Flickering lights on skybox with translucent material

Development Rendering
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Hi there!

I’m running into a persistent flickering issue with my skybox in Unreal Engine 5.6.1, and I’ve exhausted a bunch of troubleshooting steps. Hoping you or the community might have insights.

Setup Details:

  • Using a custom skybox with a translucent material (for a transparent sky using unreal’s default sun and atmosphere).
  • Main light is a Directional Light acting as the sun, with Virtual Shadow Maps (VSM) enabled for shadows.
  • Lumen is active for global illumination.
  • The skybox is a large dome/mesh enclosing the scene, with some internal lights (e.g., spotlights) placed inside it.

Problem:
The skybox starts flickering intensely whenever there’s at least one spotlight active in the scene, especially when the spotlight’s rays intersect with the sun’s directional light and the skybox is in the player’s view. It seems tied to shadow interactions – if I disable the spotlight or remove shadow casting, the flickering stops, but that’s not ideal for my lighting setup. This happens even with only 1 directional + 1 spotlight; no other lights exacerbate it further.

What I’ve Tried (No Luck So Far):

  • Increased shadow-related console commands like r.Shadow.Virtual.MaxPhysicalPages to 8192 and r.Shadow.Virtual.ResolutionLodBiasDirect to higher values.
  • Enabled and debugged shadow frustums (via stat shadowfrustums) – flickering persists when shadows overlap.
  • Tested on high-end hardware (RTX 5090) and lower-end PCs to rule out GPU/memory issues.
  • Checked world positions (player/skybox below Z=0) – not the cause.
  • Adjusted light properties: Made lights stationary, tweaked shadow bias/resolution, disabled Lumen temporarily.
  • Ensured no overlapping geometry or material issues; skybox material is set to translucent with proper blending.

At the skybox material, going to Translucency > Lighting mode > Surface ForwardShading solves the problem. But it’s too heavy for a skybox and it looks like glass with this option, which is bad.

This only triggers with VSM + spotlight + directional intersection visible on the skybox. Any ideas on how to solve this glitch @EnduringStephenK ??

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Is there a reason the skybox is translucent? Have you tried opaque with unlit?

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Why do I feel like this is an AI Generated fake question that’s later gonna be edited to be a phishing post?

4

Does this happen with r.TranslucentLightingVolume=0?

It’s possible to get flickering on translucent objects from movement in a scene where VSM shadows are cast because of the low resolution of the translucency volume.

I’ve also heard disabling batching with r.TranslucencyLightingVolume.Batch=0 fixes the issue in some cases, though we don’t have a repro yet to verify this.

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I’m using translucent because my skybox is actually a sphere, and with a directional light + Lumen I want it to react to the light color when I change its position. I did try opaque with unlit, but then the skybox doesn’t shift colors with the time of day (golden hour, night, etc.) and it ends up looking flat with a shadow, but the flicker stops

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It’s not :frowning: that’s just because english is not my native language and i wanted to summarize all the problems and tests effectively

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Thanks a lot!
Setting r.TranslucencyLightingVolume.Batch=0 solved the issue for me, and I didn’t notice any visual quality loss so far.

Do you know if there are plans to fix this behavior in a future Unreal release, so we won’t need to rely on this console variable workaround?

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Sorry for assuming, just tired of the AI Spam here. Glad you got it resolved

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I get what you’re saying, and I’m glad you fixed it, but generally you wouldn’t want translucent unless you wanted to be able to see things through it :slight_smile:

If you’re getting shadows, remember to mark the material ‘sky’, and the mesh as ‘no shadows’.

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Glad that worked for you! We’re still trying to identify the root cause of the issue but don’t have an ETA on a fix.

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