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.
IK solvers are specialized controllers that apply IK solutions procedurally across a range of frames. The IK solver that ships with gmax is the IK Limb Solver.
In general, all IK solvers:
Work on any hierarchy or bone structure
Calculate IK solutions for all frames in real-time as you make changes
Allow you to create multiple or overlapping IK chains within a single hierarchy
Graphically display active joint axes and joint limits
Use a goal or end effector to animate the end of a chain
An IK Limb solver places keyframes only on the IK goal, as opposed to forward kinematics (or the non-solver IK methods), which place rotation keyframes on the bones themselves.
In addition to the IK Limb solver, gmax provides two other 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 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, just like bones. The chain can also be terminated using the tools in the Auto Termination rollout.
With Applied IK, you animate follow objects and the program 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 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 makes it difficult to adjust the animation later.