The original 1993 Doom is a 2D game that uses clever mathematics to create the illusion of a 3D world.

Released on December 10, 1993, by id Software, Doom revolutionized gaming through a technique called Binary Space Partitioning (BSP). Lead programmer John Carmack implemented BSP to overcome the computational limitations of 486-class processors. Since the hardware couldn't calculate 3D coordinates in real-time, the engine treated the game world as a 2D map where every object had a fixed vertical height.The BSP tree works by recursively dividing 2D space into convex sets using lines. This allowed the engine to quickly determine which walls were visible to the player and the order in which to draw them. By pre-calculating these visibility structures, the game avoided the need for complex depth-sorting during gameplay. This limitation is why the original engine couldn't support rooms stacked directly on top of each other—the 2D map could only store one floor and ceiling height per coordinate.To create the 3D effect, the engine used raycasting to project the 2D lines onto the screen, scaling wall heights based on their distance from the player's viewpoint. This "2.5D" approach was so efficient that Doom could run at high frame rates on standard home computers of the era. The success of this mathematically elegant solution paved the way for the future of first-person shooters and real-time 3D graphics.