How to Create Trigger Volumes in Ammo.js

In physics-based applications and game development, you often need to detect when an object enters a specific 3D region without causing a physical collision response, such as a bounce or blockage. This article guides you through implementing a trigger volume (also known as a sensor area) in Ammo.js, the JavaScript port of the Bullet physics engine. You will learn how to configure a collision object with special flags to disable physical reactions while still generating collision events during the simulation tick.

Step 1: Create the Trigger Object

To create a trigger volume, you instantiate a standard btCollisionObject (or a btRigidBody with zero mass) and assign it a collision shape. The key step is setting the collision flag to CF_NO_CONTACT_RESPONSE. This flag tells the physics solver to detect overlaps but skip the resolution step that pushes objects apart.

// Initialize the shape of the trigger (e.g., a 2x2x2 box)
const triggerShape = new Ammo.btBoxShape(new Ammo.btVector3(1, 1, 1));

// Create a standard collision object
const triggerObject = new Ammo.btCollisionObject();
triggerObject.setCollisionShape(triggerShape);

// Set position and orientation
const transform = new Ammo.btTransform();
transform.setIdentity();
transform.setOrigin(new Ammo.btVector3(0, 5, 0)); // Positioned at (0, 5, 0)
triggerObject.setWorldTransform(transform);

// Retrieve existing flags and add the CF_NO_CONTACT_RESPONSE flag
const CF_NO_CONTACT_RESPONSE = 4; // Flag value for Ammo.btCollisionObject.CF_NO_CONTACT_RESPONSE
triggerObject.setCollisionFlags(triggerObject.getCollisionFlags() | CF_NO_CONTACT_RESPONSE);

// Add the object to your physics world
physicsWorld.addCollisionObject(triggerObject);

Step 2: Set Up Collision Filtering (Optional)

By default, the trigger will check for overlaps with all physical bodies. If you only want the trigger to detect specific types of objects (like the player or projectiles), define collision groups and masks when adding the object to the world:

const TRIGGER_GROUP = 1 << 4; // Custom group for triggers
const PLAYER_GROUP = 1 << 1;  // Custom group for player

// Trigger only collides with the player group
physicsWorld.addCollisionObject(triggerObject, TRIGGER_GROUP, PLAYER_GROUP);

Step 3: Process Overlaps in the Physics Loop

Because the trigger volume does not produce a physical response, you must manually query the dispatcher inside your simulation loop to identify overlapping bodies. You do this by iterating through the contact manifolds after stepping the physics world.

function updatePhysics(deltaTime) {
    // Step the simulation
    physicsWorld.stepSimulation(deltaTime, 10);

    // Read contact manifolds to find overlaps
    const dispatcher = physicsWorld.getDispatcher();
    const numManifolds = dispatcher.getNumManifolds();

    for (let i = 0; i < numManifolds; i++) {
        const manifold = dispatcher.getManifoldByIndexInternal(i);
        const numContacts = manifold.getNumContacts();

        // Skip manifolds with no actual contact points
        if (numContacts === 0) continue;

        const body0 = manifold.getBody0();
        const body1 = manifold.getBody1();

        // Check if one of the bodies involved is our trigger
        const isBody0Trigger = (body0.ptr === triggerObject.ptr);
        const isBody1Trigger = (body1.ptr === triggerObject.ptr);

        if (isBody0Trigger || isBody1Trigger) {
            // Identify the other body that entered the trigger
            const collidingBody = isBody0Trigger ? body1 : body0;
            
            handleTriggerOverlap(collidingBody);
        }
    }
}

function handleTriggerOverlap(collidingBody) {
    // Implement your custom logic here (e.g., deal damage, play sound, load level)
    console.log("An object has entered the trigger zone:", collidingBody.ptr);
}

Cleanup and Memory Management

Because Ammo.js runs on WebAssembly/C++, manual memory management is required. When you destroy the trigger volume, ensure you clean up its associated pointers to prevent memory leaks:

function destroyTrigger(triggerObject) {
    physicsWorld.removeCollisionObject(triggerObject);
    const shape = triggerObject.getCollisionShape();
    Ammo.destroy(shape);
    Ammo.destroy(triggerObject);
}