zdoom portals in wads?

I've read about this being possible in zdoom but never seen it implemented. Plenty of teleporters, but no portals you can step through. Also you can apparently see the part of a map that a portal leads to before you step in it.

I found this quite some time ago, but I cannot remember who made it.

It's possible, but it has a small glitch.
You have to take a run at it.

[edit]
That didn't let me rest until I found the original post
http://forum.zdoom.org/viewtopic.php?f=3&t=19822&p=386430#p386426

ZDoom's portals are horizontal, and you can't cross them. They're also called "stacked sectors".

Anything else will be a teleport line. You can kinda fake portalness with a camera texture on it, but it'll be obvious the viewpoint is not moving, so it'll be like looking at a TV rather than a window to a different part of the map.

Eternity's the port with the vertical portals, and even there they're not completely finished yet (which is why, if I'm not mistaken, they weren't used in the Vaporware demo).

Gez said:

Eternity's the port with the vertical portals, and even there they're not completely finished yet (which is why, if I'm not mistaken, they weren't used in the Vaporware demo).

They were used, just in a limited context that avoided any game-breaking issues.

Gez said:

ZDoom's portals are horizontal, and you can't cross them. They're also called "stacked sectors".

Is this how they did that upper floor at the start of KDiZD Z1M1, where that crate is hanging?