Empty Volumetric Lightmap

Development Rendering
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Hello,
I am in 5.5 working on a baked-lighting project using the regular Build menu (not GPU Lightmass) and have come up against a weird issue. My volumetric lightmap seems to be completely empty. When I visualize it in the viewer I get no samples at all and the fog/movable actors within the interior space light as if they are fully exposed to the sun/sky. I have a LM Importance Volume in the level and it is correctly sized around the area. I even tried adding LM density volume set to 0,0 to see if I could force more detail but nada. And yes, I have built the lighting numerous times for iteration and testing.

Is this a known issue or just something I have overlooked?

Thanks for any help you can offer!

UPDATE:
I did check the output log after running a build and found these lines that seem related. However, I don’t know what would be considered expected sizes/counts and so don’t know how this would compare.

LogVolumetricLightmapImport: Imported Volumetric Lightmap in 0.014s
LogVolumetricLightmapImport:      Indirection Texture 16x16x16 = 0.0Mb
LogVolumetricLightmapImport:      BrickData (all levels) 0.6Mb
LogVolumetricLightmapImport:      Bricks at depth
LogVolumetricLightmapImport:          0: 0.6% covering 100.0% of volume
LogVolumetricLightmapImport:          1: 3.7% covering 9.4% of volume
LogVolumetricLightmapImport:          2: 95.7% covering 3.8% of volume
LogVolumetricLightmapImport:      Bricks in each level
LogVolumetricLightmapImport:          Scifi_Lab_Blockout              164 bricks      100.0% (Persistent Level)
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UPDATE 2:
Some further testing:

  • Swarm Agent logs show that it is finding the LMIV and adjusting the area and samples accordingly
  • Halving the VLM detail cell size in World Settings shows an increase in brick counts/sizes in the logs
  • Testing with GPU Lightmass instead of CPU does display the samples as I would expect and movable actors begin lighting more appropriately although the resulting bake is much messier

So it seems like it is finding the volume and accounting for it but the resulting volumetric lightmap is not loading or not being used in the level afterwards when CPULM is involved.

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UPDATE 3:
I went so far as to create a brand new, empty 5.5 project, made sure all project settings were set, and then migrated the level from the other project over to the new one fresh. Ran a CPU Lightmass bake and still getting absolutely nothing in the LM Importance Volume, not a single sample.

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Yep. Exact same problem. First time in a new level it builds Volumetric Lightmaps. After that - nope, absolutely empty Volumetric Lightmap. Sometimes with some magic, using “build lights only” it works, but only sometimes and not consistent. In a new clean project, so it’s just broken in 5.5.

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Oh wonderful. I mean glad to hear it’s not just me but it never worked once using CPULM in this project. “Build Lights Only” is how I was running it.

I even had a brand new blank project, migrated over the assets from the other and tried: nada.

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you may have to show some things you’re trying to bake. i can bake just fine in a generic blockout build in 5.5.3. argueably there are some issues with geometry detection and brick resolution in supposed to be more detailed areas, but it works for the low res gi bake i aim for. visualizer shows properly generated probes.

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Well that’s the crux of the issue: the bake works fine and bakes my light maps as expected. It’s the probes that are completely absent after the bake. With the visualizer on there is nothing shown and movable actors react as if they are fully lit by sun and sky, even in airtight interiors. Both those combined indicate to me the VLM is just absent, either being reduced to nothing or removed entirely. I tried tightening up the cell size, increasing the brick memory, and placing a detail volume to try and force the full rez VLM in a test area.

I am working with modular wall, ceiling, and floor parts that are built into a series of hallways opening out into a larger room.

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This is how I solved the issue. Create a new empty level and set it as a Lighting Scenario under your persistent level. Rebake.

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This worked for me, thank you!

Since the VLM wasn’t appearing at all, none of the corresponding settings (e.g., Cell Size) made any difference. I switched Volume Lighting Method to Sparse Volume Lighting Samples which DID generate as expected, but that is obviously not ideal.

For me, I had been working on a single level, and I didn’t even have a persistent level. (At least, when I went to the Levels editor, it was blank. Bug?) I added my level and an empty level, set the empty as a lighting scenario, and bam, VLM showed up immediately after baking again.

I played around some more with the level settings and discovered that enabling lighting scenario on ANY level made VLM work as expected – even if you just have a single level. I’m not terribly familiar with Unreal yet, so I have no idea if this is a bug or working as intended due to some other settings. I suspect though that it’s a bug considering SVLS worked without jumping through such hoops.

TL;DR: Toggle Lighting Scenario on any of your levels in the Levels editor. If the Levels list is empty, add your current level (which I think becomes the persistent level after restarting Unreal).

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Hmm interesting that adding a lighting scenario works, I would have to give that a try. Good find @Vergil ! But there is nothing in the docs indicating that is required for it. In fact, baked lighting a VLM/SVLS both existed long before scenarios did. And what’s more the VLM populates fine with GPULM, just not CPULM. I’m curious to find out why that would be.