Select an object in a hierarchy where you want IK to start. > Animation menu > IK Solver > Apply an IK solver. > Click the object in the hierarchy where you want the IK chain to end.
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.
Each type of IK solver has its own behavior and workflow, as well as its own specialized controls and tools that display in the Hierarchy and Motion panels. IK solvers are plug-ins, so programmers can expand the software's capabilities by customizing or writing their own IK solvers.
Autodesk VIZ ships with one IK solver.
An IK solver generally operates in this way: an inverse kinematic chain is defined on part of the hierarchy, say from the hip to the heel, or the shoulder to the wrist of a robot assembly. At the end of the IK chain is a gizmo, called the goal. The goal can be repositioned or animated over time in a variety of ways, often using linkage, parameter wiring or constraints. No matter how the goal is moved, the IK solver attempts to move the pivot of the last joint in the chain (also called the end effector) to meet the goal. The IK solver rotates the parts of the chain to stretch out and reposition the end effector to coincide with the goal.
Frequently, the end effector is constrained to the ground plane. For example, you might "pin" the toes as the heels lift in a character walk cycle. Then the movement of the root of the chain poses the legs up from the toes.
The IK solver that ships with Autodesk VIZ is:
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The HD Solver is a solver well-suited to use for animating machines, especially ones with sliding parts that require IK animation. It lets you set up joint limits and precedence. It has performance problems on long sequences, so ideally use it on short animation sequences. It is good for animating machines, especially ones with sliding parts.
Because this solver's algorithm is history dependent, it works best for short animation sequences. The later in the sequence it is solving, the longer it takes to calculate a solution. It allows you to bind the end effector to a follow object, and it uses a system of precedence and damping to define the joint parameters. It allows for sliding joint limits combined with IK animation.
Note: Autodesk VIZ also provides two other methods of inverse kinematic manipulation of hierarchies, which don't depend on a solver: Interactive IK and Applied IK.
You can use Display Links and Link Replaces Object to display the links instead of the object. These settings are found in the Link Display rollout on the Display panel. This can be useful if you have a geometrically intensive hierarchy to animate. The interactive viewport response will be much quicker when the geometry is hidden and displayed only at links.
You apply the IK solver to a hierarchical chain after it has been created by using Animation menu > IK Solvers. You must select the node where you want the IK chain to start, then choose Animation menu > IK Solvers and select the HD solver. Then click the node where you want the chain to end. The chain will be created between the pivot points of the two nodes. If you want the solver to affect the last node in a chain, add an additional node to the end, then apply the solver from the root to that last node.
You adjust IK solver settings in the Motion and Hierarchy panels:
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When the goal of an IK chain is selected, the motion panel displays the rollouts for the individual IK solver.
HD Solver—Motion panel parameters assign, remove, and edit the end effector for the currently selected joint. You can parent the end effector to another object, and return the skeleton to an initial pose. Changing IK controller parameters in the Motion panel affects the entire HD IK chain, even when you've selected only a single object.
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HD Solver—Select the end effector of an HD IK chain. In the Hierarchy panel, click IK. The controls that appear affect the HD Solver. You will also find the tools to bind to follow objects, and set precedence and joint limits, damping and spring back.