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I often come across very impressive mods for PC games that don't have an official editor, other development tools or its source code publicly available. (Take this amazing Multiplayer mod for Just Cause 2, for example.)
How do you go about creating mods for such games?
I'm not talking about replacing the odd texture or 3D model—that sort of thing seems fairly easy given tools to pry them out of game files and put them back in—but more along the lines of adding game behavior. (Tweaking settings files also doesn't count.)

Note that I'm not asking "how to create a mod", I just want to know where to start or where to go to learn.

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Here is the source code for Just Cause 2's Multiplayer Mod. code.google.com/p/jc2mp It's open source so you can look at it and see how they did it. –  eazimmerman Jul 10 '12 at 14:13
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@ea: Hey, that's great! –  Protector one Jul 10 '12 at 14:17
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In the source code of the Just Cause 2 mod linked to by @eazimmerman, there's also a list of tools used to create the mod. This provides some hints as to how this particular mod was created. (code.google.com/p/jc2mp/wiki/Development) –  Protector one Jul 10 '12 at 14:31
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My answer is kind of two parts.

The first is that generally, you're best off making mods for games that have a high level of support for mod making. Sometimes this stuff can be extremely involved and frankly you want all the help you can get.

Maybe there was a crack team out there that was able to decompile a game and insert their own assets. More likely they got help from the developers and access to their utilities.

Even if you do have those kind of abilities? It's a whole lot easier when someone has built tools for you.

That out of the way, I would suggest looking at the Unreal Development Kit.

Turns out someone really has made the tools for you.

I'm not really sure how to elaborate on this other than to say it's probably far beyond what you need. There's a LOT of material out there on the UDK. A lot of things you can read. And a lot of tutorials.

The development team I'm with is using it ourselves and while I can't speak for everyone I'm finding it to be excellent.

I've been working with it for about two weeks now, and not too long ago I was able to make a rocket launcher that fire rockets that when it exploded left a fire behind. A fire which both spread in a line, and dealt damage to anything close by.

Lasers that bounce off walls, cloaking devices? Pretty easy to make a mod for UT3 that includes things like that. Killer Robot Monkey that chases after enemies and breathes swarms of bees at your enemies shouldn't be too hard to program either.

It's kind of only really limited by imagination and performance issues. We're expecting to be able to make a pretty good burning down buildings system going.

I'm currently working on a stealth system. Enemies won't see you if you're in dark areas.

I only mention all of the above to give you an idea of what's feasible (which is a lot). There might be a better game and better tools for making mods, but the UDK is probably one of the best. And the fact of the matter is that again, there is a lot of documentation. It's a big part of why we chose it.

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Even though UDK is very cool, you still have to make the game yourself. Sometimes you just want to add a little thing here or there to an existing game. I've come to fully agree on your point about high level of support. Since asking this, I've implemented my idea more or less as I imagined it in ArmA II. Excellent level of support, those Bohemia Interactive guys and their community. Unfortunately, the vanilla game itself is a bit crap! :P Which leads me back to my point: sometimes you just want to change an existing game. One that's already good to begin with. –  Protector one Jun 20 '13 at 14:23
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