Glass reflection flicker with Lumen

[Temby]

Hello!

I’ve been trying to solve this glass flickering issue for a few days.

https://youtu.be/8IYjYMbgpF4

As the video shows if I disable Lumen Reflections in Post Processing, the issue stays. However if I hide Lumen Reflections via Show > Lighting Features > Lumen Reflections, it goes away. I don’t know what this option does extra, that disabling it via Post Processing is not.

Reproducing the issue is quite easy. Open the Demonstration level from the free Assetville Town pack, enable Lumen + Lumen Reflections, and look inside the bank. Flickering is also bad when 2 pieces of glass are lined up in view, so 1 is seen through the other.

Things I have tried:

1. Setting the glass to 1-sided (Improvement in some cases but doesn’t fix, and not suitable for things like building windows)

2. Setting reflections to Cinematic (Also improves but doesn’t fix)

3. Disabling all light sources except the sun and sky

4. Different transparent materials (Translucent lit, additive, subsurface, clear coat). Single layer water works perfectly, probably because it’s on a different render pass. However it has a slight blur that can’t be removed and can’t be made 2-sided. Dithering an opaque metallic object also works, but is heavily blurred when the camera moves.

5. The following settings (and many others but I didnt track them all):

   Project settings > Translucency > Separate Translucency
   r.Lumen.Reflections.ScreenTraces 0
   r.Lumen.ScreenProbeGather.ScreenTraces 0
   r.Lumen.Visualize.MinTraceDistance 500
   r.Lumen.DiffuseIndirect.MinTraceDistance 500
   r.Lumen.Reflections.HierarchicalScreenTraces.MaxIterations 4

6. Moving the object far away from everything else in the level. The flickering stops, but from what I can see, no single actor is responsible.

Thanks!

[TheKJ]

Experiment with the downscale and temporal accumulation Lumen reflection CVARs.

Also just in case, test the radiosity Lumen Cvars too.

[Temby]

Thank you for your suggestions!

I was going through the downsampler parameters as you suggested and noticed the “r.Lumen.TranslucencyReflections” options. So I went through those 1 by 1 and found the following fix!

r.Lumen.TranslucencyReflections.RadianceCache 0

I can only assume that the issue is caused by complex translucency reflections. It seems silly to call a low-poly scene complex, but from a reflections standpoint not many projects seems to have many layered translucent glass materials with lumen.

I hope this helps others in future. Thank you again SOLID_SNAKE!