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I have a question regarding Client Side prediction.

Most tutorials and explanations assume that the Client sends messages like "Move my player up by 1 Position", but what if I send messages like "Set my player's velocity to x"?

Since it's hard to explain with text, I made a graphic explaining my problem.

Graphic (Ignore the 5.5cm):

enter image description here

The main problem is, that the player sets his own velocity, due to Client Side Prediction, earlier than the Server. So if two different velocities overlap, the Server would get out of sync.

How can I tackle that problem?

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1 Answer

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You should send redundant data, which here means send the position and the velocity. Even if you are out of sync, the fact that you have the position and the velocity allows you to correct the trajectory using an interpolation function.

Then using some tricks like delayed animations, accelerations, etc. allows to hide the latency.

Edit: I assume that the server is authoritative.

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Thanks for your answer. Yes, the Server is authorative but even if i send redundant data with my velocity, the client could correct his errors but i often read things like "If the client/server use shared code for their entities, prediction errors will never really occure" but in my case, they will ALWAYS occure slightly which would cause stuttering all over. – user13842 Oct 24 '12 at 12:24
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@user13842 the client will always get out of synch, you should correct the client side position and speed, like Thelvyn said, according to what server sends you. that way you don't get the stuttering movement unless your client is completely out of synch then you'll have to snap the values because correction would take way too much time. – dreta Oct 24 '12 at 12:45
This is the main reason to use an interpolation function. You cannot avoid latency, so you have to correct the movement in a smooth way. The client and the server simulate the world but the client have to listen to the server and perform corrections. Some techniques used in Source Engine networking : developer.valvesoftware.com/wiki/Source_Multiplayer_Networking – Thelvyn Oct 24 '12 at 12:49
it's briefly mentioned in this google talk too, it should get the idea across youtube.com/watch?v=Prkyd5n0P7k#t=18m47s – dreta Oct 24 '12 at 12:58
Ive found another solution for this particular problem. I add the time the client applied his input with each packet. Meaning #1 would have time:0 and the second time:100. Now the Server knows that the client pressed its button at time:100 and not before. – user13842 Oct 24 '12 at 22:03

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