A ghast fireball that hits a roof made of (bottom) cobblestone slabs can do damage on the other side of the slab, even though it does no damage to the slabs themselves. That is, if a ghast fireball impacts such a roof, the contents of the room beneath it can sustain blast damage even if the roof is left intact.
This has been confirmed with cobblestone slabs (stone_slab), andesite slabs (stone_slab3) and stone slabs (stone_slab4), so presumably the bug affects every kind of stone bottom slab.
Top slabs (upper slabs) do not manifest this bug. Neither do full-size stone blocks.
Other sources of explosions also apparently do not manifest this bug (tested with TNT, beds in the Nether, and end crystals).
Steps to reproduce:
Import and open the updated attached world MCPE-106860.mcworld. You will find yourself in front of a button labeled Start with a stairway to its left and a room to the left of that with a ceiling consisting of cobblestone bottom slabs and a layer of orange stained glass below that.
Press the Start button. This repairs the ceiling (in case it is damaged from a previous test) and applies strong, long-lasting Regeneration and Fire Resistance effects to you.
Go up the stairs and out onto the roof. You will see two ghasts above you. Note: If either of the ghasts is missing, there is a button at the top of the stairs that will respawn it.
Move around the roof, especially into the corners, for 10–20 seconds while the ghasts shoot fireballs at you.
Go back inside, down the stairs and into the room. Examine the ceiling for missing glass blocks. If there are none the first time, repeat from step 2.
Expected result:
There are no glass blocks missing under the ceiling because cobblestone blocks are resistant to blast damage from ghast fireballs.
Actual result:
Some of the glass blocks were destroyed by fireballs even though the slabs are all intact.
Original Description:
I have a netherrack cooking facility and a collocated netherwort farm in the nether. The building is made of netherbrick and is fully enclosed. It is roofed with cobblestone slabs. I often hunt ghasts from the roof while cooking netherrack and waiting for a new crop of netherwort to grow. Ghasts are firing at me, hitting the roof and tearing up the netherwort that is 4 blocks below.
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?? The issue is ghasts shooting through slabs. I have a building, it has a roof (cobblestone slabs), netherwort is growing inside the building. I have my bow out and make a sideline of potting at the gasbags while waiting for another crop to grow. The ghasts don't like me. They shoot back. I dodge and the fireball hits the roof.
What I would expect: The slabs would stop the fireball
What happens: At least part of the fireball is penetrating the slabs and tearing up my netherwort.
I hope this is clearer.
Ok, my mistake. I forgot for the moment that cobblestone is supposed to be blast resistant and I was imagining the fireballs were breaking the slabs, therefore penetrating to the interior, so I wasn't clear about what was wrong.
I need to try to reproduce this, but it might be sensitive to details like how the slabs are arranged and whether they're top or bottom slabs. It would be very helpful if you could provide a screenshot from which I could build something like what you have. Even better if you could upload a small world in which you can reproduce it yourself. Meanwhile, I'll play with it a little in case it's simply that cobblestone slabs aren't blast resistant at all (I think they ought to be, though).
I can confirm that the roof is indeed bottom slabs and is not itself being harmed by the fireballs but they or at least part of their damage is going through and damaging things below.
Thank you. This was submitted to the developers back in February after I found a way to reproduce it. I should have mentioned it in a comment at the time, but apparently I forgot.
This bug may have the same underlying cause as MCPE-155956. It seems that the calculation of slab collision allows certain trajectories or vectors that should be blocked to pass through. Here is one example from that report:
https://bugs.mojang.com/secure/attachment/492083/Trident%20goes%20through%20slab.mp4
I retested in 1.19.22 using the demo world. The demo world has two ghasts, one that shoots at a high angle to the surface, and one that shoots at a lower angle. The fireballs from either of them can, but don't always, break the glass below the slabs, so it would seem the trajectory angle isn't that important, but I can't rule out that some other property of the trajectory makes the difference, perhaps its length or direction.
Using a particle command in a repeating command block to mark the fireball's position, I have found that this issue occurs when the fireball gets ticked inside the top of the bottom slab's block space (the part that would be filled by a top slab). When this happens, the fireball gets ticked one more time on the the opposite side of the bottom slab, as if it had passed through, and the explosion center is set to that location. This happens even if there is only air on the other side of the slab, which implies that the game correctly identifies the slab as the impacted block. The bug is only that the location of the explosion is on the other side of the block. This issue is closely related to MCPE-160494: in both cases the expected behavior is that the position of the explosion would be at the leading edge of the block that the fireball impacts.
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