Map decompilers sound like absolute voodoo at first. Usually, when you compile something, you turn it from something nice and readable to absolute gibberish that only machines understand. Usually, it's stuck like that too. Being able to take any level and turn it back into something you can edit? Whatever next?
Map decompilers have been around since the Quake days. They work by prying open a BSP and recreating the original editor file it came from through a variety of methods. It'll never be a perfect recreation, and in some cases, it'll be messy, broken, and require a lot of fixing before it'll work again.
For Quake maps and GoldSrc games, you should look at two in particular, BSPTwoMap and WinBSPC. Both will take apart most GoldSrc maps you throw at them, with the notable exception of Half-Life: Blue Shift maps, which WinBSPC won't work on. Settled, then, isn't it? Just use BSPTwoMap for everything?
Not so fast. Unlike Source maps, GoldSrc maps don't store the original brush data inside the compiled level. HLBSP takes a brush and splits it into six faces, before deleting the ones you can't see in the game. (In the case of brushes touching one another, the surviving faces often get split up even further.) As a result, the decompilers have to recreate the brushwork from scratch. There are noticeable differences between the outputs of both. We tested them against four maps:
c1a0, the opening map of the "Anomalous Materials" chapter in Half-Life and chosen for its recognizability, as well as its source being freely available with the Half-Life SDK
crossfire, the infamous "airstrike" map from Half-Life's deathmatch mode an d chosen for its chunky terrain brushwork
op4ctf_gunyard, one of Half-Life: Opposing Force's capture-the-flag maps and chosen for its thin brushwork and to test Opposing Force support
rc_arena, one of the pack-in maps for Ricochet, because it's Ricochet
Test one: Half-Life's
Here are three stills taken from roughly the same location of the lobby of Sector C. All
scripted_sequence entities and triggers have been removed. All entities copied over well and functionally matched the original source. First, the original source as a control:
Now, the decompiles:
BSPTwoMap did a rather poor job recreating the brushwork. You'll notice in all BSPTwoMap decompiles that the brushwork has been chopped up into pieces. Brush entities, rather than being one brush, would become six paper-thin brushes merged together into a box. Note the desk; gaping holes in otherwise solid surfaces are a trademark of BSPTwoMap. Texture axis errors and invalid solid structure errors plagued the brushwork of every BSPTwoMap decompile. Needless to say, these maps would require a lot of work before they became usable again.
WinBSPC's defect is that the level is carved from a box the size of the entire map, which may or may not result in usable brushwork. Here, it at least resembles the original
c1a0 better, but still with invalid solids and some texture rotation issues. You'll also notice in that screenshot that it got the texture of the blast doors wrong.
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Test two: Half-Life's
BSPTwoMap's brushwork could charitably be described as inaccurate, but is closer to downright unusable. In addition to the split faces, many brushes (including the railings and ceilings) jut out in triangles, completely unlike the original map. Even worse is the sky; a sky brush that did not exist in the original map cut through the top half of the playfield. Note that we had to split that sky brush in half to let you see the ground of the map. BSPTwoMap's razor-thin brushwork is even more apparent here.
WinBSPC's is more accurate and usable, even getting the odd, uneven terrain correct, but note the texture errors on the walkways. Signs, again, were oriented upside down.
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Test three: Half-Life: Opposing Force's
BSPTwoMap tripped around any sort of thin brushwork; as you can see from the screenshot, there's more than a few invalid faces from when the railings became concave.
WinBSPC gets it closer, aside from some texturing issues and a few broken solids. Amusingly, the health chargers got flipped upside down.
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Test four: Ricochet's
Okay, this test was just for fun. Ricochet isn't quite as heavy on stunning level design as other GoldSrc games. At least, we thought it was for fun, when it turns out this one fared the worst out of the lot:
The BSPTwoMap decompile is an actual disaster. Not one of the platforms survived the decompile in any sort of a recognizable shape. We could barely navigate the map in the editor well enough to take a screenshot. The WinBSPC decompile didn't come out so hot either; though its shape is more recognizable, most of the platforms were split into 5-6 brushes each, and the jump arrows were similarly splintered and with texture alignment issues.
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So what have we learned today? Neither work miracles. BSPTwoMap ends up with many razor-thin, broken solids, and WinBSPC carves brushwork out of a box and often gets the texture alignment wrong. We leave it up to you to decide which one is more tolerable. (You can download the results of our tests using the download link below.)
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