Wind and External Forces

This guide explains how to set up wind and external forces in KawaiiPhysics.

External forces in KawaiiPhysics come in two flavors: direct AnimNode properties (the Force category) and the more flexible External Force Presets (the ExternalForces array). This page focuses on the former and explains how to use both together.

Wind System

You can integrate with Unreal Engine's Wind Directional Source.

Enabling

Enabling bEnableWind in the node's Force category makes the bones respond to Wind Directional Sources placed in the level.

Property	Type	Default	Description
bEnableWind	bool	false	Receive influence from WindDirectionalSource
WindScale	float	1.0	Wind influence (when bEnableWind is enabled)
WindDirectionNoiseAngle	float	0.0	Noise added to the wind direction (degrees, when bEnableWind is enabled)

Wind Directional Source Support

Version Info

Added in v1.17.0 (WindDirectionNoiseAngle added in v1.21.0)

Setup:

1. Place a Wind Directional Source in the level
2. Enable bEnableWind on the KawaiiPhysics node
3. Adjust the influence with WindScale
4. Optionally add variation to the wind direction with WindDirectionNoiseAngle

The strength and direction of the Wind Directional Source are reflected on the KawaiiPhysics bones. Because the same wind source can be shared with Cloth and SpeedTree, it is easy to keep the look consistent.

WindDirectionNoiseAngle = 0 (no noise)

Preset-based Wind

The FKawaiiPhysics_ExternalForce_Wind preset you can add to the ExternalForces array can also drive wind. The preset version has a bone-length-ratio correction curve (ForceRateByBoneLengthRate) for finer control. See External Force Presets for details.

Custom External Forces

To apply force in an arbitrary direction, you have the following options.

SimpleExternalForce (Direct Property)

Setting SimpleExternalForce (FVector) in the Force|External Force category applies that force continuously. bUseWorldSpaceSimpleExternalForce (default true) switches between world and component space.

// AnimNode property (C++ access example)
Node->SimpleExternalForce = FVector(100, 0, 0);  // constant force along X
Node->bUseWorldSpaceSimpleExternalForce = true;

External Force Presets (Recommended)

When you need direction, time variation, or per-bone modulation, use External Force Presets. Basic (directional force) / Curve (time variation) / Gravity / Wind are provided, and you can add your own presets in C++.

Use Cases

• Wind pressure when jumping
• Knockback from explosions (apply FKawaiiPhysics_ExternalForce_Basic as a one-shot)
• Resistance underwater

Gravity

Default Gravity

Gravity (FVector) in the Force category applies gravity to the bones. The default is (0, 0, 0) (no gravity).

Property	Type	Default	Description
Gravity	FVector	(0, 0, 0)	Gravity vector
bUseLegacyGravity	bool	false	Application method (false = AnimDynamics compatible, true = legacy compatible)
bUseDefaultGravityZProjectSetting	bool	false	Multiply by the project setting's DefaultGravityZ
bUseWorldSpaceGravity	bool	true	Treat gravity in world space

// Downward gravity
Node->Gravity = FVector(0, 0, -980.0f);

Custom Gravity

For advanced control such as freely changing the gravity direction or following the Character's gravity direction, see Custom Gravity and the FKawaiiPhysics_ExternalForce_Gravity preset.

Force Composition

Multiple forces are added together.

Final force = Gravity + Wind + SimpleExternalForce + each ExternalForce preset

Gravity, Wind, and ExternalForce combined and applied to the physics bones

Dynamic Control

To change forces in real time during gameplay, use UKawaiiPhysicsLibrary (BlueprintThreadSafe).

// Vary wind scale with a sine wave (via the node reference)
float WindStrength = 1.0f + FMath::Sin(GetWorld()->GetTimeSeconds()) * 0.5f;
KawaiiPhysics = UKawaiiPhysicsLibrary::SetWindScale(KawaiiPhysics, WindStrength);

// Change gravity direction
KawaiiPhysics = UKawaiiPhysicsLibrary::SetGravity(KawaiiPhysics, FVector(0, 0, -980.0f));

For transient forces such as impacts, add an external force preset with AddExternalForcesToComponent(..., bIsOneShot = true).

For more details, see Runtime Control.