Deterministic Physics in Ammo.js Across Devices

Achieving deterministic physics across different devices using ammo.js—the JavaScript port of the C++ Bullet Physics engine—presents unique challenges due to platform-specific hardware and browser runtime variations. This article explores how ammo.js processes physics calculations, why floating-point discrepancies occur across different devices, and the specific strategies developers must use to enforce consistent simulation results.

The Foundation of Ammo.js Determinism

ammo.js is compiled directly from the C++ Bullet Physics source code using Emscripten. Because Bullet is designed to be a deterministic physics engine, ammo.js inherits its mathematical predictability. If you feed the exact same inputs and state into the engine under identical conditions, it will theoretically produce the exact same output.

However, because ammo.js runs in web browsers via WebAssembly (Wasm) or JavaScript, cross-device determinism is not guaranteed out of the box. True determinism requires controlling how the engine calculates time, handles floating-point math, and compiles instructions on different CPU architectures.

The Challenge of Cross-Device Floating-Point Math

The primary barrier to cross-device determinism in ammo.js is how different devices handle floating-point arithmetic.

• IEEE 754 Differences: While modern devices conform to the IEEE 754 standard for floating-point representation, compilers and hardware architectures (such as x86 vs. ARM) process these numbers differently.
• Fused Multiply-Add (FMA): Some modern CPUs combine multiplication and addition into a single instruction (FMA) to increase performance, which reduces rounding errors. Older CPUs execute these as two separate steps. This microscopic difference in rounding cascades over thousands of physics steps, causing simulations on an iPhone (ARM) to rapidly diverge from those on a desktop PC (x86).
• Browser Engines: Different JavaScript/WebAssembly engines (V8 in Chrome, Spidermonkey in Firefox, JavaScriptCore in Safari) apply different optimizations to float operations, further contributing to drift.

Enforcing Determinism in Ammo.js

To mitigate these hardware differences and achieve consistent physics simulations across client devices, developers must implement the following architectural practices:

1. Lock the Simulation Timestep

By default, games run with variable frame rates. If one device runs at 60 FPS and another at 120 FPS, passing the variable delta time directly to the physics engine will destroy determinism.

To prevent this, you must use ammo.js’s internal fixed-timestep accumulator. When calling the step function:

// dynamicsWorld.stepSimulation(timeElapsed, maxSubSteps, fixedTimeStep);
dynamicsWorld.stepSimulation(deltaTime, 10, 1 / 60);

• fixedTimeStep (third argument): Set to a constant value (usually 1/60 or 1/120 seconds). This ensures the engine only advances the physics state in exact, uniform increments.
• maxSubSteps (second argument): Allows the engine to run multiple physics substeps per render frame to catch up if the frame rate drops.

2. Avoid Native JavaScript Math

Any calculations that affect physics inputs (like forces, velocities, or spawns) must avoid native JavaScript Math functions (like Math.sin or Math.random) if they need to be deterministic. Instead, use deterministic pseudo-random number generators (PRNGs) with a fixed seed, and ensure any pre-calculations use consistent WebAssembly-compatible math structures.

3. Maintain Consistent Object Addition Order

Bullet’s constraint solvers solve contact points and constraints sequentially. The order in which rigid bodies and constraints are added to the ammo.js world determines their memory layout and execution order. If Device A adds Object X before Object Y, and Device B does the opposite, the engine’s solver will yield slightly different results. Always instantiate and add physical bodies to the simulation in a strict, identical sequence on all devices.

4. Enable Deterministic Compilation Flags

When building or selecting an ammo.js build, ensure it is compiled with strict floating-point compliance. Emscripten compiles WebAssembly with optimizations that can sometimes sacrifice mathematical precision for execution speed. Utilizing WebAssembly builds rather than pure asm.js is highly recommended, as Wasm enforces stricter IEEE 754 compliance across modern browser environments.

The Ultimate Solution: Server-Authoritative State

Because 100% perfect cross-device client-side determinism is nearly impossible to guarantee in WebAssembly due to hardware-level CPU variations, real-time multiplayer applications should not rely solely on ammo.js determinism.

Instead, run a headless version of the simulation on a server (e.g., using Node.js and an ammo.js port). Treat the server’s simulation as the single source of truth, and use client-side ammo.js instances strictly for prediction and interpolation. If a client’s local simulation drifts due to hardware differences, snap or smoothly interpolate the client back to the server-provided state.