Multiplayer Physics and Synchronization

# CHAPTER 14

Multiplayer Physics and Synchronization #

1. Introduction #

2. Learning Objectives #

• Explain why running independent physics simulations causes "Desync."

• Understand the Authoritative Server model for physics calculations.

• Implement Network Transforms to synchronize Position and Rotation.

• Understand the visual illusion of Interpolation (smoothing out lag).

• Understand the concept of Client Prediction for responsive gameplay.

3. The Desync Problem (Determinism) #

4. Server Authority #

• Only the Server runs the true physics simulation.

• The Clients (Players) turn their rigidbodies to isKinematic = true. They do not simulate physics at all!

• The Server calculates the ball bouncing, and 30 times a second, it sends the exact X, Y, Z coordinates to the Clients. The Clients simply teleport the ball to match the Server.

5. Interpolation (Smoothing the Lag) #

• The Fix: We use Interpolation (Lerp). Instead of instantly teleporting the ball to the new Server coordinate, the Client smoothly glides the ball from its old position to the new position over the course of a few frames. It is a visual trick to hide network lag.

// Conceptual Client-Side Interpolation
Vector3 targetNetworkPosition;
float lerpSpeed = 15f;

void Update()
{
    // Smoothly glide the visual object toward the server's authoritative position
    transform.position = Vector3.Lerp(transform.position, targetNetworkPosition, lerpSpeed * Time.deltaTime);
}

6. Client Prediction (The Illusion of Control) #

• Client Prediction: The moment the Player presses 'Jump', the Client *predicts* what the Server will say and instantly moves the player upward locally.

• Reconciliation: When the Server's data finally arrives 100ms later, the Client checks if its prediction was correct. If it was, nothing happens. If the Server says "You hit a ceiling," the Client instantly snaps the player back to the correct position (Rubber-banding).

7. Visual Learning: The Multiplayer Pipeline #

[ CLIENT A ] (Presses 'Push Box')
    |
(100ms Ping) Sends Input
    |
    v
[ SERVER ] (Calculates Physics)
   Box Rigidbody moves forward 2 meters.
   Sends new Coordinates (X=5, Z=10) to ALL Clients.
    |
(100ms Ping) Sends Data
    |
    v
[ CLIENT B ] (Receives Coordinates)
   Lerps (glides) the box visually to X=5, Z=10.

8. Best Practices #

• Network Transform Components: Do not write synchronization code from scratch. Use built-in components (like Unity's NetworkTransform or Mirror's NetworkTransform). These components automatically handle the heavy lifting of packing position/rotation data, compressing it to save bandwidth, and applying interpolation.

9. Common Mistakes #

• Syncing Velocity instead of Position: A beginner will try to save network bandwidth by saying, "I'll just tell the clients the ball's velocity, and let them calculate the movement." This leads to massive desyncs because a micro-stutter on a client's PC will alter the time calculation, causing the ball to fly off-course. Always sync the absolute, authoritative Position!

10. Mini Project: Networked Box Setup #

1. 1. The Object: A heavy crate.

1. 2. The Server: The crate has a standard Rigidbody. The Server applies AddForce when players push it.

1. 3. The Component: Attach a NetworkTransform to the crate. Check "Sync Position" and "Sync Rotation". Check "Interpolate".

1. 4. The Client: Ensure the Client's C# script sets crateRigidbody.isKinematic = true on startup. The Client will now peacefully watch the server's flawless physics simulation.

11. Practice Exercises #

1. 1. What is the definition of "Desync" in a multiplayer physics game?

1. 2. Why must standard physics objects (like bouncing balls) have their Rigidbodies set to isKinematic = true on the Client's machine?

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the concept of Client Prediction in competitive multiplayer games (like *Counter-Strike* or *Rocket League*). Why is it required, and how does Reconciliation handle incorrect predictions?

• Q: Why is floating-point determinism impossible to guarantee across different hardware architectures? How does this force game developers into the Authoritative Server model?

• Q: Contrast the bandwidth usage of syncing 100 physics objects at 60 updates per second versus syncing them at 10 updates per second with heavy Client-Side Interpolation. What are the visual tradeoffs?

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #