2 portals overlaying each other does not render correctly, shows up as invisible. Portal particle effects still occur and you can still go through them.
Edit: Further testing shows that the graphics settings (Fancy or Fast) and Advanced OpenGL still causes this bug to happen.
What should happen is at least the portal closest to you is rendered while everything behind is not (due to the rendering engine in Minecraft).
Steps to Reproduce:
1. Build 2 portals overlaying each other
2. Stare at it
Linked issues
is duplicated by 8
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Comments 6
https://www.youtube.com/watch?feature=player_detailpage&v=tkIotMkFrXY
Also it seems that having such big portals may increase the spawn rate of the zombie pigmen, which in my opinion isn't intentional.
Can confirm also video shows a example of the zombie pig man bug
See MC-36103.
Can confirm. When placing two portals directly next to each other this occurs.