Coding in the SDK

...also known as greebo's random notes about coding for The Dark Mod.

This article is constant WIP. When coding in the SDK, there are thousands of little lessons to be learned - some of them include things I wish I would have known earlier, some might be common knowledge. Anyway, I plan include all the miscellaneous things in this article.

General

When the id people designed Doom 3, they roughly divided their code into two parts: the engine and the gameplay code. The engine code is closed source, i.e. we don't have the code, only the compiled binary in the form of DOOM3.exe.

The engine contains all the "important" stuff which id software is making money with, by licensing it to third party companies. This is where the real brainpower is in, so we don't get that part (yet - John Carmack repeatedly stated that he wants to release all the source under the GPL soon).

The gameplay code is fully available. This includes the entities, game logic, AI, physics, weapon and animation code. This leaves a whole lot of headroom for modders like you and me, and we probably should thank id software for that. There are all kinds of mods out there which change the gameplay code to create their own game so to say. Some of them are "mini-mods" (introducing a new weapon or double-jump-mods), some of them are larger, aiming to change the nature of the game and finally there's The Dark Mod, which changed almost everything that can be changed in the first place. The term Total Conversion comes close - when playing TDM, you won't believe it's the same game as Doom 3.

What parts of the code are inaccessible?

All of the code id are earning money with by licensing their engine to others. I don't know all the parts, but these are the ones from the top of my head:

• the rendering engine
• the sound engine
• the collision model manager
• the Map and AAS compilers
• the declaration manager (although we can implement our own parsers)
• the network system
• the keyboard and mouse handlers
• anything platform-specific
• the command system
• the virtual file system
• the CVAR system plus some special CVARs
• the editor (D3Ed), but we have DarkRadiant

What parts of the code are left for us to change?

A whole lot, and in most cases it is sufficient:

• gameplay code (entities, logic)
• physics
• AI
• custom declarations
• custom CVARs
• the scripting system
• the animation code
• plus everything outside the SDK like scripts, defs, materials and models.

Coding Style

This deserves its own article: TDM Coding Style