to this position when pulled away. The Spring Tension sets the amount of force that the object uses to resist being moved from its rest position. The Damping value sets the friction in the joint, which is the value with which the object resists any motion.

Note

As you enter values in the From and To fields, the object moves to that value to show visually the location specified. You also can press and hold the left mouse button on the From and To values to cause the object to move temporally to its limits. These settings are based on the current Reference Coordinate system. •

Copying, pasting, and mirroring joints

Defining joint constraints can be lots of work—work that you wouldn't want to have to duplicate if you didn't have to. The Copy and Paste buttons in the Object Parameters rollout enable you to copy Sliding Joints or Rotational Joints constraints from one IK joint to another.

To use these buttons, select an IK system and click the Copy button;