Max Light Level monster spawn

[GrayR0ot]

Expected behavior

Mobs should spawn if light level is below or equals to configured light level.

Observed/Actual behavior

Entities only spawn at night with a vanilla (0) light level

Steps/models to reproduce

Time set day
Set server max-light setting to 15 or greater

Plugin and Datapack List

No plugin

Paper version

Latest

Other

Here was the pull request that created that feature.
Regarding our recent discord discussion, it's appear to be bugged?

#7129

[lynxplay]

Basically, the config value right now only overwrites monster_spawn_block_light_limit and not monster_spawn_light_level.

I am unsure if this is intended behavior, but the concept (if intended) would need further description at least in the docs.

[Machine-Maker]

This is directly configurable with datapacks, just by setting monster_spawn_light_level in the overworld dimension type. Unsure if we want to add a setting to address this.

Either way, I do not think the existing setting should be changed, its initial behavior seems correct, only controlling the block light level, not the ambient light level from the sky.

[DavidTs93]

The issue is not just monster_spawn_light_level - I tested by changing all the values to max (monster_spawn_block_light_limit to 15, monster_spawn_light_level to 0-15) and no monster spawns.

I checked the code, and it seems like the problem is (NMS) Mob#checkSpawnRules - for all "sentient" mobs it checks if PathfinderMob#getWalkTargetValue is bigger or equal to 0, but for monsters (except: Giant, Guardian, Pillager, Silverfish, Warden) the value is -(LevelReader#getPathfindingCostFromLightLevels(pos)), which equals LevelReader#getLightLevelDependentMagicValue - 0.5, and as far as I can tell, the value is "proper" ONLY when the light level (in the Overworld) is 7.5 or lower.

The order of calling:

1. MinecraftServer#tick
2. ServerLevel#tick
3. ServerChunkCache#tick
4. ServerChunkCache#tickChunks
5. NaturalSpawner#spawnForChunk
6. NaturalSpawner#spawnCategoryForChunk
7. NaturalSpawner#spawnCategoryForPosition
8. NaturalSpawner#isValidPositionForMob
9. Mob#checkSpawnRules -> PathfinderMob#checkSpawnRules
10. PathfinderMob#getWalkTargetValue -> Monster#getWalkTargetValue
11. LevelReader#getPathfindingCostFromLightLevels
12. LevelReader#getLightLevelDependentMagicValue

The last one, in the Overworld, yields the current light level at a location (sky and block) divided by the max light level (15), and going up this needs to be 0.5 or more to spawn the monster, which only happens when the combined light level is 7.5 or less, i.e. 7-.

Edit: this might not be 100% right, because it seems that monsters DO spawn in higher light levels when changed (with a datapack) but ONLY when not directly under the sky (i.e. blocks above). This doesn't explain why they spawn in ALL light levels even though the range was set to be 12-15...

[Machine-Maker]

This should be a vanilla issue then correct? Did you search on the mojira tracker to see if you could find one?

[DavidTs93]

This should be a vanilla issue then correct? Did you search on the mojira tracker to see if you could find one?

Yes, just tested and same behavior in vanilla as well.
Checked the MC Jira and nothing is reported about this.

Edit: create a ticket just in case.