Matter.js Ragdoll Dismemberment System

Creating a ragdoll dismemberment system in Matter.js involves constructing body segments connected by physics constraints, monitoring collision events for high-impact forces, and detaching limbs dynamically when those forces exceed defined thresholds. This guide covers how to set up the ragdoll's skeletal hierarchy, track joint stress using impact velocity, and cleanly break constraints to simulate realistic limb detachment.

1. Constructing the Ragdoll Hierarchy

A basic ragdoll consists of individual rigid bodies (head, torso, upper/lower arms, and upper/lower legs) connected by Matter.Constraint instances.

To facilitate dismemberment, assign unique identifiers or custom metadata to each constraint, including a breakForce threshold and the connected body references.

const { Bodies, Body, Composite, Constraint, World } = Matter;

function createLimb(x, y, width, height, label) {
  return Bodies.rectangle(x, y, width, height, {
    label: label,
    collisionFilter: { group: Body.nextGroup(true) }, // Prevent self-collision initially
    density: 0.001
  });
}

function createBreakableJoint(bodyA, bodyB, pointA, pointB, breakForce) {
  const joint = Constraint.create({
    bodyA,
    bodyB,
    pointA,
    pointB,
    stiffness: 0.6,
    damping: 0.1,
    render: { visible: true, lineWidth: 4, strokeStyle: '#555' }
  });

  // Custom property to define how much force the joint can handle
  joint.breakForce = breakForce;
  return joint;
}

Assemble the ragdoll by anchoring limbs to the torso and lower limbs to upper limbs, then add both bodies and constraints to the Matter.World.

2. Measuring Collision and Impact Forces

Matter.js does not calculate continuous tension on constraints automatically. The most reliable method to detect excessive force is by listening to collision events (collisionStart or collisionActive) and calculating the relative impact impulse of the colliding bodies.

Impact force can be estimated using the relative velocity and mass of the colliding objects:

\[\text{Impact Force} \approx |\vec{v}_A - \vec{v}_B| \times \text{mass}\]

Matter.Events.on(engine, 'collisionStart', (event) => {
  event.pairs.forEach((pair) => {
    const { bodyA, bodyB } = pair;

    // Calculate relative velocity
    const relativeVelocity = Matter.Vector.sub(bodyA.velocity, bodyB.velocity);
    const speed = Matter.Vector.magnitude(relativeVelocity);

    // Calculate approximate impact force
    const impactForce = speed * Math.min(bodyA.mass, bodyB.mass);

    // Check if either body should detach from its constraints
    checkJointBreakage(bodyA, impactForce);
    checkJointBreakage(bodyB, impactForce);
  });
});

3. Detaching the Limbs

When a body experiences an impact force higher than its connected joint's breakForce, the corresponding constraint must be removed from the physics simulation.

Maintain a list or look up constraints associated with the impacted body in engine.world.constraints:

function checkJointBreakage(body, force) {
  const allConstraints = Composite.allConstraints(engine.world);

  for (let i = allConstraints.length - 1; i >= 0; i--) {
    const joint = allConstraints[i];

    if (joint.breakForce && (joint.bodyA === body || joint.bodyB === body)) {
      if (force >= joint.breakForce) {
        // Remove constraint to dismember the limb
        Composite.remove(engine.world, joint);

        // Adjust collision filters so the severed limb collides normally
        body.collisionFilter.group = 0;

        // Apply a small residual impulse to the severed limb
        Body.applyForce(body, body.position, {
          x: (Math.random() - 0.5) * 0.05,
          y: (Math.random() - 0.5) * 0.05
        });
      }
    }
  }
}

4. Handling Cascading Dismemberment

When an upper limb (such as an upper arm) is severed from the torso, the lower limb (forearm) should remain attached to the upper arm unless it also experiences an excessive impact.

By structuring the ragdoll hierarchically:

Breaking Constraint A automatically leaves Constraint B intact. The upper arm and forearm continue to behave as a two-piece swinging composite independent of the torso.

5. Tuning Stability and Performance