Networking a Physics Co-op Game: LEGO Voyagers

Light Brick Studio built LEGO Voyagers as a local co-op physics game — then retrofitted online co-op after the game was essentially finished. Any brick connects in countless ways, and two players manipulating the same structure simultaneously creates a networking problem that doesn't have an obvious solution.

The team used Unity Netcode for GameObjects with host-authoritative validation and Relay for matchmaking transport. They implemented a hybrid physics model — local simulation runs normally, but positions are nudged toward network-authoritative values. Fast events like catapult launches use deterministic trajectories: PhysX is disabled, the arc is computed analytically, and physics re-enables on landing.

On Switch, physics runs at 30Hz with a reduced geometry set, model collapse, and simplified shadows and water. Automatic scene conversion tools let designers keep working without rewriting scenes for NGO compatibility. The game shipped on PS4/PS5, Xbox, PC, Switch, and Switch 2.

The biggest challenge was timing: online co-op was added when the game was already finished. The attachment system — where any brick connects in countless ways — had to be networked while two players could simultaneously grab and connect pieces. Designers had also built mechanics inconsistently, with three different lever scripts that all needed to behave identically across clients.

Physics determinism across clients is notoriously unreliable. Even with identical inputs, floating-point differences and PhysX internals cause simulations to diverge. On Switch, the hardware limits physics to 30Hz with a different geometry set than other platforms, adding another layer of inconsistency to reconcile.

The team adopted a hybrid physics approach: local simulation handles frame-to-frame movement, but positions are nudged toward network-authoritative values each tick. This keeps physics feeling responsive locally while staying synchronized across clients. For fast, deterministic events like catapult launches, they disabled PhysX entirely, computed the trajectory analytically, and re-enabled physics on landing — guaranteeing both clients see the same result.

The core architectural principle was "separate execution from validation" — clients execute physics locally, the host validates and corrects. Automatic scene conversion tools scanned designer-built scenes and rewrote them for NGO compatibility, so the team didn't need to manually retrofit every level. On Switch, model collapse, simplified shadows, and reduced water detail kept the physics-heavy game running at 30fps within the hardware's memory constraints.

Online co-op was successfully retrofitted onto a finished game — a rare feat in multiplayer development. The game shipped on PS4/PS5, Xbox, PC, Nintendo Switch, and Switch 2, with stable online play across all platforms. Switch maintains 30fps physics despite its hardware limitations.

The automatic scene conversion tools let designers continue iterating without needing to understand NGO internals, and the hybrid physics model kept gameplay feeling responsive without requiring full determinism — a practical compromise that delivered a shippable product.