Production > Animation > Cut-out Animation > Animating using Inverse Kinematics > When to use the Inverse Kinematics Tool

When to Use the Inverse Kinematics (IK) Tool

Harmony provides different tools for animating cut-out characters and trajectories. The two main tools used to animate characters are the Transform and Inverse Kinematics tools.

Here, we will compare the usage of these tools and explain the Inverse Kinematics philosophy.

Transform Tool

Also known as forward kinematics, the Transform tool is the main tool to use for cut-out animation. This tool rotates, scales, moves and skews selected elements as one global element.

Forward kinematics means that the limb is animated from the parent down to the last child as a single piece. In other words, if the shoulder is animated, then the arm, forearm and hand follow as a complete arm drawing.

Inverse Kinematics Tool (IK)

This assistance tool helps you achieve complex motions, such as sitting down or knee bending. The Inverse Kinematics tool moves and rotates every selected element as a chain.

Inverse kinematics means that the element is animated from the child up to the parent. In other words, if the hand is animated, then the forearm, arm and shoulder follow the hand in a fairly natural way in terms of the movement, rotation and bending.

IK Tool and Rigging

The IK tool cannot be used on every type of cut-out character. It depends on the way the pieces and parts are attached to one another. In other words, it depends on the rigging type.

There are several ways to rig a puppet. These are the three main rigging techniques:

The Basic Rig Technique

This is the simplest rig technique.
There are no connections between the parts.
They are free to move, rotate and scale independently one from the other.
The Inverse Kinematics tool CANNOT be used on this type of rigging.

The Hierarchy Rig Technique

This is the complex rig technique.
All the parts are connected one to the other.
As they move, rotate and scale, they will influence all the other parts.
The Inverse Kinematics tool can be used on this type of rigging.

The Mixed Rig Technique

This is the best of the basic and hierarchy rig.
Some parts are independent such as the torso.
Some parts are set up into a hierarchy such as the arms and legs.
Independent parts are rigged in hierarchy using parent pegs.
The Inverse Kinematics tool can be used on this type of rigging.

Animating with the Tools

Puppets are animated by using a combination of the Transform and Inverse Kinematics tools.

You will mainly use the Transform tool (forward kinematics) as it has all the main motions: rotate, scale, skew, move and select. You can also control the exact position of the parts while using the Transform tool. As well, the Transform tool creates a temporary global pivot on the selection that can be moved around for the animation purpose.

To complete complex motions, such as bending the knees while keeping the feet on the ground, you can use the Inverse Kinematics tool as an assistance tool. By adding IK constraints (nails), it is possible to lock the feet or any other part to a particular spot and move the rest of the body, which will react to the constraints.

Inverse Kinematics Philosophy

It is important to keep in mind that the Inverse Kinematics tool is an assistance tool. You will not be using it to animate all the time.

Also, the Inverse Kinematics tool can be used to do the character's posing faster. Posing is a crucial part in the animation process. The IK tool can be used to pose the character more naturally, depending on your own preferences.

An IK nail can be added anywhere on the character to animate only a small part of the hierarchy chain, such as an arm or a portion of the arm.

Tips and Tricks for Inverse Kinematics

Here are some tips and tricks to help you to use the Inverse Kinematics tool.

IK on Mixed Rigs

The Inverse Kinematics tool can be used on a full puppet's body even if it is connected as a mixed rig, meaning some parts are rigged in a hierarchy and others not. For example, you can add IK nails on the puppet's feet, select the full body master peg and move the character as if it was connected in a full hierarchy rig. This technique works on most mix rigs.

IK Constraints

It is important to remember that you can add and remove IK constraints (nails, hold orientation, etc.) at any time during the animation. They hold a certain part on the spot while you animate the rest of the body. Because they are not constricting one part to another object, you can add or remove them without affecting any of the animation you have already done.

Useful Shortcuts
While using the IK tool, you can Ctrl + click (Windows/Linux) or ⌘ + click (Mac OS X) to select any part.
Hold down Altto rotate the selected part without affecting the IK chain.
Press Shiftand click in a part's pivot to add or remove an IK Nail.
Press Shiftand click on a bone to add or remove and Hold Orientation constraint.