How is this effect done?

There are numerous maps throughout Doom 64 in which a sector that is taller than one adjacent to it is visible, whereas in Vanilla Doom, the lower sector would normally cut it off. Is this an effect exclusive to Doom 64's modified Doom 2 engine, or is this possible in Zdoom or Gzdoom modding? Is it possible at all in Vanilla?

GoatLord said:

Is this an effect exclusive to Doom 64's modified Doom 2 engine, or is this possible in Zdoom or Gzdoom modding? Is it possible at all in Vanilla?

The way it is achieved in Doom64 is exclusive to Doom64. Rendering compatibility with vanilla Doom means that this method is not possible in vanilla or any other PC Doom port.

However, the effect can be replicated in some advanced source ports, including ZDoom and GZDoom, by using 3D floors or portals.