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dummynet

lag-fighting...

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Hello! Can anybody give an understandable explanation of what is interpolation and extrapolation in terms of fighting lags when coding network communication for a game with instant action (like cs, etc...)? I know it is about time and different states of objects on both server and client sides, but i still don't get it clearly... Assuming, we have to synchronize player movement, please give a chart of what is happening on both client and server at different time moments...

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okay, consider this

one player will get the current position of another player... from a half second ago

there isn't a way to reduce that lag, so this client will always either show the position half a second behind [not good], or try to guess where that other player is at the current time [tolerable]. It guesses the current position by showing the player as moving continously, with the same direction and speed. When the client then recieves an update about how wrong it is [the other player isn't moving in a continous straight line are they?], the client then interpolates between the position it is currently showing, and the new guess as to where the current position is [remember, it is late again]. This way the players don't see the other players disappear and show up elsewhere. In order to deal with different players seeing eachother in different positions first person shooters sometimes will register a hit from the view of the one shooting giving good feeback to the players and making the lag even less apparent.

sorry, no charts for you [from me]

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Ok, so interpolation means smoothing the movement from the previous "wrong" position to the current "wrong" position. And that imitates the visually "correct" movement if the updates are not too rare... And what about extrapolation ?

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Quote:

try to guess where that other player is at the current time [tolerable].


That's the extrapolation, when the client tries to predict a "next" location, from a current location and velocity.

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Hmmm... I'm getting confused a little...
Ok, let's say client is tracking a moving unit (a character, or somewhat like that).
It is now time T-1.
At T-1 client has received an update packet from the server. The server told the client, that at (T-2) the unit was at sx(T-2), sy(T-2), sz(T-2) (i won't overcomplicate the situation with such parameters as acceleration, etc..., i suppose the velocity is constant here). Now, the client knows the position of the unit for (T-2) and can make an assumption (relying on the fact, that the velocity is constant and the movement is direct) that at current moment (T-1) the unit's location is at cx(T-1), cy(T-1), cz(T-1). Correct me if i'm mistaking, but i call that an "extrapolated" position (the predicted one).
Now time T comes.
The client receives another update packet, and the newly received packet contains info on the situation that took place in the server-side world at time (T-1). The unit is at sx(T-1), sy(T-1), sz(T-1). And that can be different from what we've tried to guess earlier, sx(T-1) != cx(T-1), sy(T-1) != cy(T-1), sz(T-1) != cz(T-1)... Ofcourse, we could be lucky, and the coordinates could match, but in general we're usually not fortunate :)
How is the client going to correct its mistake without "jittering" the unit?
The client is making another assumption. It supposes that the unit is at cx(T), cy(T), cz(T). And it starts moving the unit from cx(T-1), cy(T-1), cz(T-1) to cx(T), cy(T), cz(T)... Is that what you call the "interpolation" ?But doesn't client take sx(T-1), sy(T-1) and sz(T-1) into consideration ? And does that mean, that client is always rendering the out-of-date world with a L lag (where L is the time it takes for an update packet to reach the client from server)? And if it is, isn't it the issue we were to fight when doing networking? Our goal was to render the actual position of the unit, and we are still rendering one-moment-before, even though we are interpolating and extrapolating... That doesn't make any sense, and i'm totally confused now. Please make it clear anybody!

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Case 1: the client actually draws the unit at position T-2 (instead of T-1), so that, when receiving position for T-1 (at time T), the client can draw the unit along the correct path. This leads to a 2*OWTT latency in display updates. This is the vastly easiest thing to do, and is quite sufficient for things like RPGs. This is interpolation.

Case 2: when receiving the position for T-1 at time T, the client estimates a new position for time T, by extrapolating from actual positions at T-2 and T-1. It then starts moving the displayed character towards this position, from the currently displayed position (i e, previously guessed position for T-1). This means that the unit may be drawn in places it's never been, but for units that move mostly straight, it shows a more up-to-date position. This is extrapolation.

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hplus0603, so, in case 1, when time T+1 arrives, the client should "jitter" the unit to its position at time T, without taking into consideration the previous interpolation ? But it whould make the unit "jump" visually, wouldn't it?

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No, the client never jitters the unit. In both cases, the drawing position changes smoothly. Read through the algorithms again.

You have the choice of always rendering the unit late, or rendering the unit at an estimated position that may not be correct. Note that you can forward extrapolate the position by more than one T if you need and want to, too.

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