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Hi hunterbooth,

There wasn't really too much information about impossible spaces back then. There were a few papers that described the idea (also it's where the term was coined) but I wasn't even aware of them then.

It's better right now as currently there is more information available about rendering portals (using Unreal or Unity or just as general info). There are also two other devs that work on impossible spaces:

One is Impossible Robert working on TraVRsal (). He wrote a post about portals: https://blog.wetzold.com/2020/01/04/non-euclidean-stencil-portals-in-virtual-rea...

The second dev is Triangular Pixels working on Unseen Diplomacy 2 - Unseen Diplomacy was the first actual game, not a research project, that was published that used the impossible spaces. It's worth checking out. It doesn't use portals but that also means that if you're stuck with portals for any reason, you can still do impossible spaces (you just need doors and unloading/loading parts of the level). For Unseen Diplomacy 2 they'll be using portals. John wrote two posts that show the idea: https://unseendiplomacy2.com/tag/portals/

Impossible Robert shows in details how to deal with the most problematic thing with portals - intersecting near plane. My approach is different as instead of using a box around it, I prefer to create a near plane cap. Which comes with its own set of problems but also gives more freedom and allows to place portals anywhere in the world (box approach may require holes behind portals. It might be easier to implement the box approach. The near plane cap requires a little bit of math and depending on the target platform and used libraries, some, uhm, creative solutions.

I use OpenGLES to render stuff and it misses depth clamping. Without it, I have to render very close to the near plane. But in certain situations, intersection of geometry that's right behind the portal and the near plane will make a narrow line visible. To tackle this, I render the world using 99.9% of the depth buffer, leaving that 0.1% purely for the near plane cap.