3D middle texture
While the practice of representing a bridge through vertically-offset middle textures surrounding an InvisibleBridge has long been a staple of ZDoom modding, the possibility of discarding the bridge thing and relying just on the middle texture being there has been pioneered by the Eternity Engine, which could not use bridge things at all given its lack of support at the time for setting a thing's Z-height (only allowed by the Hexen and textual map formats). Adopting this feature in ZDoom required overcoming the limitations of the linedef structure, whose flag field was already fully-used, and this was not a priority given the perfectly serviceable bridge things available.
However, they were eventually added anyway. There are three different ways to set a linedef as being a 3D middle texture, depending on the choice of map format. From simplest to most complicated:
- UDMF: All that is needed is checking its "midtex3d" flag (named "Walkable middle texture" in DB2 and GZDB and "3D MidTex" in Slade 3).
- Hexen format: Line_SetIdentification has to be used, with extraflag 32.
- Doom format: Flag bit 10 (Value 1024 or 0x400) represents the 3DMidtex flag (Same value as in Eternity.)