Smooth Network Corrections for Ammo.js Physics

Synchronizing multiplayer physics using ammo.js often results in visual jitter or teleportation due to network latency and packet loss. This article covers the essential techniques used to smooth out network corrections for ammo.js rigid bodies, including visual interpolation, client-side prediction, error decay, and the use of kinematic proxies to ensure a seamless multiplayer physics experience.

Visual and Physics State Separation

The most effective way to prevent jitter in ammo.js is to separate the rendering representation (the visual mesh) from the actual physics representation (the btRigidBody).

Instead of directly snapping the rigid body to the server’s authoritative position, update the physics body immediately (or let the client-side prediction handle it) and smoothly interpolate the visual mesh toward the physics body’s position. This ensures that even if the physics state snaps abruptly due to a server correction, the player only sees a smooth transition.

Linear and Spherical Interpolation (Lerp and Slerp)

When a server correction packet arrives, do not teleport the entity. Instead, interpolate the position and rotation over time.

In ammo.js, you can extract the transform using getWorldTransform() on the rigid body, convert the origin and rotation to your WebGL renderer’s format (such as Three.js vectors and quaternions), and apply the interpolation step during the render loop.

Client-Side Prediction and Reconciliation

To eliminate input lag, the client should run its own local ammo.js simulation ahead of the server.

  1. Predict: Apply user inputs locally to the ammo.js rigid body immediately.
  2. Store: Keep a history of inputs and resulting physics states (positions, velocities, and rotations) alongside a timestamp or frame number.
  3. Reconcile: When the server sends an authoritative state update, compare the client’s historical state at that specific timestamp with the server’s state. If the difference exceeds a certain threshold, snap the local physical body to the server state and re-simulate all inputs that occurred after that timestamp.

Error Decay (Delta Smoothing)

When client-side reconciliation triggers a correction, the difference between the predicted position and the server position (the error) can be visually jarring if corrected instantly.

Instead of snapping the physics body, calculate the vector difference between the predicted position and the corrected position. Store this as an “error offset.” In your update loop, add this error offset to the visual mesh, and then decay (reduce) the offset to zero over a short duration (e.g., 100 milliseconds) using exponential decay. This blends the correction seamlessly into the ongoing local simulation.

Kinematic Interpolation Proxies

For non-player controlled dynamic bodies (like boxes, barrels, or other players), running local physics simulation can be expensive and prone to diverging from the server.

Instead of treating these objects as active dynamic rigid bodies on the client, configure them as kinematic bodies (CF_KINEMATIC_OBJECT). Kinematic bodies do not respond to gravity or forces automatically but can still collide with dynamic objects. You can continuously interpolate the kinematic body’s transform using server update packets. This allows other client-controlled dynamic objects to collide realistically with network-synced objects without causing network-induced desynchronization.

Jitter Buffering

Network packets rarely arrive at perfectly consistent intervals. To prevent stuttering caused by packet clumps or gaps, implement a jitter buffer (or interpolation delay).

By delaying the rendering of network-controlled objects by a small buffer time (typically 50–100 milliseconds), you ensure the client always has at least two server states to interpolate between. This creates a smooth, continuous path of movement at the cost of a slight, uniform delay in rendering remote objects.