Rollback netcode in Godot (part 1): What is rollback and prediction?

Summary

In this tutorial series, David Snowpeck from Snowpack Games explains the concepts of rollback netcode and prediction for peer-to-peer games using the Godot engine. Focusing on the Godot Rollback Netcode add-on, the video covers the problem of latency in online multiplayer games and how rollback and prediction techniques help to synchronize gameplay. Through a detailed frame-by-frame breakdown, Snowpeck demonstrates how both players’ inputs are predicted and corrected, ensuring a smooth gameplay experience despite network delays. The video also discusses the pros and cons of this technique, its application in competitive games, and its implementation challenges.

Takeaways

• 😀 Rollback and prediction are techniques used to reduce latency and improve the smoothness of online multiplayer games.
• 😀 The Godot Rollback Netcode Add-On is a reusable open-source library that facilitates the implementation of rollback netcode in Godot-based games.
• 😀 Rollback netcode works by predicting what the other player is doing, rendering those predictions immediately, and correcting them when the real input is received after a delay.
• 😀 A key challenge in online multiplayer is that players' inputs may not arrive simultaneously due to network latency, causing delays in synchronization.
• 😀 Rollback and prediction solve input delay by allowing players to see immediate movement, even if they are not receiving the actual data from the other player yet.
• 😀 The process of rollback involves reverting to an earlier game state and re-executing ticks with the newly received inputs, ensuring synchronization.
• 😀 In rollback netcode, frames are rendered based on predicted inputs, but when the actual inputs arrive, the game state may 'rollback' to re-simulate the earlier frames.
• 😀 Determinism is crucial in rollback and prediction, meaning that with the same inputs, the game must produce the same results, ensuring synchronization across peers.
• 😀 Rollback and prediction are ideal for small multiplayer games (2-8 players) that are session-based, such as fighting or strategy games, where quick action and input-based synchronization are key.
• 😀 One advantage of rollback is that players experience immediate responsiveness to their inputs, but a major con is that it’s complex to implement correctly, even with tools like the Godot rollback netcode add-on.
• 😀 Rollback and prediction also offer a unique form of fairness by ensuring that latency doesn't give any player an unfair advantage, although it can be impacted by extreme lag on a single player’s connection.

Q & A

What is the main focus of the tutorial series presented by David Snowpeck?

The tutorial series focuses on implementing rollback netcode in games built with the Godot game engine using the Godot Rollback Netcode add-on.

What is the Godot Rollback Netcode add-on, and where can it be found?

The Godot Rollback Netcode add-on is an open-source library under the MIT license that enables rollback and prediction netcode in Godot games. It can be found on GitLab and in the Godot Asset Library by searching for 'Godot Rollback Netcode.'

Which game developed by David Snowpeck demonstrates the use of the rollback netcode add-on?

The game 'Retro Tank Party,' a 2–4 player top-down tank battle game available on itch.io and Steam, demonstrates the use of the Godot Rollback Netcode add-on.

What problem does rollback and prediction aim to solve in online multiplayer games?

Rollback and prediction aim to address latency issues that cause delays between player inputs and what other players see, ensuring smoother and more synchronized gameplay experiences.

How does rollback netcode work in principle?

Rollback netcode keeps a buffer of past game states and inputs. When new input data arrives late due to network latency, the system rolls back to the corresponding past state, re-simulates the game with the correct inputs, and updates the current frame to stay synchronized.

What is the difference between a 'frame' and a 'tick' in the context of rollback netcode?

A 'frame' refers to a rendered image displayed on the screen, usually 60 times per second, while a 'tick' represents a single iteration of the game’s simulation logic. Multiple ticks can occur within one frame during rollback and re-simulation.

Why must a game implementing rollback and prediction be deterministic?

Determinism ensures that the same starting state with the same inputs will always produce the same results. This is essential because rollback netcode only synchronizes inputs, not full game states.

What types of games are best suited for rollback and prediction netcode?

Rollback and prediction work best for small-session games with 2–8 players, such as fighting games and real-time strategy games, especially those that are peer-to-peer and involve rapid, responsive gameplay.

What are some advantages of rollback and prediction netcode?

Advantages include immediate player input responsiveness, continued motion through prediction even when inputs are delayed, fairness regardless of latency, and eventual accuracy of match results based on true inputs.

What are the main drawbacks of using rollback and prediction netcode?

The main drawbacks are implementation complexity and the potential for a single player with high latency to negatively impact the entire match’s performance.

What is meant by 'eventual accuracy' in rollback netcode?

Eventual accuracy means that while the in-game experience may include artifacts or rollbacks, the final match result will be perfectly accurate to all player inputs on their respective ticks.

Why is rollback and prediction commonly used in fighting games?

Rollback and prediction allow for highly responsive controls and maintain fairness, which are critical in fast-paced, competitive fighting games like Mortal Kombat or Smash Bros.