: Maybe he meant it was Bungie's first 3D FPS, although that's still only
: partly true. PID was strictly a 2D game, made to look 3D. Marathon was
: Bungie's first true 3D game engine although it still didn't allow slanted
: walls. We call it 2.5D, but in reality it's probably closer to 2.9D.
Graphically, Marathon is still a 2D game, about as 2D as PiD was. It has no vertical perspective; looking up isn't actually changing a camera angle or anything, it's some sort of texture skewing that just gives the illusion of looking up. (IIRC - someone correct me here). The file format is completely 2D with floor and ceiling elevations applied to completely 2D polygons - there is no such thing as a true 3D surface to be slanted in Marathon, just two identical copies of a 2D polygon with some textured, extruded lines between them. Overlapping spaces isn't 3D, as 5D space illustrates; it's just an illusion allowed by using a portals-based engine.
Even the physics are 2D, though turned on their side; they will calculate that something should follow a nice parabolic curve through the air, but that curve is only accounting for distance relative to your facing (one dimension) and height (a second dimension); you can turn that "vertical plane" any way you like while something is travelling it, which is why things (most notably players, but I suppose a guided projectile affected by gravity too) can turn in mid-air and "fly" back the way they came, with no inertia relative to the rest of the world. Sprites of course are completely 2D and don't even skew like wall textures do.
So no, Marathon is far more 2D than it is 3D; it's just got a lot more neat tricks that make it SEEM more 3D than PiD.
