An inverse kinematics (IK) solver transforms the bones of a skeleton in order to achieve a desired pose. For instance, a LookAtIKSolver rotates a bone (e.g. a head) to look into a desired direction. A TwoJointIKSolver can be used to bend/stretch a leg so that it touches the ground, or to make an arm reach a certain target.

An IKSolver instances modifies a SkeletonPose instance. And the goal is to point to or reach the Target position. The solver works in model space - not in world space. Therefore, Target and other positions and orientations must be specified in model space.

If Solve(Single) is called and MaxAngularVelocity is PositiveInfinity (default), the SkeletonPose is changed instantly. To avoid this instantaneous change, a MaxAngularVelocity limit can be set, then the bones will rotate slowly to the target pose over several Solve(Single) calls. MaxAngularVelocity defines the maximal rotation velocity for each bone. (However, limiting MaxAngularVelocity is not ideal if the model is also animated, because the animation might reset the previous IK pose.)

The solver also has a Weight parameter. If the weight is 0, the solver is disabled. If the weight is 1, the solver tries its best to make the skeleton reach the target. A weight less than 1 can be used to blend the target skeleton pose with the unmodified skeleton pose.

Note that the IKSolver implements IAnimatableObject which means that it has properties which can be animated: The only property that can be animated is the Weight, the other properties are not animatable. Animating the weight can be useful to fade an IK pose in or out.

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