Limit Angular Velocity in Ammo.js

This article explains the precise methods used to restrict and control the angular velocity of rigid bodies in the ammo.js physics engine. You will learn how to limit rotational speed using manual velocity clamping, restrict rotation to specific axes using angular factors, and apply angular damping for realistic physical resistance.

Manual Angular Velocity Clamping

The most precise way to enforce a strict upper limit on an object’s rotational speed in ammo.js is through manual clamping during the physics simulation’s update loop. Ammo.js does not feature a built-in “max angular velocity” property on rigid bodies, meaning developers must check and cap the velocity vector manually.

To implement this, retrieve the angular velocity vector, check its magnitude, and scale it back if it exceeds your defined threshold:

// Run this inside your physics update loop, before or after world.stepSimulation()
function limitAngularVelocity(rigidBody, maxSpeed) {
    const angularVelocity = rigidBody.getAngularVelocity();
    const speed = angularVelocity.length();

    if (speed > maxSpeed) {
        // Normalize the vector and scale it to the maximum allowed speed
        angularVelocity.normalize();
        angularVelocity.op_mul(maxSpeed);
        
        // Apply the restricted velocity back to the body
        rigidBody.setAngularVelocity(angularVelocity);
    }
}

This method guarantees that the object never rotates faster than the specified radian-per-second limit, ensuring physical stability and preventing rendering glitches caused by extreme speeds.

Restricting Rotational Axes with Angular Factors

If your goal is to precisely restrict rotation along specific coordinate axes (for example, locking a character so they only rotate around the Y-axis), you should use the setAngularFactor method.

By passing a btVector3 to this method, you dictate the influence of forces on each axis. A value of 1 allows free rotation, while 0 completely blocks it.

const angularFactor = new Ammo.btVector3(0, 1, 0); // Only allow rotation around the Y-axis
rigidBody.setAngularFactor(angularFactor);

Using an angular factor of (0, 0, 0) completely prevents the object from rotating due to collisions or external forces, effectively reducing its angular velocity to a permanent zero.

Applying Angular Damping

To prevent objects from spinning indefinitely without hard-clamping their speed, you can apply angular damping. Damping acts as rotational friction, gradually slowing down the object’s rotation over time.

const linearDamping = 0.0;
const angularDamping = 0.5; // Higher values cause faster deceleration
rigidBody.setDamping(linearDamping, angularDamping);

While damping does not set a hard limit on maximum velocity, it naturally opposes acceleration, stabilizing the physics simulation and preventing run-away angular velocities.