About Inverse Kinematics

The Inverse Kinematics (IK) tool allows you to pull on a character's extremities, such as the hands and feet, and have the rest of the body follow. It can be used on any piece connected in a hierarchy. However, you don't have to use IK every time you have a hierarchy rig. This tool is useful when you want to bend a character's knees, make it sit and move the rest of the body, and so on. It will also assist you with posing difficult moves.

In most 3D animation software, Inverse Kinematics is implemented as a skeleton system. Harmony uses the rigging hierarchy already created in the Timeline view, so there is no need to place bones in the character. Inverse Kinematics in Harmony uses the connection between each of the parts' pivots.

Inverse Kinematics will not work on basic rigs without hierarchy because there is no hierarchy chain present. IK only works on a hierarchy where the pivots are properly set.

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'll 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 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.

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 from each other.
  • The Inverse Kinematics tool CANNOT be used on this type of rigging.
  • The Hierarchy Rig Technique
  • This is a complex rig technique.
  • All the parts are connected to each 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 in 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.

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.

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 create the character's posing faster. Posing is a crucial part in the animation process. The Inverse Kinematics 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.

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

  • Inverse Kinematics on Mixed Rigs

The Inverse Kinematics tool can be used on a full puppet's body even if it's 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.

  • Inverse Kinematics Constraints

It's 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're not constricting one part to another object, you can add or remove them without affecting any of the animation you have already completed.

  • Useful Shortcuts
  • While using the Inverse Kinematics tool, you can Ctrl + click (Windows/Linux) or ⌘ + click (macOS) to select any part.
  • Hold Alt to rotate the selected part without affecting the IK chain.
  • Press Shift and click in a part's pivot to add or remove an IK Nail.
  • Press Shift and click on a bone to add or remove and Hold Orientation constraint.