Transition Portal - CD4017BE/VerticallyStackedDimensions GitHub Wiki

Transition Portal blocks can't be crafted, they are generated at the top or bottom most block layer of a dimension (see Dimension stacking). They are also unbreakable and completely invulnerable to explosions and boss monsters.

Behavior

A Transition Portal can have different states that mimic the existence (or non existence) of solid blocks in the first two layers adjacent to the matching portal in the other dimension. Therefore from appearance and collision behavior it acts as two blocks: one at its actual location and the other one already by one block outside of the world's build limit.

It can show any combination of the first and second block each being either filled or empty. But only in its fully empty state it will let Entities pass through and teleport them to the other dimension as soon as their bounding box is fully behind the portal's front face. This ensures that players (and other up to 2 block tall Entities) won't suffocate in a wall on the other side.

The Portal itself is unbreakable (even in creative mode), but when attempting to break it, instead the block in the other dimension represented by the current state is broken in exactly the same way the player attempted to break the portal block. In the same way also right-clicking the portal in one of its empty states with a block in hand will place the block on the other side.
This way you can dig or pillar your way through the portal.

Survival Tips:

Since the portals can only show whether the blocks on the other side are solid or not, you can never be sure what blocks you would actually encounter. This is especially dangerous if there happens to be a lava lake on the other side for example. So better not trust tunnels you haven't dug yourself.

Also since the portals only show the first two blocks, you can't be sure that there will be a block to stand on when traveling down. If you're unlucky there is a very deep pit.

However, if you ever need to travel down trough an open sky (only air on the other side) portal layer without having any flight equipment you can do the following:
Place a solid block into one portal and place a ladder into the open portal directly next to it. Then stand on the portal with the block filled and carefully let yourself fall over the edge where you placed the ladder before. Immediately press the sneak key as you beginn to fall so when you spawn in the other dimension you keep holding on the ladder you placed. From there you can use the standard waterfall trick to progress further down.

Synchronization

Because the chunks on the other side aren't always loaded, the portal blocks store the necessary information in their block state and synchronize whenever it changes. This synchronization causes the adjacent chunk in the other dimension to force load and happens when one of the two blocks in front of the portal changes between a solid and a non solid block. Synchronization also happens just before teleporting Entities or breaking/placing blocks on the other side.

Specialties of "top open" dimensions:

If the terrain of a dimension does not reach a chunks upper most 16x16x16 segment at ceiling portal height so that this region contains nothing but air blocks then the upper portal layer won't be generated in that chunk.

Instead, the portals are later generated "on demand" as soon as a block is placed at or above 3 blocks below the portal height. This will then generate the portals in a 2x2 chunk area centered around the placement event. (this will also trigger if the block placement wasn't caused by a player)

The reason for this special behavior is that certain things like spawn point and surface generated features could otherwise get confused by the portal layer at the ceiling. Also does this improve performance for worlds that are mostly empty or have very low surface level. (see #7 for details)