Description
Spiders can currently spawn in one block high spaces if the top block in transparent. In more detail, the current spawn code requires there to be a space of 3x3x1.5, however, the extra half block required gets ignored if the top block is transparent (glass for example).
Original report
I was trying to create a spawning platform limited to spiders, and placed a platform of opaque blocks, and two blocks above that I placed glass blocks. I expected this to still allow spiders to spawn, but they did not. I then changed the glass blocks to another transparent block, bottom slabs, and spiders subsequently spawned.
Steps to reproduce:
Open a new world.
Create a platform with opaque blocks.
Leave a one block high air gap.
Create a platform with glass blocks above the air gap.
Attachments
Comments 5
This is not what is happening. The transparent block is not ignored if it is a glass block, but is ignored if it is for example a bottom slab or soul sand. The spawning condition is not consistent with all transparent blocks.
The description is actually not accurate. Spiders can only spawn if the second block from the ground is a non-full block. In the image I just uploaded, spiders spawn below bottom half slabs, which is not a full block, but do not spawn below glass, which a full block, despite being transparent. They also spawn below soul sand, chests, etc. which are all non-full blocks, but do not spawn below transparent full blocks like leaves and glass.
Would MC-113981 describe your issue? While the glass is transparent, it is considered "full cube" unlike the slab.
I have changed your report to actually show what is happening - the current spawning condition is 3x3x1.5, though it ignored transparent blocks for the height, which is either intentional or a bug.