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The Minecraft Wiki says carpet doesn't prevent mob spawning. But does it affect mob spawning on blocks mobs normally don't spawn on? If I put carpet on glass or slabs, can mobs spawn there?

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This thread might help people work out a way to test this. Duplicating that guy's problem (a house near to a mine dark enough to spawn a mob) could allow someone to test the effects of adding carpet to slabs or glass. –  Brythan Jul 18 '13 at 22:39
@Brythan That's not necessary. It's very easy to test mob spawning; you don't need to try to duplicate weird glitches. –  SevenSidedDie Jul 19 '13 at 1:19
Mobs do spawn on upside-down slabs. Just sayin'. :-) –  Orc JMR Jul 19 '13 at 3:34

2 Answers 2

up vote 9 down vote accepted

Update for 1.8 and after

Update 1.8 added a 1/16 meter high hitbox to carpets, which made natural mob spawning on carpets impossible. Carpets are now safe.
Thanks @TomášZato for bringing this up!

(There still are some types of "unnatural" spawning that are able to force mobs into carpets, like Village Sieges.)

Before 1.8

Carpet both does not prevent and does not facilitate mob spawning, but it does help in a way.

Carpet is a non-solid block, so, in terms of algorithm, mobs do not spawn on carpets, but rather in. Basically, you can replace all carpets with signs for the same, but more obvious, situation.

Mob spawning alrotihm, simplified, is the following:

  1. Pick a totally random place (in 3D, not only on the surface) to spawn a pack. It must be a block of air (not carpet).
  2. Try to spawn several mobs around the pack on the same height. Block underneath must be solid, and the block mob spawn in may be any non-solid block, like a carpet.

What it means regarding carpets: if you have a level floor in a room, and room is surrounded by solid stone, no mobs will spawn even in the dark, as there are no air blocks on floor level. If you have a building surrounded by air, though, mobs will be able to start spawning outside and "leak" onto your carpet floors.

So, glass floors are as safe as they ever were.

Detailed algorithm is on the wiki.

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Perhaps the first line should be "Carpet both does not prevent and does not facilitate mob spawning, but it does prevent pack spawning", or something like that? It reads confusingly until you get almost to the bottom of the post, as written. –  SevenSidedDie Jul 19 '13 at 4:00
Am I reading this right when I understand him to have said that mobs can't spawn on carpets, but part of a mob PACK can spawn on a carpet if the pack's center point is off the carpet? –  Ender Jul 19 '13 at 15:17
@Ender It's saying mob packs won't spawn a pack centre point on carpet, but a nearby pack centre point already successfully spawned can then spawn actual mobs on carpet (assuming the block underneath is legal for spawning). –  SevenSidedDie Jul 19 '13 at 17:15
@Ender As SevenSidedDie said. And mobs never spawn not in a pack, just that a pack can consist of only a single mob. –  Orc JMR Jul 20 '13 at 11:53
If carpet is non solid, how can I walk on it? –  Tomáš Zato Jun 27 at 21:38

No carpets are basically an invisible block. All mobs can spawn on carpet. However there is an exception. Slimes cannot spawn on carpets even in the swamp biome because the "biome floor is covered"

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Could you mention your source? –  SevenSidedDie Jul 19 '13 at 3:06
This is wrong on several points. All mobs can spawn on carpet under specific conditions, slimes spawn on carpets just like any other mob, and there is no such thing as "biome floor". A biome is a property of whole space, not only floors. –  Orc JMR Jul 19 '13 at 14:54
Update: 1.8 gave carpets a hitbox, so nothing spawns on them anymore. –  Orc JMR Jun 29 at 3:49

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