Remove or Break Constraints Dynamically in Ammo.js

This article explains how to systematically break and remove active constraints in the Ammo.js physics engine during runtime. You will learn the correct APIs for removing constraints from the dynamics world, how to safely deallocate WebAssembly memory to prevent leaks, and how to implement a force-based breaking threshold for dynamic joint destruction.

Removing a Constraint from the Dynamics World

To remove an active constraint (such as a hinge, point-to-point, or slider joint) from your physics simulation, you must deregister it from the dynamics world and then manually deallocate its memory. Because Ammo.js is a WebIDL wrapper around the C++ Bullet Physics library, JavaScript’s automatic garbage collection will not free the underlying C++ memory.

Use the following systematic steps to safely remove a constraint:

// 1. Remove the constraint from the physics world
dynamicsWorld.removeConstraint(constraint);

// 2. Delete the C++ object from memory to prevent memory leaks
Ammo.destroy(constraint);

// 3. Nullify your JavaScript reference
constraint = null;

If you do not call Ammo.destroy(), the memory occupied by the constraint will remain allocated in the WebAssembly heap, eventually leading to a crash.

Dynamically Breaking Constraints Based on Applied Force

To make a constraint “breakable” (for example, a rope snapping or a joint breaking under heavy load), you must monitor the forces acting on the constraint during your physics update loop. Ammo.js provides the getAppliedImpulse() method to measure the total impulse applied to the joint in the last physics step.

Step 1: Track the Constraint

Maintain an array or set of active breakable constraints in your simulation loop:

const breakableConstraints = [];

function registerBreakableConstraint(constraint, maxImpulse) {
    breakableConstraints.push({
        ammoConstraint: constraint,
        maxImpulse: maxImpulse
    });
}

Step 2: Check Impulses in the Simulation Loop

After calling dynamicsWorld.stepSimulation(), iterate through your active breakable constraints, check their current impulse, and destroy them if they exceed your threshold.

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

    // Check breakable constraints
    for (let i = breakableConstraints.length - 1; i >= 0; i--) {
        const item = breakableConstraints[i];
        const appliedImpulse = item.ammoConstraint.getAppliedImpulse();

        // If the force exceeds the defined threshold, break the joint
        if (appliedImpulse > item.maxImpulse) {
            // Remove from the physics world
            dynamicsWorld.removeConstraint(item.ammoConstraint);
            
            // Free WebAssembly memory
            Ammo.destroy(item.ammoConstraint);
            
            // Remove from our tracking array
            breakableConstraints.splice(i, 1);
            
            console.log("Constraint broken due to excessive force!");
        }
    }
}

Deactivating Bodies After Constraint Removal

When a constraint is dynamically broken, the two rigid bodies previously connected by the joint may remain in a “sleeping” state if they were not moving before the break. To ensure the bodies react immediately to the loss of the constraint, force them to wake up by activating them:

// Retrieve the rigid bodies attached to the constraint
const bodyA = constraint.getRigidBodyA();
const bodyB = constraint.getRigidBodyB();

// Remove constraint and destroy it
dynamicsWorld.removeConstraint(constraint);
Ammo.destroy(constraint);

// Force both bodies to wake up (activate)
if (bodyA) bodyA.activate(true);
if (bodyB) bodyB.activate(true);