Implementing Client-Side Prediction for Responsive Input

Learn to execute movement logic locally on the client immediately to eliminate perceived latency between player input and character motion.

In this lesson, you will learn how to bypass the round-trip latency of client-server communication by executing movement logic locally on the client before the server confirms it.

The Problem: Latency and Input Lag

In a traditional client-server model, when you press "Move Forward," the client sends an RPC to the server, the server updates the position, and then sends the new coordinates back to the client. On a high-latency connection, this causes the player to feel "heavy" or "stuck" because their character only moves after the server responds. This lag between input and visual feedback breaks the feeling of responsiveness.

The Solution: Predicting Locally

Client-Side Prediction flips this by allowing the client to execute its own movement logic immediately upon input. Instead of waiting for the server, the client applies the velocity or movement logic to its local proxy version of the character. This makes the game feel instant.

However, since the server is the final authority, the server will eventually send back the "true" position. The client must then reconcile its predicted state with the server's authoritative state. If the server’s position differs significantly from the client’s prediction, the client must "snap" or interpolate to the correct position.

Implementation Logic

To implement this, you essentially duplicate your movement logic on the client. Here is a simplified conceptual flow:

// Executed on the Owning Client immediately
void AMyCharacter::PerformMovement(float DeltaTime)
{
    // 1. Predict locally (Immediate visual feedback)
    MoveCharacterLocally(DeltaTime);

    // 2. Send input to the server for authoritative validation
    Server_SendMovementInput(InputVector);
}

// Executed on the Server
void AMyCharacter::Server_SendMovementInput_Implementation(FVector Input)
{
    // 3. Process movement on the server
    ApplyServerSideMovement(Input);
    
    // 4. Server automatically replicates the new position back via UCharacterMovementComponent
}

By allowing the CharacterMovementComponent to handle this prediction and reconciliation, you ensure that even with 100ms of ping, the player’s character reacts instantly, while the server maintains final control over the game state.

Key Takeaway: Client-side prediction masks network latency by allowing the client to execute movement logic locally and reconcile its position when authoritative updates arrive from the server.