ue-animation-system

UE Animation System

You are an expert in Unreal Engine's animation system.

Context Check

Read .agents/ue-project-context.md first. Note which plugins are enabled (Control Rig, Motion Matching, Full Body IK), whether GAS is in use, and the skeleton/character hierarchy.

Information to Gather

1. What animation need? (locomotion, ability, cinematic, IK, procedural)
2. C++ only, Blueprint only, or mixed?
3. Does the character use ACharacter with an existing USkeletalMeshComponent?
4. Is GAS active? (affects montage replication)
5. Multiplayer? (determines replication strategy)
6. Does the project use modular linked anim layers?

---

Architecture

ACharacter / AActor
  └── USkeletalMeshComponent
        └── UAnimInstance subclass
              ├── NativeInitializeAnimation()           [setup, game thread]
              ├── NativeUpdateAnimation(float dt)       [game thread]
              ├── NativeThreadSafeUpdateAnimation(dt)   [worker thread]
              ├── FAnimInstanceProxy                    [worker thread eval]
              └── Montage API / Linked Layers

Animation updates run in two phases. Game thread: NativeUpdateAnimation — safe to read gameplay state. Worker thread: blend tree evaluation. Write all shared state as UPROPERTY() Transient members in NativeUpdateAnimation; read those cached values in NativeThreadSafeUpdateAnimation.

---

AnimInstance

Subclass Pattern

// MyAnimInstance.h
UCLASS()
class MYGAME_API UMyAnimInstance : public UAnimInstance
{
    GENERATED_BODY()

    virtual void NativeInitializeAnimation() override;
    virtual void NativeUpdateAnimation(float DeltaSeconds) override;
    virtual void NativeThreadSafeUpdateAnimation(float DeltaSeconds) override;

protected:
    UPROPERTY(Transient) TObjectPtr<ACharacter> OwningCharacter;
    UPROPERTY(Transient) TObjectPtr<UCharacterMovementComponent> MovementComp;

    UPROPERTY(Transient, BlueprintReadOnly, Category="Locomotion")
    float Speed = 0.f;

    UPROPERTY(Transient, BlueprintReadOnly, Category="Locomotion")
    float Direction = 0.f;

    UPROPERTY(Transient, BlueprintReadOnly, Category="Locomotion")
    bool bIsInAir = false;
};

// MyAnimInstance.cpp
void UMyAnimInstance::NativeInitializeAnimation()
{
    Super::NativeInitializeAnimation(); // ALWAYS call super
    OwningCharacter = Cast<ACharacter>(TryGetPawnOwner());
    if (OwningCharacter)
        MovementComp = OwningCharacter->GetCharacterMovement();
}

void UMyAnimInstance::NativeUpdateAnimation(float DeltaSeconds)
{
    Super::NativeUpdateAnimation(DeltaSeconds);
    if (!OwningCharacter || !MovementComp) return;

    const FVector Velocity = MovementComp->Velocity;
    Speed    = Velocity.Size2D();
    bIsInAir = MovementComp->IsFalling();

    if (Speed > 3.f)
    {
        const FRotator ActorRot    = OwningCharacter->GetActorRotation();
        const FRotator VelocityRot = Velocity.ToOrientationRotator();
        Direction = UKismetMathLibrary::NormalizedDeltaRotator(
            VelocityRot, ActorRot).Yaw;
    }
}

void UMyAnimInstance::NativeThreadSafeUpdateAnimation(float DeltaSeconds)
{
    Super::NativeThreadSafeUpdateAnimation(DeltaSeconds);
    // Only read UPROPERTY members written in NativeUpdateAnimation above.
    // Do NOT call any UObject functions not marked BlueprintThreadSafe.
}

FAnimInstanceProxy — Thread-Safe Access

Heavy animation logic can run on worker threads via NativeThreadSafeUpdateAnimation. Access shared data through the proxy:

void UMyAnimInstance::NativeThreadSafeUpdateAnimation(float DeltaSeconds)
{
    FMyAnimInstanceProxy& Proxy = GetProxyOnAnyThread<FMyAnimInstanceProxy>();
    Proxy.Speed = Proxy.Velocity.Size();
    Proxy.bIsFalling = Proxy.MovementMode == EMovementMode::MOVE_Falling;
}

// FAnimInstanceProxy declaration — worker thread data container
USTRUCT()
struct FMyAnimInstanceProxy : public FAnimInstanceProxy
{
    GENERATED_BODY()
    FMyAnimInstanceProxy() = default;
    explicit FMyAnimInstanceProxy(UAnimInstance* Instance) : FAnimInstanceProxy(Instance) {}

    float Speed = 0.f;
    FVector Velocity = FVector::ZeroVector;
    TEnumAsByte<EMovementMode> MovementMode = MOVE_None;

    virtual void PreUpdate(UAnimInstance* AnimInstance, float DeltaSeconds) override;
    virtual void Update(float DeltaSeconds) override;
};

// In UMyAnimInstance: override CreateAnimInstanceProxy to return your proxy
virtual FAnimInstanceProxy* CreateAnimInstanceProxy() override
{ return new FMyAnimInstanceProxy(this); }

The engine copies data between game thread and worker thread at safe sync points.

---

Montages

Source: AnimMontage.h, AnimInstance.h

Key API (UAnimInstance):

float Montage_Play(UAnimMontage*, float PlayRate=1.f,
    EMontagePlayReturnType=MontageLength, float StartAt=0.f, bool bStopAll=true);
void  Montage_Stop(float BlendOut, const UAnimMontage* Montage=nullptr);
void  Montage_Pause(const UAnimMontage* Montage=nullptr);
void  Montage_Resume(const UAnimMontage* Montage);
void  Montage_JumpToSection(FName Section, const UAnimMontage* Montage=nullptr);
void  Montage_SetNextSection(FName From, FName To, const UAnimMontage* Montage=nullptr);
bool  Montage_IsActive(const UAnimMontage*) const;
bool  Montage_IsPlaying(const UAnimMontage*) const;
FName Montage_GetCurrentSection(const UAnimMontage* Montage=nullptr) const;
float Montage_GetPosition(const UAnimMontage*) const;

Playing + Delegate Pattern

void UMyComponent::PlayAttackMontage(UAnimMontage* Montage)
{
    UAnimInstance* AnimInst = GetMesh()->GetAnimInstance();
    if (!AnimInst || !Montage) return;

    // Play FIRST — Montage_SetEndDelegate calls GetActiveInstanceForMontage()
    // internally, which returns nullptr until Montage_Play creates the instance.
    if (AnimInst->Montage_Play(Montage) <= 0.f) return;

    FOnMontageEnded EndDelegate;
    EndDelegate.BindUObject(this, &UMyComponent::OnAttackEnded);
    AnimInst->Montage_SetEndDelegate(EndDelegate, Montage);

    FOnMontageBlendingOutStarted BlendOutDelegate;
    BlendOutDelegate.BindUObject(this, &UMyComponent::OnAttackBlendingOut);
    AnimInst->Montage_SetBlendingOutDelegate(BlendOutDelegate, Montage);
}

void UMyComponent::OnAttackEnded(UAnimMontage* Montage, bool bInterrupted) { }
void UMyComponent::OnAttackBlendingOut(UAnimMontage* Montage, bool bInterrupted) { }

Dynamic Slot Montage

UAnimMontage* DynMontage = AnimInst->PlaySlotAnimationAsDynamicMontage(
    SomeSequence, FName("UpperBody"), 0.25f, 0.25f, 1.f, 1);

Multiplayer Replication

• With GAS: use UAbilitySystemComponent::PlayMontage() — GAS handles replication via FGameplayAbilityRepAnimMontage.
• Without GAS: replicate a montage pointer or a custom rep struct; server calls Montage_Play, clients play on OnRep_.
• Never call Montage_Play independently on all net roles without sync.

GAS Integration — PlayMontageAndWait

// GAS ability task — PlayMontageAndWait (requires GameplayAbilities module)
UAbilityTask_PlayMontageAndWait* Task =
    UAbilityTask_PlayMontageAndWait::CreatePlayMontageAndWaitProxy(
        this, NAME_None, AttackMontage, 1.f);
Task->OnCompleted.AddDynamic(this, &UMyAbility::OnMontageCompleted);
Task->OnInterrupted.AddDynamic(this, &UMyAbility::OnMontageInterrupted);
Task->ReadyForActivation();  // must call to start the task

---

Blend Spaces

Blend spaces are data assets sampled in the AnimGraph. Drive them by setting UPROPERTY members on the AnimInstance that the AnimGraph reads.

• 1D (UBlendSpace1D): single axis, typically Speed (0–600). Use FInterpolationParameter with InterpolationType=SpringDamper, InterpolationTime=0.15, DampingRatio=1.0.
• 2D (UBlendSpace): two axes — Direction (-180 to 180) and Speed (0–600). Cardinal direction samples at each speed tier.
• Aim Offset (UAimOffsetBlendSpace): additive blend space for Yaw/Pitch aiming, placed after the base locomotion pose in the AnimGraph.

See references/locomotion-setup.md for complete axis configuration and sample placement.

---

State Machines

State machines live in the AnimGraph. Bind native C++ logic to transition rules and state entry/exit without Blueprint:

// In NativeInitializeAnimation()
AddNativeTransitionBinding(
    FName("LocomotionSM"), FName("Idle"), FName("Walk/Run"),
    FCanTakeTransition::CreateUObject(this, &UMyAnimInstance::CanStartMoving),
    FName("IdleToMoving"));

AddNativeStateEntryBinding(
    FName("LocomotionSM"), FName("Land"),
    FOnGraphStateChanged::CreateUObject(this, &UMyAnimInstance::OnLandEntered));

Query state machine at runtime:

const FAnimNode_StateMachine* SM =
    GetStateMachineInstanceFromName(FName("LocomotionSM"));
float RunWeight = GetInstanceStateWeight(
    GetStateMachineIndex(FName("LocomotionSM")), SM->GetCurrentState());

Conduit Nodes

Conduits evaluate a single boolean rule and fan out to multiple destination states — replacing duplicated transition logic. Add a Conduit in the AnimGraph editor; its CanEnterTransition runs once and all outgoing transitions share the result. Use conduits when three or more states need the same entry condition (e.g., "is grounded?"). For simple A-to-B transitions, a direct rule is clearer.

---

Anim Notifies

Source: AnimNotify.h, AnimNotifyState.h

UAnimNotify — Point-in-Time

UCLASS(meta=(DisplayName="Footstep"))
class MYGAME_API UFootstepNotify : public UAnimNotify
{
    GENERATED_BODY()
public:
    // Always override the UE5 3-argument signature
    virtual void Notify(USkeletalMeshComponent* MeshComp,
        UAnimSequenceBase* Animation,
        const FAnimNotifyEventReference& EventReference) override;

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Footstep")
    FName FootSocket = FName("foot_l");
};

UAnimNotifyState — Duration (Begin/Tick/End)

UCLASS(meta=(DisplayName="Weapon Collision Window"))
class MYGAME_API UWeaponCollisionState : public UAnimNotifyState
{
    GENERATED_BODY()
public:
    virtual void NotifyBegin(USkeletalMeshComponent*, UAnimSequenceBase*,
        float TotalDuration, const FAnimNotifyEventReference&) override;
    virtual void NotifyEnd(USkeletalMeshComponent*, UAnimSequenceBase*,
        const FAnimNotifyEventReference&) override;
};

BranchingPoint (Synchronous)

Set bIsNativeBranchingPoint = true in the constructor. Override BranchingPointNotify() instead of Notify(). Fires synchronously during Montage_Advance — use for section jumps and precise timeline control. All other notifies are queued (fire after tick completes, safe for VFX/SFX).

Named Notify Delegate

AnimInst->OnPlayMontageNotifyBegin.AddDynamic(
    this, &UMyComponent::HandleNotifyBegin);

void UMyComponent::HandleNotifyBegin(FName NotifyName,
    const FBranchingPointNotifyPayload& Payload)
{
    if (NotifyName == FName("EnableHitbox")) ActivateHitDetection();
}

See references/anim-notify-reference.md for built-in notify catalog and more custom patterns.

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IK and Procedural

Foot IK with Line Traces (NativeUpdateAnimation — game thread)

FVector UMyAnimInstance::GetFootTarget(FName SocketName) const
{
    const FVector Foot = GetOwningComponent()->GetSocketLocation(SocketName);
    FHitResult Hit;
    FCollisionQueryParams P(SCENE_QUERY_STAT(FootIK), true);
    P.AddIgnoredActor(OwningCharacter);
    if (GetWorld()->LineTraceSingleByChannel(
            Hit, Foot + FVector(0,0,50), Foot - FVector(0,0,75),
            ECC_Visibility, P))
        return Hit.ImpactPoint;
    return Foot;
}

Feed results into a Control Rig asset (UE5 recommended) or a Two Bone IK skeletal control node in the AnimGraph.

Skeletal control nodes (AnimGraph):

Node	Purpose
Two Bone IK	Two-joint IK (arm, leg) — effector + joint target
FABRIK	Multi-bone chain IK — tip bone + effector
Look At	Single bone tracks target location with clamp
Copy Bone	Copies transform components between bones
Spline IK	Bones follow a spline curve (spine, tail)

Layered Blend Per Bone (Upper/Lower Split)

In the AnimGraph:

1. Connect state machine output to Base Pose.
2. Connect UpperBody slot output to Blend Poses 0.
3. Layer Setup: Bone=spine_01, Depth=0, MeshPoseBlendFactor=1.0.

Attack montages use the UpperBody slot; locomotion plays uninterrupted below.

Aim Offset

// In NativeUpdateAnimation:
const FRotator Delta = UKismetMathLibrary::NormalizedDeltaRotator(
    OwningCharacter->GetBaseAimRotation(),
    OwningCharacter->GetActorRotation());
AimYaw   = FMath::Clamp(Delta.Yaw,   -90.f, 90.f);
AimPitch = FMath::Clamp(Delta.Pitch, -90.f, 90.f);

Place an Aim Offset node after the base pose in the AnimGraph, feeding AimYaw and AimPitch.

---

Linked Anim Graphs and Layers

Source: AnimNode_LinkedAnimGraph.h, AnimNode_LinkedAnimLayer.h

Linked Anim Layer (Recommended)

1. Create a UAnimLayerInterface Blueprint with layer function signatures.
2. Main AnimInstance has Linked Anim Layer nodes referencing the interface.
3. Separate AnimInstance subclasses implement the interface per mode.
4. Swap at runtime:

// Switch locomotion implementation
AnimInst->LinkAnimClassLayers(UClimbingLocomotionLayer::StaticClass());
// Reset all layers to defaults:
AnimInst->LinkAnimClassLayers(nullptr);
// Retrieve a linked instance:
UAnimInstance* Layer =
    AnimInst->GetLinkedAnimLayerInstanceByClass(UClimbingLocomotionLayer::StaticClass());

Linked Anim Graph (by Tag)

AnimInst->LinkAnimGraphByTag(FName("CombatGraph"), UMyCombatAnimInstance::StaticClass());
UAnimInstance* Sub = AnimInst->GetLinkedAnimGraphInstanceByTag(FName("CombatGraph"));

Notify Propagation

AnimInst->SetReceiveNotifiesFromLinkedInstances(true);
AnimInst->SetPropagateNotifiesToLinkedInstances(true);
// UE5.2+: let linked layers share main instance montage evaluation:
// AnimInst->SetUseMainInstanceMontageEvaluationData(true);

---

Root Motion

// Options: NoRootMotionExtraction, IgnoreRootMotion,
//          RootMotionFromMontagesOnly, RootMotionFromEverything
RootMotionMode = ERootMotionMode::RootMotionFromMontagesOnly;

// Per-montage disable
FAnimMontageInstance* Inst = AnimInst->GetActiveInstanceForMontage(Montage);
if (Inst) { Inst->PushDisableRootMotion(); /* ... */ Inst->PopDisableRootMotion(); }

For networked root motion, set the movement component's smoothing mode to ENetworkSmoothingMode::Exponential and enable bAllowPhysicsRotationDuringAnimRootMotion if rotation comes from animation. The server runs root motion authoritatively; clients predict and correct via FRootMotionMovementParams.

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Common Mistakes

Anti-Pattern	Fix
Reading gameplay state in NativeThreadSafeUpdateAnimation	Cache values as UPROPERTY Transient in NativeUpdateAnimation (game thread)
Polling Montage_IsActive in a loop	Bind FOnMontageEnded delegate before calling Montage_Play
Two montages sharing the same slot group	Use distinct slot names (UpperBody, LowerBody) or bStopAllMontages=false
Skipping Super::NativeInitializeAnimation()	Always call super — it initializes the proxy and skeleton
Calling non-thread-safe UObject methods in thread-safe update	Only read primitive UPROPERTY copies; never call GetOwningActor() from worker thread

---

Build.cs

PublicDependencyModuleNames.AddRange(new string[] {
    "Core", "CoreUObject", "Engine", "AnimGraphRuntime"
});
// Optional:
PrivateDependencyModuleNames.Add("ControlRig");        // Control Rig IK
PrivateDependencyModuleNames.Add("GameplayAbilities"); // GAS montage tasks

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Related Skills

• ue-gameplay-abilities — PlayMontageAndWait task, GAS montage replication.
• ue-actor-component-architecture — SkeletalMeshComponent setup, Character hierarchy, component tick ordering.
• ue-cpp-foundations — delegate binding, UPROPERTY specifiers, TWeakObjectPtr.

Reference Files

• references/anim-notify-reference.md — built-in notify catalog and custom notify implementation patterns.
• references/locomotion-setup.md — complete locomotion blend space and state machine configuration guide.