The original BioShock ran on Unreal Engine 2.5, but Irrational's upcoming BioShock Infinite will run on Unreal Engine 3. Like its predecessor, Infinite's development team has made some serious tweaks (via Joystiq) to Epic's engine for their purposes.

We already knew that the studio would be tweaking its engine, but now we've got some concrete details. Irrational's technology director Chris Kline has detailed the changes in a blog post on the studio's official forums.

All major FPS engines (CryEngine 3, UE3, id Tech 5) are designed and optimized for static environments that the player moves through, which is a reasonable choice because, even if there are trees moving in the wind, the ground under the player's feet isn't going anywhere. Unfortunately for our tech team (but good for you) everything in Columbia is capable of moving.The very ground beneath your feet could fall out of the sky at any moment, which makes for some awesome gameplay and visuals but required us to create a completely new technology that we're calling "Floating Worlds". You saw a little of this in the gameplay demo video (in the part right after Saltonstall jumps on the Sky-Line) and you'll be seeing a lot more of it in the future.

To get the best performance out of the game engine, Irrational "built a whole new parallel processing framework (a 'job architecture', in programmer lingo) that lets the engine take advantage of as many cores as you can throw at it. This will let us eke out all the power of the PS3 and 360, and also give hardcore PC gamers something to show off their rigs with."

A new renderer based upon deferred lighting was also required. This type of rendering is used in Uncharted 2: Among Thieves, Crysis 2, and Killzone 2. "On top of that they've developed a proprietary per-pixel dynamic relighting scheme that allows characters and dynamic objects to receive global illumination." Fancy!

Artificial intelligence was another aspect of UE3 that wasn't right for Infinite "out of the box." The studio "needed an entirely new AI system that was both more efficient and gave designers the ability to author their own behaviors," due to "the intelligence and sheer number of AI we planned to throw at the player."

A new animation system was built on top of NaturalMotion's Morpheme middleware as "smooth and complex animation is the key to looking intelligent." The sound engine also got work, building off of AudioKinetic's WWise to support "5.1 with adjustable dynamic range and a fully dynamic mixing system. The team went a step further and "implemented both a custom sound propagation system (so voices properly echo down corridors and around buildings) and a dynamic wind audio system that reinforces the dynamic weather in the world."

With all of that going on, it's no wonder that the game is eying a 2012 release! If you want to see some of these changes in action, simply watch this nine-minute gameplay demonstration. See if you can spot any per-pixel dynamic relighting!