IK Rigs can be used to create animation retargeting between different Skeletal Meshes. This differs from Unreal Engine's traditional Animation Retargeting feature in that you can transfer animation accurately between skeletons that may or may not match, while providing a simplified workflow.
Retargeting animations provides a way to share animation data between multiple different skeletons, without needing to create and manage new animations outside of Unreal Engine.
This page provides an overview of the IK Rig retargeting process using IK Rigs and the IK Retargeter.
Prerequisites
- You are familiar with creating and using IK Rig Assets. Refer to the IK Rig Editor page for how to do this.
- Your project has two different Skeletal Meshes to evaluate the retargeting process.
Source and Target Setup
Retargeting with IK Rig is achieved by specifying a source and target Skeletal Mesh, which are defined by an IK Rig Asset for each of those meshes. Therefore, you should ensure you have created an IK Rig for each of your characters.
Retarget Root
Within both IK Rig Assets, you must define the root for each character, which is typically the pelvis or hip Bone. This is done in order for the character's root motion to be defined and transferred in a proportional way.
In the Hierarchy panel, right-click on the Bone and select Set Retarget Root. Do this in both IK Rig Assets.
This will denote the Retarget Root in the Hierarchy panel and also in the IK Retargeting panel.
Retarget Chains
Limbs and other appendages that you want to transfer in the retarget process must also be defined on both the source and target IK Rigs. This is a process similar to "characterizing" your rig as seen in other applications like Autodesk MotionBuilder or Maya. The main difference is that you define it by joint chains, rather than by individual Bones. This provides flexibility in retargeting characters with vastly different bone structures.
For example, if your target character contains more arm joints than your source, the retargeting behavior will still work correctly because you are defining the entire arm chain.
- Source arm chain.
- Target arm chain.
To create a chain, click Add (+) New Chain in the IK Retargeting panel, which will create an empty chain.
Chains require the following parameters to be set:
| Name | Description |
|---|---|
| Chain Name | The name of this chain. This name can be arbitrary, but it should match with the intended retarget chain in the other IK Rig. The matching chain name process is determined by a Fuzzy string match. Therefore, while your Chain Names in each IK Rig do not have to match perfectly, you should still endeavor to match them. For example, a Chain Name of |
| Start Bone | The starting Bone of the retarget chain. If you are retargeting an arm, then typically you would select the upper arm Bone here. |
| End Bone | The ending Bone of the retarget chain. If you are retargeting an arm, then typically you would select the hand Bone here. |
| IK Goal | You can optionally select an IK Goal here in order to stabilize limbs or chains which may not be retargeting with good accuracy. This means you will also need to create Solvers for those goals, which will execute their solve after the retargeting process. |
Chains must be created for all major limbs that you want to retarget, with minor limbs such as fingers being optional. In this example, the following chains will be created in both IK Rigs:
- Spine
- Left Arm
- Right Arm
- Neck
- Head
- Left Leg
- Right Leg
You can also create chains by selecting every Bone in the intended chain, right-clicking on them in the Hierarchy panel, and selecting New Retarget Chain from Selected Bones.
IK Retargeter
Once you have created both IK Rigs and created the necessary Bone chains, you can now begin the retargeting process in the IK Retargeter. The IK Retargeter is an asset and editor that references both source and target rigs, providing you with functionality to customize the retargeting results.
To create an IK Retargeter, click Add (+) in the Content Browser, then select Animation > IK Retargeter. A dialog window will appear where you must specify the source IK Rig asset.
Preview Retargeting
Once the IK Retargeter is open, the source Skeletal Mesh will be visible in the viewport and denoted in the Source IKRig Asset property. You can add the target mesh by assigning the target IK Rig in the Target IKRig Asset property. Both characters will now display, with the source character affecting the target.
You can select different animations from the Asset Browser panel to preview the retargeting effect on different assets.
The Chain Mapping panel displays the source and target chains, and their mapping relationships. Using the dropdown menus under Source Chain, you can specify different chain mappings or correct mismatches.
Pose Editing
Depending on the reference poses of the characters being retargeted, it may be necessary to edit this pose in the form of a base Retarget Pose. Typically you may need to do this if your target character's reference pose is different from the source, such as being in a T-pose, rather than an A-pose. Matching these retarget poses will increase the accuracy of the retargeting.
- Source character's reference pose is an A-Pose.
- Target character's reference pose is a T-Pose.
You can resolve reference pose discrepancies by editing or creating different Retarget Poses from these toolbar controls.
| Name | Description |
|---|---|
| Current Retarget Pose | Displays a list of the current retargeting poses used for the target character. By default this will only list Default Pose, but will populate when you create new poses by clicking New Pose.  |
| Edit Pose | Edits the Current Retarget Pose, which enables the Bones to be manipulated in the viewport. Enable this mode to modify your target character's pose to closely match the pose of the source. 
Only the Bones defined in the retargeting chains will be visible and selectable. |
| New Pose | Creates a new retarget pose which will be selectable from the Current Retarget Pose drop-down menu. Clicking this will cause a dialog window to appear where you name the new pose. 
Having multiple retarget poses is useful if you intend to use different target characters by changing the Target IKRig Asset property. Additionally, certain animations may retarget more accurately than others depending on their retarget pose. Because of this, it may be necessary to create unique retarget poses to account for specific animations. |
| Delete Pose | Deletes the Current Retarget Pose. |
| Reset Pose | Clicking this resets any changes you have made to the pose and returns the character to its base reference pose. |
Retargeting with IK Goals
On characters with larger size and bone differences, simple bone chain retargeting may produce unwanted motion on bones. For example, these target legs can exhibit extra movement, causing undesired clipping.
IK Goals and Solvers from the target IK Rig can be used to stabilize these limbs and reduce this problem. This is done by creating similar IK Goals and Solvers for these problem joints and assigning the goals to their respective chain in the IK Retargeting panel.
For example, a Full Body IK Solver is created on the target IK Rig. This Solver links to IK Goals on the feet, which are used to provide IK for both legs.
These goals are then assigned to their respective retarget chain under the IK Goal column. This will cause the Solver to execute after the retargeting process, using these goals.
Now when previewing the retarget results in the IK Retargeter, the stabilization has improved. You can also preview the differences of this IK step by enabling or disabling Retarget IK in the Details panel.
Export Retargeting Results
Once you are satisfied with your retargeting results on the target character, you can export the animations to compatible Animation Sequences for that character's Skeleton. To do this, select the animations you want to export in the Asset Browser panel, then click Export Selected Animations.
Specify your export path in the dialog window and click OK to export the animations. The retargeted animations will now be available in the output folder with the postfix _Retargeted.
IK Retargeting Editor Settings
The IK Retargeting Editor Details panel contains the following properties:
| Name | Description |
|---|---|
| Source IKRig Asset | The source IK Rig to copy animation from. This value cannot be changed. |
| Target IKRig Asset | The target IK Rig to copy animation to. |
| Target Preview Mesh | You can specify a Skeletal Mesh here to preview the retargeting behavior on alternate meshes. |
| Retarget Root | Enables the translation movement of the root bone defined in the target IK Rig. Usually you do not disable this unless you are debugging root retargeting behavior. |
| Retarget FK | Enables the retargeting of all bone chains. Usually you do not disable this unless you are debugging bone chain retargeting behavior. |
| Retarget IK | Enables the IK retargeting pass if IK Goals are being used in the retarget chains. Usually you do not disable this unless you are debugging IK retargeting behavior. |
| Target Actor Offset | The lateral distance of the target character from the source character. For larger characters you may want to increase this.  |
| Target Actor Scale | A preview scale modifier which you can use to increase or decrease the target character's scale. This is useful if your target is very small or large relative to the source, where changing this value can align their sizes more closely for convenience.  |
| Bone Draw Size | Controls the visual size of the target character's bones when Edit Pose is enabled. |
Batch Retargeting
Most animation assets, such as Animation Sequences, Blend Spaces, or Animation Blueprints, can be retargeted quickly from the Content Browser using batch retargeting.
In the Content Browser, select the animation assets you want to retarget, then right-click them and select Retarget Animation Assets > Duplicate and Retarget Animation Assets.
A batch retargeter window will appear where you can customize the retargeting behavior and results before starting the batch process.
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Source and Target Skeletal Mesh preview. These panels will not be active until you specify an IK Retargeter. Once active, you can change the Target Skeletal Mesh if you want to change the character being retargeted to.
Changing the Target to a completely different skeleton from the one defined in the IK Retargeter may produce incorrect results.
- The IK Retargeter Asset to inform the Source and Target Skeletal Meshes and retargeting batch process.
- Text fields where you can specify new Prefix and Suffix modifiers to the output asset names, as well as search and replacing text. Old Name and New Name will display a preview of your text entries.
- The output folder where the retargeted assets will be saved to.
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Remap References Assets will enable any assets that have references to other retargeted assets (such as a Blend Space referencing an Animation Sequence) to remap to the newly created retargeted results.
- Disabling this will maintain the original references and may cause issues with assets referencing animations that are no longer compatible with the new Skeleton.
- Enabling this may cause reference issues if you are not retargeting all assets being referenced.
Click Retarget to start the batch retargeting process.
