Physics Parameters

Parameters that control the basic behavior of physics simulation.

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About This Page

This is the complete reference for physics parameters. For how to tune the sway, see Adjusting the Sway; to learn the concepts first, see Basic Concepts; for other task-oriented usage, see Bone Chain Setup / Curve Editor / Bone Subdivision.

FKawaiiPhysicsSettings

A struct that defines the basic physics control settings. It is assigned to the node via the PhysicsSettings property. Damping (how easily it sways) and Stiffness (how strongly it returns) are the core of the physics behavior — the balance between the two determines the overall "softness." Each member can be modulated by bone-length ratio using the curves described later.

USTRUCT(BlueprintType)
struct KAWAIIPHYSICS_API FKawaiiPhysicsSettings
{
    float Damping = 0.1f;               // Damping
    float Stiffness = 0.05f;            // Stiffness
    float WorldDampingLocation = 0.8f;  // Suppression of movement reflection
    float WorldDampingRotation = 0.8f;  // Suppression of rotation reflection
    float Radius = 3.0f;                // Collision radius
    float LimitAngle = 0.0f;            // Rotation limit angle
};

Damping

Damping Coefficient - Controls the intensity of swaying. Smaller values reflect more acceleration in the physics behavior, so it sways more. Larger values make the motion duller and heavier.

Property	Value
Type	float
Default	0.1
Range	0.0 or higher
Category	KawaiiPhysics

Stiffness

Stiffness - Higher values maintain the original shape (the animated pose) more strongly and return to it quickly. Lower values let it sway more freely.

Property	Value
Type	float
Default	0.05
Range	0.0 or higher
Category	KawaiiPhysics

WorldDampingLocation

Suppression of component movement reflection - Controls how much of the Skeletal Mesh Component's world-space movement is reflected into the sway.

• 0 = movement fully reflected (maximum sway)
• 1 = follows the component exactly (not reflected into sway)
• Actual reflection factor = 1 - WorldDampingLocation

Property	Value
Type	float
Default	0.8
Range	0.0 or higher
Category	KawaiiPhysics

WorldDampingRotation

Suppression of component rotation reflection - The rotation counterpart of the above. Controls the Skeletal Mesh Component's world-space rotation in the same way (actual reflection factor = 1 - WorldDampingRotation).

Property	Value
Type	float
Default	0.8
Range	0.0 or higher
Category	KawaiiPhysics

note

Because they are named "Damping (suppression)," WorldDampingLocation / WorldDampingRotation are counter-intuitive: higher values mean less sway (1 follows the component exactly = no sway). Set them lower when you want the hair/cloth to trail behind character movement and rotation.

Radius

Collision radius for each bone - The radius of the collision sphere each bone carries. Used in the push-out calculation against colliders (SphericalLimit, etc.). Its editor display name is Collision Radius.

Property	Value
Type	float
Default	3.0
Range	0.0 or higher
Display Name	Collision Radius
Category	KawaiiPhysics

LimitAngle

Rotation limit angle - The maximum rotation angle (in degrees) per step caused by the physics behavior. 0 means unlimited. Setting it appropriately suppresses erratic, jittery behavior.

Property	Value
Type	float
Default	0.0 (unlimited)
Range	0.0 or higher
Category	KawaiiPhysics

Simulation Settings

SimulationSpace

Simulation Space - Specifies the coordinate system for physics control.

Property	Value
Type	EKawaiiPhysicsSimulationSpace
Default	ComponentSpace

Value	Description
ComponentSpace	Simulate in component space
WorldSpace	Simulate in world space. Can avoid influence from sudden Root bone movement/rotation
BaseBoneSpace	Simulate in specified bone space

note

Using anything other than ComponentSpace causes minor performance degradation, but can avoid influence from sudden Root bone movement/rotation.

TeleportDistanceThreshold

Teleport Distance Threshold - If the SkeletalMeshComponent movement per frame exceeds this value, that movement won't be reflected in physics control.

Property	Value
Type	float
Default	300.0

TeleportRotationThreshold

Teleport Rotation Threshold - If the SkeletalMeshComponent rotation per frame exceeds this value, that rotation won't be reflected in physics control.

Property	Value
Type	float
Default	10.0

PlanarConstraint

Planar Constraint - Fixes each bone on a plane according to the specified axis.

Property	Value
Type	EPlanarConstraint
Default	None

Value	Description
None	No planar constraint
X	Constrain to X axis
Y	Constrain to Y axis
Z	Constrain to Z axis

SkelCompMoveScale

Component Move Scale - Scale applied when reflecting SkeletalMeshComponent movement in physics behavior.

Property	Value
Type	FVector
Default	(1, 1, 1)

Bone Settings

RootBone

Control Root Bone - The specified bone and all bones below it become control targets.

Property	Value
Type	FBoneReference
Category	Bones

ExcludeBones

Exclude Bones - Removes the specified bones and all bones below them from control targets.

Property	Value
Type	TArray<FBoneReference>
Category	Bones

DummyBoneLength

Dummy Bone Length - If greater than 0, adds a dummy bone at the end of control bones. Adding dummy bones improves physics control of terminal bones.

Property	Value
Type	float
Default	0.0
Range	0.0 or higher

BoneForwardAxis

Bone Forward Direction - Forward direction of bones. Affects physics control and dummy bone placement position.

Property	Value
Type	EBoneForwardAxis
Default	X_Positive

Value	Description
X_Positive	+X direction
X_Negative	-X direction
Y_Positive	+Y direction
Y_Negative	-Y direction
Z_Positive	+Z direction
Z_Negative	-Z direction

Bone Subdivision

Inserts virtual dummies between real bones inside the simulation (without modifying the real skeleton) to raise collision resolution. See Bone Subdivision for details.

BoneSubdivisionCount

Dummy Subdivision Count - Minimum number of dummy bones to insert between adjacent bones. Improves collision detection (e.g. prevents skirts from penetrating legs). 0 to disable.

Property	Value
Type	int32
Default	0
Range	0 - 10
Category	Bones|Bone Subdivision

bBoneSubdivisionCollisionOnly

Collision Only - Skips velocity integration (gravity/wind/etc.) for inter-bone dummies and lets them participate only in collision/constraints from interpolated positions (lightweight). Does not affect the dummy count.

Property	Value
Type	bool
Default	true
Edit Condition	BoneSubdivisionCount > 0
Category	Bones|Bone Subdivision

bBoneSubdivisionDensifyByRadius

Densify By Radius - Adds dummies based on radius so collision spheres roughly cover the gap between bones. Uses BoneSubdivisionCount as a minimum and places more where bones are far apart (up to 50 per segment).

Property	Value
Type	bool
Default	false
Edit Condition	BoneSubdivisionCount > 0
Category	Bones|Bone Subdivision

BoneConstraintSubdivisionCount

Horizontal Subdivision Count - Number of collision-proxy dummies (bridge dummies) inserted along each horizontal BoneConstraint. Fills gaps between adjacent chains (columns) to prevent penetration. 0 to disable. Independent of BoneSubdivisionCount.

Property	Value
Type	int32
Default	0
Range	0 - 10
Category	Bones|Bone Subdivision

BoneConstraintSubdivisionFeedbackScale

Feedback Scale - Strength of transferring a bridge dummy's collision displacement to its endpoint bones (0 = none, 1 = standard). Lower it if the result is too stiff.

Property	Value
Type	float
Default	1.0
Range	0.0 - 2.0
Edit Condition	BoneConstraintSubdivisionCount > 0
Category	Bones|Bone Subdivision

Warm-up Settings

WarmUpFrames

Warm-up Frame Count - Number of physics idle runs. Used when you want to start/display after physics processing settles.

Property	Value
Type	int32
Default	0
Range	0 or higher

bNeedWarmUp

Enable Warm-up - Flag to enable warm-up.

Property	Value
Type	bool
Default	false

bUseWarmUpWhenResetDynamics

Warm-up on Reset - Flag to run physics idle during ResetDynamics.

Property	Value
Type	bool
Default	true

Curve Control

The following parameters can be controlled by curves. The curve value at "length from RootBone to specific bone / length from RootBone to terminal bone" (0.0-1.0) is multiplied to each parameter.

Curve	Description
DampingCurveData	Adjusts Damping parameter
StiffnessCurveData	Adjusts Stiffness parameter
WorldDampingLocationCurveData	Adjusts WorldDampingLocation parameter
WorldDampingRotationCurveData	Adjusts WorldDampingRotation parameter
RadiusCurveData	Adjusts Radius parameter
LimitAngleCurveData	Adjusts LimitAngle parameter

tip

Using curves allows settings like making the root stiff and the tip soft.

Advanced Settings

bUpdatePhysicsSettingsInGame

Update Parameters During Game - Flag to update physics parameters for each bone every frame. Disabling slightly improves performance, but makes it impossible to change physics parameters during runtime.

Property	Value
Type	bool
Default	true

ResetBoneTransformWhenBoneNotFound

Reset When Bone Not Found - Flag to reset Transform when control target bone is not found. Generally recommended to keep disabled.

Property	Value
Type	bool
Default	false