'There's a lot of cool things that, if you think too hard about it, they should never have worked': Fallout 3's pathfinding system had to be completely reworked over several months to account for Liberty Prime, but it was worth it because he's 'awesome'

You may remember the march of Liberty Prime, a set piece from Fallout 3 where the massive communist-hating robot stampedes across the wasteland and terrorizes everything in his path. Even climactic moments like the faceoff against big bad Mehrunes Dagon in Oblivion, which released two years before Fallout 3, seem static compared to Prime; Dagon is big and all, but Prime trundles across the open world like any other NPC.

You might think it's as simple as dropping a 3D model into the world and letting him run around, but the specifics of Prime's march took endless hours to get right. As Bethesda's studio and production director Angela Browder recalled in an interview with PC Gamer's Ted Litchfield, "Liberty Prime is just one of those ones where we spent months trying to get him to walk this very specific path. Months."

"We just want you to walk down this very specific path and do this very specific thing. It's all we're asking you to do, sir," Browder said. "Sometimes when you're developing, you're like, 'Could you just stop?' You'll get Liberty Prime really close, and then some random NPC will run in his way. And then next thing you know Liberty Prime is over here shooting … But it's a cool moment that everyone remembers. So it's worth the effort, for sure."

Browder reckons working with a custom engine means the sky is the limit, though it occasionally feels like a house of cards. "There's a lot of cool things that, if you think too hard about it, they should never have worked," she said.

The fuel for all this extra work was simple: "You never want to say no to an idea that's awesome without at least trying to make it happen."

Former Bethesda programmer Jean Simonet dug into some of the specifics of the Liberty Prime headache on his website. "Navmeshes were all the rage," he wrote, but that was a problem because navmesh-based pathfinding "doesn't work very well with agents of varying sizes." Liberty Prime, being several stories tall, was a magnet for aberrations.

"He broke everything!" Simonet's blog continues. "All the clever tricks I had used until then didn't work. And Liberty prime was crucial to the game, he HAD to walk a pretty long distance while blowing stuff up and throwing nukes and spreading capitalist propaganda." After several months spent rewriting the pathfinding code specifically to account for Prime, the march finally played out as intended.

Simonet ends his post echoing Browder's retrospective thoughts: "It was all worth it! Liberty Prime is awesome!"

You can read all sorts of insights from various Fallout developers in the latest issue of PC Gamer's print magazine, out now.