Ue Control Rig And Ik

Control Rig & IK

When baked clips are insufficient — feet must land on uneven terrain, hands must grip a moving weapon, or animation must transfer between different skeletons — reach for Unreal's procedural animation suite: Control Rig, the IK Rig, and the IK Retargeter. Both systems are plugins that extend the core animation system (ue-animation-system).

When to use this skill

• Foot/leg placement on uneven terrain, stairs, or slopes (foot IK, pelvis adjustment).
• Hand/weapon IK, two-handed grips, look-at or aim offsets.
• Procedural secondary motion, runtime pose corrections, or physics-driven adjustments.
• Retargeting an animation library from one character skeleton to another at import time (offline) or at runtime via FAnimNode_RetargetPoseFromMesh.
• Authoring a Control Rig asset for Sequencer cinematics or building a custom rig in C++.

The tools — pick by problem

Enable plugins: Control Rig and IK Rig (both ship with UE; disabled by default).

Control Rig

Mental model

A Control Rig asset (UControlRig) contains a RigVM graph that runs node-based logic over a URigHierarchy — a tree of typed elements (bones, controls, nulls, curves). The graph executes on two events: Forward Solve (runtime pose evaluation) and Backwards Solve / Construction (for editor keyframing and rig setup).

FRigUnit is the base struct for every rig graph node. Custom C++ rig units subclass it. UControlRig subclasses URigVMHost and exposes GetHierarchy() to read/write bone and control transforms at runtime.

Placing Control Rig in an AnimGraph

The AnimGraph node FAnimNode_ControlRig (AnimNode_ControlRig.h:21) integrates a UControlRig into an Animation Blueprint. It accepts a source pose, drives the rig's Forward Solve each frame, and outputs the modified pose.

Key node properties:

• Control Rig Class — the TSubclassOf<UControlRig> that the node instantiates.
• Transfer Input Pose — lets the rig read the incoming animated pose (required for overlay/fixup rigs; disable for fully procedural rigs).
• Alpha / LOD Threshold — blend strength and max LOD to run the rig.

Expose rig variables as pins on the node (mark them instance-editable in the rig graph), then wire AnimBP logic (traces, curve reads) into those pins each frame.

Runtime access via UControlRigComponent

For actors that need Control Rig outside an AnimBP (e.g., a prop driven by game code), attach a UControlRigComponent (ControlRigComponent.h:175). It hosts the rig, maps scene components to rig elements, and ticks the solve each frame independently.

cppCopy code
// Attach component and assign rig class in the constructor or BeginPlay
UControlRigComponent* CRC = NewObject<UControlRigComponent>(this);
CRC->SetControlRigClass(UMyFootIKRig::StaticClass());
CRC->RegisterComponent();

Access the running instance with CRC->GetControlRig() and call ControlRig->SetControlValue(...) / ControlRig->GetControlValue(...) to drive controls from game code. GetHierarchy() returns the live URigHierarchy for direct bone queries.

Modular Control Rig (UE 5.5+)

UModularRig (ModularRig.h) composes multiple rig modules into one character rig. Each FRigModuleInstance is a named slot in the modular hierarchy. An agent assembling a character rig should prefer modular rigs when the character has swappable limb rigs.

Sequencer integration

Animators keyframe Control Rig controls directly in Sequencer via a Control Rig track added to a skeletal mesh actor. The rig's Backwards Solve event maps keyframed control transforms back to bones. From C++, retrieve the running rig via UControlRig::FindControlRigs(Outer, UControlRig::StaticClass()).

IK Rig + solvers

Mental model

An IK Rig (UIKRigDefinition, IKRigDefinition.h:206) is an asset bound to a skeleton that holds:

• Goals (UIKRigEffectorGoal) — named effector points with position/rotation targets.
• Solvers — ordered stack of FIKRigSolverBase subclasses that pull bones toward goals. Each solver runs in sequence; results cascade.
• Retarget chains (FBoneChain) and a pelvis bone — used by the retargeter.

Available solvers in Plugins/Animation/IKRig/Source/IKRig/Public/Rig/Solvers/:

Driving IK goals at runtime via UIKRigComponent

Attach a UIKRigComponent (IKRigComponent.h:14) to the character actor. It implements IIKGoalCreatorInterface, so FAnimNode_IKRig automatically discovers and reads its goals.

cppCopy code
// Drive the left foot goal from game code
UIKRigComponent* IKComp = GetComponentByClass<UIKRigComponent>();
if (IKComp)
{
    // Position in component space of the skeletal mesh; alpha 1 = fully IK-driven
    IKComp->SetIKRigGoalPositionAndRotation(
        FName("LeftFootGoal"),
        LeftFootHitLocation,   // from line trace
        LeftFootHitNormal.Rotation().Quaternion(),
        1.0f,  // position alpha
        1.0f   // rotation alpha
    );
}

FIKRigGoal (IKRigDataTypes.h) carries the goal name, position, rotation, alphas, and the space enum (EIKRigGoalSpace: Component, Additive, or World).

AnimGraph: FAnimNode_IKRig

Add the IK Rig AnimGraph node, assign the UIKRigDefinition asset, and wire goal transforms from pins or from a UIKRigComponent on the actor. The node runs the IK solve on top of the incoming base pose each frame.

Copy code
[Base Pose] → [IK Rig node (RigDefinitionAsset=MyFootIKRig, Goals=...)] → [Output Pose]

Place the IK Rig node after the state machine / locomotion blend so it modifies an already-computed base pose.

IK Retargeter

Mental model

An IK Retargeter (UIKRetargeter, IKRetargeter.h:70) links a source UIKRigDefinition to a target UIKRigDefinition. It matches named retarget chains (spine, arm-left, leg-right…) by fuzzy name and transfers motion proportionally, compensating for different bone lengths and proportions.

Workflow:

1. Create an IK Rig for the source skeleton; define retarget chains and set the pelvis.
2. Create an IK Rig for the target skeleton; define matching chains.
3. Create an IK Retargeter asset linking source→target.
4. Edit the Retarget Pose to align T-pose vs A-pose discrepancies.
5. Either export baked Animation Sequences for the target, or use runtime retargeting.

Runtime retargeting: FAnimNode_RetargetPoseFromMesh

FAnimNode_RetargetPoseFromMesh (AnimNode_RetargetPoseFromMesh.h:28) retargets the pose of a source skeletal mesh component onto the current character in real time. The source mesh component must tick before the target's AnimBP.

cppCopy code
// Expose on the AnimBP's AnimGraph node (set from game code or a property)
// RetargetFrom = ERetargetSourceMode::CustomSkeletalMeshComponent
// SourceMeshComponent = (set to the source character's USkeletalMeshComponent)
// IKRetargeterAsset = MyRetargeter

Key properties: LODThreshold (disable retarget above this LOD), LODThresholdForIK (skip IK but keep FK retarget at high LODs for perf), CustomRetargetProfile (override settings at runtime without changing the asset).

See references/ik-retargeter.md for the retarget chain setup, retarget pose workflow, and the Retarget Operation Stack.

Foot placement pattern (common)

A robust foot IK pattern using IK Rig:

1. In the AnimInstance's NativeUpdateAnimation, line-trace downward from each foot bone (world space). Store hit location, hit normal, and a ground-clearance flag.
2. Pass results into the AnimBP thread as UPROPERTY members.
3. Add a UIKRigComponent with a foot IK setup (Limb IK or FBIK, one goal per foot, one Set Transform for the pelvis).
4. Each tick, call SetIKRigGoalPositionAndRotation for each foot goal and the pelvis.
5. In the AnimGraph, the IK Rig node at the end of the graph reads those goal values and solves the legs and pelvis in the correct space.

For pelvis adjustment (keep both feet planted): use a Set Transform solver on the pelvis goal, lowering it by the maximum foot-sink distance so legs can reach both feet.

Simple AnimGraph IK (no Control Rig asset required)

Built-in AnimGraph skeletal control nodes for single-limb needs:

These nodes require no external assets. Set the effector target location each frame from the AnimBP (typically from a socket or a line-trace result). Place them after the base pose blend, before output.

Gotchas & edge cases

• Node order — Control Rig / IK nodes must appear after the base pose they modify. Putting IK before the state machine produces zero-pose artifacts.
• Invalid goal position — a goal at FVector::ZeroVector in world space snaps limbs to the character origin. Always validate trace results before passing them as goals.
• Wrong goal space — EIKRigGoalSpace::Component vs World vs Additive must match how you computed the target. Additive offsets relative to the current bone; Component is relative to the skeletal mesh component; World is absolute.
• Retarget chain mismatch — unmatched chain names silently fall back to FK-only retargeting; confirm chains are matched in the IK Retargeter editor's Hierarchy panel.
• Retarget pose discrepancy (T vs A pose) — the single most common source of broken retargeting; edit the retarget pose for each skeleton to match the source pose.
• FBIK on many characters — Full Body IK is iterative and CPU-heavy; budget using LOD thresholds (LODThresholdForIK on FAnimNode_RetargetPoseFromMesh; LODThreshold on FAnimNode_IKRig and FAnimNode_ControlRig).
• Root motion + IK fighting — validate locomotion + IK together; IK on an unconstrained root can cause visible sliding if the root motion is also changing pelvis height.
• ModularRig path deprecation — GetRigModuleNameSpace() was deprecated in 5.6; use GetRigModulePrefix() instead.

Version notes

• UModularRig and the modular rig system landed in UE 5.5; before that, only monolithic UControlRig existed.
• The IK Retarget Operation Stack (Op Stack) replaced the older per-chain FK/IK settings in UE 5.5; the UIKRetargeter::GetRetargetOps() accessor reflects this.
• FAnimNode_RetargetPoseFromMesh serialization changed in 5.5 (see WithSerializer trait); bUseAttachedParent_DEPRECATED is the old field, removed from active use.
• FRigUnit C++ unit upgrade guide for 5.2 API changes: see the official doc link below.

References & source material

Engine source (UE 5.8 — all paths under Engine/Plugins/Animation/):

• ControlRig/Source/ControlRig/Public/ControlRig.h:63 — UControlRig class declaration.
• ControlRig/Source/ControlRig/Public/ControlRig.h:203 — GetHierarchy().
• ControlRig/Source/ControlRig/Public/ControlRig.h:277 — Execute(const FName&).
• ControlRig/Source/ControlRig/Public/ControlRig.h:331 — SetControlValue<T>().
• ControlRig/Source/ControlRig/Public/AnimNode_ControlRig.h:21 — FAnimNode_ControlRig.
• ControlRig/Source/ControlRig/Public/ControlRigComponent.h:175 — UControlRigComponent.
• ControlRig/Source/ControlRig/Public/ModularRig.h — UModularRig, FRigModuleInstance.
• ControlRig/Source/ControlRig/Public/Units/RigUnit.h:59 — FRigUnit base struct.
• ControlRig/Source/ControlRig/Public/Rigs/RigHierarchy.h:167 — URigHierarchy.
• ControlRig/Source/ControlRig/Public/Rigs/RigHierarchy.h:2181 — GetLocalTransform().
• ControlRig/Source/ControlRig/Public/Rigs/RigHierarchy.h:2229 — SetLocalTransform().
• ControlRig/Source/ControlRig/Public/Rigs/RigHierarchy.h:2277 — GetGlobalTransform().
• IKRig/Source/IKRig/Public/Rig/IKRigDefinition.h:134 — FBoneChain.
• IKRig/Source/IKRig/Public/Rig/IKRigDefinition.h:165 — FRetargetDefinition.
• IKRig/Source/IKRig/Public/Rig/IKRigDefinition.h:206 — UIKRigDefinition.
• IKRig/Source/IKRig/Public/Rig/Solvers/IKRigFullBodyIK.h:15 — FIKRigFBIKGoalSettings (FBIK solver settings).
• IKRig/Source/IKRig/Public/Rig/Solvers/IKRigLimbSolver.h:34 — FIKRigLimbSolver.
• IKRig/Source/IKRig/Public/Retargeter/IKRetargeter.h:32 — FIKRetargetPose.
• IKRig/Source/IKRig/Public/Retargeter/IKRetargeter.h:70 — UIKRetargeter.
• IKRig/Source/IKRig/Public/AnimNodes/AnimNode_IKRig.h:22 — FAnimNode_IKRig.
• IKRig/Source/IKRig/Public/AnimNodes/AnimNode_RetargetPoseFromMesh.h:28 — FAnimNode_RetargetPoseFromMesh.
• IKRig/Source/IKRig/Public/ActorComponents/IKRigComponent.h:14 — UIKRigComponent.
• IKRig/Source/IKRig/Public/Rig/IKRigDataTypes.h — FIKRigGoal, EIKRigGoalSpace.

Official docs (UE 5.8, confirmed live):

• Control Rig overview — https://dev.epicgames.com/documentation/unreal-engine/control-rig-in-unreal-engine
• Control Rig in Animation Blueprints — https://dev.epicgames.com/documentation/unreal-engine/control-rig-in-animation-blueprints-in-unreal-engine
• IK Rig editor — https://dev.epicgames.com/documentation/unreal-engine/ik-rig-in-unreal-engine
• IK Rig Retargeting — https://dev.epicgames.com/documentation/unreal-engine/ik-rig-animation-retargeting-in-unreal-engine

Deep-dive references in this skill:

• references/control-rig-graph.md — RigVM graph concepts, FRigUnit authoring, hierarchy element types, solve events.
• references/ik-retargeter.md — retarget chain setup, retarget pose workflow, Op Stack, runtime retargeting with FAnimNode_RetargetPoseFromMesh.
• references/animgraph-ik-nodes.md — Two Bone IK, FABRIK, CCDIK, Leg/Foot IK node details and performance guidance.

Related skills: ue-animation-system, ue-sequencer-and-cinematics.