Reconciling Differing Client Predictions In Fast-Paced Multiplayer Games

The Core Problem of Inconsistent States

The fluidity and responsiveness of fast-paced multiplayer games depends on each player’s game client having an accurate understanding of the current state of the game world. However, latency caused by network delays means player actions take time to transmit to the server and other clients. During this transmission delay, the game state observed by each client diverges, resulting in a temporary inconsistency as the clients render different versions of the world and player actions.

For example, in a multiplayer first-person shooter, when Player A fires their weapon at Player B on their screen, that action takes time to reach the server and other clients. During that latency period, Player B may have moved position on their local game client. The server must then reconcile the divergent realities – did Player A’s shot hit Player B based on its trajectory, or did it miss based on their new position? Resolving these temporary inconsistencies is key to maintaining a playable real-time experience.

Strategies to Minimize Prediction Errors

Game developers utilize various strategies to minimize the visual artifacts and playability issues arising from state divergence between client predictions and final server authorizations.

Using Client-Side Prediction and Validation

Client-side prediction allows each player’s game to render player actions locally without waiting for server round trips. Locally predicted actions are validated later when the server’s authoritative game state updates arrive. If the predictions diverge too far from reality, the game smoothly reconciles the differences.

Smoothing Out Inconsistencies with Interpolation

Sophisticated interpolation techniques can ease visual jitteriness arising from inaccurate predictions prior to server updates. By smoothing the transitions between predicted and actual game states, the multiplayer experience feels more fluid.

Handling Edge Cases with Advanced Reconciliation

While relatively straightforward reconciliation strategies handle most common prediction error scenarios, more advanced techniques are required to handle edge case predictions that diverge strongly from server reality. These include temporarily rolling back out-of-date client states and fast-forwarding to resync with the server.

Client-Side Prediction JavaScript Code Example

Here is a sample client-side prediction implementation in JavaScript:

let clientPositionX = 0;
let clientPositionY = 0; 

let serverPositionX = 0;
let serverPositionY = 0;

function predictMovement(input) {

  // Locally update position based on input  
  clientPositionX += input.xChange;
  clientPositionY += input.yChange;

  // Optimistically render local prediction
  renderCharacter(clientPositionX, clientPositionY); 
  
  // Package up and send input to server
  sendInputToServer(input);
}

function renderCharacter(x, y) {
  // Render character to screen based on position
} 

function reconcilePositions(serverPosition) {
  // Server has acknowledged our input, let's reconcile

  // Find difference between our prediction and server reality
  let xDiff = serverPositionX - clientPositionX;
  let yDiff = serverPositionY - clientPositionY;

  // Smoothly transition to server's position over time
  clientPositionX += xDiff * 0.2;  
  clientPositionY += yDiff * 0.2;
  
  // Rerender scene based on reconciled position
  renderCharacter(clientPositionX, clientPositionY); 

}

// Update reconciliation loop
setInterval(function() {

  // Get latest acknowledged position from server  
  serverPositionX = getServerPositionX();
  serverPositionY = getServerPositionY();
  
  reconcilePositions(serverPosition);

}, 100);

This demonstrates a basic client-side prediction approach, optimistically updating the local client state on each player input while waiting for the authoritative reconciliation from the server on the acknowledged positions. Any prediction errors are smoothed out for a graceful visual transition.

Advanced Server Reconciliation Techniques

While client-side prediction handles many basic reconciliation needs, games with more precision gameplay require the server to take a more active role in resolving complex edge cases and keeping clients tightly synchronized.

Timestamp Synchronization

Each client game instance and the server must align their timesteps precisely to properly reason about latency delays and reconcile chronology of events. NTP time synchronization, though adding implementation complexity, is essential for fast-paced games.

Lag Compensation with Rollback States

Lag compensation techniques simulate player actions as if there was no latency, then roll back and resimulate when lag is detected. This prevents delays giving some players an unfair advantage in fast exchanges of fire or actions.

Machine Learning Assisted Reconciliation

As machine learning advances, server-side simulations can extrapolate likely player movements and actions during latency periods to generate tighter approximations before confirmations arrive from clients. This allows more accurate preliminary reconciliation decisions.

Achieving Fluid Multiplayer Experiences

Creating compelling multiplayer experiences across a range of genres and platforms requires choosing the reconciliation approach tailored to gameplay needs and engineering constraints.

Choosing the Right Reconciliation Strategy

Less precision-critical multiplayer genres like massively multiplayer RPGs can utilize basic client prediction with occasional server corrections. However, fast-paced shooters require much tighter reconciliation techniques to avoid playability and fairness issues.

Optimizing Based on Genre and Platform

Mobile platforms will constrain techniques to minimize battery and bandwidth consumption, while PC and consoles allow more advanced algorithms. The game genre also affects the frequency and types of reconciliations required.

Ensuring Fairness and Playability Comes First

While visually smooth transitions between client and server states is important, the reconciliation system’s top priority is ensuring gameplay fairness and fun. Even if visual hitches occur, the player experience must remain logically consistent and enjoyable.