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Handling inputs server-side

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Hi there,

I'm working on a 2d platform game using the quake 3 networking model. The client sends inputs 20 times a second, i.e. three inputs per packet. To avoid problems with "bursty" connections (see here), I process the received inputs directly on the server during a single frame. Since the inputs as well as the physics (gravity etc) of the game affects the player entities, I essentially run the entire physical update for the specific entity three times in one frame when the server gets the packet.

Now, to my problem. The above model worked very well before I added gravity to the mix, but since then I realized that I need to update the player entity on the server even when there aren't any player inputs queued up. Otherwise, lagging players would just hang in the air, as they do in my current implementation. Running physics without inputs has proven to be troublesome because depending on the timing when inputs are received, the server may be a few physics frames ahead or behind of the client. It may start off with another starting point , causing a misprediction when returning the updated position.

I've read a lot of articles and many dance around this subject, for example:
- The physics is only updated when input is received, ignoring the problem what to do when inputs are missing for a longer period.

- No physics is applied in this example; all state change depends on input so the problem does not exist.

I see some alternatives:
1. Keep track on when the server starts "extrapolating" the entity, i.e. runs updates without fresh client input. When new inputs arrive, reset to the original position and re-simulate the entity with the new inputs for the duration in which it was previously extrapolated.
2. The server stops updating the entity if it runs out of client inputs. Instead, the other clients goes on to extrapolate the movement of the client that is missing inputs.
3. Something entirely different.

Number 1 is attractive since it seems the "correct" way to go about this, but I'm having trouble getting it exactly right because of the jitter; i.e. I can't fix the mispredictions entirely. Also, I feel it's somewhat over-complicated.

Number 2 is nice since it's basically the model I have, but with the additional extrapolation added on. A problem with this model is that the clients would see a very old position of the lagging player, and since all hit detection etc is done server-side the laggers would become ghosts.

Anyone got a number 3? How do you usually solve this?

EDIT: Actually, I realized while writing this that #2 is a pretty acceptable solution to this. I'll keep the post up in case someone has a better idea or someone wants to read this for reference. Edited by Ollhax

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I would recommend dividing your simulation into two chunks: one that is "autonomous" and happens regardless of player input, and one that is based entirely on player input. The typical solution is to have your input-driven simulation add forces/impulses/etc. to the physics engine that powers the autonomous simulation.

Then you run the autonomous aspect of the simulation on both the client and server. When the server receives an input, it adjusts the simulation, possibly accounting for latency along the way. It then reports the result back to the client, which corrects its own simulation to match that of the server.

If you want, you can also have the client simulate the input-driven side to help it feel a bit more responsive. You may want to be careful with how you do this as it can leave the game open to a variety of cheats and exploits.

The key is to keep the server authoritative and always control the client based on the results of the authoritative simulation, while doing your best to do the simulation in advance on the client to mask latency.

One last trick is to use extrapolation as you noted to fill in the gaps if the server doesn't hear from the client before the end of a tick. You can easily apply damping or other falloff to this extrapolation so that a lagged out player doesn't just keep running forward infinitely.

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