Only a few of those statements are true

by uberfoop , Seattle-ish, Wednesday, June 12, 2013, 21:54 (3963 days ago) @ ZackDark

A-hem

Just to make sure you know, when Cody said "2.5D FPS", he was referring to games like Doom and Marathon. The Z axis is semi-faked in those engines. Modern FPS engines based on fully 3d geometry can easily be used to emulate Doom and Marathon, but the reverse isn't true. Marathon may feel very 3d in-game, but there's a lot of stuff it flat-out can't process; you'll notice that no Marathon levels have bridges in them, for instance (or at least not bridges where the people on the bridge can look out at people walking under the bridge).

These ancient 2.5D portal-based engines do have one advantage, in that they have completely native support for placing rooms on top of each other. This allows you to do some silly "non-Euclidean" stuff if you so choose. But there are a number of reasons that people very rarely use that functionality, and you can emulate it seamlessly in modern engines anyway.