Wither farms improved: Nether fort mobs spawning outside nether fort bounding boxes!
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EDIT: in case you want to find the area in which nether fort mobs can spawn over nether brick in your worlds, use this mod: https://youtu.be/_gz31yqyFzE
This works in Minecraft 1.8 and above, not in lower versions. Feel free to ask your doubts.
Sorry if you don't like the music I used in this vid, I put music to make the video less boring and more interesting.
The way this works is actually pretty similar to defanive's discovery, but it's more or less like a bugfix. (sorry if you don't understand the explainig, it's a bit messy).
Mojang limited structures' generation to y = 127 and below and removed the algorithm which added structures to the hashmap, but now they're only added to the volume the bounding box cover... But what does that mean? Well, nether fort mobs now spawn inside the nether fort (segment)'s bounding box and in the areas within the bounding box of then nether fort itself (not its segments/parts) and the block below is nether brick.
Contributors to this discovery:
Initial_Y (TW: @ylbmessasid):
First discoverer of this, made a farm using this concept (http://www.mediafire.com/download/1a62s37pv5kocno/Low+Y+wither+skeleton+farm.zip)
Me, ElB0ludo (TW: @El80ludo):
Discovered this (though he wasn't the first one), made a video with all the information he got about it
TDL (TW: @thedudelisby):
Read the code, explained me how this works.
Ragou42 (TW: @ragou42):
Read the code, explained how this works in this video's comments.
MOD used: http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2345401-bounding-box-outline-reloaded-mod-for-minecraft
Music used:
Digimon Adventure 02: Yuukan ni Tachimukau zo!! - https://youtu.be/Hig3z2bioWM
Digimon frontier: Attack the frontier - https://youtu.be/aiJhH0Y_r0Q
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