Starlight (Forge) — a mod that completely reworks and optimizes Minecraft's lighting engine

Starlight (Forge) — a mod that completely reworks and optimizes Minecraft's lighting engine

Starlight (Forge) is an optimization mod for Minecraft that completely rewrites the internal lighting engine. The vanilla implementation calculates block and sky light propagation slowly: on servers with dozens of players this causes TPS drops during mass block changes, while in singleplayer it leads to dark seams at chunk borders after generation or /reload. Starlight replaces the whole system with its own implementation, tied directly to the chunk object. 

What the lighting engine changes

Light propagation is a chain of calculations in which a block with a given light level passes its value to its neighbors, they pass it further, and so on down to the falloff limit (15 for a full source). The vanilla engine keeps light sections separate from chunks and processes them through a queue with a large number of intermediate structures. In this scheme, updates often stall or get repeated twice for the same point.

In Starlight, light sections exist only together with ChunkAccess — the chunk object itself. If a chunk isn't loaded, no lighting updates can exist for it. This eliminates a whole class of bugs with "dangling" sections: empty chunks with light data, dark spots after reloading the world, unrefreshed sky light at borders. This also reduces the scheduler's workload — the engine no longer checks states that no longer make sense to check.

Lighting at chunk borders right after /reload  in a flat world — a vanilla run shows dark seams along the borders, while with Starlight active the seam is uniform 

Launch and behavior

The jar file is placed in the mods folder — it doesn't require any additional mod libraries and doesn't create any extra files in config/ on startup. Behavior is the same on client and server: on first load, an existing world rebuilds its lighting data chunk by chunk as they load, so during the first seconds after entering an old world you may notice brief micro-stutters when moving quickly.

Starlight can be installed selectively — only on the client or only on the server. If the mod is only on the server, clients can connect with vanilla or with Starlight on the player's side. It also works the other way around: a client with Starlight can connect normally to a vanilla server.

Conclusion

Starlight (Forge) solves one task — fast and correct lighting calculation on heavily loaded servers and in worlds with frequent mass block changes. For survival or building on an average setup the difference in FPS is barely noticeable, so it really makes sense to install the mod on a Forge server with an active redstone-driven economy, in a large modpack, or as a way to get rid of dark seams at chunk borders.