Nether portal

The Portal is part of the October 31st, 2010 Halloween Update. It consists of a frame of Obsidian, four blocks wide, by five blocks tall. The corners of the frame are not required, and only serve for aesthetic purposes. Once the frame is constructed, the player then sets the space inside the frame on fire, using a flint and steel, or Lava and a flammable block. This creates 6 portal blocks inside the frame, resembling a vortex.

Prior to Beta 1.6.0, Portals could be created in SMP servers but do not function to teleport players to The Nether thus SMP servers required modding to access the Nether. As of 1.6, Portals now work in SMP.

Behavior


When the player stands in a portal block for a few seconds, the player is taken to The Nether. The player can step out of a portal before it completes its animation to abort the teleport. A portal will also be created in the place where you enter The Nether, which you can again enter to be returned to the normal world. Building multiple portals on Earth within a certain proximity will all lead to the same portal in The Nether, and the same might happen vice versa.

The purple portal blocks emit light, so passive mobs may crowd around active portals at night. Like Bedrock, portal blocks cannot be broken (although they can disappear); however, they can be placed with modding. It only takes one block of portal to teleport to The Nether. Through the use of an inventory hack, you can place these portal blocks anywhere, but when a non-portal, non-obsidian block is set down adjacent to it, it will remove itself. The ambient portal music can always be heard from these modded blocks, even without an obsidian frame.

Portals can deactivate if there is fire adjacent to it (started by the player or Ghasts). TNT or an exploding Creeper can also disable a portal, but cannot destroy the surrounding Obsidian. It is possible to "re-ignite" portals by setting the space inside the frame on fire once again. If a portal is deactivated, and the player dies without activating it again, the next time they enter the nether a new portal will be created.

Pre-release, Notch suggested it would be possible for Ghasts to rarely appear on Earth near a portal. but Jeb later stated that the feature has not been implemented.

Also, it is possible to create a portal without possessing Diamond, by placing lava with buckets and converting the lava spawn blocks into obsidian with water. This can be done block by block within a mold, or by placing the lava spawn blocks under or directly next to water to create obsidian blocks in place. Any blocks within the portal need to be removed. This way you never have to destroy obsidian, which would require a diamond pick-axe.

Linkage between Overworld and Nether
Nether World is related to the Normal World in the 1:8 ratio in terms of distances. When you move 1 block in the Nether, it corresponds to 8 blocks in the Normal World. This does not apply on the Y-Axis, the Nether Realm has the full 128 layers and is 1:1. Portals do not "remember" what portal they are linked to in the other world, they perform the following whenever a portal is used by a player:




 * 1) Calculate the destination coordinate based on the entry coordinate. (X, Y, Z) <---> (X*8, Y, Z*8). That is, the X and Z coordinates are multiplied or divided by 8 (rounded towards 0) depending on direction of travel. The Y coordinate is not modified.
 * 2) At destination, the game looks for the closest active portal block within a 128 block radius of the player (257x257x128 tall area centered on destination coordinate). An active portal is defined as a portal block that does not have another portal block below it, thus only the 2 lowest portal blocks in the obsidian frame are considered. If one exists, teleport the player to the closest one.
 * 3) If no active portal blocks exists in the above search region, the game creates one by looking for the closest possible valid position within a 16 radius column (33x33x128 tall area centered on the destination coordinate) that has enough space to spawn a portal and is on solid ground. The game prefers to create the exit portal with the same facing orientation as the entry portal, but will check the other 3 directions as well. Regardless of orientation, the closest valid position is always picked.
 * 4) And if there are no valid spawn locations within the spawn region above, the game will finally create a portal at the destination coordinate (and clamp the Y-coordinate to between 70 and 118), converting any blocks (including air blocks) in the way into a portal. Such a portal has 4 extra obsidian blocks placed on both sides of the portal to prevent the player from falling. (See image on right.)

This is described in more detail at http://www.minecraftforum.net/topic/345806-nether-portal-science/.

Implications

 * Likelihood of 2 overworld portals linking to the same nether portal - Normal World portals that are within 1024 distance of each other on either X or Z axis are almost always going to link to the same Nether realm portal on initial construction because 1024 translates to a distance of 128 in the Nether Realm, and the game checks for existing Portals within 128 radius around the destination (the 257x257x128 box).


 * Pairing portals - To setup pairs of portals properly so that they reliably travel to each other, it is best to build both portals manually. Build at desired location X,Y,Z in the Normal World. Then travel to the Nether World. And then dig your way to X/8, Y, Z/8, and build a portal there.


 * Zone of exclusions - The portal spawning algorithm can only spawn portals that is within a 33x33 block column centered on the destination. This will often cause it to spawn a portal at a Y-level that is far higher or far lower than the "exact matching" level. If this height difference is 60, in order to maintain this pair of portals to link to each other reliably, no other portal must be constructed within a 60 distance radius of the expected true portal location in the Nether world, otherwise it would be considered "nearer" and link to that one instead. This in effect creates a 480 block exclusion zone in the Normal World (centered on the Nether->Normal expected coordinate) whereby a new pair of portals cannot be made without breaking the existing pair. Manually pair portals as close as possible in all 3 coordinate axes to prevent portal pairing conflicts. It doesn't have to be exact, or even all that close, if the player knows/ensures that no other portals will be constructed in the exclusion zone created by the difference.


 * 1-way long distance teleport - The portal choosing algorithm can be used for long distance travel by manual construction at carefully selected coordinates. If the player has a Portal in Normal world at (0,64,0) but makes a Nether Portal at (127,64,0) with its perfect Normal World pair at (1016, 64, 0), then the portal at (0,64,0) will go to the Nether Portal correctly (1-way trip) because it is the only portal available within the 128 radius of the expected Nether portal position of (0,64,0). In about 15 seconds, the player can then travel 1016 blocks in the Normal World. Fast travel by Portal is One-Way, and usually used to go from bedrock to surface in Normal World. This is done by constructing a pair of near-perfect surface portals, then making sure the portal near the bedrock in the mines selects the nether portal of the surface pair as its closest.


 * 2-in-1 Nether Portals - It is possible to end up in a situation where a Nether Portal "randomly" places the player in 1 of 2 possible Normal World destination portals. This is simply because the Nether Portal has two effective coordinates as it is 2 blocks wide, say (X, Y, Z) on the left, and (X+1, Y, Z) on the right. If the player entered on the left side, (X, Y, Z) translates to (X*8, Y, Z*8) in the overworld and the game picks the portal closest to that. If the player entered on the right side, (X+1, Y, Z) translates to (X*8+8, Y, Z*8) and the game picks a portal closest to that point instead. This situation occurs when the Nether Portal's location is roughly equidistant between the 2 Normal World portals (within 8 blocks overworld distance difference). However, building 2 Nether Portals side by side is probably better for destination clarity than building a 2-in-1 portal.


 * Spawning a portal in the air - It is possible for a destination Nether portal to spawn floating in the air above a lava lake. This can only occur if there is no possible spawn location in the entire 33x33x128 column of search region to find a suitable spot to place a fresh new portal AND there are no existing portals within the 128 block radius to link to.

Trivia

 * If you warp from world to world, the direction you are facing is maintained. (i.e. if you were facing east on the overworld and you warped to the Nether, you would still be facing east in the Nether)
 * An automatically generated portal may be built at a 90 degree angle to the one you entered. The direction you are facing is still maintained. (i.e. you may find yourself staring at the side of the portal)
 * If you run out of Flint and Steel and disable all of your portals while in The Nether, it is still possible (although difficult) to reactivate a portal by having a Ghast's fireball hit the portal instead of you. Conversely, an active portal hit by a Ghast's fireball will be deactivated.
 * When you look through an activated portal with water behind it, the water will not be visible which will make squid easier to see. Likewise, if you look through a portal with water in front of it, the portal blocks will be invisible.
 * Portal blocks are invisible if they are looked at from behind another portal block, mimicking glass.
 * Even when a portal is built with only 10 blocks of Obsidian (by leaving out the corners), the portal frame spawned on the other side will have the full 14 blocks.
 * It may be a glitch, but the player can strike portal blocks with tools, fists, and blocks, but objects will pass through them. This also explains why you can't put blocks and fire inside the portal frame while activated.
 * It is not possible to use a portal while riding anything such as a minecart or a pig. Prior to Beta 1.6, this crashed the game.
 * Zombie Pigmen grunts can be heard through portals.
 * If you are in your inventory and holding an item while traveling through a portal, the text and the loading bar will be gray.
 * If TNT is struck before a player enters a portal, but does not explode, the TNT will explode upon the player's return to the Overworld. Likewise, furnace burning or redstone circuitry is suspended while in the Nether, and vice-versa.
 * If you open a chest when waiting in a portal to leave, you can access the items in the chest even after spawning in the other realm until you close the inventory.
 * If a mob from the real world enters a portal, they will float in the portal as if it was water. Mobs cannot use portals to teleport to the Nether, however they can walk through them.
 * Prior to 1.6, portals did not function in SMP and servers required modding to access the Nether.
 * If the game crashes while traveling through a portal, your inventory will be deleted and you will be re-spawned at your spawn point.
 * Lightning can activate portals because it lightning causes fire.
 * You can travel to the nether via a portal block placed on any tile using an inventory editor.
 * If you place a portal block obtained through Hacking beside another user-placed portal block, the first one will disappear.
 * The self-destructive nature of a portal block makes it a useful tool for Multiplayer as a spacer as it makes placing blocks one off a surface significantly easier. This is most useful for taller fences which cannot be placed atop one another.  Another use of a portal block is to revert grass to dirt.  Hoe the grass to farmland, then place a portal block on top of it.  The farmland block will detect that the block above is not air and revert to dirt, and the portal will detect that the block below has changed and self destructs.
 * Between Beta patch 1.6.0 and 1.6.4, when the player used a portal in SMP, the player's health indicator is restored to 10 hearts, but not their actual health. The health indicator reset to the correct value only after the player took any damage.
 * The sound emitted by the portal decreases in volume and frequency with the distance from the portal, and appears to fade entirely with at least 15 blocks between the player and a portal block along any axis. The effect is radial.

Media

 * On 29 October 2010 PC Gamer released this video, showing a portal being constructed and used.
 * On 1 April 2011 Think Geek released this video to advertize one of their annual fake April Fools product the Minecraft USB Desktop Nether Portal.