Anchor Soft Body to Rigid Body in Ammo.js

In physics simulations using Ammo.js (the JavaScript port of the Bullet Physics engine), attaching soft bodies like ropes, capes, or flags to standard rigid bodies is a common requirement. This article provides a direct, step-by-step guide on how to securely anchor a specific node of a soft body to a rigid body using the native appendAnchor method, ensuring a stable and realistic physical connection in your 3D application.

Step 1: Identify the Target Node Index

A soft body in Ammo.js consists of a collection of nodes (vertices). To anchor the soft body, you must first determine the index of the node you want to attach. For example, if you are anchoring a rope, you will typically want to anchor the first node (index 0) or the last node. For a cloth, you might anchor the corner nodes.

// Example: Selecting the first node of the soft body
const nodeIndex = 0;

Step 2: Prepare the Rigid Body

Ensure your target rigid body (btRigidBody) is already created and added to the physics world. The rigid body acts as the parent anchor point; as it moves or rotates, the anchored soft body node will follow it.

Step 3: Use the appendAnchor Method

The connection is established using the appendAnchor method belonging to the btSoftBody class. This method links a specific node index to a rigid body.

The syntax for the method in Ammo.js is as follows:

softBody.appendAnchor(nodeIndex, rigidBody, disableCollision, influence);

Complete Code Implementation

Here is a practical JavaScript example of how to configure and apply the anchor:

// 1. Create your rigid body (e.g., a kinematic or dynamic box)
const rigidBody = createMyRigidBody(); 

// 2. Create your soft body (e.g., a rope or cloth)
const softBody = createMySoftBody();

// 3. Define anchor parameters
const nodeIndex = 0;           // Anchor the very first node of the soft body
const disableCollision = true; // Disable collisions between the two linked bodies
const influence = 1.0;         // 100% influence for a secure, unbreakable bond

// 4. Append the anchor
softBody.appendAnchor(nodeIndex, rigidBody, disableCollision, influence);

Best Practices for Stability