Assistance with Lumen lighting/reflection issue, appears as if screen space

Development Rendering
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UE5.7, lumen for GI and reflections, nanite, hardware RT.

Please see links to video and project below. When moving the camera in certain areas in this scene, lighting/reflections change like screen space, but Lumen lighting and reflections are used and screen space is disabled (as far as I’m aware. Can someone explain the cause please?

Video: https://f.io/QhCZnraS

Project: https://f.io/XlgUu8uZ

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Steps to Reproduce

  1. Load linked project.
  2. Snap camera to “CameraPos” actor location.
  3. Move camera per video ref.
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Hi,

Thanks for the repro project. The issue you see is a common side effect of Screen Tracing. Lumen uses a hybrid approach where it uses screen space traces as the first step for reflections (tracing rays through the depth buffer which is very fast but only works for objects visible on screen) and falls back to global tracing when the screen space data is not available (for objects that are off-screen or outside the view frustum).

Global tracing traces against the available Lumen scene representation which is represented by SDF (signed distance fields for approximating objects) for software tracing or triangles for hardware raytracing, which provide a much more accurate representation.

The resulting color of the trace might still be what is on the screen if the hit depth is visible on the screen, but if the hit is behind something or off-screen it will return the color from the Lumen scene. The Lumen reflection sharpness can be increased somewhat by setting the Ray Lighting Mode (under Lumen Global Illumination in the PostProcessVolume settings) to hit Lighting for Reflections, but it will not affect the reflected scene color, which still depends on the mechanism outlined above. To get more even lighting (avoiding the change from screenspace to global lighting), you can disable Screen Traces under Lumen Reflections (there is also a Screen Traces setting under Lumen Global Illumination). To better visualise how these changes affect your scene, you can place a plane with a mirror material (metallic to 1.0 and roughness to 0.0) flat on the floor.

Hopefully that clarifies things, but let me know if you have more questions.

Thanks,

Sam

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Hi Sam, thanks for the detailed explanation, I’ve fixed the issue now. This ticket can be closed.

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Great to hear, glad I could help.

Thanks,

Sam

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