Build a Ballista with Constraints in Matter.js

This guide explains how to construct a functioning, physics-driven ballista in Matter.js using flexible constraint arms and elastic bowstrings. You will learn the structural architecture required for the weapon, how to configure rotational limbs and spring-loaded constraints, and how to harness stored tension to launch high-velocity projectiles within a 2D physics simulation.

Core Physics Architecture

A functional ballista in Matter.js relies on four interconnected components:

  1. The Base and Stock: A static or heavy rigid body acting as the mounting point and guide track.
  2. The Pivot Arms (Limbs): Rigid dynamic rectangles anchored to the stock via rotational constraints.
  3. Torsion Springs: High-stiffness constraints pulling the arms forward into their resting position.
  4. The Bowstring: Elastic constraints running from the tips of each arm to the projectile or a central carriage.

Step 1: Initializing Matter.js Modules

Import the required modules from the Matter.js engine:

const {
    Engine,
    Render,
    Runner,
    Bodies,
    Composite,
    Constraint,
    Vector,
    Body
} = Matter;

const engine = Engine.create();
const world = engine.world;

Step 2: Creating the Base and Limbs

The stock anchors the arms and guides the projectile. Define the stock as a static body and attach two symmetric arms using pivot constraints.

// Stock / Base
const stock = Bodies.rectangle(400, 300, 250, 20, { isStatic: true });

// Limbs
const limbWidth = 10;
const limbLength = 100;

const leftArm = Bodies.rectangle(350, 250, limbWidth, limbLength, {
    collisionFilter: { group: -1 }
});

const rightArm = Bodies.rectangle(450, 250, limbWidth, limbLength, {
    collisionFilter: { group: -1 }
});

// Pivot joints (Revolute Constraints)
const leftPivot = Constraint.create({
    bodyA: stock,
    pointA: { x: -60, y: 0 },
    bodyB: leftArm,
    pointB: { x: 0, y: 30 },
    stiffness: 1,
    length: 0
});

const rightPivot = Constraint.create({
    bodyA: stock,
    pointA: { x: 60, y: 0 },
    bodyB: rightArm,
    pointB: { x: 0, y: 30 },
    stiffness: 1,
    length: 0
});

Using negative numbers for collisionFilter.group prevents the moving components from colliding with each other during operation.

Step 3: Adding Spring Tension to the Limbs

To replicate torsion bundles, attach forward-pulling springs to the tips of each limb. These constraints pull the arms forward toward anchors on the stock.

// Torsion Springs (Stiffness provides firing power)
const leftTorsionSpring = Constraint.create({
    bodyA: stock,
    pointA: { x: -100, y: -40 },
    bodyB: leftArm,
    pointB: { x: 0, y: -40 },
    stiffness: 0.15,
    damping: 0.05,
    length: 20
});

const rightTorsionSpring = Constraint.create({
    bodyA: stock,
    pointA: { x: 100, y: -40 },
    bodyB: rightArm,
    pointB: { x: 0, y: -40 },
    stiffness: 0.15,
    damping: 0.05,
    length: 20
});

Increasing the stiffness value increases the forward acceleration of the arms when drawn back.

Step 4: Rigging the Bowstring and Projectile

The bowstring links both arm tips to the projectile. Using a light projectile ensures maximum energy transfer from the limbs.

// Bolt (Projectile)
const bolt = Bodies.rectangle(400, 280, 80, 10, {
    density: 0.005,
    frictionAir: 0.001
});

// Bowstring - Left Side
const stringLeft = Constraint.create({
    bodyA: leftArm,
    pointB: { x: 0, y: -45 },
    bodyB: bolt,
    pointA: { x: 0, y: 0 },
    stiffness: 0.8,
    damping: 0.01,
    render: { strokeStyle: '#ffffff', lineWidth: 2 }
});

// Bowstring - Right Side
const stringRight = Constraint.create({
    bodyA: rightArm,
    pointB: { x: 0, y: -45 },
    bodyB: bolt,
    pointA: { x: 0, y: 0 },
    stiffness: 0.8,
    damping: 0.01,
    render: { strokeStyle: '#ffffff', lineWidth: 2 }
});

Composite.add(world, [
    stock,
    leftArm,
    rightArm,
    leftPivot,
    rightPivot,
    leftTorsionSpring,
    rightTorsionSpring,
    bolt,
    stringLeft,
    stringRight
]);

Step 5: Implementing the Draw and Release Mechanism

A ballista operates by pulling the projectile back along the stock to load potential energy into the torsion springs, then releasing the string constraints.

function drawBallista(drawDistance) {
    // Move the bolt back along the stock track
    Body.setPosition(bolt, { x: 400, y: 300 + drawDistance });
    Body.setVelocity(bolt, { x: 0, y: 0 });
}

function fireBallista() {
    // Listen for the point where the projectile passes the resting threshold
    Matter.Events.on(engine, 'afterUpdate', function releaseMechanism() {
        if (bolt.position.y <= 240) {
            // Sever the string constraints to let the bolt fly free
            Composite.remove(world, stringLeft);
            Composite.remove(world, stringRight);
            
            // Remove listener after trigger
            Matter.Events.off(engine, 'afterUpdate', releaseMechanism);
        }
    });
}

Optimization and Tuning Tips