It's a Minecraft mod that provides see-through portals and seamless teleportation. It also can create "Non-Euclidean" (Uneuclidean) space effect.
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This is one of the most complex Minecraft mods. Implementation Details This mod eliminates the limitation that only one dimension can be loaded and rendered on client, eliminates the limitation that only the chunks nearby player can be synchronized to client, eliminates the limitation that client can only load and render chunks nearby the player. The portal rendering is recursive. The portal can have rotation, scale and mirror transformations. The teleportation is seamless, creating the feel that the space is connected.
This mod also provides some API for other mods to create portals. Also includes other utilities. API description.
https://fabricmc.net/wiki/tutorial:setup