🧿 Retargeting with the IK Retargeter

Why Retargeting Has to Exist

Animation in Unreal is never truly generic. Every Animation Sequence is authored for one specific Skeleton, and it stores its motion as rotations on bones addressed by name. A walk cycle does not say "rotate the upper arm." It says "on frame 12, set upperarm_l to this rotation." Hand that clip to a Skeleton with no bone called upperarm_l and there is simply nothing for the data to land on.

That is the exact situation Lesson 1 set up with two real characters living in one project. The Kat figure (SK_Anya) came from Daz, so it carries Daz-style bone names on its own SK_Anya_Skeleton. Unreal's standard mannequin (SK_Manny) uses Epic's names on SK_Mannequin. Here is the real bone tree of the Daz character again, so the names are concrete:

Figure: The real bone tree of the Daz-derived SK_Anya in the ClaudeTest project (the same capture you met in Lesson 1). The root is literally Kat-for-Genesis-9, and the joints read hip, pelvis, l_thigh, l_shin · a mannequin animation looking for pelvis/thigh_l/calf_l will not find these.

The name clash is only half the trouble. Even if you renamed every bone to match, the two bodies are built to different proportions: Kat is a different height, her arms and legs are different lengths, her shoulders sit at a different width. Copy a mannequin's raw bone rotations onto her and her hands would punch through her hips or float away from her sides. Real retargeting has to solve both problems at once, remapping names and re-fitting the motion to the target's proportions.

The Two-Asset System: IK Rig and IK Retargeter

Unreal solves retargeting with two cooperating assets. It is worth being precise about which one does what, because their names sound similar and beginners routinely confuse them.

The division of labor is clean. Each IK Rig speaks only about its own character: "on this skeleton, the left arm is these three bones." The IK Retargeter is the bilingual translator that sits between two rigs and says "the thing your source calls LeftArm is the thing your target calls LeftArm, so route the motion accordingly." You build the rigs once per character; you build a retargeter once per source-and-target pair.

Figure: The retargeting system. Each character gets one IK Rig that labels its retarget root and chains in a shared vocabulary (Spine, LeftArm, LeftLeg…) · the single IK Retargeter in the middle pairs a source rig with a target rig and matches chains by that shared name, regardless of the underlying bone names.

Because both rigs describe themselves with the same chain vocabulary, the retargeter never has to know that one skeleton says upperarm_l and the other says l_upperarm. It only sees "LeftArm on the source" and "LeftArm on the target," and connects them. That shared vocabulary is the whole trick, and building it is the job of the IK Rig.

flowchart LR
    A[Build IK Rig
on source] --> C[Create IK Retargeter]
    B[Build IK Rig
on target] --> C
    C --> D[Map chains
source to target]
    D --> E[Fix retarget pose
align stances]
    E --> F[Preview an anim
on the target]
    F --> G[Export to a new
Animation Sequence]
    style C fill:#fff3cd,stroke:#ffc107
    style G fill:#e8f5e9,stroke:#4CAF50
                    

Building an IK Rig: Root and Chains

An IK Rig is created from a Skeletal Mesh (right-click the mesh, Create → IK Rig) and opened in its own editor, where the character stands next to its Skeleton hierarchy. Setting it up for retargeting is two jobs: naming the root, and drawing the chains.

The retarget root

The retarget root is the single bone that carries the character's overall position and facing in the world, almost always the pelvis or hip. It is special because it is the one bone whose translation retargets, not just its rotation: it is what moves the whole body forward during a walk. Get this wrong and your character will animate its limbs correctly but slide around, or stay pinned to the origin. On the mannequin the retarget root is pelvis; on Kat it is hip.

Retarget chains

A retarget chain is a named run of bones from a start joint to an end joint that represents one limb or region. You define one chain per body part and give it a standard name. The names are what the retargeter matches on, so using consistent names (Epic's own conventions like Spine, LeftArm, RightLeg, Head) is what makes the pairing in the next asset automatic.

Figure: An IK Rig on the Daz character. The hip bone is set as the retarget root (red), and the limbs are grouped into named chains · note the chains carry standard names (LeftArm, RightLeg, Spine) even though the bones inside them use Daz names like l_upperarm and r_thigh.

Epic ships a ready-made IK Rig for its mannequin (IK_Mannequin) with all of these chains already defined, so for the mannequin side you usually reuse it rather than build one. For a Daz or Mixamo character you build the rig yourself: create it from the mesh, set the root, then add one chain per limb by picking its start and end bone and typing the standard name. A humanoid needs about eight chains: Spine, Head, two arms, two legs, and often the two clavicles.

The Retarget Pose: Making Skeletons Agree

Here is the step beginners skip, and it is the one that causes the ugliest results. Retargeting compares the source's motion against a reference pose and applies the difference to the target's reference pose. For that comparison to make sense, the two reference poses have to describe the same stance. If the mannequin's reference pose is a wide-armed T-pose and your Daz character's is a relaxed A-pose with arms down, the retargeter thinks the source is holding its arms 45 degrees higher than it really is, and every retargeted clip inherits that twist.

You fix this directly in the IK Retargeter editor, which shows both characters side by side. You select the target, switch to Edit Retarget Pose, and rotate its arms (or legs, or spine) until its stance matches the source's. You are not animating anything; you are correcting the two skeletons' idea of "neutral" so they line up. Once the poses agree, the actual motion transfers cleanly.

Figure: Why the retarget pose matters. On the left the source stands in a T-pose while the target rests in an A-pose, so the retargeter bakes a 45 degree arm twist into every clip · on the right the retarget pose has been edited so both characters share the same neutral stance, and the animation transfers without distortion.

The IK Retargeter: Map, Preview, Export

With a rig on each character, you create the IK Retargeter (right-click the source IK Rig, Create → IK Retargeter, then assign the target IK Rig). Its editor is where everything comes together: source on the left, target on the right, a shared timeline, and a chain-mapping panel.

Chain mapping

The retargeter auto-pairs chains whose names match, so if you used consistent names in Section 3, the mapping panel is already mostly filled in: Spine to Spine, LeftArm to LeftArm, and so on. You scan the list, fix any chain that did not pair, and leave anything you do not want transferred unmapped (a common choice for finger chains on a clip that has no finger motion).

flowchart LR
    subgraph SRC[Source: Mannequin]
      S1[Spine
spine_01..05]
      S2[LeftArm
upperarm_l..hand_l]
      S3[LeftLeg
thigh_l..foot_l]
    end
    subgraph TGT[Target: Kat]
      T1[Spine
spine1..4]
      T2[LeftArm
l_upperarm..l_hand]
      T3[LeftLeg
l_thigh..l_foot]
    end
    S1 --> T1
    S2 --> T2
    S3 --> T3
    style SRC fill:#e6faf7,stroke:#4fd1c5
    style TGT fill:#fdf3e0,stroke:#e9c07a
                    

Preview, then export

Load any Animation Sequence that belongs to the source skeleton and the target character immediately performs it in the preview, retargeted live. This is where you catch problems: sliding feet, popping, an arm clipping the body. You tune the retarget pose or a chain's settings until it reads correctly. But a live preview is not an asset yet. To get something you can drop into a game or a Sequencer shot, you export.

Exporting has two flavors, and choosing between them is a real design decision:

flowchart TD
    R[IK Retargeter
source to target] --> Q{How will you
use it?}
    Q -->|Fixed set of clips,
cinematics| BAKE[Batch export to
Animation Sequences]
    Q -->|Chosen at runtime,
gameplay| LIVE[Retarget Pose From Mesh
in an Anim Blueprint]
    BAKE --> LIB[A retargeted library
on the target skeleton]
    style BAKE fill:#e8f5e9,stroke:#4CAF50
    style LIVE fill:#fff3cd,stroke:#ffc107
                    

Hands-On: Retarget a Walk

You need two different characters in one project. The easiest pairing is Epic's mannequin (which arrives with a finished IK Rig and a library of walks and idles) plus any second character: a free Mixamo download, a Fab character, or a Daz-derived mesh like the one in this lesson. You will move one of the mannequin's walks onto that second character.

Knowledge Check

Summary

You turned Lesson 1's idea into a working pipeline. Here is the arc:

The problem. Animation is stored on named bones, authored for one skeleton. A different character has different bone names and different proportions, so a clip cannot transfer directly. Retargeting fixes both the names and the fit.

Two assets. An IK Rig describes one character by marking a retarget root (the pelvis or hip) and grouping bones into named retarget chains. An IK Retargeter pairs a source rig with a target rig and maps chains by name, so it never has to know the underlying bone names differ.

The pose and the export. Before motion transfers cleanly, the two rigs' retarget poses must describe the same stance, or a constant offset twists every clip. Once aligned, you preview, then either batch-export baked Animation Sequences for a fixed library or retarget live in an Anim Blueprint for dynamic motion.

Where this fits

Retargeting is the step that fills the animation library the rest of the track draws on. With clips retargeted onto your character, Cinematics with Sequencer is where you drop them into a shot and point a camera, and Rendering Output turns that shot into final frames. In the wider Story-to-Screen pipeline, this is Phase 4, sitting between character finalization and shot assembly.