Bone Chain Setup

This guide explains how to set up bone chains in KawaiiPhysics.

note

This page is a setup guide. For full details of the RootBone / ExcludeBones / AdditionalRootBones / DummyBoneLength properties, see Physics Parameters.

Selecting Root Bone

Root Bone specifies the parent bone of the bone chain to apply physics to.

warning

Physics is not applied to the Root Bone itself. Physics is applied to its child bones and below.

Auto-detection

KawaiiPhysics automatically detects child bones from the Root Bone and constructs the chain.

spine_03 (Root Bone - no physics)
├── hair_front_01  ← Physics applied
│   └── hair_front_02  ← Physics applied
└── hair_back_01   ← Physics applied
    └── hair_back_02   ← Physics applied

DummyBoneLength (Improving End-Bone Sway)

The end bone of a chain has no child bone, so it lacks a reference for its rotation and its physics tends to be unstable. Setting DummyBoneLength to a value greater than 0 adds one invisible dummy bone at the tip, giving the end bone a "child" and stabilizing its rotation and sway. It is an important parameter that directly affects how things sway.

With DummyBoneLength = 0 (left), the end bone is unstable. With a value greater than 0 (right), an invisible dummy is added at the tip and the end bone sways stably and naturally. (Diagram labels are in Japanese.)

Property	Value
Type	float
Default	0.0 (disabled)
Range	0 or higher
Category	Bones

Tips:

• Useful when the end bone's sway is weak, jerky, or unnatural
• The value is the dummy bone's length (distance from the end bone). Start with roughly the same length as the end bone
• At runtime it can be changed with UKawaiiPhysicsLibrary::SetDummyBoneLength (the bone structure is rebuilt when changed)

note

This adds a single dummy at the tip of the chain. It is different from BoneSubdivisionCount, which inserts dummies between adjacent bones to increase collision resolution (see Bone Subdivision).

Exclude Bones

You can exclude specific bones from physics calculation.

UPROPERTY(EditAnywhere, Category = "Bones")
TArray<FBoneReference> ExcludeBones;

warning

A bone listed in ExcludeBones removes that bone and all of its descendants from physics control. If you specify a bone in the middle of a chain, physics will no longer be applied to anything down to the tip from there.

Physics is applied to the chain starting from the Root Bone. A bone listed in Exclude Bones — together with all of its descendant bones — is removed from the physics calculation. Diagram labels are in Japanese.

Use Cases

• Parts you don't want to move, like ribbon knots
• Bones you want to control with a different KawaiiPhysics node

tip

To set a dedicated exclude list per Additional Root Bone, use OverrideExcludeBones (when bUseOverrideExcludeBones is enabled) on each FKawaiiPhysicsRootBoneSetting.

Multiple Bone Chains

You can use multiple KawaiiPhysics nodes for one character.

AnimGraph
├── KawaiiPhysics (for hair)
├── KawaiiPhysics (for tail)
└── KawaiiPhysics (for clothing)

tip

Parts with different parameters are easier to adjust when managed with separate nodes.

Additional Root Bones

Version Info

Added in v1.17.0

You can set multiple RootBones in a single KawaiiPhysics node. Useful when you want to control multiple bone chains with the same parameters.

Setup

1. Select KawaiiPhysics node
2. Add elements to Additional Root Bones array
3. Set additional RootBone for each element

FKawaiiPhysicsRootBoneSetting

Property	Type	Description
RootBone	FBoneReference	Additional control root bone
OverrideExcludeBones	TArray<FBoneReference>	Exclude bones list specific to this root bone
bUseOverrideExcludeBones	bool	Enable exclude bones override

Usage Example

// Control front and back hair with one node
RootBone: hair_front_root
AdditionalRootBones:
  - RootBone: hair_back_root
  - RootBone: hair_side_l_root
  - RootBone: hair_side_r_root

Benefits

• Reduced Node Count: Combine into one node if parameters are the same
• Performance: Slightly more efficient than multiple nodes
• Easier Management: Parameter changes apply to all at once

Comparison with Traditional Method

Method	Benefits	Drawbacks
Multiple Nodes	Different parameters for each part	More nodes
Additional Root Bones	Manage all with same parameters	Difficult to fine-tune per part

tip

Recommended to use Additional Root Bones for parts you want to animate with the same parameters like hair, and use separate nodes for parts that need different parameters like skirt and tail.

Bone Length and Radius

Each bone's length is automatically calculated. Radius can be set manually or controlled with curves.

For more details, see Curve Editor.