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There was an example of this in the classic 'Duke Nukem 3d'. It had a level by Richard "Levelord" Gray, 'Lunatic Fringe'. https://dukenukem.fandom.com/wiki/Lun

There was an example of this in the classic 'Duke Nukem 3d'. It had a level by Richard "Levelord" Gray, 'Lunatic Fringe'. https://dukenukem.fandom.com/wiki/Lunatic_Fringe https://dukenukem.fandom.com/wiki/Lunatic_Fringe This level had a circular hallway ring around the outside that had two full rotations around without intersecting, using the 'build' engine's ability to separate areas by their room connections that also drove the 'room over room' technology which was groundbreaking at the time. It made for fun multiplayer, and the illusion held well there. The central chamber has 4 entrances/exits if I recall, and you would only encounter two of them in each loop around the outside. I recall building a toy level while experimenting with the engine that "solves" the "3 houses with 3 utilities without crossing" puzzle using this trick as well.

dfox 2y ago
Room over room (as done in later build games) usually does not require the sectors to overlap in this manner as it is a different thing (it is extension of how swimable water works, where the engine renders the other sector instead of floor/ceiling). While Lunatic Fringe is pretty in-your-face example of impossible geometry in build, the duke3d maps contain many more cases of intersecting geometry. Obviously such things are impossible in Doom because there is no way to build BSP tree out of that and because doom only tracks X/Y coordinates of player(s)/monsters.
- ant6n 2y ago
  Portals allow weird stuff (non linear geometry) in a BSP level. I thought Doom had petals.
  - dfox 2y ago
    The primary thing that BSP does is that it maps coordinates onto a BSP node/sector. Thus you cannot have overlapping geometry, as this mapping would not be unique. Quake has some idea of portals (I'm not sure about Doom), but it is used only as an additional layer of optimization, the engine is not fundamentally portal-based.
  - jorvi 2y ago
    There is a very interesting Doom level named “myhouse.wad” that does a lot of clever things to seemingly allow room-over-room.
    - Narishma 2y ago
      I don't think that mod works on a normal Doom engine.
- justsomehnguy 2y ago
  > only tracks X/Y coordinates of player(s)/monsters. It does, otherwise Lost Souls and Cacodemons wouldn't able to fly.
  - dfox 2y ago
    I meant that as in contrast to build tracking the sector the thing is in. Due to that you can have two sectors that intersect not only in 2D, but even in 3D and the engine still does the right thing (ignoring the fact that the renderer gets slightly confused when there are overlapping sectors in the view)
- wizzwizz4 2y ago
  Is there no way of building a BSP out of it? I don't see why it has to be Euclidean to be partitioned, and loops are also possible (e.g. Deimos Lab). (The coordinates are definitely a blocker, so this question is academic.)
  - dfox 2y ago
    The “sectors” in the resulting level data have to be convex for the doom engine. The “asset pipeline” handles that by breaking up non-convex geometry into smaller convex sectors. So, there are no loops or holes in the actual level data, also from the cursory glance both of the large loops in Deimos Lab are actually not complete loops, but they have a place where the loop is broken. But that does not matter that much, as almost any level contains geometry that is either concave or has a hole (courtyard in E1M1, both large rooms in MAP01…)
  - immibis 2y ago
    The Doom design starts with X,Y,Z and then finds your BSP node. You can't warp to a different X,Y,Z without some type of warp portal which doesn't exist in Doom. Modern Doom forks have this feature and also have graphics portals so you can implement this (as MyHouse.pk3 does).
- formerly_proven 2y ago
  No mention of the real Prey (2006) in a discussion of weird geometry in games?
  - rompledorph 2y ago
    I was truly amazed by this game! You don’t see anything like that elsewhere
    - habitue 2y ago
      The portals in prey were cool but, definitely the games Portal and Portal 2 are places you could see it elsewhere. Also in portal you can actually choose where the portals are, whereas Prey's portals were fixed in place
- somat 2y ago
  Sure you can do this in doom, or at least it can be done in modern source ports. https://www.youtube.com/watch?v=Iq1-TZXz9xo https://www.youtube.com/watch?v=Iq1-TZXz9xo (myhouse.wad) There is nothing about the data structures ID choose to use in doom that prevent portal shenanigans. ID(John Carmack) just did not implement them.
  - anthk 2y ago
    Modern source ports are a totally different thing. If any, try to do what the former comment states with a Boom compatible engine as Crispy Doom or Chocolate Doom.
    - jalbertoni 2y ago
      You're confusing things a bit here, Boom is already an enhanced port and it got the kind of line-to-line teleport that allows you to do this. Crispy and Chocolate Doom are what the community calls "vanilla" Doom, with no enhancements at all. That said, if you are willing to hex-edit the map data, you can have non-euclidean geometry in vanilla Doom: https://doomwiki.org/wiki/Linguortal https://doomwiki.org/wiki/Linguortal
      - anthk 2y ago
        Crispy Doom will play most Boom compatible levels just fine. I tried it with some Megawads, and I had no issues, even with FreeDoom as the IWAD. No, I'm not confused. To me, Boom it's pretty close to vanilla. Zdoom and the rest are pretty much another league, closer to Quake/HL than Doom.
whoopdedo 2y ago
> the 'room over room' technology which was groundbreaking at the time. Bungie's Marathon from 1994 could also do this and demonstrated it in the deathmatch map 5-D Space[1]. It was really only Doom that bugged out on overlapping sectors. [1] https://www.lhowon.org/level/marathon/30 https://www.lhowon.org/level/marathon/30
SJC_Hacker 2y ago
> also drove the 'room over room' technology which was groundbreaking at the time. Descent did this in 1994.
- ChrisClark 2y ago
  Descent was a polygonal game though, Duke3D was a raycasting engine. It was the first four a raycasting engine, and quite impressive.
  - mysterymath 2y ago
    Duke3D isn't a raycaster; it's a portal-based quad rasterizer. Wall quads are forced vertical, and floor quads are rasterized using a neat trick. The renderer computes successive lines of constant distance along slooped floor surfaces, then draws textures using affine scaling, which is correct along those lines. The portal walls clip the draw of successive sectors, which solves the hidden surface problem. https://fabiensanglard.net/duke3d/build_engine_internals.php https://fabiensanglard.net/duke3d/build_engine_internals.php
jandrese 2y ago
I remember a Descent PVP level that was a big room with a corridor that came off of one end and went to the other by looping back through the main room but was not visible inside of the main room. Was a bit of a mindbender in a game that already stretched the players spatial awareness.
lucianbr 2y ago
In what sense is the Duke Nukem thing "an example of this"? The duke thing is an internally consistent programmed behaviour, this is... just random errors caused by a random change in the source code. Duke is maybe non-euclidean geometry, or something. This doom pi thing is... nothing to do with geometry. More an example of "garbage in, garbage out" maybe.
- ColinDabritz 2y ago
  It's an example of "non-euclidean" space, and yes, it is a bit different than the article.
- snthpy 2y ago
  The DN example would be an instance of non-Euclidean topology whereas the other one is presumably non-Euclidean geometry.
- deleted 2y ago
  [deleted]
Synaesthesia 2y ago
This video does a good job explaining it, yes it really is quite mind-bending. https://www.youtube.com/watch?v=UitzmhJe578 https://www.youtube.com/watch?v=UitzmhJe578
immibis 2y ago
I wrote a little engine with this capability a long time ago. I didn't know about the Build engine at the time. I divided the world into convex sectors and allowed arbitrary links (portals) instead of following a BSP tree. Rendering is front-to-back and clips at the portal boundary. If you can rasterize the inside of a convex shape, you can rasterize a sector-portal-world by marking certain faces as portals, and when rendering one, setting the clipping region (or stencil buffer) to where the face would be, then rendering the sector behind it with an appropriate transformation (which is part of the portal's data, alongside the ID of the other sector). Collisions through portals are much harder than rendering through portals.
thaumasiotes 2y ago
> the "3 houses with 3 utilities without crossing" puzzle In graph theory's terminology, this is "K3,3", one of two irreducible nonplanar graphs. The other one is K5. You can also make all the connections without crossing any edges if you embed the graph in a torus, which is equivalent to building a bridge over some set of edges that other edges are allowed to take.
Andrew_nenakhov 2y ago
There is an interesting VR game called "Tea For God" that puts you in your play area, and cleverly creates new corridors and rooms as you make it around the corner or lift so there is an illusion that you are in a very vast place, despite never leaving the same room, all without using the joystick or teleporting.
eek2121 2y ago
I spent so much time back in the day working with build. In many ways it was super easy (and fun!) to use. Thanks for the throwback!

dfox 2y ago

Room over room (as done in later build games) usually does not require the sectors to overlap in this manner as it is a different thing (it is extension of how swimable water works, where the engine renders the other sector instead of floor/ceiling). While Lunatic Fringe is pretty in-your-face example of impossible geometry in build, the duke3d maps contain many more cases of intersecting geometry. Obviously such things are impossible in Doom because there is no way to build BSP tree out of that and because doom only tracks X/Y coordinates of player(s)/monsters.

ant6n 2y ago
Portals allow weird stuff (non linear geometry) in a BSP level. I thought Doom had petals.
- dfox 2y ago
  The primary thing that BSP does is that it maps coordinates onto a BSP node/sector. Thus you cannot have overlapping geometry, as this mapping would not be unique. Quake has some idea of portals (I'm not sure about Doom), but it is used only as an additional layer of optimization, the engine is not fundamentally portal-based.
- jorvi 2y ago
  There is a very interesting Doom level named “myhouse.wad” that does a lot of clever things to seemingly allow room-over-room.
  - Narishma 2y ago
    I don't think that mod works on a normal Doom engine.
justsomehnguy 2y ago
> only tracks X/Y coordinates of player(s)/monsters. It does, otherwise Lost Souls and Cacodemons wouldn't able to fly.
- dfox 2y ago
  I meant that as in contrast to build tracking the sector the thing is in. Due to that you can have two sectors that intersect not only in 2D, but even in 3D and the engine still does the right thing (ignoring the fact that the renderer gets slightly confused when there are overlapping sectors in the view)
wizzwizz4 2y ago
Is there no way of building a BSP out of it? I don't see why it has to be Euclidean to be partitioned, and loops are also possible (e.g. Deimos Lab). (The coordinates are definitely a blocker, so this question is academic.)
- dfox 2y ago
  The “sectors” in the resulting level data have to be convex for the doom engine. The “asset pipeline” handles that by breaking up non-convex geometry into smaller convex sectors. So, there are no loops or holes in the actual level data, also from the cursory glance both of the large loops in Deimos Lab are actually not complete loops, but they have a place where the loop is broken. But that does not matter that much, as almost any level contains geometry that is either concave or has a hole (courtyard in E1M1, both large rooms in MAP01…)
- immibis 2y ago
  The Doom design starts with X,Y,Z and then finds your BSP node. You can't warp to a different X,Y,Z without some type of warp portal which doesn't exist in Doom. Modern Doom forks have this feature and also have graphics portals so you can implement this (as MyHouse.pk3 does).
formerly_proven 2y ago
No mention of the real Prey (2006) in a discussion of weird geometry in games?
- rompledorph 2y ago
  I was truly amazed by this game! You don’t see anything like that elsewhere
  - habitue 2y ago
    The portals in prey were cool but, definitely the games Portal and Portal 2 are places you could see it elsewhere. Also in portal you can actually choose where the portals are, whereas Prey's portals were fixed in place
somat 2y ago
Sure you can do this in doom, or at least it can be done in modern source ports. https://www.youtube.com/watch?v=Iq1-TZXz9xo https://www.youtube.com/watch?v=Iq1-TZXz9xo (myhouse.wad) There is nothing about the data structures ID choose to use in doom that prevent portal shenanigans. ID(John Carmack) just did not implement them.
- anthk 2y ago
  Modern source ports are a totally different thing. If any, try to do what the former comment states with a Boom compatible engine as Crispy Doom or Chocolate Doom.
  - jalbertoni 2y ago
    You're confusing things a bit here, Boom is already an enhanced port and it got the kind of line-to-line teleport that allows you to do this. Crispy and Chocolate Doom are what the community calls "vanilla" Doom, with no enhancements at all. That said, if you are willing to hex-edit the map data, you can have non-euclidean geometry in vanilla Doom: https://doomwiki.org/wiki/Linguortal https://doomwiki.org/wiki/Linguortal
    - anthk 2y ago
      Crispy Doom will play most Boom compatible levels just fine. I tried it with some Megawads, and I had no issues, even with FreeDoom as the IWAD. No, I'm not confused. To me, Boom it's pretty close to vanilla. Zdoom and the rest are pretty much another league, closer to Quake/HL than Doom.

whoopdedo 2y ago

> the 'room over room' technology which was groundbreaking at the time. Bungie's Marathon from 1994 could also do this and demonstrated it in the deathmatch map 5-D Space[1]. It was really only Doom that bugged out on overlapping sectors. [1] https://www.lhowon.org/level/marathon/30 https://www.lhowon.org/level/marathon/30

SJC_Hacker 2y ago

> also drove the 'room over room' technology which was groundbreaking at the time. Descent did this in 1994.

ChrisClark 2y ago
Descent was a polygonal game though, Duke3D was a raycasting engine. It was the first four a raycasting engine, and quite impressive.
- mysterymath 2y ago
  Duke3D isn't a raycaster; it's a portal-based quad rasterizer. Wall quads are forced vertical, and floor quads are rasterized using a neat trick. The renderer computes successive lines of constant distance along slooped floor surfaces, then draws textures using affine scaling, which is correct along those lines. The portal walls clip the draw of successive sectors, which solves the hidden surface problem. https://fabiensanglard.net/duke3d/build_engine_internals.php https://fabiensanglard.net/duke3d/build_engine_internals.php

jandrese 2y ago

I remember a Descent PVP level that was a big room with a corridor that came off of one end and went to the other by looping back through the main room but was not visible inside of the main room. Was a bit of a mindbender in a game that already stretched the players spatial awareness.

lucianbr 2y ago

In what sense is the Duke Nukem thing "an example of this"? The duke thing is an internally consistent programmed behaviour, this is... just random errors caused by a random change in the source code. Duke is maybe non-euclidean geometry, or something. This doom pi thing is... nothing to do with geometry. More an example of "garbage in, garbage out" maybe.

ColinDabritz 2y ago
It's an example of "non-euclidean" space, and yes, it is a bit different than the article.
snthpy 2y ago
The DN example would be an instance of non-Euclidean topology whereas the other one is presumably non-Euclidean geometry.
deleted 2y ago
[deleted]

Synaesthesia 2y ago

This video does a good job explaining it, yes it really is quite mind-bending. https://www.youtube.com/watch?v=UitzmhJe578 https://www.youtube.com/watch?v=UitzmhJe578

immibis 2y ago

I wrote a little engine with this capability a long time ago. I didn't know about the Build engine at the time. I divided the world into convex sectors and allowed arbitrary links (portals) instead of following a BSP tree. Rendering is front-to-back and clips at the portal boundary. If you can rasterize the inside of a convex shape, you can rasterize a sector-portal-world by marking certain faces as portals, and when rendering one, setting the clipping region (or stencil buffer) to where the face would be, then rendering the sector behind it with an appropriate transformation (which is part of the portal's data, alongside the ID of the other sector). Collisions through portals are much harder than rendering through portals.

thaumasiotes 2y ago

> the "3 houses with 3 utilities without crossing" puzzle In graph theory's terminology, this is "K3,3", one of two irreducible nonplanar graphs. The other one is K5. You can also make all the connections without crossing any edges if you embed the graph in a torus, which is equivalent to building a bridge over some set of edges that other edges are allowed to take.

Andrew_nenakhov 2y ago

There is an interesting VR game called "Tea For God" that puts you in your play area, and cleverly creates new corridors and rooms as you make it around the corner or lift so there is an illusion that you are in a very vast place, despite never leaving the same room, all without using the joystick or teleporting.

eek2121 2y ago

I spent so much time back in the day working with build. In many ways it was super easy (and fun!) to use. Thanks for the throwback!