How the heck did they do this in 1996?

[StarSpun5000]

So in Memento Mori 2 Map01, there has been one small thing about that map that has stuck with me ever since I first played it, and that's this door. It is not simply a regular see through door, no no no, you can SHOOT through this door, and enemies on the other side can shoot through it at you. How is this even possible in vanilla doom? I've seen see through doors in WADs like Alien Vendetta and even Memento Mori 1, and while I don't know the exact specifics of how those were done, I do have a general idea, but this is on an entirely different level. How?

[dististik]

I don't know for sure but my guess would be that the actual "door" is only low enough that it blocks movement and a midtexture extends to make it seem like a complete door.

[StarSpun5000]

10 minutes ago, dististik said:

my guess would be that the actual "door" is only low enough that it blocks movement and a midtexture extends to make it seem like a complete door.

Oh trust me, that's what I thought at first, but I tested that theory and well....no

You can shoot through the top part of this door as well as any other part of the door.

[Sneezy McGlassFace]

Oh, you can shoot though the entire door? I hope somebody explains it but I'll be looking it up in the editor when I get home from work. 

Edited February 16, 2023 by Sneezy McGlassFace

[Kappes Buur]

 

In those days, and since really, control sectors and linedefs referencing control sectors were/are used for some incredible effects.

 

 

The secret in this case is the double control sector to the right, and the doublesided linedef in the "door"  frame referencing sector 47.

 

 

When the linedef iin the upper hallway is triggered, the ceiling of the upper control sector raises to the height of the lower control sector, and thus the "door" linedef, allowing the player to walk through.

Edited February 16, 2023 by Kappes Buur

[Foxpup]

To explain further (and actually answer the original question), all line of sight checks (which includes hitscan attacks and explosive blast damage) ignore linedefs with both sides in the same sector, while collision checks for actors and projectiles do not (observe that projectiles are blocked by this door if they're higher than 48 units, eg when fired from the top of the stairs). This is normally a correct optimisation when the linedef is actually part of its surrounding sector, but fails when it's a different sector entirely (generally termed a self-referencing sector).

 

Since self-referencing sectors break many assumptions made by the rendering and life-of-sight code, they are a common trick used to create transparent walls, doors, and platforms, but they're by no means the only one; transparent doors created by other methods will exhibit different behaviour.