A vertical slice is a type of MVP in the world of game development, in which the game is fully playable and includes all the necessary features to function as a game. This generally means all the systems of the game are built, as well as all the graphics, animation, and cinematics. Usually the demo only spans for a very short amount of gameplay but includes complete implementations of that piece of gameplay, hence the name vertical slice. This lets players (and investors) get a quick feel for what the completed game would be like to play. This is a type of low rank approximation.
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