Slopes are an editing feature supported by some Doom source ports which allows the designer to make surfaces that are at angles other than horizontal or vertical. Sloped surfaces in a 2.5D system have issues that are mostly irrelevant in true 3D engines such as Quake.
In a 2.5D map system, depending on the implementation, it can be either floors or walls that are sloped. Doom source ports usually prefer to slope the floors and ceilings, just like, for example, Duke Nukem 3D, as it is technically more efficient. Regardless of the implementation, the greater the angle between the surface and its "normal" (horizontal or vertical) alignment, the more stretched its texture. Source ports may provide mechanisms to alleviate the problem, such as the Sector_Set*Scale specials in ZDoom.
A full implementation of sloped surfaces includes changes to both the renderer and the physics code. The former is relatively simpler than the latter, since even slight changes in the physics code risk breaking demo compatibility.
Ports with slopes
- CDoom 2.07 (99% of the bugs fixed, since cdoom uses fixed point arithmetic)
- EDGE (renderer only)
- Eternity Engine (renderer only)
- Using slopes at the ZDoom wiki
- Slope linedef types at the Eternity Engine wiki
- EDGE line types 567-569 at the EDGE editing guide