BspDowngradeTool (CS:GO -> Alien Swarm)

This executable hackily modifies maps compiled with CS:GO's VBSP to work within Alien Swarm. This gives code modders access to the newest VRAD and tools, complete with AO and perf fixes!

Usage

1. Symlink your game directories in the Counter-Strike Global Offensive folder. This is identical to the setup required to get Hammer++ to work in non-CS:GO games.
2. Add a stage to call the executable in Hammer's Run Map [Expert] dialogue, either before or after before copying to your game's maps folder. Pass your .bsp as the only argument.

Technical

This is possible as BSPs haven't changed all that much in CS:GO.

The most drastic difference is the v11 static prop lump; identical to Alien Swarm's v9, save for 8 extra bytes at the end of each entry. Discarding this and changing the version number is trivial!

Some BSP lumps (namely 22 and 23) hold different contents in CS:GO, though appear to be redundant in Alien Swarm, permitting their omission. However, Source is a complex engine with many obscure features. It's possible this data is vital in some immediately unapparent way. Unfortunately, CS:GO's format is closed-source and not well-documented. Time (and rigorous testing) will likely reveal further issues.

This BSP format will be well-supported by tools in the Source Engine community - it's very close to Portal 2's. In the future, Portal 2 could also be supported, though it may require running VBSP twice and splicing lumps together.