Skylight Light

Virtual skylight for underground farms and Dynamic Trees — without burning mobs or faking open sky.

Tag any block as a skylight source (ceiling lamps, sky blocks, custom blocks via datapack or KubeJS). The mod flood-fills an open region beneath each source and raises sky light there so plants grow as if they had daylight, while canSeeSky() stays false at default settings.

Features

Tag-driven — no hardcoded block list; use #skylightlanterns:skylight_source
Dynamic Trees compatible — boosts LightLayer.SKY (DT leaf survival needs ≥ 12)
Vanilla crops & saplings — enough light for normal growth underground
Safe defaults — virtual sky level 14 (grow everything; canSeeSky needs 15)
No undead burning — mobs do not sun-burn in skylight regions (configurable safety net)
Performance-minded — bounded flood-fill, per-tick chunk budget, node caps; no rewriting the vanilla light engine every tick
Server config — radius, depth, sky level, burn prevention, optional hostile spawn blocking

Requirements


Minecraft	1.20.1
Mod loader	Forge 47+ (tested on 47.4.13)
Side	Server logic; install on client for singleplayer

Optional (not required):

Dynamic Trees — benefits from sky-light boost
KubeJS — convenient way to add blocks to the tag

How to use

Install the mod in your mods folder.
Add your lamps or sky blocks to the block tag skylightlanterns:skylight_source.

Datapack

data/<namespace>/tags/blocks/skylight_source.json:

{
  "replace": false,
  "values": [
    "yourmod:ceiling_lamp",
    "blockofsky:sky_block"
  ]
}

KubeJS

ServerEvents.tags('block', event => {
  event.add('skylightlanterns:skylight_source', 'kubejs:ceilling_lamp')
  event.add('skylightlanterns:skylight_source', 'kubejs:ceilling_lamp2')
  event.add('skylightlanterns:skylight_source', 'kubejs:ceilling_lamp_red')
  event.add('skylightlanterns:skylight_source', 'blockofsky:sky_block')
})

Place a tagged block on the ceiling of an enclosed room.
Plant crops, saplings, or Dynamic Trees rooty dirt underneath.
Growth should work as if the area had open sky light — without holes to the surface.

Configuration

After the first run, edit config/skylightlanterns-server.toml on the server.

Option	Default	Description
`maxRadius`	12	Horizontal spread from each source (blocks)
`maxDepth`	24	Maximum depth below a source (blocks)
`nodeCap`	6000	Max lit blocks per source (performance limit)
`skyLevel`	14	Virtual sky brightness (14 = DT + crops; 15 enables `canSeeSky`)
`attenuateByDepth`	false	If true, light falls off with distance from source
`preventBurning`	true	Undead never sun-burn inside a skylight region
`blockHostileSpawns`	true	Block monster spawns inside the region

Tip: Keep skyLevel at 14 unless you know you need full daylight behavior.

How it works (technical)

When a tagged block is placed or a chunk loads, the mod scans for sources and flood-fills air/passable space below them, stopping at solid blocks and config limits.
On the server, SKY light reads are boosted inside cached regions via a mixin on the sky light engine — the same path Dynamic Trees uses in hasAdequateLight().
The mod does not override canSeeSky() at default settings, so undead and weather behave normally outside the virtual light values.

Testing

Build a sealed underground room (solid roof, no sky access).
Place a tagged ceiling lamp or sky block.
Plant a Dynamic Trees sapling or crops below.
Leaves should stay green and the tree should grow; crops should mature.

Spawn a zombie under the lamp in daytime — it should not burn (with default config).

Credits

Concept inspired by the older Skylight Lanterns project. This is an independent reimplementation for Minecraft 1.20.1 / Forge, not a port of that jar.

License

MIT — see LICENSE in the release package.

Skylight Lights