I’ve recently started looking into modding for Half Life 2 / the Source engine. On the whole, Valve provides quite a lot of help for modders. There is an official Source SDK that can be downloaded through Steam, a number of official forums on Steampowered.com that Valve developers are (occasionally) active in, and a wiki with relatively up to date information for developers on a wide range of topics. There are a number of active third party sites devoted to Source modding, the best of which appears to be Interlopers.net. All this is good. what is not so good is that the architecture Source and Steam appears is rather unfriendly to beginners.
Any beginner to modding a game has to start somewhere. The usual place to start is by making a small modification to the original game’s behaviour. For example, a beginner to Doom modding might take the game’s source code (or rather, a stable windows port such as ZDoom), make a change to a small aspect such as the player’s maximum health, recompile and play the game. Similarly, someone using the Quake engine might make a small change to game’s QuakeC code, perhaps to increase the number of gibs spawned when a monster is killed, recompile the code and test out their change. This approach, of tweaking a small aspect of an existing game, and quickly seeing the results is a standard and very useful one. It allows new developers to get in at the shallow end, so to speak, by gradually exploring the existing code base, learning to make more significant alterations at their own pace.
With the Source SDK, there is an option to create a blank mod, or to make one based on Half Life 2 singleplayer. You might imagine that choosing the latter option would produce a mod that replicated the Half Life 2 singleplayer game, while allowing the developer to start making code changes, but this is not the case. This option produces a mod with the source code from HL2 singleplayer, but no access to the resources of the game. Instead it uses what is called the “Source SDK Base”. This is a shared set of basic resources that is available to anyone who owns a Source engine game (e.g. Half Life 2, CounterStrike Source, Day of Defeat, etc). Getting the mod to a state where it simply modifies the default Half Life 2 singleplayer game is possible, but it involves quite a bit of work copying configuration and resources from the original game and modifying the code to mount the original HL2 resources within the engine’s internal file system.
The thinking behind this is clear enough, Valve want to enable and encourage people to make mods standalone mods that work across their product range, rather than reusing a load of resources from a single game and therefore being tied to it. However, this assumes the developer is ready and able to make a large standalone mod, whereas I would argue this is only a stage reaches after a large intermediate period of learning their craft and forming or joining a dedicated mod team. Until then, most people will want to product basic mods of a single game. If Valve wants to improve the learning curve for Source modding, thus increasing the size of the community and the associated revenue it drives, then they should enable a better route within the SDK for beginners. The option to create a HL2 singleplayer mod should do exactly what it suggests, creating a mod that replicates the normal behaviour of the game and can be played immediately, allowing the developer to start making changes at their leisure.