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Have you played Valve Software's Half Life? It's one of my favorite games. The gameplay itself is the standard for the genre (first person shooter), but the atmosphere and style of the game - the dirty, lonely bowels of the facility; the wonderful little scripted sequences (like when the scientist gets killed by the tentacle monster in Blast Pit - I love that part); the times you're left cowering in a corner, hiding from the marines and aliens engaged in a massive firefight. If it were a book, it would be un-put-downable; I came to realize this when I noticed I was chasing the Gonarch through the tunnels of Xen with my RPG, growling.

No? Perhaps, then, you like the related games, Opposing Forces or Blueshift? Guiding lost marine Adrian Shepherd past even nastier monsters than Freeman encountered, but with an enhanced arsenal at your disposal to help you out. Or taking on another day on the job as security guard Barney Calhoun, risking life and limb to get those scientists out safely (although those who *have* played it know that the scientists always meet with.. unexplained accidents, hmm?).

Prefer multiplayer? Counter-Strike is an absolute phenomenon in the online gaming world - it's one of (if not the) most popular online games worldwide. Terrorist versus Counter-terrorist in games of hostage rescue, sabotage, assassination... or more recently, I've heard of a game called Natural Selection, which manages to combine elements of Real-Time Strategy games with the popular team deathmatch.

OK, enough advertising (I take cash only, guys). All these games have a major thing in common. In each game, around 50% of the code is exactly the same as the others.

No, really. If you strip out all the 'content' - not just the maps and models, but the AI, physics, and 'gameplay' - you'll be left with pretty much the same thing in each case. A system for rendering small, urban environments, with a skeletal animation system, a bunch of special effects, networking, input, and sound. And that, ladies and gentlemen, is Valve's Half-Life engine.

Think about what I'm saying here, for a moment - Half-Life and Counterstrike are, beneath the differing models and weapons, essentially the same program. As are They Hunger and Sven Co-op, Todesangst and Day of Defeat.. a fairly large amount of diversity stemming from a single code base, a single engine

Now, apply that to your own projects. If you had a reusable engine like that, most games would just become a question of creating new maps and models (or the equivalent for your projects); menial tasks like initializing DirectX or OpenGL and reading in settings from storage would become a thing of the past, as you'd just use the code you wrote previously.

I hate to break it to you, but all that knowledge of the Win32 API you've built up probably won't be very useful in the industry; as more and more companies lean towards the professional 'software factory' technique, they'd need to you write the Windows portion of an engine once, and after that you'd be useless. Many projects never touch underlying APIs - they use an engine to do it for them, either bought in or developed in-house. Valve, originally, bought the Quake source code to turn into the Half-Life engine - as a result they never had to touch things like OpenGL, because iD Software's code did it all for them. They just knuckled down and integrated their specific game content (AI, physics, etc) into Quake's engine interfaces. (Well, ok, so they actually *did* go in and modify the engine to support more advanced features like 16-bit textures, but my point is that they didn't *have* to).

So, in this series of articles, I'm going to do my level best to teach you how to build an engine, from scratch. I'm not just going to present you with code and say 'call function X to do funky things;' by the end of this series, you're not just going to have an engine - you're going to understand it. So listen up, maggot! :-P

It's important to realize that in practical terms, an engine can't do *everything* - the more things the engine has to support, the more low-level it becomes, until you're just writing a wrapper for DirectX/OpenGL. No, when you write an engine, you have to lay down some rules about what games can build on it - you can't build Doom 3 on the Half-Life engine, for example, because Doom 3 requires some things (like per-pixel lighting) that the Half-Life engine simply doesn't provide. More obviously, you couldn't use the HL engine if you were planning a game for GameCube, because HL doesn't support the platform.

However, it's my belief that this process of 'specialization' happens step-by-step - an engine starts, at it's lowest level, supporting every possible game, and then as you add more to it, it 'converges' on a set of limitations. You can picture it as a pyramid - the base is wide and supports the rest, while the top is a finely focused point, allowing only a very small set of games to be built on top. If we remove the top half of the pyramid, we're left with a much wider base to build a larger set of games on - those games will, however, need to do more of the building work themselves. So an engine can be progressively built up in this way, adding layer by layer.

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The Series
  Part I
  Part II
  Part III
  Part IV
  Part V