How to Simulate an Air Cushion in Matter.js

Simulating an inflatable air cushion in Matter.js requires a combination of soft-body physics, tuned damping constraints, and custom force calculations to mimic internal pneumatic pressure. This guide walks through constructing a deformable cushion using Matter.js composites, configuring physical properties to absorb high-velocity impacts without excessive bouncing or tunneling, and applying custom collision events to simulate progressive air compression and venting.

1. Build the Deformable Membrane

Matter.js does not feature a native pneumatic fluid-body type, so an air cushion is best modeled as a soft-body composite. A soft body consists of a grid of rigid circular bodies connected by elastic distance constraints.

Use Matter.Composites.softBody to generate the structure:

const cushion = Matter.Composites.softBody(x, y, columns, rows, columnGap, rowGap, crossBrace, particleRadius, particleOptions, constraintOptions);

Grid dimensions: A grid of 8 columns by 3 rows generally provides enough surface resolution to deform around falling objects.
Particle options: Set density high enough to resist immediate collapse, but keep individual particle mass modest so the structure remains compliant.
Cross-bracing: Set crossBrace: true to prevent the cushion from shearing completely flat on impact.

2. Configure Restitution and Damping

An air cushion must dissipate kinetic energy rapidly. In standard physics engines, high-velocity collisions produce either an extreme rebound (high restitution) or a stiff collision that acts like concrete (low restitution with rigid bodies).

To achieve safe deceleration:

Zero Restitution: Set the restitution (bounciness) of both the cushion particles and the falling body to 0. Energy absorption should come from the constraints, not surface bounce.
Constraint Damping and Stiffness: In constraintOptions, set stiffness to a moderate range (e.g., 0.2 to 0.5) and damping to a high range (e.g., 0.1 to 0.3). This allows the cushion to give way upon impact while bleeding off the projectile's kinetic energy as heat-like mechanical loss.

3. Simulate Internal Air Pressure

While the outer membrane constraints hold the cushion together, a real inflatable cushion resists compression non-linearly: the more it is compressed, the higher the internal pressure pushes back.

To simulate this dynamic pressure, attach a beforeUpdate event to apply an upward counter-force proportional to the cushion's displacement:

Matter.Events.on(engine, 'beforeUpdate', () => {
    cushion.bodies.forEach(body => {
        // Calculate displacement from the resting Y position
        const displacementY = body.position.y - body.initialY;

        if (displacementY > 0) {
            // Apply progressive restorative force (Hooke's Law variation)
            const pressureForce = displacementY * 0.005;
            Matter.Body.applyForce(body, body.position, { x: 0, y: -pressureForce });
        }
    });
});

Store the original Y coordinate (body.initialY = body.position.y) upon creation. When an object drives the cushion downward, the counter-force pushes back progressively to stop the descent before the object hits the floor.

4. Implement Air Venting Dynamics

True stunt cushions allow air to escape through vents when struck, preventing the falling body from rebounding. You can mimic air venting by dynamically increasing air friction during active compression:

Matter.Events.on(engine, 'collisionActive', (event) => {
    event.pairs.forEach(pair => {
        if (pair.bodyA.label === 'fallingObject' || pair.bodyB.label === 'fallingObject') {
            const object = pair.bodyA.label === 'fallingObject' ? pair.bodyA : pair.bodyB;
            
            // Emulate air displacement resistance
            object.frictionAir = 0.15;
        }
    });
});

Matter.Events.on(engine, 'collisionEnd', (event) => {
    event.pairs.forEach(pair => {
        if (pair.bodyA.label === 'fallingObject' || pair.bodyB.label === 'fallingObject') {
            const object = pair.bodyA.label === 'fallingObject' ? pair.bodyA : pair.bodyB;
            
            // Reset to default aerodynamic resistance
            object.frictionAir = 0.01;
        }
    });
});

5. Prevent Tunneling on High-Velocity Impacts

High-velocity objects can pass straight through thin bodies in discrete physics steps—an issue known as tunneling. To ensure stability during extreme falls:

Increase Engine Iterations: Increase the solver accuracy in the engine configuration:
```
engine.positionIterations = 10;
engine.velocityIterations = 10;
```
Anchor the Base: Anchor the bottom row of particles in the soft body by setting isStatic: true on those specific bodies, or bind them to a static ground segment with fixed constraints.
Thicken the Collider: Avoid paper-thin falling objects. Use thicker bounding geometry for falling entities to maximize the contact frames available to the collision solver.