Has anyone ever used a linguortal in a map?

I tried building one for kicks once, didn't really grasp the concept and it didn't work properly.

I did make a mikoportal setup for a dbp map (I think it was a 67 one), then promptly realized it'd instantly break in the port 99% of dbp player use and scrapped it. Fun mapping experiment yho.

I mean, this is just a very limited visual trick, it makes sense that it is rarely used. Since it messes with the BSP tree it probably causes some issues that weren't mentioned there.
Mikoportals though might be usable for some more interesting stuff.

I don't believe this trick has ever been used in a map. It's simply too specialized and narrow. I see it almost in the same category as the ACE Engine where you're exploiting outside the normal bounds of mapping tricks.

It's not easy to understand the trick even at a conceptual level, but I'll give it a try.

Suppose you are in a dark room. You can't see anything. You want to leave but if you start moving you might bump into a wall or fall into a pit, or something worse.

You are given a laser pointer which won't create enough light to move around but will create a laser "dot" on any solid objects that it hits (assume there's no glass or mirrors or anything tricky in the room). The problem with this laser pointer is it doesn't stay on: it clicks on and then immediately turns off. Then you have to press the button to turn it back on again, and repeat.

You can still move around the room, but it's tedious. You have to "scan" the room with your pointer, one dot at a time, and move a little bit, then do it again, and so on.

This is how Wolfenstein 3D worked: scanning the room with a laser pointer, one "dot" at a time. This is, incidentally, one reason why Wolfenstein 3D is a "flat" world: you don't have to move the laser pointer up and down if you are guaranteed the world is flat and all the walls are the same height, which saves a ton of time.

For Doom, Carmack needed a better method. So, the idea was, beforehand, to send a "traveler" through the room who could produce a special kind of map. This special map allows you, from any point on the map, to determine which portions of the level are visible to you. No more scanning with your laser pointer!

The trick here is to edit the special map and put a lie into it. When facing a certain wall, which is in reality a flat, dead-end window, the trick will be to sneakily edit in a reference to part of the special map that is nowhere near that wall and put that in the wall's place. Now when facing that window, the special map will say "refer to page 51" which is wrong. And, the whole scene in "page 51" will be drawn where the flat wall should be. But this is not really there: it's only an illusion created by editing this special map in exactly this way.

Some engines don't trust the special map and rebuild it themselves, or otherwise try to fix up mistakes, so the trick is very flaky. It fully relies on the renderer using the map as-is to construct the player's view.

I think I sort of get it now. Still, imo it's cooler as a tech hacking / experiment sort of thing: on the practical side, my takeaway is that it's basically a parlor trick. I can't imagine ever reliably using a system that requires a more basic port, considering telling people *not* to play stuff in GZ and similar is a doomed effort 99% of the time.

Isn't this in some ways similar to what "portal-based" (if that's the name) engines work? (Build / Descent) Except there it's defined in a sort-of rigorous way?

In Build, at least, a trick like shown in the first post is exactly possible, and not just visually. You can actually walk into this portal and seamlessly enter a completely different area, not confined by the original geometry. For an example - check Duke Nukem 3D E3L10 - Tier Drops.