Sync Bone

Version Info

Added in v1.20.0. Significantly enhanced in v1.21.0 with apply direction, distance attenuation, scale curves, and more.

The Sync Bone feature applies the movement and rotation of a specified bone (the sync source) to physics-controlled bones. It is effective for preventing skirts from penetrating through legs.

Overview

Previously, collisions were used to prevent penetration of skirts and similar meshes, but it could be difficult to handle complex shapes and movements. With the Sync Bone feature, you can reflect the movement of bones such as legs onto physics bones without using collisions.

How It Works

Leg Bone (Sync Source = Bone)
    ↓ Get position/rotation
[Sync Bone Processing]
    ↓ Apply to physics bones (considering apply direction, scale, and distance attenuation)
Skirt Bone (Sync Target = TargetRoots)

When the source bone moves or rotates, that movement is reflected onto the physics bones, preventing penetration.

How Sync Bone prevents penetration

Setup

1. Select the KawaiiPhysics node
2. Add elements to the Sync Bones array (Force category)
3. Set the sync source bone (Bone) and the sync targets (TargetRoots)

FKawaiiPhysicsSyncBone

Property	Type	Description
Bone	FBoneReference	Sync source bone (e.g., a leg bone)
TargetRoots	TArray<FKawaiiPhysicsSyncTargetRoot>	Sync target bones/chains (see below)
GlobalScale	FVector	Overall amount of movement applied (0.0-1.0, default (1,1,1))
ScaleCurveByDeltaDistance	FRuntimeFloatCurve	Curve that scales the correction amount based on the source's movement distance (X: movement distance, Y: scale)
ApplyDirectionX / Y / Z	ESyncBoneDirection	Direction in which movement is applied for each axis (default Both)

FKawaiiPhysicsSyncTargetRoot (Specifying Sync Targets)

Each element of TargetRoots configures the sync target bone and how the influence is applied to the chain below it.

Property	Type	Description
Bone	FBoneReference	Bone to apply the sync to
bIncludeChildBones	bool	Whether to also target all child bones of this bone (default true)
ScaleCurveByBoneLengthRate	FRuntimeFloatCurve	Applies a scale to the influence based on the length ratio from the TargetRoot (X: LengthRate, Y: Scale)

When bIncludeChildBones is enabled, the child bones of the specified bone are automatically expanded as sync targets (ChildTargets). Using ScaleCurveByBoneLengthRate, you can apply the influence gradually, for example stronger on bones closer to the TargetRoot and weaker toward the tip.

ESyncBoneDirection (Apply Direction)

For each axis, you can specify which direction of movement is applied.

Value	Description
Both	Apply movement in both directions (default)
Positive	Apply only positive-direction movement
Negative	Apply only negative-direction movement
None	Do not apply movement

For example, when you only want to push out the skirt when the leg moves outward, you can limit the axis and direction.

Distance Attenuation

This setting attenuates the apply amount (Alpha) based on the distance between the sync source bone and the sync target. It helps suppress excessive influence on distant bones.

Property	Type	Description
bEnableDistanceAttenuation	bool	Enable distance attenuation (default false)
AttenuationInnerRadius	float (cm)	No attenuation within this distance
AttenuationOuterRadius	float (cm)	Maximum attenuation is applied beyond this distance
MaxAttenuationRate	float	Maximum attenuation amount (0 = no attenuation, 1 = apply amount becomes 0)

Distance ≤ Inner          : No attenuation (full apply)
Inner < Distance < Outer  : Interpolated based on distance
Distance ≥ Outer          : Maximum attenuation (MaxAttenuationRate)

Usage Example: Preventing Skirt Leg Penetration

Set the leg bones as the sync source (Bone) and nearby skirt bones as the sync targets (TargetRoots).

Sync Source: thigh_l / thigh_r (leg bones)
Sync Target: skirt_front_l_01, skirt_side_l_01 ... (skirt bones)

• First set GlobalScale to a low value (0.3-0.5), then gradually increase it to fine-tune.
• Enabling bIncludeChildBones lets you apply the sync to the entire skirt chain at once.
• If you only want to reflect the movement when the leg moves toward the body, limit the ApplyDirection.

Combining with Collision

By combining the Sync Bone feature with Collision, more reliable penetration prevention is possible.

Penetration prevention effect of combining SyncBone and Collision

Combining with BoneConstraint

By combining the Sync Bone feature with the BoneConstraint feature, more natural skirt expression is possible.

A complete demo combining SyncBone, BoneConstraint, and Collision

• Sync Bone: Prevents penetration with the legs
• BoneConstraint: Maintains the distance between skirt bones

[BoneConstraint] ← Maintains distance between bones
       ↓
Skirt Bones
       ↑
   [Sync Bone] ← Reflects leg movement

Notes

warning

• If GlobalScale is too high, physics movement may become unnatural. Start with low values (0.3-0.5) and adjust from there.
• Using distance attenuation together helps suppress excessive influence on distant bones.

Comparison with Collisions

Method	Benefits	Drawbacks
Collision	Physically accurate	Complex setup, high computational cost
Sync Bone	Lightweight, easy setup	Less physically accurate

In many cases, combining Sync Bone and collisions achieves the best results. Combining it with Bone Subdivision enables even finer penetration prevention.

Related Pages

• Collision Setup
• Bone Subdivision
• Limits Parameters
• Data Assets