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Article: Input Playback

by Jesse Chounard

Lately I’ve noticed a trend in games to allow players to save and play back recordings of their play upon completion of a match.  This allows players to analyze their mistakes, as well as learn new tactics and techniques from their opponents.  This feature is becoming common in all sorts of games.  You can find it in real time strategy games like StarCraft, first person shooters like Halo 3, and fighting games like Super Street Fighter 4.

In many cases, these gameplay recordings are often uploaded to a server on the internet, and are available for download by anyone.  The recordings don’t require buffering or waiting for a big download to finish.  Instead, the playback just starts right up.  How is this possible?  Great looking high definition video files are gigantic.  Also, video compression takes a huge amount of computation, so wouldn’t it slow the gameplay down to record everything?

It turns out that there’s a really simple trick.  We’re not recording video.  Instead, we just store all of the user input, and then later we can use those inputs to exactly recreate the gameplay in our game engine.  So while it looks like we’re watching a stored video, instead we’re just watching the computer playing the game using our stored inputs instead of using the game controller for input.

In this tutorial, I’ll show you a simple method that you can use to achieve gameplay recording and playback, and also list a few other uses for this technique.