Ordinary portals are flat, but it's not an axiom. And I tell you about such shapes of portals, which are difficult to guess, not to mention to imagine. In particular, here is a portal in the form of: a cylinder, a Mobius strip, a trefoil knot and much more. Some of the portals shown were opened by myself, and some by other people.

Chapters:

00:00 Cylindrical Portal
02:08 Introduction
02:45 Surface portal
04:16 Linear Transformations
05:10 Möbius strip
10:01 Monoportal
12:17 Möbius Monoportal
13:35 Triple portal
17:34 N-monoportal
18:30 Shifting monoportal
19:11 Scaling monoportal
21:02 Tilings
23:09 Chain
23:36 Knot
32:29 Other
34:16 Other projects
35:08 Conclusion

References:
[1]: https://optozorax.github.io/portal/ - my demo
[2]:   / 1080438078852947969   - avatar
[3]:    • Что будет если поместить портал в пор...  
[4]: https://sirxemic.github.io/wormhole/ - online wormhole simulation
[5]: https://optozorax.github.io/portal/?s... - nonlinear motion scene
[6]: https://raytracing.github.io/books/Ra... - book about raytracing
[7]: https://kenbrakke.com/polycut/polycut... - PolyCut
[8]:    • ТЫ БЫ НИКОГДА ТАКОЕ НЕ ЗАГУГЛИЛ  
[9]:    • How Does a Non-Reversing Mirror Work?  
[10]:    • Portal Reloaded - Gameplay Trailer  
[11]:    / @onigiriscience  
[12]:    • Blocky Knot Portal v2  
[13]:    • Self-Hiding Knot Portal  
[14]:    • Thurston, Knots to Narnia  
[15]: https://www.imaginary.org/program/kno...
[16]:    • The Insane Math Of Knot Theory  
[17]: https://contest.zenmagnets.com/56-5-c... - Cubic Eight Knot
[18]: http://paulbourke.net/geometry/borrom... - Borromeo Rings
[19]:    • Non-Euclidean recursive house  
[20]:    • Portals to Non-Euclidean Geometries  
[21]: https://unasanu.xyz/
[22]: https://t.me/optozorax_dev - my telegram channel
[23]:    • Portal Teaser Trailer  
[24]:    • Portal 2 Physics - Portal Mirrors  

Places where I write:
Telegram - https://t.me/optozorax_dev