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In order to understand what a game engine is, it is necessary to take a moment and understand what games are made of. It ain't sugar and spice.

Games are composed of two things: code and data. Code is what makes a game function, and data is what makes it fun. Take Super Mario Bros. as an example. The game code defines the rules of the game. That Mario can jump, run, how fast he runs, that there are monsters, some monsters can be stomped on, etc. The game's data defines what Mario looks like, how he animates, and most importantly the arrangement of terrain and monsters that form levels.

ROM hacking Super Mario Bros. can lead to many Mario-like games. You can replace Mario's sprite, what fireballs look like, and give it a total makeover. But unless you change the code itself, the game will have certain Mario-isms:

The game will always be to some degree Super Mario Bros.

A game engine is game code that is designed to be data-driven. Unlike the above example, a proper game engine would allow the behavior of in-world entities to be defined in almost every way through data. Virtually all games have some data component, but only relatively recently has this component become flexible enough that two games built from the same engine can be very different from one another. That is, data defines both the function and the fun, while the code is just there to make the data do its job.

One of the first cases of a true game engine was Quake. It was a first-person shooter, but the game engine was much more flexible. It did not even make the assumption that the game was first-person; a user of the engine could pull the camera away to a third-person perspective. And the main character would be rendered there in third person with all of the controls intact and functional. With some work, Quake could have run a game like Super Mario 64, all without directly changing the code of the engine.

Please note that this is a simplification of a complex topic. Many game engines don't quite fit this definition, as the method of customizing an engine often involves writing code in addition to data. Half-Life 2's game engine, the Source engine, is modified by loading DLLs, which are compiled C/C++ code. Even Quake mods, written in so-called QuakeC, were compiled directly into virtual machine assembly code. Scripting throws a wrench into this as well, as scripts are code that is loaded like data. In that case, a game engine is basically just a library or module that handles 80% of the grunt-work in making a game: collision detection, that things can move, rendering things, etc. It is then up to the user to add the 20% that makes the game unique, whether through data and code or purely data.

Also making this more complex are people who sell licenses to engines that involve handing over source code, which is pretty much any commercially available engine. No game developer worth his salt is going to build his game based on your assurance that your engine is bug-free. This allows developers using the engine to actually change the engine code itself, not just layering new code on top of the engine. And developers frequently avail themselves of this opportunity. So a game that claims that they are using the X engine probably made some changes to X.

The general rule of thumb is this: You know it's an engine if you don't have to actually change the engine's core code to make a game that is substantially different game from other games made with this engine.

As a bonus, games that are significantly "enginified" are also very easily modded, and games using the same engine are easy to port if other games using the same engine have already been ported.

Engines can be internal, restricted to a particular development company, or external, which can be licensed for use by others. In-house "engines" are really just common codebases that multiple development teams in a company share. It is impossible to know whether these truly fit the definition of "game engine" because the different teams modify the source code for their own needs. Unless an engine is external, it is difficult to know what you can do with it without modifying the source code.

This may sound like a Game Maker, but there is a difference. A Game Maker is limited to a specific style within a genre. This limitation is what allows them to be easier to use. Game Engines cover a wide range of possible game types. Super Mario World level editing is effectively a Mario-style Game Maker. You can never make anything other than that kind of game. The line between the two, of course, is somewhat fuzzy. And when you touch that fuzzy, it can get dizzying.

External Game Engines:

  • BYOND: A 2D tile-based multiplayer game engine. Used by several games with articles on this wiki.
  • Serious Engine by Croteam (named for Serious Sam):
    • Serious Engine 1: Serious Sam (The First Encounter and The Second Encounter), Carnivores: Cityscapes, Deer Hunter 2003, Bird Hunter 2003: Legendary Hunting, Nitro Family, Alpha Black Zero: Intrepid Protocol
    • Serious Engine 2: Serious Sam II
    • Serious Engine 3: The HD remakes of Serious Sam (The First Encounter and The Second Encounter), Serious Sam III: BFE

Internally Developed Game Engines:

Things That Are Commonly Called Engines But Aren't:

  • The Havok "Engine". This is not an engine; it is a physics library.
  • Renderware, developed by Criterion Games and bought out by Electronic Arts (notable for making Play Station 2 development a lot easier). This is not in the most technical sense an engine, because it does not provide a means for using it without modifying the source code. It is a codebase, a large code library that serves as a useful starting point for making games, but it has no inherent extensibility built into it besides writing code. Games made using this codebase include Burnout (Criterion's flagship series), Grand Theft Auto (III, Vice City, and San Andreas), Sonic Heroes and SpongeBob SquarePants: Battle for Bikini Bottom.
  • Unity 3D (2006-present) and Cocos2d (2008-present): These are commonly used for making IOS Games, but like Renderware above, they are not engines in the above definition.