UE 5.6 – Lumen dark blotchy shadows after upgrading from 5.3 to 5.6 (Diffuse Indirect issue?)

Development Rendering
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1

After upgrading our project from UE 5.3 to 5.6, I started seeing dark blotchy artifacts on surfaces when using Lumen. These weren’t present in 5.3 and seem tied specifically to Diffuse Indirect lighting in 5.6.

Can confirm it happens on every map we have, its not map unique related.

What I’ve Tried

  • Verified project runs with DX12 and HWRT (r.Lumen.HardwareRayTracing=1).

  • Toggled all of these CVars with no effect:

    • r.Lumen.ScreenProbeGather.ScreenTraces

    • r.Lumen.ScreenProbeGather.RadianceCache.NumClipmaps

    • r.Lumen.ScreenProbeGather.RadianceCache.Quality

  • Changing AO methods and toggling GTAO settings → no improvement.

  • Disabling mesh distance fields, cast shadows, etc. → no improvement.

  • Updating GPU drivers → no effect.

  • Normal maps seem to intensify the blotches.

  • The only commands that completely eliminate them:

    • r.Lumen.DiffuseIndirect 0 (kills blotches but also removes diffuse indirect lighting entirely).

    • r.Lumen.ScreenProbeGather.DownsampleFactor 0 (less visible) vs 32 (worse).

Some Screenshots
This is now at 5.6

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This is how it was at 5.3

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Some more Examples on other maps

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The question
Is this a known regression in UE 5.6 related to Lumen’s Diffuse Indirect lighting or Ambient Occlusion?

  • If intended, is there a recommended way to configure Lumen in 5.6 to avoid these artifacts without disabling DI entirely?

  • If a bug, what’s the best way to report/reproduce it for Epic?

2

what do the Lumen debug views look like?

3

Looks like this

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4

Yeah that’s certainly bad, basically everything has its surface cache culled. Don’t know if it’s the cause of your issue but it is worth trying to solve that.

Do you have Affect Dynamic Indirect Lighting disabled on your meshes?

Looks like you have a bunch of meshes that are also just missing, either not generating distance fields or set to not affect distance field lighting.

5

Can confirm they are all using Affect Dynamic Indirect Lighting and also the distance fields seem to still be generated correctly

Distance field ambient occlusion

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Global Distance Fields

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Mesh Distance Fields

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Its a really weird situation, I’m really clueless at this point

6

Are they ISMs?

I would suggest applying the default grid material to one of the assets, if the artifact doesn’t go away then that at least rules out the material.

7

They are HISM right now but still happening after converting them back to SM and any other SM already in the level

Also confirming it seems not about material, applying the default grid material looks the exact same

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8

Alright! Great news finally got the culprit.

It was our DefaultEngine.ini config, after commenting out some settings to let the 5.6 take over Lumen GI worked as expected again!

Thanks for your help @Arkiras, your suggestions are much appreciated.

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9

Been narrowing down the issue and can add what is exactly causing those black splotches in our case, it was about the blue noise defined at DefaultEngine.ini, exactly those lines:

BlueNoiseScalarTextureName=/Engine/EngineMaterials/STBlueNoise_scalar_128x128x64.STBlueNoise_scalar_128x128x64 BlueNoiseVec2TextureName=/Engine/EngineMaterials/STBlueNoise_vec2_128x128x64.STBlueNoise_vec2_128x128x64

Since UE 5.6 updated Lumen’s sampling/LUTs, by pinning the 5.3 assets at DefaultEngine.ini, our final gather used mismatched blue-noise/volume textures, which biased the GI sampling and showed up as black, blotchy diffuse indirect.

So the fix was just commenting those out, and if we ever need overrides, we’ll copy the current 5.6 paths from BaseEngine.ini rather than the old ones.

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