Fighting Game Over Network

texhnolyzze · 2019-09-06T14:26:25

Hello! I want to make an online game using libGDX and kryonet. For the last two days I have been studying all sorts of concepts for creating online games. Server reconcillation, Interpolation between two states on the client side are understandable to me now. But there are two things that I don’t understand to the end. The first is the lag condensation. To quote Gabriel Gambett https://www.gabrielgambetta.com/lag-compensation.html: When you shoot, client sends this event to the server with full information: the exact timestamp of your shot, and the exact aim of the weapon. Here’s the crucial step. Since the server gets all the input with timestamps, it can authoritatively reconstruct the world at any instant in the past. In particular, it can reconstruct the world exactly as it looked like to any client at any point in time. This means the server can know exactly what was on your weapon’s sights the instant you shot. It was the past position of your enemy’s head, but the server knows it was the position of his head in your present. The server processes the shot at that point in time, and updates the clients. Ok, basically everything is clear. But this moment: "Since the server gets all the input with timestamps, it can authoritatively reconstruct the world at any instant in the past. In particular, it can reconstruct the world exactly as it looked like to any client at any point in time.". I do not understand how to implement this. Clients send timestamps to our server along with events. But after all, clients clocks are not synchronized. How can I understand from a given timestamp what state I should roll back the world to? I can only roll back that particular client, right? Ok, my second question is how to implement animation over the network? My game will use regular sprite animation (Animation class from libGDX). It is clear that it is impossible to synchronize it between two clients. But how do I interpolate the animation of another client? What data should be transmitted over the network for this? Thanks.

Networking and Multiplayer Programming

Started by texhnolyzze August 23, 2019 09:23 AM

14 comments, last by NetworkDev19 4 years, 7 months ago

hplus0603

11,916

August 24, 2019 02:36 AM

GGPO is described in a Gamasutra article (that somewhat overpromises on how general it is): https://www.gamasutra.com/view/news/177508/The_lagfighting_techniques_behind_GGPOs_netcode.php

Global ordering between actors is somewhat possible, but I would say that it's always the "previous state" of an actor that affects the "next state" of another actor.

Thus, if A and B shoot each other on the same step, it doesn't matter who gets processed first. Both of the "previous" (alive) versions will "shoot" and generate "dead" targets in the next state. I e, the function is "stepWorld(oldState, newState)" where you area only allowed to read oldState, and only allowed to write newState. (And that will recursively hold for all entities in the world.)

If you keep copies of actor state for older time steps to implement lag compensated hitscan weapons, then this becomes pretty easy, because you "read" state at time step "stepFrom" and you "output" state at time step "stepTo."

enum Bool { True, False, FileNotFound };

hplus0603

11,916

August 24, 2019 02:46 AM

Also, regarding clock sync -- assuming latency is not too variable, you should be able to make the client send messages so they arrive at the server slightly before they're needed, which is really all you need.

You don't need to clocks to be 100% in sync, as long as the messages from the client arrive at the server within an acceptable window (say, between 1 and 4 simulation steps before they're needed -- your specific requirements may differ.)

enum Bool { True, False, FileNotFound };

texhnolyzze

Author

August 24, 2019 03:12 PM

hplus0603

Okay, thank you for the answers.
Just the last question. Do you think I need to keep a snapshot buffer? Let’s say I received a batch of events from two players on the server, and say in the batch from player 1 there is an event “Punch”. This is a just an usual event, it has a timestamp (local timestamp of player 1). How do I handle this event on my server? Should I just apply it to the previous snapshot just like all the other events? Or should I apply this event in another snapshot, which is closest to the state of the game that player 1 saw while he pressed the "Punch" button. What if I detect the fist of player 1 in player 2? What should I do in this case? Thank.

Sorry for so many questions, I hope this is the last

hplus0603

11,916

August 25, 2019 02:56 AM

I would apply all events that affect some particular object, in the time step for that event.

If you have more than two players, you may end up with a situation where two players end up both dealing the "killing blow" in the same tick -- you're going to have to figure out what to do about cases like that.

enum Bool { True, False, FileNotFound };

NetworkDev19

September 06, 2019 02:26 PM

On 8/23/2019 at 10:34 AM, texhnolyzze said:

On this link:

Source-LagCompensation

There is such formula:

Command Execution Time = Current Server Time - Packet Latency - Client View Interpolation

Can you explain me please what is Client View Interpolation? Is this some constant tunable in Source Engine? I mean, if our server do 30 snapshots a second and sends them to clients, then Client View Interpolation will be 1000 / 30 = 33.333ms, cause client interpolates other players states within this interval, right?

I just figured this out myself.

Client view Interpolation is the amount of time the client has interpolated between 2 world states it has received.

Think of it as the client is running the game world in the past. It knows about 2 worldstates, and is waiting for a 3rd. It interpolates between the 2 worldstates while it waits for the 3rd otherwise players would "teleport" around visually. The server needs to know what 2 worldstates the player is interpolating between and *how far they've interpolated between them* in order to accurately recreate the state the player is seeing when they did the shoot command. Or at least, as close as possible of an estimate.

Hope that helps

Fighting Game Over Network

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