How to Set Identity Transform in Ammo.js

This article explains how to definitively reset a rigid body’s position and rotation to the default identity state in the Ammo.js physics engine. You will learn the exact API calls required to update the body’s world transform, synchronize its motion state, clear any lingering velocities, and force the physics simulation to recognize the change immediately.

To set a physics object’s transform to the identity state (zero translation and zero rotation) in Ammo.js, you must update both the rigid body’s world transform and its motion state, then clear its velocities and reactivate the body. Simply modifying the transform is often not enough, as the physics engine may overwrite the changes during the next simulation step if the body is asleep or if the motion state is out of sync.

Here is the complete, step-by-step implementation:

// 1. Create a temporary Ammo transform helper if you don't have one cached
const transform = new Ammo.btTransform();

// 2. Set the transform to identity (Position: 0,0,0 | Rotation: 0,0,0,1)
transform.setIdentity();

// 3. Apply the transform directly to the rigid body
rigidBody.setWorldTransform(transform);

// 4. Update the motion state (crucial if you are syncing with a renderer like Three.js)
const motionState = rigidBody.getMotionState();
if (motionState) {
    motionState.setWorldTransform(transform);
}

// 5. Clear linear and angular velocities to prevent the object from keeping its momentum
const zeroVector = new Ammo.btVector3(0, 0, 0);
rigidBody.setLinearVelocity(zeroVector);
rigidBody.setAngularVelocity(zeroVector);

// 6. Force the physics body to activate so the engine processes the change
rigidBody.activate(true);

// Clean up temporary pointers if created locally
Ammo.destroy(transform);
Ammo.destroy(zeroVector);

Why These Steps Are Necessary