How to Set btBoxShape Collision Margin in Ammo.js

This article explains how to adjust the default collision margin of a btBoxShape in Ammo.js, the WebAssembly port of the Bullet physics engine. You will learn the exact API methods required to modify this margin, why it is important for your 3D physics simulation, and how to implement it using a practical JavaScript code example.

In Ammo.js, collision shapes use an internal margin to make collision detection faster and more stable. By default, Bullet physics assigns a collision margin of 0.04 units to shapes. For a btBoxShape, the engine automatically compensates for this margin by shrinking the collision box slightly and expanding it by the margin size. However, you can manually adjust this value to fine-tune the collision behavior of your 3D objects.

To adjust the collision margin of a btBoxShape, you must call the setMargin() method directly on your shape instance.

Here is a practical code example demonstrating how to initialize a box shape and change its default margin:

// Assuming Ammo.js is already initialized and loaded

// 1. Define the half-extents of the box (width, height, depth divided by 2)
const halfExtents = new Ammo.btVector3(1.0, 1.0, 1.0);

// 2. Create the btBoxShape instance
const boxShape = new Ammo.btBoxShape(halfExtents);

// 3. Define your custom margin value (e.g., 0.01 for tighter collisions)
const customMargin = 0.01;

// 4. Apply the margin to the shape
boxShape.setMargin(customMargin);

// Optional: Verify the margin has been updated
console.log("New collision margin:", boxShape.getMargin());

While you can reduce the margin to 0.0 for highly precise boundaries, doing so is generally discouraged. Removing the collision margin completely can lead to physics instability, causing objects to jitter, penetrate one another, or fall through solid ground. For optimal performance and realism, keep the margin at a small positive value, typically between 0.01 and 0.04.