How to Create a Hanging Rope in Ammo.js

This article provides a step-by-step guide on how to create a realistic hanging rope using the soft body helpers in the Ammo.js physics engine. You will learn how to configure the physics world for soft bodies, utilize the rope helper function, customize physical properties for realistic behavior, and anchor the rope ends to other objects in your 3D scene.

Setting Up the Soft Body World

To use soft bodies in Ammo.js, you must instantiate a soft-rigid dynamics world instead of a standard rigid-only dynamics world. This requires a specific collision configuration and a soft body solver.

const collisionConfiguration = new Ammo.btSoftBodyRigidBodyCollisionConfiguration();
const dispatcher = new Ammo.btCollisionDispatcher(collisionConfiguration);
const broadphase = new Ammo.btDbvtBroadphase();
const solver = new Ammo.btSequentialImpulseConstraintSolver();
const softBodySolver = new Ammo.btDefaultSoftBodySolver();

const physicsWorld = new Ammo.btSoftRigidDynamicsWorld(
    dispatcher,
    broadphase,
    solver,
    collisionConfiguration,
    softBodySolver
);
physicsWorld.setGravity(new Ammo.btVector3(0, -9.81, 0));

Generating the Rope with Soft Body Helpers

Ammo.js includes a utility class called btSoftBodyHelpers which simplifies the creation of complex soft bodies like ropes and clothes. The CreateRope function builds a soft body rope by defining its start position, end position, and segment resolution.

const softBodyHelpers = new Ammo.btSoftBodyHelpers();

const start = new Ammo.btVector3(0, 10, 0);
const end = new Ammo.btVector3(5, 10, 0);
const segments = 20; // Number of points along the rope
const fixeds = 1;    // 1 pins the start, 2 pins the end, 3 pins both, 0 pins neither

const ropeSoftBody = softBodyHelpers.CreateRope(
    physicsWorld.getWorldInfo(),
    start,
    end,
    segments,
    fixeds
);

Configuring Rope Physics for Realism

By default, the generated rope might stretch or react unrealistically. You can fine-tune its behavior by adjusting its total mass, position iterations, and damping coefficients.

// Set the total mass of the rope
const ropeMass = 1.5;
ropeSoftBody.setTotalMass(ropeMass, false);

// Retrieve and modify the soft body configuration
const sbConfig = ropeSoftBody.get_m_cfg();
sbConfig.set_piterations(4); // Position solver iterations (higher increases stiffness)
sbConfig.set_kDP(0.01);      // Damping coefficient (reduces bounce/jitter)

// Add the soft body to the physics world
physicsWorld.addSoftBody(ropeSoftBody, 1, -1);

Anchoring the Rope to Other Bodies

If you want the rope to hang from or connect to dynamic rigid bodies (such as a crane hook or a swinging anchor), you can link specific nodes of the rope to those bodies using the appendAnchor method.

const nodeIndex = 0; // 0 represents the start of the rope; (segments - 1) represents the end
const targetRigidBody = externalBoxPhysicsBody;
const disableCollisionBetweenLinkedBodies = true;
const influence = 1; // Connection influence strength (typically 1)

ropeSoftBody.appendAnchor(
    nodeIndex, 
    targetRigidBody, 
    disableCollisionBetweenLinkedBodies, 
    influence
);

Updating the Visual Mesh

To render the rope in a 3D graphics library like Three.js, you must read the positions of the soft body nodes during your application’s physics update loop and apply them to your visual geometry.

function updateRopeVisuals(visualMesh) {
    const nodes = ropeSoftBody.get_m_nodes();
    const positions = visualMesh.geometry.attributes.position.array;

    for (let i = 0; i < nodes.size(); i++) {
        const node = nodes.at(i);
        const nodePosition = node.get_m_x(); // Returns a btVector3

        const stride = i * 3;
        positions[stride] = nodePosition.x();
        positions[stride + 1] = nodePosition.y();
        positions[stride + 2] = nodePosition.z();
    }

    visualMesh.geometry.attributes.position.needsUpdate = true;
}