I’m currently working on a very large-scale world in UE5, and for performance reasons I decided to use a traditional baked lighting workflow instead of Lumen.
For this project, being able to bake lighting for only a small area independently is absolutely essential. Otherwise, changing a single light or small indoor area would require rebuilding lighting for the entire world, which is not practical for a huge open-world project.
I’ve already tried many suggested solutions, including:
Data Layers
Levels / Sublevels
World Partition loading/unloading
Different baking workflows
I also asked several AI tools for help and experimented with many setups, but none of them truly achieved isolated light baking. In my tests, lighting from different areas still affects each other during CPU Lightmass baking.
At this point I’m honestly not sure whether:
I’m using the systems incorrectly,
UE5 simply does not properly support localized CPU Lightmass baking with World Partition,
or there is some workflow I’m still missing.
I’ve spent a long time researching this issue and couldn’t find a clear solution, so I’m hoping experienced UE5 developers can help clarify the correct workflow.
My goal is something like:
Bake lighting for one building/floor/interior only
Rebuild only that area when changes are made
Avoid rebaking the entire city/world every time
Is this actually possible in UE5 with CPU Lightmass + World Partition?
Or is GPU Lightmass / another workflow currently the only realistic solution?
Any advice or real production workflow examples would be greatly appreciated.
Checking around the community, and the available UE resources, the answer would be that, at least per UE5.7, localized, limited baking is not yet possible, as the Lightmass still builds lighting for the level as a whole.
And you are correct, the closest alternative available at the moment, is to work with GPU Lightmass, as it should compute your lightmaps only for the loaded actors. Any unloaded cells would not be reached. You can review it’s UE doc for full details, here:
Anyone should want to be using GPU lightmass anyway. For any non-trivial scene it is incredibly faster to build. Every with swarm running our scenes were taking 20+ minutes with CPU lightmass.
This is a valid concern for large worlds, and from what I’ve seen, CPU Lightmass does not behave like a truly localized per-area baker in UE5, especially once World Partition is involved. Even if you isolate Data Layers, sublevels, or loaded cells, Lightmass can still consider surrounding geometry, indirect lighting, and shared lighting data depending on how the scene is built. For a production-scale open world, I’d probably avoid relying on CPU Lightmass for small iterative interior bakes unless those interiors are separated into their own lighting scenario/streamed level setup with strict light and geometry isolation. GPU Lightmass, separate interior levels, or a hybrid workflow with baked interiors and dynamic/exterior lighting may be more realistic than expecting World Partition to rebuild only one small area cleanly.