Inverse kinematics builds on the concepts of hierarchical linking. To understand how IK works, you must first understand the principles of hierarchical linking and forward kinematics. See Animating with Forward Kinematics and Inverse Kinematics (IK).
IK solvers are specialized controllers that apply IK solutions procedurally across a range of frames.
Four different IK solvers ship with 3ds Max:
An IK solver places keyframes only on the IK goal or end effector, as opposed to forward kinematics (or the non-solver IK methods), which place rotation keyframes on the bones or hierarchical objects themselves.
In addition to the IK solvers, 3ds Max provides two non–solver methods of inverse kinematic animation: Interactive IK and Applied IK. These IK methods do not apply an IK solver.
Interactive IK gives you the ability to use IK manipulation on hierarchies without applying an IK solver. You animate your IK structure by activating Interactive IK (found in the IK panel of the Hierarchy panel and on the Hierarchy toolbar) and manually animating the position of end effectors. IK solutions are calculated only for the keyframes you set. All other motion is interpolated as set by the object's controllers. Moving the end of the chain simply adds rotational keys to the objects in the chain. The objects can have joint limits assigned, for additional control. The chain can also be terminated using the tools in the Auto Termination rollout.
With Applied IK, you animate follow objects and 3ds Max calculates the solution on every frame of a specified range. The IK solution is applied as standard transform animation keys. Applied IK works with any linked hierarchy of objects except for bones that use the HI Solver or the IK Limb Solver. It can combine forward kinematics with inverse kinematics on the same objects. You can apply it automatically to a range of frames, or interactively to single frames.
Applied IK is fast and accurate, but it creates keys for every object in the kinematic chain, on every frame. The large number of keys can make it difficult to adjust the animation later. Use Applied IK repeatedly to adjust the animation.
An IK solver creates an inverse kinematic solution to rotate and position links in a chain. It applies an IK Controller to govern the transforms of the children in a linkage. You can apply an IK solver to any hierarchy of objects. You apply an IK solver to a hierarchy or part of a hierarchy using commands on the Animation menu. Select an object in the hierarchy, choose an IK solver, and then click another object in the hierarchy to define the end of the IK chain.
Interactive IK and Applied IK are alternative methods to the IK Solver methods.
