External Force Presets

Version Info

Added in v1.16.0

The External Force preset system allows you to apply various external forces in a modular way. Multiple presets are available that inherit from the base struct, and you can also add custom presets in C++.

Overview

Types of presets and application flow

Architecture

FKawaiiPhysics_ExternalForce (Base)
├─ FKawaiiPhysics_ExternalForce_Basic (Generic directional force)
├─ FKawaiiPhysics_ExternalForce_Curve (Time-based curve force)
├─ FKawaiiPhysics_ExternalForce_Gravity (Gravity)
└─ FKawaiiPhysics_ExternalForce_Wind (Wind)

View Source

Base Class: FKawaiiPhysics_ExternalForce

The base struct for all external force presets.

Common Parameters

Parameter	Type	Default	Description
bIsEnabled	bool	true	Enable/disable the force
bDrawDebug	bool	false	Enable/disable debug drawing
ApplyBoneFilter	TArray<FBoneReference>	Empty	Bones to apply force to (all if empty)
IgnoreBoneFilter	TArray<FBoneReference>	Empty	Bones to exclude from force
ExternalForceSpace	EExternalForceSpace	WorldSpace	Simulation space
RandomForceScaleRange	FFloatInterval	(1.0, 1.0)	Random scale range for force

Simulation Space (EExternalForceSpace)

Value	Description
ComponentSpace	Simulate in component space. Mesh movement doesn't affect velocity
WorldSpace	Simulate in world space. Mesh movement affects velocity
BoneSpace	Simulate in another bone's space

Bone Filter Usage Example

// Apply force only to specific bones
ApplyBoneFilter.Add(FBoneReference("hair_01"));
ApplyBoneFilter.Add(FBoneReference("hair_02"));

// Exclude specific bones
IgnoreBoneFilter.Add(FBoneReference("hair_root"));

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Basic: Simple Directional Force

Applies a constant directional force continuously or at intervals.

Parameters

Parameter	Type	Default	Description
ForceDir	FVector	(0, 0, 0)	Force direction
ForceRateByBoneLengthRate	FRuntimeFloatCurve	-	Force correction curve by bone length ratio
Interval	float	0.0	Interval between force applications (seconds). 0 = every frame

Usage Example

// Constant force to the right
FKawaiiPhysics_ExternalForce_Basic BasicForce;
BasicForce.ForceDir = FVector(100, 0, 0);
BasicForce.Interval = 0.0f; // Apply every frame

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Curve: Curve-Based Force

External force that changes over time according to a curve. Ideal for animation and timeline-based effects.

Parameters

Parameter	Type	Default	Description
ForceCurve	FRuntimeVectorCurve	-	Force curve over time (X: time, Y: force)
CurveEvaluateType	EExternalForceCurveEvaluateType	Single	Curve evaluation method
SubstepCount	int	10	Number of time subdivisions (for non-Single modes)
TimeScale	float	1.0	Time scale factor
ForceRateByBoneLengthRate	FRuntimeFloatCurve	-	Force correction curve by bone length ratio

Curve Evaluation Type (EExternalForceCurveEvaluateType)

Value	Description
Single	Evaluate at a single point
Average	Average of multiple points
Max	Maximum of multiple points
Min	Minimum of multiple points

Usage Example

// Oscillating force
FKawaiiPhysics_ExternalForce_Curve CurveForce;
CurveForce.TimeScale = 2.0f; // Play at 2x speed
CurveForce.CurveEvaluateType = EExternalForceCurveEvaluateType::Average;

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Gravity: Gravity Force

Simulates gravity. Can integrate with Character settings.

note

Gravity is always fixed to WorldSpace. Space selection is not available.

Parameters

Parameter	Type	Default	Description
ForceRateByBoneLengthRate	FRuntimeFloatCurve	-	Force correction curve by bone length ratio
bUseCharacterGravityDirection	bool	false	Use Character's CustomGravityDirection (UE5.4+)
bUseCharacterGravityScale	bool	false	Use Character's GravityScale
bUseOverrideGravityDirection	bool	false	Whether to override gravity direction
OverrideGravityDirection	FVector	(0, 0, 0)	Override gravity direction value

Integration with Character

In UE5.4 and later, you can automatically integrate with Character's gravity settings.

// Use Character's gravity settings
FKawaiiPhysics_ExternalForce_Gravity GravityForce;
GravityForce.bUseCharacterGravityDirection = true;
GravityForce.bUseCharacterGravityScale = true;

Overriding Gravity Direction

// Horizontal gravity (e.g., space station)
FKawaiiPhysics_ExternalForce_Gravity GravityForce;
GravityForce.bUseOverrideGravityDirection = true;
GravityForce.OverrideGravityDirection = FVector(1, 0, 0);

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Wind: Wind Force

Uses wind from WindDirectionalSource. Designed to work with Cloth and SpeedTree.

note

Wind is always fixed to WorldSpace. Space selection is not available.

Parameters

Parameter	Type	Default	Description
ForceRateByBoneLengthRate	FRuntimeFloatCurve	-	Force correction curve by bone length ratio
WindDirectionNoiseAngle	float	0.0	Noise angle for wind direction from WindDirectionalSource (degrees)

WindDirectionalSource Setup

1. Place a WindDirectionalSource in the level
2. Configure wind strength and direction
3. Add Wind preset to KawaiiPhysics node

FKawaiiPhysics_ExternalForce_Wind WindForce;
WindForce.WindDirectionNoiseAngle = 15.0f; // 15 degree random variation

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Custom External Forces (CustomExternalForces)

Experimental Feature

This feature is experimental. It may not work properly unless you click on the AnimNode or compile the Animation Blueprint.

You can implement custom force logic in Blueprint or C++.

Base Class: UKawaiiPhysics_CustomExternalForce

UCLASS(Abstract, Blueprintable, EditInlineNew, CollapseCategories)
class UKawaiiPhysics_CustomExternalForce : public UObject

Parameters

Parameter	Type	Default	Description
bIsEnabled	bool	true	Whether this custom force is enabled
bDrawDebug	bool	false	Whether to draw debug visualization

Overridable Functions

Function	Description
PreApply	Pre-apply processing (called once for all bones)
Apply	Apply force to each bone (called per bone)

C++ Implementation Example

UCLASS()
class UMyCustomForce : public UKawaiiPhysics_CustomExternalForce
{
    GENERATED_BODY()

public:
    virtual void Apply_Implementation(FAnimNode_KawaiiPhysics& Node,
        int32 ModifyBoneIndex, const USkeletalMeshComponent* SkelComp,
        const FTransform& BoneTransform) override
    {
        // Custom logic
    }
};

Blueprint Implementation

1. Create a Blueprint class inheriting from UKawaiiPhysics_CustomExternalForce
2. Override PreApply and Apply events
3. Add to KawaiiPhysics node's CustomExternalForces

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Common Features

ForceRateByBoneLengthRate

A force correction curve based on distance from root, available in all presets.

• X-axis: Position in bone chain (0.0 = root, 1.0 = tip)
• Y-axis: Force scale factor

// Example: Increase force toward the tip
ForceRateByBoneLengthRate.GetRichCurve()->AddKey(0.0f, 0.5f);
ForceRateByBoneLengthRate.GetRichCurve()->AddKey(1.0f, 1.5f);

Debug Drawing

Set bDrawDebug = true to visualize force vectors in the viewport.

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Blueprint API

External forces can be dynamically controlled from Blueprint.

Adding External Force

UFUNCTION(BlueprintCallable, Category = "Kawaii Physics")
static bool AddExternalForce(const FKawaiiPhysicsReference& KawaiiPhysics,
                             FInstancedStruct& ExternalForce, UObject* Owner, bool bIsOneShot = false);

Adding External Forces to Component

UFUNCTION(BlueprintCallable, Category = "Kawaii Physics")
static bool AddExternalForcesToComponent(USkeletalMeshComponent* MeshComp,
                                         TArray<FInstancedStruct>& ExternalForces, UObject* Owner,
                                         FGameplayTagContainer& FilterTags,
                                         bool bFilterExactMatch = false,
                                         bool bIsOneShot = false);

For more details, see UKawaiiPhysicsLibrary.

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Related Pages

• External Forces Parameters - AnimNode external force parameters
• AnimNotify - External force control during animation
• UKawaiiPhysicsLibrary - Blueprint API