Light Leak Issue in Modular Buildings (Lumen / HZBTraversal)

Development Rendering
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1

Hello,

In the project I’m currently working on, we’ve encountered a strange light leak issue in modular buildings. The project has r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal set to 0, which we initially did to reduce noisy artifacts on foliage. The noise likely occurs because the foliage isn’t modeled as full geometry but rather uses cards.

When r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal is set to 1, the light leak disappears - however, the noisy foliage shadows return.

I’ve already checked several potential causes, such as wall thickness and mesh merging (combining meshes into a single building), but none of these resolved the problem.

To further investigate, I tested the affected building across multiple Unreal Engine versions and found that the issue only occurs in UE 5.6 and 5.7. I also tested simple rooms made of basic cubes to confirm whether the problem persists in a minimal setup - and it does. Additionally, I used ToggleForceDefaultMaterial to observe if different material setup affect the bug and the bug was still present no matter the material.

Could you please share any recommended workarounds?

Thank you,

Kornelia

2

Steps to Reproduce

  1. Create a new Unreal engine 5.5.4 blank project
  2. Create a new basic level
  3. Add a cube to the level and scale it to (50, 50, 2)
  4. Duplicate the cube to create six walls, forming a closed room (no doors, windows, or holes)
  5. Now go inside the “room” and change r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal to 0. Observe no change.
  6. Open that project in 5.6.1 as a copy
  7. Change r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal to 0 again
  8. Observe weird light leak in the corners, test using high and epic scalability
3

Hello,

Thank you for reaching out.

I’ve been assigned this issue, and we will be looking into this light leaking for you.

4

Hello Stephen,

I have some additional informations for you that might be helpful. Our engineer went through changelists on main branch and he found out that the issue didn’t occur until CL 39898483.

About the foliage issue - we noticed a new ticket that might be related. The noisy artifacts I mentioned look exactly the same as those from the ticket below. It happens once r.Lumen.ScreenProbeGather.ScreenTraces.HZBTraversal is set to 1.

[Content removed]

Hope that helps.

Best regards,

Kornelia

5

Hello,

Thank you for reporting this. I have been able to reproduce the light leaking on the latest CL, and am opening a bug report.

We will get back to you when we have a public tracker for this.

If you have any further questions in the meantime, please let us know.

6

Hello,

We have opened a bug report:

The tracker will be visible after it is approved internally at Epic Games and is publicly accessible.

If you have any further questions, please let us know.