Weird Reflections of Translucent Surfaces in Lumen UE 5.1 (in Standalone Ray Trace ok)

[Ivvvvveeeee]

Hi everyone! Maybe a newbie question, but I am trying to get a reflection of translucent surface and getting weird results with broken geometry. Using UE 5.1 Lumen

The translucent material is this:

The reflective is this:

What’s even weirder is that the geometry becomes correct as soon as I switch from Lumen to Standalone Ray Traced (Deprecated)

Does anyone know if there is a logical explanation to this?

Thanks!

[ZacD]

screen space reflections.

[Ivvvvveeeee]

Oh, I see, thanks. Screen Space produces this effect, but is there a reason for it and is it possible to make Lumen work in this environment?

[jblackwell]

probably with lumen.reflections.screentraces 0.

There’s a brillance to lumen’s architecture with how it trades off reflection techniques every time a hit fails- screen trace to ray trace to fallback. Amazing for performance and general versatility, but it does mean that anything screen traces struggle with can break parts of the pipeline.

General/helpful rule of thumb: Check the path-tracer’s result. Whatever it’s showing is the radiometrically correct result, so it lets you know if your materials are broken, or your lighting technology is.

[Ivvvvveeeee]

Thanks so much for your answer, it’s getting clearer:)
Path tracing is actually producing a perfectly correct image, while r.Lumen.Reflections.ScreenTraces 0 switches off the reflection. Does this mean that reflection is screenspace but for some reason erroneous?

Could you point me to the direction of how to troubleshoot this?

[jblackwell]

That’s good to know. If path tracing is giving you the desired image, then it definitely means lumen/the overall lighting pipeline is the problem. If disabling screen traces means the reflection disappears entirely, then it means that the reflection is only being computed in screen-space. If I had to guess, it’s because your surface is entirely translucent, and lumen reflections don’t actually support viewing translucent objects in reflections (currently).

Depending on what your art goals are, you could change the blend mode? If it’s masked or opaque, it will work correctly.