IK Rig Retargeting

You can use IK Rigs to create animation retargeting between different skeletal meshes. This differs from Unreal Engine's traditional Animation Retargeting feature in that you can transfer animation between skeletons with varying numbers of bones, bone names, and orientations, while optionally maintaining precise hand or foot contact points using IK.

Retargeting animations provide a way to share animation data between multiple different skeletons, without needing to create and manage new animations outside of UE.

This page provides an overview of the IK Retargeter.

Prerequisites

• Your project has two different skeletal meshes to evaluate the retargeting process.

• You already created IK Rig assets and defined retargeting chains within them. Refer to the Retargeting Bipeds with IK Rig page for how to do this.

Creation and Overview

To create an IK retargeter, click Add (+) in the Content Browser, then select Animation > IK Rig > IK Retargeter. A dialog window appears where you must select the IK rig that you want to retarget animation from. Once selected, name and open the IK Retargeter asset.

The IK Retargeter contains the following tools and options:

1. Toolbar: Here you can save changes, browse associated assets, and more.

   1. Run Retarget and Show Retarget Pose toggle switch; toggle between retarget pose options.

   2. Asset Settings; click to display selected operations and the total execution cost directly in the editor.

2. Op Stack and Hierarchy panel:
   

   • When set to Op Stack a default operation stack appears in the panel. Add new operations by selecting Add New Op. Highlighting an operation in the stack opens the operation settings in the Details panel. 

   • When set to Hierarchy a list of character bones appear. Select a character and filter bones and their assigned chains from the list of bones.

3. Asset Browser: Here you can select the animations from the source to preview in the editor and export the selected animations.

4. Retarget Output Log: Displays debug information, warnings, and errors indicating the current status of your IK Retargeter.

5. Details and Preview Settings panel:

   • When set to Details, a list of settings appear in the panel. 

   • When set to Preview Settings, different previews of the retargeted animations appear in the editor and the viewport.

6. Viewport: Here you can preview and debug the source and target characters being retargeted.

Retarget Chains

Limbs and other appendages you want to transfer in the retarget process must 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 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 still works correctly because you are defining the entire arm chain, no matter the number of bones.

1. Source arm chain.

2. Target arm chain.

Chain Creation

Retargeting chains are automatically created from the IK Rig Editor  toolbar by clicking Auto Create Retarget Chains. To create a chain, open the IK Rig asset for both characters, navigate to the IK Retargeting panel, then do the following:

1. Click Add New Chain (+).

   

2. In the Add New Retarget Chain dialog window, ensure Chain Name is set correctly, then click Add Chain. In most cases, the IK rig automatically assigns this value from its list of common chain names.

   

   Typically you do not need to add IK goals unless your retargeting requires additional IK adjustments, such as Speed Planting, Stride Warping, or Blend to Source. Add IK goals to your new retarget chain by selecting Add Chain and Goal. Afterward, you’ll be prompted to create an IK solver and determine how the solver behaves.

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…

Chain Properties and Names

Chains require the following parameters to be set:

The Chain Name property automatically fills in itself depending on the name of the selected bones used when creating the chain. The system looks for common bone names used, and then picks a name that best matches the selected bones. This mapping list is as follows:

For symmetrical chains, such as arms and legs, the auto-naming feature is determined by comparing the average location of the bones in the chain, and then assigning a prefix of Left or Right. If the selected bones are mostly located on the negative X side, they are "Left", positive X is "Right", and if they are relatively centered on the X axis, then they are considered "Center" and no prefix is applied.

Pelvis

In addition to defining chains, you must also define the pelvis bone. This is done so the character's root motion can be defined and transferred in a proportional way.

Like chains, the pelvis is defined in the IK Rig Editor. To do this, open the IK Rig asset for both characters, navigate to the Hierarchy panel, right-click the bone and select Set Pelvis.

Once done, the pelvis indicates with the selected bone in the Hierarchy and the IK Retargeting panels.

Retargeting Stack Framework

The retarget stack framework is a collection of modular operations which are evaluated on a bone chain selected from the bone hierarchy. The retargeting operations are a more performant way to edit skeletal meshes and retarget animations. 

You add custom operations for the retargeter by selecting and arranging only the operations necessary for a select bone chain to retarget a pose or animation. Each operation can be reordered within the stack.

Some operations have a sub-stack of operations for custom logic and evaluations that need to be applied in a certain order or method to expand an existing operation. 

All retargeting operations support setting custom LOD thresholds. Custom LOD thresholds are set per-operation, this provides a way to skip costly steps automatically at higher LODs. Some operations have debugging capabilities in their settings.

Visit the Retarget Operation Stack page to learn more about the retargeting operation stack and the individual operations.

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  Retargeting Operation Stack

  Use operations in the Retarget Stack to get faithfully retargeted poses and animations.

Retarget Pose

Depending on the reference poses of the characters being retargeted, you might need to edit this pose in the form of a base Retarget Pose. Typically you 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 increases the accuracy of the retargeting.

1. Source character's reference pose is an A-Pose.

2. Target character's reference pose is a T-Pose.

You can resolve reference pose discrepancies by toggling to Editing Retarget Pose in the toolbar, then edit, import, or export different poses using the Retarget Pose tools in the Hierarchy panel.

Create and Edit Pose

Although you can edit the Default Pose for any of your retargeted characters, we recommend creating new retarget poses if you need to make adjustments. To do this, click Create (+) > Create, name the new retarget pose, then click Ok.

Next, ensure Current Retarget Pose is set to your new pose, then enable Edit Mode. You can now select and modify bones in the viewport and match the poses. When finished, disable Edit Mode.

Importing and Exporting Poses

Rather than manually creating new retarget poses, you can also import them from Animation Sequences or Pose Assets.

To import from an Animation Sequence, click Create (+) > Import from Animation Sequence. In the dialog window, select the sequence to import from and set the following parameters:

• Sequence Frame defines a specific frame of the animation sequence to base the imported pose on. By default this will be set to 0, which will use the first frame of the animation.

• Pose Name defines the name of the Retarget Pose upon importing.

To import from a Pose Asset, click Create (+) > Import from Pose Asset. In the dialog window, select the pose to import from, click the dropdown menu to set the specific pose name to use, then select Import Retarget Pose.

You can also export retarget poses as a Pose asset to share with other IK Retargeting assets in your project. To do this, click Create (+) > Export Pose Asset, give your asset a name, then click Save.

Miscellaneous Workflows

The Retarget Pose area also contains the following miscellaneous tools to assist with creating and editing your Retarget Pose:

Name	Description
Current Retarget Pose	Displays the current Retarget Pose used by either the Source or Target character. Initially this will only list Default Pose, and populates when you create new poses. You can click the dropdown menu to select different poses.
Running Retargeter / Edit Retarget Pose	When enabled, this engages the retargeter so you can preview the results of the current retarget pose using animation sequences. Disabling this switches back to Edit Retarget Pose mode, where you can continue to make further refinements if necessary. The viewport highlights in a blue outline to indicate that the Edit Retarget Pose mode is enabled.
Retarget Pose Blend	When making several edits to a retarget pose, it may be helpful to preview what changes have occurred on which bones. To help this, you can edit the Retarget Pose Blend slider. Setting it to 0 changes the pose back to the Skeletal Meshes default reference pose. Setting it to 1 changes the pose to the current Retarget Pose.
Reset	You can use the Reset menu to reset the retarget pose back to the skeletal meshes default reference pose. You can select the following options: Reset Selected Bones resets only the bones you have selected in the viewport or Hierarchy. Reset Selected and Children Bones resets only the bones you have selected and their children in the viewport or Hierarchy. Reset All resets all bones.
Auto Align	Automatically aligns bones on the source skeleton to target, or aligns the bones of the target skeleton to the source. You can select the following options:  Align All Bones resets the retarget pose and then auto aligns all retargeted bones using the specified method. Align Selected aligns selected bones using the specified method. Align Selected and Children auto aligns select bones and their children (recursive) using the specified method. Alignment Method aligns bones based on the selected method: Direction aligns the direction of the bone to match that of the equivalent bone in the other skeleton. Uses the chain hierarchy to define a direction vector. Local Rotation Axes aligns the local axes of the bone to match those of the equivalent bone in the other skeleton. May produce nonsensical results on skeletons with different rotation directions. Global Rotation Axes aligns the global axes of the bone to match those of the equivalent bone in the other skeleton. May produce nonsensical results on skeletons with different rotation directions. Mesh generates a direction vector for the bone based on the principal axis of the vertices weighted to the bone. Snap Character to Ground translates the whole skeleton vertically to restore original height from the ground. Uses the selected bone as the reference point for a grounded limb. Otherwise searches the bone for the lowest retargeted bone.
Duplicate	You can duplicate the current retarget pose by clicking Create (+) > Duplicate Current, then naming the new pose.
Rename	Rename the current retarget pose by clicking Rename, entering a new name, then clicking Ok.

Hierarchy Display

The Hierarchy panel displays the skeletal hierarchy of either the source or target character, depending on which is enabled. Bone names indicate if they are used in Retarget Chains by highlighting in addition to the retarget chain column displaying the name of the chain being used.

You can search to locate and filter specific bone names or retarget chains.

You can also click the filter dropdown menu to set the following filters:

• Hide Bones Not in a Chain: Hides bones not used within a chain, regardless if the IK retargeter is using the chain.

• Hide Bones Not Retargeted: Hides all bones not used by the IK Retargeter.

• Hide Retargeted Bones: Hides bones used by the IK retargeter

Preview Animation and Export

The Asset Browser panel is used to preview and export the retargeting effect on different assets. Double-click an asset to play it.

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.

In the export dialog window, choose the export folder, then optionally specify the following rename properties:

• Add Prefix, which adds text before the new asset name.

• Add Suffix, which adds text after the new asset name.

• Search for and Replace with, which searches for existing names in the file name and replaces it with the name specified. Search and replace is not case sensitive.

Click Export to save your retargeted Animation Sequences.

Retargeter Properties and Settings

Asset Settings

The runtime for each operation is visible in the Details panel when the Asset Settings button is pressed and Profile Ops is enabled from the Details panel.

Name	Description
Source IKRig Asset	The source IK rig to copy animation from.
Source Preview Mesh	The source skeletal mesh. You can change this, and the IK retargeting system will use name matching to attempt to make the IK rig compatible with a different mesh and skeleton.
Target IKRig Asset	The target IK rig to copy animation to.
Target Preview Mesh	The target skeletal mesh. You can change this, and the IK retargeting system will use name matching to attempt to adapt the IK rig to the provided skeleton and proportions. Incompatibilities are printed as warnings or errors in the retarget output log.
Target Mesh Offset	A positional offset that can be applied to the target skeletal mesh to offset it from the source. This is useful to help with previewing both characters, or if the target character is very large, requiring a larger offset.
Target Mesh 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.
Source Mesh Offset	Similar to Target Mesh Offset, this is a positional offset you can apply to the source Skeletal Mesh to offset it from the target.
Show Source Mesh	Show or hide the source skeleton mesh in the editor viewport.
Show Target Mesh	Show or hide the target skeleton mesh in the editor viewport.
Show Source Skeleton	Show or hide the source skeleton in the editor viewport. The viewport must be showing bone to see the skeleton.
Show Target Skeleton	Show or hide the target skeleton in the editor viewport. The viewport must be showing bone to see the skeleton.
Override Source Skeleton Color	Override the source skeleton color in the editor viewport.
Source Override Color	Select a new skeleton color.
Override Target Skeleton Color	Override the target skeleton color in the editor viewport.
Target Override Color	Select a new skeleton color.
Ignore Root Lock in Preview	When set to true, animation sequences with Force Root Lock enabled act as though the setting is disabled. This affects only the preview in the retarget editor. Use ExportRootLockMode to control exported animation behavior. This setting has no effect on runtime retargeting where root motion is copied from the source.
Debug Draw	Provides a way to target chains. Tools are individualized by operation and controlled through the op stack.
Profile Ops	Toggle performance profiling of the op stack.