Can you use ammo.js for 2D physics?

While ammo.js is fundamentally a 3D physics engine, it is not strictly limited to 3D and can be configured to run 2D physics simulations. By constraining rigid body movement and rotation to a single two-dimensional plane, developers can successfully simulate 2D environments. This article explains how ammo.js achieves 2D physics, the technical limitations of doing so, and whether it is the right choice for your project.

How ammo.js Simulates 2D Physics

Because ammo.js is a direct JavaScript port of the Bullet Physics engine, it inherits Bullet’s robust 3D mathematics. To force a 3D rigid body to behave as if it is in a 2D space, you must restrict its degrees of freedom.

In ammo.js, this is accomplished using “linear factors” and “angular factors” on the rigid bodies. By setting these factors, you can lock specific axes of movement and rotation:

• Linear Factor: To restrict movement to the X and Y axes, you set the linear factor to (1, 1, 0). This prevents the body from moving along the Z-axis.
• Angular Factor: To restrict rotation so objects only rotate around the Z-axis (which is necessary for 2D top-down or side-scrolling physics), you set the angular factor to (0, 0, 1).

By applying these constraints to all rigid bodies in your simulation, the physics world effectively operates on a flat 2D plane.

Performance and Overhead

While simulating 2D in ammo.js works, it is not highly optimized for pure 2D games. Because the engine is inherently 3D, it still performs 3D collision detection and mathematical calculations behind the scenes, even if the result on one axis is always zero.

This introduces unnecessary CPU overhead compared to a dedicated 2D physics engine. If your project features hundreds of simulated objects, using ammo.js for 2D can result in lower performance than using an engine built from the ground up for 2D mathematics.

When to Use ammo.js for 2D

Using ammo.js for 2D physics is ideal in specific scenarios:

• 2.5D Games: If your game uses 3D graphics and complex 3D meshes for collisions, but restricts gameplay to a 2D plane (like a side-scrolling platformer with 3D depth), ammo.js is a great fit.
• Unified Physics Pipeline: If your project already uses ammo.js for 3D elements and you need to implement a minor 2D minigame or UI physics layer, sticking to one engine simplifies your codebase.

Better Alternatives for Pure 2D

If your project is entirely 2D, you should avoid ammo.js in favor of dedicated 2D JavaScript physics engines. These engines are faster, easier to configure, and consume fewer resources because they only calculate two axes of data. Popular alternatives include:

• Matter.js: A popular, feature-rich 2D physics engine for the web.
• Planck.js: A JavaScript rewrite of the industry-standard Box2D engine.
• P2.js: A high-performance, full-featured 2D physics engine.