Reflections are a great example of this. Real, calculated, reflections are a relatively new concept in videogames, but lots of old videogames are able to replicate this effect by creating an "upside down" world, with duplicates of everything, and reflective surfaces are actually windows peering into the upside-down. Its an OK facsimile, but requires specific conditions to be met and can't be applied broadly

[–] kuberoot@discuss.tchncs.de 2 points 1 week ago

It's not just an OK facsimile, it's basically almost perfect - the only thing it's missing is interaction with lighting. That said, it absolutely is limited to only perfectly flat surfaces, and limited in terms of how many different planes you can have, usually just one for water or a big mirror in a bathroom.

I will mention, consider the game Portal - every portal in that game is effectively the work needed to create a realtime planar reflection, by having an extra camera rendering the world from a transformed point of view, with adjusted clipping.