Summary
I reproduced broad interior wall illumination in a fresh Blank Blueprint project using six overlapping opaque stock Cube meshes and one exterior Directional Light. The closed room has no skylight, atmosphere, fog, emissive lighting, or interior light.
With Lumen’s default Screen Probe Gather, an interior wall has a broad filled glow. The same camera and fixed exposure in Path Tracing show a black room. Changing only r.Lumen.FinalGatherMethod from 1 to 0 removes the broad glow; restoring 1 brings it back. This reproduces without custom game assets, procedural generation, or game code.
What Type of Bug are you experiencing?
Rendering (Graphics / Niagara)
Steps to Reproduce
- Create a UE 5.8.3 Blank Blueprint project using Desktop / Maximum. Use DirectX 12 / SM6, Lumen GI and reflections, hardware ray tracing enabled, Virtual Shadow Maps, and Epic (3) GI scalability. Open an Empty Level.
- Assemble a closed 20 x 20 x 5 metre room from six separate /Engine/BasicShapes/Cube meshes using /Engine/BasicShapes/BasicShapeMaterial. Use 12.5 cm thick overlapping slabs with the exact transforms listed in Additional Notes. Set each slab to Static mobility, Cast Shadow=true, and Visible in Ray Tracing=true.
- Add one Movable Directional Light at (0, -1000, 2000) cm, with rotation pitch=0, yaw=0, roll=0. Set intensity to 51706.296875 lux, Indirect Lighting Intensity=1, Source Angle=0.349809 degrees, and Cast Shadows=true. Do not add other lights or sky/environment actors.
- Add an Unbound Post Process Volume: Manual exposure, Apply Physical Camera Exposure=false, Exposure Compensation=0, Bloom Intensity=0, and Local Exposure Highlight/Shadow Contrast Scale=1.
- Add a CameraActor at (500, 10500, -50) cm with pitch=0, yaw=225, roll=0, FOV=90 degrees, and camera Post Process Blend Weight=0. Pilot it. Use Lit mode, Realtime, and Game Settings for viewport exposure. Keep the camera fixed and let the image become visually stable.
- With r.Lumen.FinalGatherMethod=1, observe the broad glow across the left interior wall. Capture with HighResShot 1.
- Switch the same view to Path Tracing without changing camera or exposure. Wait for the accumulation progress bar to finish. The corresponding interior wall is black. Return to Lit.
- Enter r.Lumen.FinalGatherMethod 0, allow compilation/lighting updates to finish, and capture the stable view. The broad wall glow disappears. Restore r.Lumen.FinalGatherMethod 1 and capture: the broad glow returns.
Expected Result
The completely enclosed opaque room should not receive broad interior illumination from its exterior Directional Light. The black Path Tracer result provides the reference under the same scene, camera, and fixed exposure.
Observed Result
Screen Probe Gather produces broad filled illumination across an interior wall of the sealed room. Path Tracing shows a black interior. Irradiance Field Gather (r.Lumen.FinalGatherMethod 0) removes the broad wall glow and spill in this view; restoring Screen Probe Gather (1) brings them back.
Affects Versions
5.8
Platform(s)
Windows
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Additional Notes
I’m seeing this in the installed UE 5.8.3 build, 5.8.3-58210709+++UE5+Release-5.8 (CL 58210709), on Windows 11 25H2, build 10.0.26220.9587. GPU: NVIDIA GeForce RTX 3070, driver 610.88 (32.0.16.1088). CPU: AMD Ryzen 5 5600X. RHI: DirectX 12 / SM6. The fresh project has Substrate enabled and static lighting disabled.
Exact large-room slab transforms, in centimetres (all rotations zero; scales apply to the stock 100 cm Cube):
| Slab | Location (cm) | Actor scale |
|---|---|---|
| X-minus | (-1006.25, 10000, 0) | (0.125, 20.25, 5.25) |
| X-plus | (1006.25, 10000, 0) | (0.125, 20.25, 5.25) |
| Y-minus | (0, 8993.75, 0) | (20.25, 0.125, 5.25) |
| Y-plus | (0, 11006.25, 0) | (20.25, 0.125, 5.25) |
| Z-minus | (0, 10000, -256.25) | (20.25, 20.25, 0.125) |
| Z-plus | (0, 10000, 256.25) | (20.25, 20.25, 0.125) |
The room’s interior centre is (0, 10000, 0), and the slabs overlap at every edge. A 2 x 2 x 2 metre comparison enclosure with the same thickness and construction is nearly black in Lit mode and does not show the broad filled wall glow.
Recorded baseline values:
r.DynamicGlobalIlluminationMethod=1
r.ReflectionMethod=1
r.RayTracing=1
r.Lumen.HardwareRayTracing=1
r.Lumen.HardwareRayTracing.LightingMode=0
r.Lumen.ScreenProbeGather.ScreenTraces=1
r.Lumen.ScreenProbeGather.RadianceCache=1
r.Lumen.ScreenProbeGather.RadianceCache.GridResolution=48
r.Lumen.ScreenProbeGather.RadianceCache.ProbeResolution=32
r.Lumen.RadianceCache.ShowBlackRadianceCacheLighting=0
r.Lumen.FinalGatherMethod=1
r.EyeAdaptation.CachedLightingPreExposure=4
r.Shadow.Virtual.Enable=1
All comparisons use the same large-camera position, geometry, sun, exposure, and 1368 x 769 screenshot dimensions. Captures were taken approximately 3-10 seconds after diagnostic changes, once the view appeared visually stable. The black-cache and resolution-64 views remained unchanged during later observation. All diagnostic settings were restored afterward.
These results show dependence on the final-gather path and a substantial contribution from radiance-cache lighting. They do not establish the cache as the sole source or identify a particular faulty shader operation. Method 0 is an observed workaround for this reproduction; I have not established equivalent quality elsewhere. This report focuses on broad wall illumination; narrow bright perimeter lines are a separate issue.
Only this engine build, GPU, and driver were tested. No cross-vendor test, packaged-game check, GPU capture, or renderer source modification was performed. The effective GPU backend of each individual Lumen pass was not separately captured.
I have a saved L_LightLeakRepro.umap, the exact BuildLightLeakTest.py reconstruction script, ten unchanged PNG screenshots, and relevant log excerpts available as a reproduction packet. Its dependencies are stock engine assets. The packet has not been imported into another project as a further validation step.