Coping With Unpredictable Lag In Multiplayer Physics Games
The Core Issue – Desync Caused by Variable Network Latency
A key challenge in multiplayer physics games is keeping the game state synchronized across players’ devices when latency in the network causes delays in communication. This unpredictable lag can lead to desynchronization or “desync” – where the physics simulation diverges between clients. Players may then experience glitchy movement and inaccurate collision detection.
Defining the problem more specifically – lag in transmitting a player’s inputs and the server’s physics updates can cause one player’s simulation to advance ahead of another’s. For example, if Player A performs a jump action, but the server’s confirmation of this arrives slowly to Player B, their simulations will differ. When the jump updates finally reach Player B, the position update will seem sudden or “teleporting.”
The impact on gameplay can be jarring, with character actions and physics interactions playing out differently for each player. At best, this disadvantages players with slower connections. But it can make games completely unplayable if desyncs trigger erroneous collisions or fall damage.
Strategies to Mask Lag
Games have adopted various methods to smooth over desync issues and mask underlying lag. These cope with the delays by adding tolerance and cushioning time gaps in the physics simulation.
Extrapolation
Extrapolation means predicting a player’s upcoming actions or physics state based on current trajectory and inputs. For example, predicting a jumping player’s vertical position some milliseconds into the future. This anticipated state gets rendered and simulated speculatively while waiting for the next update.
When the actual update arrives, any discontinuities get smoothed over – reducing glitchiness. Players experience constant responsive visuals and gameplay even when updates arrive late.
Interpolation
Interpolation works similarly but fills in states between old and newly arrived data. Rather than speculate future states, it incrementally blends between the last rendered position and newly updated location. This prevents sudden teleporting when updates arrive late.
Buffering Inputs
Buffering queues player inputs on their local client while awaiting the server’s response. On confirmation, inputs get replayed sequentially to catch up. This fits actions retroactively into the authorized game timeline.
Input buffering allows play to continue unhindered despite lag. But the eventual input resolution can still cause sudden deviations between clients.
Allowing Rollbacks
When desyncs become too severe, rollback reconciliation can reset clients to an earlier synchronized state. Each client re-simulates events to resume aligned gameplay.
Rollbacks can cause disorienting jumps backwards in time. But they serve as a last resort before desyncs cascade irreconcilably.
Optimizing the Physics Engine
Fine-tuning a game’s physics engine and simulation parameters can also reduce desynchronizing effects of lag.
Using Fixed Time Steps
Physics engines simulate advancing time in discrete steps. Using fixed length steps, rather than variable ones, ensures updates remain aligned between clients. This common advancing timeline makes synchronizing easier.
Locking Frame Rates
Likewise, a locked frame rate that exactly fits the time step provides regular opportunities to exchange data. With reliable timing, delays translate more predictably into numbered missed frames rather than fuzzy windows of uncertainty.
Deterministic Simulation
A deterministic physics model without randomness will behave reliably given the same inputs. This makes state changes easier to replicate on remote clients, advancing predictably when given the same seed data.
Avoiding Cascading Reactions
Minimizing chained reactions and physics dependencies isolates desyncs rather than letting them spiral. For example, a missed collision should avoid triggering excessive subsequent effects.
Handling Problem Cases
Despite best efforts, some types of events reliable trigger desync issues due to their sensitivity or complexity. These require specialized handling.
Fast Projectiles
The high velocities of bullets or arrows exacerbate positioning differences across disconnected simulations. Extrapolating their trajectories is unreliable.
Instead, tentative projectile impacts get visualized while confirming actual hits on the server. This disguises the lag but avoids inaccurately simulating ricochets or penetrations.
Player Triggers
When player interactions trigger effects like opening doors or detonating explosives, the action needs authority before clients simulate the results. Unsupported desynced triggers can crash game state.
Inputs queue locally while an autonomous server process validates timing and preconditions before allowing resolution.
Complex Collisions
Intricate real-time collisions with long contact times and interdependencies quickly desynchronize. Simplifying physics geometry and reducing iteration counts constrains divergence.
Key events like impacts or section fractures can embed serial numbers to track desyncing progressions across clients.
Smoothing Animation and Movement
Additional smoothing of visible sprites and motion reduces perceptual glitchiness stemming from lag compensation.
Blending Animations
Quickly alternating actions due to rebuffering or recorrection can create choppy transitions. Incrementally crossfading animation states disguises rapid switches.
Damping Velocity Changes
Instant velocity changes from corrected positions or rewound states manifest as abrupt pixel shifting. Blending velocities over several frames hides jerky transitions.
Occlusion Culling
Skipping rendering for distant players or particles omitted due to lag conceal the visual artifacts. Limits on simulated entities in view also curb overhead.
