Help, can't visualize a fantasy worldbuilding mechanic

[AtlasSniperma]

Hi
I have a problem related to an aspect of fantasy worldbuilding. I'll do my best to explain the issue but it has been rattling around in my head for about a decade so I'm sorry if some things are missed or misexplained as aspects of it are very obvious to me and therefore may be lacking in explanation.

Basic caveats;
) This fantasy world is a flat-earth. An endless plane with terrain etc but it's not a sphere.
) There is no sun, the sky simply gets brighter and darker(/warmer and colder) periodically. So light does not factor in here.

In world description:
There are sections of this world that, as perceived by those in the setting, have a finite perimeter and infinite area. Inside the area there are two main axis you can look; further in, or sideways. Further in continues 'endlessly', but sideways appears to wrap on itself; if you turn 90degrees from "in", you would see yourself in the distance. You see objects repeated periodically in a strip as if the entire place is a hall lined on both sides by a mirror.

Space is a fabric description(3d and 4d):
If we imagine the 3d reality of this "planar" world as a 2d plane, the 'hall' is a cylinder rising perpendicular from the surface. You can move and see around on the plane as normal. On the cylinder, the light "bends" around the surface. Of course, both surfaces aren't 2d in a 3d, they're 3d in a 4d space; there is an up angle in the "plane" and the up angle in "hall" would work exactly as normal, bending where necessary to follow the bent shape of the 'cylinder'. I'm sorry, I'm a lot more experienced with describing the world version above.
Special note; the cylinder only wraps along one direction, it's not a torus. And you can't look "up" from the plane to see the length of the cylinder; that's looking through the 3rd dimension, which the 2d analogous surfaces don't have.

I have absolutely no problem imagining the hall/cylinder. That's fine.
The problem I face, and have faced for years, is where the two shapes meet.
Obviously(I hope, intuitively at least), the meeting point between the two surfaces would look like a sphere in the planar world, and a wall in the cylinder world. But how does that *look*?

My best current conceptualization of how I would be able to see this, is with a raycaster; where a ray hits the sphere determines where the ray "emits" from the wall. The angle of the ray against a plane tangent to the sphere at the position of incidence is the angle of emission from the wall. This (intuitively to me) would likely result in some kind of pincushion distortion, but mapped onto the surface of the sphere(?)
What would the edges of the sphere's shape in your perspective look like? How do things distort? How is the distortion affected by your distance from the sphere? If you're distant and raised compared to the sphere, will it look like the ground inside is curving up like a false hill, or down like some kind of sinkhole? etc etc etc.
Not to mention questions from in the hall looking out.

I've talked about raycasters here, as I feel they'd be the absolute best way to handle getting myself a chance to see this beast of a thing. The issue is I've tried over and over and haven't been able to program one for the life of me. Using game engines I just run into problems getting the mapping connection from sphere to wall etc. I'm at a point where I feel like I need help finding some way to set this up in order to get a visceral resolution of the concept.
Until I see it, I'm afraid it will forever be rattling around inside my head, unwilling to let go.

Please,

 

[Algr]

It sounds like this might be more complicated than it needs to be for your story. You might want to try ditching the 4d aspect, and instead imagining a cubic volume with a border that simply becomes the opposite edge when you move through it.

It's like old video games used to do. The space that Asteroids existed in was a 2d plane folded into a torus. Yours might extend infinitely in two dimensions, and loop in one. One of the dimensions would include the ground, so you need to define what happens if people dig too far or launch a rocket. And you'd need to resolve an issue with infinite mass. You don't want infinite gravity.

If the sky just gets lighter and darker, then where does the light actually come from? Are there shadows? Is it always overcast? Is there a blue sky?