Lumen GI and Reflections feedback thread

Hi everyone, I’m starting this thread for feedback and discussion about Lumen GI and Reflections in UE5 Early Access.

Lumen documentation
Lumen technical details documentation

We’re excited for you to use Lumen for the first time, but Lumen is also under heavy development as this is only an Early Access release. We have big plans for improving performance, full Hardware Ray Tracing mode, less noise and ghosting, Landscape, and many more.

We’re under a lot of pressure with internal deadlines so can’t promise responses, but we will read everything here.

Lumen team is:
Daniel Wright
Krzysztof Narkowicz
Patrick Kelly


I hope you get successful! This seems promising, truly.

i have feedback!

It’s glorious. Miraculous even. This has been one of the holy grails of real time graphics for a long time.

How long are we going to have to wait for lumen in stereo? I’m dying to see it in VR.


You can figure out performance issues using ProfileGPU:


In this case ray tracing shadows for rect light is quite slow. Try disabling shadow ray tracing in project settings, it will make everything faster (we will take a look why it’s so slow). You can also try Lumen without HW ray tracing (toggle in project settings).


ok i’ll try that out. i’m really hoping that simple box-y doom style maps could get easier to render in the engine as UE5 matures. thank you!

Performance issues aside, it looks fantastic and very promising. Great job!

1 Like

So far, very impressed by how all of Lumen’s systems work together.

Quality/speed balance of off-screen reflections when using DXR is particularly good.

Noticed that HISMs appears black in Lumen scene, when using hardware raytracing.

It also seems that GIreplace material expression does not work.

One suggestion so far for the the future. It would be nice and useful to have physically correct refraction on single layer shading model, that would use lumen scene and being capable of refracting off-screen.

I took a similar look at the runtime cost. The conclusion is still the same: ray-tracing cost for direct lighting in Lumen is too high.

The cost, highlighted by Krzysztof, is due to the ray-gen dispatch across the entire surface cache atlas. This is currently done per-light, and the cost stacks up quickly. I’m actively working in this area so we can perform sparse updates and significantly lower the expected runtime.

To give a brief example of in-progress work, when looking at Sun Temple, the current cost per-light in Early-Access is about 0.4ms while some preliminary work moves it down to 0.05ms. This isn’t a final report, however, as we expect batched evaluation to push the overhead down to something like 0.05ms per 16 lights, instead.

Lumen’s use of ray-traced direct lighting is tied to the use of ray-traced shadows in the scene. You can decouple this by setting the following CVar:
r.Lumen.DirectLighting.HardwareRayTracing 0

However, you might opt to work without ray-traced shadows, since virtual shadow maps are the preferred method to use with Nanite assets.



awesome! this is great to hear

i’ll throw this bug report in here because it might have to do with lumen

I think these are self-explanatory ones:

And two more…


Lumen continues to blow my mind. Performance is an issue- though I’m currently stuck with a 1050 ti, combining that with an early alpha of UE5 is a recipe for low frame rates, I get that. The biggest issues I’m seeing are significant amounts of ghosting around the edge of the screen as I move the camera, and on moving objects, as well as noise in reflections and darker areas, which aren’t dark enough to warrant noise to begin with. Despite this, I can’t believe how realistic a thrown-together gray box room can be! It looks so real, despite not even being textured! It truly is amazing.

Lumen is truly impressive and works surprisingly well in this early access version. Performance has been acceptable so far and the Reflections seem really promising.

However I really hope they can implement a version of this that works with stereo rendering. I would be happy even if it would work without the screen traced part. This could be used for VR with a few adjustments.

Screen tracing would not work well of course but having irradiance probes fed over a bunch of frames by tracing against the simplified distance field representation would be optimal to get a good result that could be used in a similar way as the volumetric light map and this could be done with manageable performance cost. Tracing for the probes could be done in worldspace/lumenscenespace instead of screenspace so it would not suffer from stereo rendering and the probe update process could be limited per frame similar to RTXGI.

I am developing a VR game with procedural generation and I have been looking into GI solutions for a long time now and I think its definitely possible. I am already going trough the source of Lumen to see if I could modify the system in that way but I would love to see a official implementation of such a system for high end VR

1 Like

Not sure if I’m missing something but my landscape is black in Lumen reflections

Also awesome work! loving lumen and all the new toys!

Lumen reflections on single layer water does not seem to be working.

Hey buddy can we see ue5 running in older hardware in future. Because lumen kind a heavy load for my PC. I am a student, and my spec is i3 4th gen gt 1030. Ue4 runs perfectly on my PC without any problem, and can handle heavy ue4 projects. Hope that you guys will take my feedback and bring out some solutions.

And how about translucency/refractions. Not only it is not working with lumen curently but it was pretty easy to break with raytracing too.
That said, I can’t wait to drop offline rendering at one point and use just Unreal!

Really impressed by what Lumen ca do for the performance cost.

If you all have the time: Since RT is depricated and eventually removed: will there be another option that does not involve ssgi or ss reflections??

As good as Lumen is, for virtual production or visualization is not good having actors receive direct light from a far over exposed spot in the background like if it’s right next to it.

1 Like