Adding Realistic Physics to Babylon.js with Cannon.js – A Practical Guide

Problem: How to Make a Babylon.js Scene Feel Real

When you drop a sphere onto a plane in a Babylon.js demo, it just falls and stops. That’s because the engine has no physics engine attached. Developers want gravity, bounce, and collision detection without writing a physics engine from scratch.

Thesis: Cannon.js is a lightweight, well‑documented solution that plugs cleanly into Babylon.js’s PhysicsEngine API.

Babylon.js exposes a PhysicsEngine abstraction that accepts a plugin. Cannon.js is the most common CPU‑based plugin because it is small, runs in the main thread, and works in all major browsers. The integration is straightforward and keeps your scene logic in one place.

Section 1 – Setting Up the Environment

1. Install the libraries – Use npm or CDN. For a quick demo:

   
   <script src="https://cdn.babylonjs.com/babylon.js"></script>
   <script src="https://cdn.babylonjs.com/cannon.js"></script>
   <script src="https://cdn.babylonjs.com/cannon.js"></script>
   

2. Create the engine and scene – Standard Babylon.js boilerplate:

   const canvas = document.getElementById('renderCanvas');
   const engine = new BABYLON.Engine(canvas, true);
   const scene = new BABYLON.Scene(engine);
   

3. Enable physics – Call scene.enablePhysics with a gravity vector and a CannonJSPlugin instance. This must run after the scene is created but before any impostors are added.

   scene.enablePhysics(new BABYLON.Vector3(0, -9.81, 0), new BABYLON.CannonJSPlugin());
   

   After this call, the scene’s physics engine is ready. You can verify it in the console:

   console.log(scene.getPhysicsEngine()); // should show a CannonPhysicsPlugin instance
   

Section 2 – Adding Physics‑Enabled Meshes

Babylon.js uses PhysicsImpostor to give a mesh physical properties. The constructor takes the mesh, the impostor type, and a configuration object.

// Ground – static plane
const ground = BABYLON.MeshBuilder.CreateGround('ground', {width: 10, height: 10}, scene);
ground.physicsImpostor = new BABYLON.PhysicsImpostor(ground, BABYLON.PhysicsImpostor.PlaneImpostor, {mass: 0, restitution: 0.2}, scene);

// Falling sphere
const sphere = BABYLON.MeshBuilder.CreateSphere('sphere', {diameter: 1}, scene);
sphere.position.y = 5;
sphere.physicsImpostor = new BABYLON.PhysicsImpostor(sphere, BABYLON.PhysicsImpostor.SphereImpostor, {mass: 1, restitution: 0.7}, scene);

// Stack of boxes
for (let i = 0; i < 5; i++) {
  const box = BABYLON.MeshBuilder.CreateBox(`box${i}`, {size: 1}, scene);
  box.position.set(2, 1 + i * 1.1, 0);
  box.physicsImpostor = new BABYLON.PhysicsImpostor(box, BABYLON.PhysicsImpostor.BoxImpostor, {mass: 1, restitution: 0.1}, scene);
}

Each impostor receives a mass (0 for static objects), a restitution (bounciness), and optionally friction. Once impostors are attached, the physics engine automatically updates the meshes each frame.

Section 3 – Running the Render Loop

Start the render loop as usual. The physics simulation runs in sync with the rendering, so no extra steps are needed.

engine.runRenderLoop(() => {
  scene.render();
});

Open the page in a browser, and you should see the sphere fall, bounce, and the boxes tumble onto the ground. Use the browser console to inspect positions:

console.log(sphere.position); // updates each frame

Section 4 – Collision Callbacks and Events

Babylon.js exposes an observable on each impostor. For example, to log when the sphere hits a box:

sphere.physicsImpostor.onCollideObservable.add((collider) => {
  console.log('Sphere collided with', collider.object.name);
});

Remember that collision callbacks fire after the physics step, so the positions are already updated.

Trade‑Offs and Limitations

• CPU Load – Cannon.js runs entirely on the main thread. A scene with dozens of high‑mass objects or very small time steps can cause frame drops. Profiling is essential: use the browser’s performance panel to monitor physics time.
• Precision vs. Speed – Lowering the solver iterations (default 10) speeds up simulation but reduces collision accuracy. Adjust scene.getPhysicsEngine().setSolverIterations(5) if you need a faster but less precise simulation.
• Future deprecation – Newer Babylon.js releases (v6.x) are moving toward Ammo.js as the default physics engine, and the CannonJSPlugin may be deprecated. Verify the current API before shipping a production build.
• WebWorker Offload – Cannon.js can be run in a WebWorker to free the main thread, but that requires a custom wrapper and more code. For most demos, the main‑thread approach is sufficient.

Actionable Next Steps

1. Confirm the plugin is available in your Babylon.js version: console.log(BABYLON.CannonJSPlugin) should not be undefined.
2. Experiment with different restitution and friction values to match the desired material feel.
3. Profile the scene with the browser’s performance tools. If physics time exceeds 10 % of the frame budget, consider lowering solver iterations or moving to Ammo.js.
4. Consider adding a PhysicsImpostor.setAngularVelocity to simulate spinning objects.
5. When ready for production, bundle the libraries with a build tool (Webpack, Vite) and include the Cannon.js source to avoid runtime CDN failures.

With these steps, you can turn a static Babylon.js demo into a dynamic, physics‑aware scene that feels natural to users. The integration is lightweight, well‑documented, and works across browsers, making it a solid choice for most web‑based 3D projects.