Client-side prediction
Encyclopedia
Client-side prediction is a network programming
Computer network programming
In computing, network programming, essentially identical to socket programming or client–server programming, involves writing computer programs that communicate with other programs across a computer network. The program or process initiating the communication is called a client process, and the...

 technique used in video games intended to conceal negative effects of high latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...

 connections. The technique attempts to make the player's input feel more instantaneous while governing the player's actions on a remote server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

.

The process of client-side prediction refers to having the client
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

 locally react to user input before the server has acknowledged the input and updated the game state. So, instead of the client only sending control input to the server and waiting for an updated game state in return, the client also, in parallel with this, predicts the game state locally, and gives the user feedback without awaiting an updated game state from the server.

The earliest first-person shooter to use this technique was Duke Nukem 3D
Duke Nukem 3D
Duke Nukem 3D is a first-person shooter computer game developed by 3D Realms and published by GT Interactive Software. The full version was released for the PC . It is a sequel to the platform games Duke Nukem and Duke Nukem II published by Apogee...

, released in January 1996. Later that year, it was implemented in QuakeWorld, the popular add-on to Quake. While network play was included in the original Quake game, it was optimized mainly for LAN
Län
Län and lääni refer to the administrative divisions used in Sweden and previously in Finland. The provinces of Finland were abolished on January 1, 2010....

 play. Having all had high-speed home connections (a rarity at the time), Quakes designers overlooked their assumptions of high bandwidth and low ping times that made playing online frustrating for dial-up users. After a series of experiments in a long private beta, id Software
Id Software
Id Software is an American video game development company with its headquarters in Richardson, Texas. The company was founded in 1991 by four members of the computer company Softdisk: programmers John Carmack and John Romero, game designer Tom Hall, and artist Adrian Carmack...

released QuakeWorld with a new predictive model that proved popular with both high and low latency players.

This reduces latency problems, since there no longer will be a delay between input and feedback due to network ping times. However, it also introduces a desynchronization of the client and server game states, which needs to be handled to keep the game playable. Usually, the desync is corrected when the client receives the updated game state, but as instantaneous correction would lead to "snapping", there are usually some "smoothing" algorithms involved.
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