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Septopus

What am I missing about multi-player game development?

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Couldn't you run physics in the client to make it "smooth" game-play wise - but then run a simplified version on the server to determine collision/interaction etc?

Not sure if that makes any sense, just woke up :P

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I can't really run ANY version on the server without a server that can run SOME physics. :(  Unity won't even load up on a system without a decent graphics card and I'm not expecting to have a budget for real servers like, ever...

I'm using my ins in the industry to get some free virtual machine access and no bandwidth charges...  Which is great, but not physics server running great.

My hope was that I could use a geometry library to handle collisions and what not on the server, but that does nothing for keeping the actual physics simulations in sync with each other...

I think the only way 'I' can make this work is to remove the physics and use deterministic movement...  I'm just not presently very optimistic about the prospect.

 

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Yeah, after sleeping a bit and working over the details in my head/on my blog..  I think the deterministic approach(where needed), and targeted(isolated) use of physics isn't going to be a problem for me. 

As long as no two physics simulations are authoritative for the same object(s), which they never are currently, I don't think I will be risking a significant paradox....

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Does the server have to be a c#/unity program? If server hardware resources are limited, I would set up a simple server program running the absolute minimal physics simulation you can get away with, on a physics engine such as Bullet. This would be authoritative over the player movement. You can then use unity, or whatever you are using to power your clients, to drive a local physics simulation on the clients for a kind of client-side prediction. These could also simulate extra visual objects that the server does not need to care about, for nicer visuals etc.

Using two physics engines like this obviously requires a bit of tinkering to get them to agree on everything, but for simple player locomotion, that should not be that much work (disclaimer: I don't know if your locomotion is indeed simple).

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On 11/22/2018 at 5:43 AM, Hodgman said:

On a slower scale, e.g. characters who walk around a world, the exact same issues apply. The jankiness is just exaggerated and more obvious in the race car example :)

I think the other thing to consider is how important a collision is. I mean if just stops a character's movement for a moment it may mot be a big deal, even if it never happened on the other client. In a car game where it might cause a catastrophic crash on one client and not the other, it's a different story. I found it very annoying in Age of Conan when you were stopped by something you couldn't see because the server ruled over the client in this way.  I would much prefer to see someone else's characters pause for a second for seemingly no reason than have mine bump into something invisible. This brings us too response. If only your character is effected when you bump into someone as in some MMOs, it doesn't really matter to any other client what happened on yours.

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7 hours ago, GuyWithBeard said:

Does the server have to be a c#/unity program?

No, but I'm trying to avoid using 3D hardware on the server, and avoid using multiple languages for development of a single project. ;)

I've looked into a C# physics engine, but there aren't many to choose from, and most will still require SOME 3D hardware.

Again though, the goal is to build the system in a distributed fashion, if I did add a server physics simulation, I would want to distribute that to multiple servers as well... ;)

Also, where collisions are concerned.

A single client will only be simulating physics(movement) for objects that are under it's control. 

i.e if a player character bumps off the collider of a networked kinematic object.  Their character will stop, or bounce or whatever is appropriate according to the physics.  The networked object will only respond(stop/bounce/etc.) if the network object on their end that represents the colliding player interacts with their physics too. ;)

It actually works pretty well in all my tests so far.  For walking/running around the world as a team or on a collision course.  Just the usual unavoidable lag delay that comes with a network between the players.  No real jankiness to report, at least since I got rid of the physics for network instantiated objects and started using pure interpolation with colliders.

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12 hours ago, Septopus said:

I'm trying to avoid using 3D hardware on the server,

That is exactly my point. You can make the server a windowless program. It won't require any graphics hardware, it won't even require a window system.

12 hours ago, Septopus said:

Again though, the goal is to build the system in a distributed fashion, if I did add a server physics simulation, I would want to distribute that to multiple servers as well...

I don't see how this is incompatible with server authoritative physics. You can have a system where server instances are spun up depending on need and player count. (Obviously this all depends on the game type). If the server is only responsible for simulating the gameplay-relevant physics and propagating that to all clients you can even attempt to run multiple server instances on a single machine.

Anyway, you seem to have a direction you want to go, so that's probably what you should do.

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15 minutes ago, GuyWithBeard said:

That is exactly my point. You can make the server a windowless program. It won't require any graphics hardware, it won't even require a window system.

I don't see how this is incompatible with server authoritative physics. You can have a system where server instances are spun up depending on need and player count. (Obviously this all depends on the game type). If the server is only responsible for simulating the gameplay-relevant physics and propagating that to all clients you can even attempt to run multiple server instances on a single machine.

Anyway, you seem to have a direction you want to go, so that's probably what you should do.

I do enjoy the discussion though.  ;) 

Making it "windowless" or "headless" as it were, doesn't necessarily negate the need for "physics/graphics" hardware for many engines.  I called it 3D hardware, but in this case it would be the physics hardware, usually supplied as part of the graphics hardware.  Many engines only work with hardware physics support.  I do not currently know of any physics engines I can code in C# that do not have this requirement.  If you know of something, I would be happy to try it out, for part of this project or future ones. :D

 

Edited by Septopus

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Well, the trick is not to use an engine for those parts. Write a "text" program in C++ (or C# or whatever you are comfortable with) and link to the physics library. In the case of Bullet this is a collection of headers/classes. Everything runs completely on the CPU.

I do recognize this might require you to write parts of the simulation code twice, ie. once for the server and once for the client, but you can be smart about it. I have worked on a project in Unity where we separated the simulation code from the presentation code. The simulation code was not allowed to access any of the unity classes, only standard C# library code. This meant writing your own Vector3 classes and other helpers you needed to run the simulation. The presentation code was allowed to reference everything in the simulation code + all of unity, and it was responsible for rendering (or "presenting") the world.

The simulation code was then easily packaged into its own C# "console program" and run on a server without Unity.

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As for simulating the server-side physics, there are several ways to do that. You can write all of your physics code yourself, which may or may not be feasible. That way the exact same code can run on the client and server. Another way is to use a physics engine. If you are using C# you may have to use a wrapper for this as most physics engines are native code. I have used a wrapper for the ODE physics engine in the past. AFAIK there are c# wrappers for Bullet as well.

Unity obviously uses PhysX under the hood so you may have to experiment a little to get your server-side physics to match the client(s). This is also true if you use PhysX directly on the server, as Unity has its own layers in between in the game code and the raw physics code. This is what I was referring to earlier (the "tinkering" part). However, with state replication, you'll be doing gradual corrections on the clients so it shouldn't be too difficult to get it working quite well.

Edited by GuyWithBeard
typos

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