5.8 Lumen Overwrites Planar Reflection Actor

General Issues and Bug Reporting (Unreal Engine)
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Summary

In 5.8, a Planar Reflection actor renders no reflection when Lumen Global Illumination is active, even with Reflections Method set to None.

The same setup works in 5.7. With only the planar actor placed (no Post Process Volume), 5.8 also shows heavy flickering and an odd shadow in the reflection. Setting GI to Screen Space makes the planar reflection return (5.8), but that obviously disables Lumen in the scene.

What Type of Bug are you experiencing?

Rendering (Graphics / Niagara)

Steps to Reproduce

  1. Create a blank project in UE 5.8 (repeat in 5.7 for comparison).
    Enable Project Settings > Engine > Rendering > Optimizations > “Support global clip plane for Planar Reflections” and restart the editor.

  2. Add a floor plane with a mirror material (Base Color white, Metallic 1, Roughness 0.05) and a few cubes above it.

  3. Place a Planar Reflection actor on the floor surface (just drop it onto the floor).

  4. Add a Post Process Volume with Infinite Extent (Unbound) enabled. Set Global Illumination Method = Lumen and Reflections Method = None.

  5. Look at the floor in the editor viewport and in PIE, with nothing selected.

  6. Set Global Illumination Method = Screen Space (Reflections = None) and look again.

Expected Result

The Planar Reflection actor renders a reflection of the scene on the floor with Lumen GI enabled, as it does in UE 5.7.

Detailed pictures of a scene in 5.7 with working setup: 5.8 Lumen disables Planar Reflection? - #2 by Cronoss95

Observed Result

UE 5.8: With GI = Lumen and Reflections = None, the floor shows no planar reflection at all. With GI = Screen Space and Reflections = None, the planar reflection renders correctly.

With only the Planar Reflection actor placed and no Post Process Volume, the 5.8 reflection flickers heavily and shows a strange shadow.

UE 5.7 with the identical scene and settings (scalability set to High in both) renders the planar reflection correctly, with no flicker. I can confirm the 5.7 reflection comes from the planar actor because the planar actor reflection is a bit distorted and that can be fixed via ExtraFOV from the planar actor.

Using Screen Space in 5.8 in GI also shows planar reflections. Why? Because they are distorted and can also be fixed via ExtraFOV.

Affects Versions

5.8

Platform(s)

Windows

Upload an image

WithNoPPV_AlreadyWeirdShadow_In_5-8.PNG
WithNoPPV_AlreadyWeirdShadow_In_5-8.PNG3440×1440 2.92 MB
PlanarReflectionsCompletelyMissing_In_5-8_SameSetUp_As_In_5-7.PNG
PlanarReflectionsCompletelyMissing_In_5-8_SameSetUp_As_In_5-7.PNG3440×1440 2.73 MB
PlanarReflectionsReturn_When_Set_ToScreenSpace_In_5-8.PNG
PlanarReflectionsReturn_When_Set_ToScreenSpace_In_5-8.PNG3440×1440 2.67 MB

For crash reports, include your callstack

It is not a crash

Additional Notes

Forum thread with the same findings: 5.8 Lumen disables Planar Reflection?
Both fresh projects use identical scalability settings (all High) and identical scene setup, with only the engine version differing. With Reflections Method = None and GI = Screen Space, the planar reflection renders correctly in 5.8, so the Lumen GI path appears to be the trigger. I first noticed the issue when upgrading an existing project from 5.7 to 5.8.