U.P.E. aka Unreal Portal Engine - is a resource engine designed to visually improve the portal(s) in Minecraft Java Edition.

Do you remember the colored portals from the 20w14infinite snapshot and jeb_ the sheep? I created resource packs using CIT (Custom Item Textures), in which I implemented the ability to paint portals using renamed leather armor and dye, and to apply absolutely any image to them.
Resource packs? Why not a resource pack? During development, I encountered difficulties due to the final resource pack size, model limitations in Minecraft, and name length limitations in older versions and in the OS itself. Therefore, I had to split the resource packs into separate components.

Core Engine (for all game versions)
Add-ons (for Optifine versions from 1.9 to 1.21.4 and for versions starting from 1.21.5 and all future versions, including snapshots)
Connector (for add-ons for game versions 1.21.5 and all future versions, including snapshots)

The core engine doesn't do anything on its own. It's just a collection of models. So, enabling it in-game won't change anything.

Addons work alongside the engine. With them, you can paint portals or use any PNG image (or GIF after proper conversion). Or you can use any image and color it with a dye. It's all up to your imagination.

Connector was created due to the priority of resource packs in the list. It's needed to allow addons to work together on versions 1.21.5+.

Where is a resource pack useful? In singleplayer and on servers.
Since these are resource packs with item renaming, all visual changes will be visible to everyone who has them installed.
You can color-code portals to help you remember which one leads where or which one is connected to which.
You can create a portal gallery.
It's all up to your imagination.

FAQ
Question 1: what does the engine do?
Answer: adds a set of .json models that, after renaming an item, can do the following:

But if you just enable the resource pack, nothing will change in the game, since it is just a set of models.

Question 2: what is needed for the engine to work?
Answer: you need to create an addon for it using CIT (Custom Item Textures) technology, which is available in Optifine, in third-party mods like CIT Resewn, and starting with 1.21.5+, in the vanilla game.

Question 3: will addons work on the server too?
Answer: yes, because CIT (Custom Item Textures) is used exclusively on the game client and not on the server.

Question 4: how to create your own addon?
Answer: you need knowledge of resource packs, scripts for mass file renaming, and Notepad++ for mass character changes.

Question 5: what do you mean by mass renaming?
Answer: portal sizes. This engine supports all portal sizes, from the impossible 1x1 to the maximum 21x21.