Full-Body IK

Use Control Rig's Full Body Inverse Kinematic (FBIK) feature to construct rigs in Control Rig with a high degree of control and flexibility. The overall solver method is built on a Position Based IK framework, which enables faster rig performance, per-bone settings, preferred angles, squash and stretch, and more. FBIK is designed to act as a procedural adjustment tool within Control Rig, such as for ground alignment, or arm reaching behavior.

This document provides an overview of how to create the FBIK node, and its various features.

Prerequisites

• FBIK is a node within the Control Rig Graph, therefore you should have an understanding of the Control Rig Editor.

• You must enable the FullBodyIK plugin. To do this, navigate in the Unreal Editor menu bar to Edit > Plugins and locate FullBodyIK. Ensure the plugin is enabled and restart the editor.

  
• You must have created a Control Rig asset for a character and know how to create controls. See the Control Rig Quick Start page for information on how to do this.

Creation and Setup

FBIK is achieved within Control Rig by creating the FBIK node, then assigning bones and effectors to the appropriate pins.

To start, right-click in the Control Rig graph, and in the context menu select Hierarchy > Full Body IK. This will create the Full Body IK node. Connect the node to the Forwards Solve node.

Rig Graph Setup

Next, set the Root bone property on the Full Body IK node. In typical full-body setups, this will likely be the hips or pelvis bone.

Create and add your chosen controls for the FBIK system. Typically these controls correspond to the limbs you want to affect, and are based on the primary ending bone of the chain. In this example, controls are created for the head, hands, and feet.

Effector Setup

You will need to add an effector pin for each control added to the graph. To do this, click the Add (+) button next to the Effectors property. Effectors will need to be created for each control.

Expanding one of the Effectors entries will reveal its properties. Within each Effector you will set the corresponding Bone value for the control, then connect the Transform pins from the corresponding Get Transform node.

Connect each effector to their corresponding Get Transform nodes in the same way until all effectors and controls are connected.

Setup Result

You should now have a basic FBIK setup for your character. You can manipulate the controls within the Control Rig viewport to see the results.

Bone Settings

Depending on your setup, the base FBIK behavior may not behave as intended. This might be because certain bone settings are not enabled. Bone Settings are properties within the Full Body IK node, and are used to control the behavior of each bone that FBIK is affecting.

Bone Settings are added to the Full Body IK node by clicking the Add (+) button next to Bone Settings, then specifying the bone you wish to affect in the Bone property. Each bone setting only affects a single bone, so you may want to add multiple bone settings depending the specific problem you are experiencing with your rig.

In a humanoid example, the following issues are visible:

• The hips are rotating and translating too aggressively.

  

• The legs are not bending enough, having a stiff appearance.

  

• The ankle is rotating too far upward, causing an unnatural pose.

  

Bone Settings can be used to resolve issues like these, in order to support your preferred IK setup.

Rotation and Position Stiffness

Rotation and Position Stiffness properties are used to control how much a bone in the IK Chain can be affected by controls and effectors. Use these properties to change the degree to which the pelvis bone will be affected by the movement of the controls. A value of 0 results in completely free movement, whereas a value of 1 completely locks the bone from movement.

In the humanoid example, the pelvis bone is specified as the bone setting, and a value of 0.8 is set for both Rotation and Position Stiffness. In this case, some movement is still preferable.

Preferred Angles

Preferred Angles can be used to prioritize joints rotating along a specific axis to reach the effector. In the humanoid example, they can be used to resolve stiffness issues in the legs and knees.

Near the bottom of the Bone Settings are properties for Preferred Angles. You must also ensure Use Preferred Angles is enabled in order to use these Preferred Angles.

The Preferred Angle properties you specify depend on the type of character, and its joint structure. For this example, the mannequin's knee should bend along the Z axis in a positive direction, therefore a value of 45 should be set on the Z property.

Limits

Limits can be used to limit the range of, or completely lock, the rotation of bone axes along the IK chain. These settings are located between the Stiffness and Preferred Angles properties.

Each axis has settings that allow for the following limits:

• Free, which allows for the bone to move freely.
• Limited, which only allows movement within a specified range. If this is chosen, then the Min/Max properties are used to define the range of movement.
• Locked, which disables movement along that axis.

In the humanoid example, these limits can be used to correct the unnatural ankle rotation issue. A value of -70 is set for Min Z and 70 for Max Z, and the Z axis is set to Limited.

Excluding Bones

Depending on the needs of your rig, there may be cases where you want to exclude bones from the FBIK solve. This can be useful when correcting unnatural-looking poses, or when simplifying FBIK behavior.

Excluding bones is recommended rather than exclusively using Bone Settings to set bones to be fully stiff or locked.

To exclude a bone, click the Add (+) button next to Excluded Bones, and select a bone from the dropdown menu to exclude.

Node Reference

The following is a list of all the pins and settings on the Full Body IK node.