Lumen GI and Reflections feedback thread

[Krzysztof.N]

@TheKJ

Why not allow reflection captures with Lumen GI?

Cutting out realtime rendered reflections could help out some game situations for studios.

We do allow that. You can disable Lumen reflections and fallback to SSR+cubemaps:
https://docs.unrealengine.com/5.3/en-US/lumen-performance-guide-for-unreal-engine/#replacinglumenreflectionswithscreenspacereflections

@Miguel1900

Based in your explanation, I think it may be a bug, as it has a similar behaviour when “normal” or when ray traced:

I think what you are missing here is screen space traces. So how Lumen works is that it first traces a screen space trace and if that trace hits then it samples color on screen. If such trace goes behind geometry or offscreen, then we continue tracing from last known point using a world space trace (Distance Field or BVH trace) and that trace will sample Lumen Scene on hit. With RayTracingQualitySwitchReplace you are able only to replace what is in the Lumen Scene, so there will be a view dependent lighting mismatch, possibly leading to more noise.

@jblackwell

FYI to @Krzysztof.N as of 5.3 lumen reflections don’t appear to be accepting reflection captures at the end of their rays; not sure what’s up with that exactly.

To make them work you need standalone Lumen reflections with hit lighting. Enable r.Lumen.HardwareRayTracing.HitLighting.ReflectionCaptures 1. Then you should see how metals in reflections sample reflection captures (also works in Lumen visualization overview view mode). They seem to work on my end, though I’m on UE5-Main latest.