Get Applied Impulse from Ammo.js Collisions

In physics-based web applications, determining the exact force of a collision is essential for features like calculating impact damage, playing dynamic audio, or triggering visual effects. This article provides a direct, step-by-step guide on how to access the physics dispatcher in ammo.js (the JavaScript port of the Bullet physics engine) to retrieve the exact applied impulse from contact manifolds during a collision event.

To retrieve the applied impulse, you must query the collision dispatcher after the physics world performs its simulation step. Ammo.js stores collision data in contact manifolds, which contain the contact points between colliding pairs of rigid bodies.

Here is the step-by-step implementation to extract this data:

Step 1: Access the Collision Dispatcher

After calling physicsWorld.stepSimulation(), retrieve the dispatcher from your dynamics world to access the current frame’s collision pairs.

const dispatcher = physicsWorld.getDispatcher();
const numManifolds = dispatcher.getNumManifolds();

Step 2: Iterate Through Contact Manifolds

Loop through all active manifolds to inspect the colliding pairs. You must cast the generic collision objects to btRigidBody to identify which specific entities are colliding.

for (let i = 0; i < numManifolds; i++) {
    const manifold = dispatcher.getManifoldByIndexInternal(i);
    
    const body0 = Ammo.castObject(manifold.getBody0(), Ammo.btRigidBody);
    const body1 = Ammo.castObject(manifold.getBody1(), Ammo.btRigidBody);

    // Identify your target objects (e.g., using userPointer or custom properties)
    if (isTargetCollision(body0, body1)) {
        const totalImpulse = getCollisionImpulse(manifold);
        if (totalImpulse > 0) {
            console.log(`Collision detected with an impulse of: ${totalImpulse}`);
        }
    }
}

Step 3: Extract the Impulse from Contact Points

Each manifold contains one or more contact points where the bodies touch. You must loop through these points and call getAppliedImpulse() on each to get the exact scalar impulse applied to resolve the collision.

function getCollisionImpulse(manifold) {
    const numContacts = manifold.getNumContacts();
    let totalImpulse = 0;

    for (let j = 0; j < numContacts; j++) {
        const contactPoint = manifold.getContactPoint(j);
        
        // Retrieve the applied impulse for this specific contact point
        const impulse = contactPoint.getAppliedImpulse();
        totalImpulse += impulse;
    }

    return totalImpulse;
}

Key Considerations