Using IK Pivots

An IK pivot is an imaginary point in three-dimensional space around which an Effector rotates. You can use these IK pivots to increase the realism of animations that involve rotating a joint around an external point in space. For example:

IK pivots are often used during walk cycles. When the heel hits the floor, an IK pivot located at the bottom of the heel causes the ankle Effector to rotate forward, simulating the way a human heel rolls forward after the back of the foot hits the ground. When the foot lifts from the ground, a different IK pivot for the ankle Effector, this time located at the toe base, can cause the ankle to rotate up and forward.
A character twisting its foot to crush out a cigarette could have an IK pivot for the ankle placed at the location of the cigarette. Rotating the character’s ankle causes the heel to rotate around the position of the pivot.

The effects of an IK pivot

When you set an IK pivot for an Effector, you provide its 3D translation in global space at the same time as you provide global translation, orientation and scaling values for the Effector itself. The IK pivot modifies the way HumanIK takes into account the translation and orientation values for the Effector, effectively creating a new IK goal point.

This final IK goal point is calculated by the following process:

The Effector is located at a translation you specify for it.
The IK pivot point is located at a translation you specify for it.
The pivot point is then rotated to the orientation you specified for the Effector.
The final goal point is located along that orientation at a distance from the pivot point equal to the difference between the translation of the pivot point and the original translation value you specified for the Effector.

Looked at in another way, the translation values you specify for the Effector and the pivot point define two points at the ends of an imaginary line with a fixed length. When the pivot point is rotated to the orientation you specify for the Effector, this imaginary line is also rotated, effectively moving the Effector to another location in 3D space.

Any IK constraints (Reach, Pull and Resist) set for an Effector apply normally when using an IK pivot, controlling how closely the character’s skeleton comes to this IK goal point in the final solution, and how much the rest of the character’s skeleton may be deformed to reach that point.

Setting and retrieving IK pivot positions

To set an IK pivot for an Effector, you can call either of the HIKSetEffectorPivotStatefv() or HIKSetEffectorPivotStateTQSfv() functions. Both of these functions expect you to provide the translation of the IK pivot in an array of four floating-point values.

For example:

const float pivotPointT[] = {124.3f, 253.8f, 604.4f, 0.0f};
HIKSetEffectorPivotStateTQSfv(myEffectorSet, LeftWristEffectorId, globalT,
                                                                  globalQ,
                                                                  globalS,
                                                                  pivotPointT);

You can retrieve the position of the IK pivot using the corresponding HIKGetEffectorPivotStatefv() and HIKGetEffectorPivotStateTQSfv() functions.

For details, see the function descriptions in the API Reference.