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Custom Gravity

This guide explains how to customize gravity direction.

Custom gravity directions

Changing the Gravity vector changes how bones swing: default (down), wall-walking (sideways), zero-G (0), and underwater/buoyancy (up). (Diagram labels are in Japanese.)

There are two ways to handle gravity in KawaiiPhysics:

  1. The AnimNode Gravity property (Force category) — a simple vector
  2. The FKawaiiPhysics_ExternalForce_Gravity preset (ExternalForces array) — can integrate with the Character's gravity direction and GravityScale

Basic Gravity Settings (Gravity Property)​

Gravity is an FVector, and its default is (0, 0, 0) (no gravity). To apply downward gravity, set it explicitly.

// Downward gravity (magnitude in world units)
Node->Gravity = FVector(0, 0, -980.0f);
tip

Enabling bUseDefaultGravityZProjectSetting multiplies the Gravity vector by the project setting's DefaultGravityZ (absolute value). In that case you only need to specify a normalized direction such as Gravity = FVector(0, 0, -1.0f) to get gravity of the appropriate magnitude.

Changing Gravity Direction​

World Coordinate Specification​

When bUseWorldSpaceGravity (default true) is enabled, Gravity is interpreted in world space.

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

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

// Diagonal down
Node->Gravity = FVector(0.5f, 0, -0.5f).GetSafeNormal() * 980.0f;

Changing It at Runtime​

To change gravity at runtime, use SetGravity from UKawaiiPhysicsLibrary (BlueprintThreadSafe).

// Use the character's down direction as gravity (e.g. wall-walking)
FVector CharacterDown = -Character->GetActorUpVector();
KawaiiPhysics = UKawaiiPhysicsLibrary::SetGravity(KawaiiPhysics, CharacterDown * 980.0f);

Following the Character's Gravity Direction (Preset)​

To automatically follow the Character Movement gravity direction (UE5.3+ custom gravity), use the FKawaiiPhysics_ExternalForce_Gravity preset instead of the Gravity property.

FKawaiiPhysics_ExternalForce_Gravity GravityForce;
GravityForce.bUseCharacterGravityDirection = true;  // follow the Character's gravity direction
GravityForce.bUseCharacterGravityScale = true;      // use the Character's GravityScale

See External Force Presets for details.

Use Cases​

Wall-Walking Character​

When a character walks on walls, align gravity with the opposite of the wall normal.

void UpdateGravity(FVector WallNormal)
{
    // Set the opposite of the wall normal as gravity
    KawaiiPhysics = UKawaiiPhysicsLibrary::SetGravity(KawaiiPhysics, -WallNormal * 980.0f);
}

Outer Space (Zero Gravity)​

Node->Gravity = FVector::ZeroVector;  // same as default (no gravity)

Underwater (Buoyancy)​

// A weak upward force expresses buoyancy
Node->Gravity = FVector(0, 0, 300.0f);

Gravity Per Body Part​

To apply different gravity to different parts, use multiple KawaiiPhysics nodes.

AnimGraph
├── KawaiiPhysics (Hair: normal gravity)
│   Gravity = (0, 0, -980)
│
└── KawaiiPhysics (Wings: upward buoyancy)
    Gravity = (0, 0, 200)

Dynamic Gravity Changes​

To change gravity during gameplay, call SetGravity from Blueprint / C++.

// Example: update gravity from a Blueprint-callable function
void AMyCharacter::SetHairGravity(FVector NewGravity)
{
    // Reference the KawaiiPhysics node in the AnimGraph via ConvertToKawaiiPhysics,
    // then call UKawaiiPhysicsLibrary::SetGravity(Ref, NewGravity)
}

For more details, see Runtime Control.