Lumen GI and Reflections feedback thread

Follow up,

It does seem to depend on the distance between instance static meshes, rather than the location of the instance static meshes in world coordinates.
I.e. is the instance static component has meshes that are 12000 units apart from each other this happens.

Any ideas on how to solve this?

Standard Lumen Reflection:

Mode 3:

As you can see, right out of the box there are some problems. First, if you’re using sky atmosphere, the skylight will be tinted based on the sun position, which will cause the scene to absorb that color incorrectly. Second, there is no GI/AO of course.

With some hand tuning, you can adjust the skylight color to match the scene…

Here I’ve adjusted the skylight color and settings to match the scene reasonably well, and also added a fake distance field based AO in the material of the raytraced view only. The missing GI can still be noticed, like the top right wall which should be bouncing orange light in the reflection, but obviously cannot and so instead the DFAO is showing up.
And for comparison again, here is lumen. Noisy, but with proper GI.

I could definitely see it being useful in certain cases but it needs special consideration.


It’s definitely good to have this sort of trickery on-hand, but I personally feel like the setup costs would outweigh the benefits relatively quickly. Perhaps if you need specific behavior for real-time, but the main advantage lumen has over the PT is interation, which this would make much more difficult. I suppose it all depends on the content.

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are there other variables required to to get lighting mode 3 to work? cause it does not work on my end. whether if i have or do not have a skylight in the scene.

In UE5-Main it’s now controlled by r.Lumen.HardwareRayTracing.HitLighting.Skylight 1. In UE5-Main PPV takes precedence, so if you enable hit lighting through PPV then you need to uncheck it for this CVar to work.

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forgot to mention: this is in 5.4.2.

(i have multiple versions installed. i know the 5.5 flag.)

edit: it works if i disable the ppv. sky boost to color match. it’s not doing well for outdoors tho cause the skylight is a direct hit and interacts with fog. sky reflection results vary. hmm.

sample fog w/o skylight

sample fog w skylight

no sample fog w skylight

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Yeah it’s not ideal but it’s definitely doable for specific cases. Architectural visualization comes to mind as a case that always wants to put perfect mirrors in all white rooms, and can probably justify spending time baking and faking a thing or two of they really needed to eliminate the noise.
For most other applications, the situation can be entirely avoided with more careful art direction.

PPV takes precedence over this CVar, so you need to uncheck ray lighting mode in PPV for this CVar to do anything.

got it. ppv is applied every frame and resets it. logic.

Hi @Krzysztof.N ,

Unfortunately I was using default Lumen settings. I have prepared a proof project for you:

PT one sided mat:

PT two sided:

Lumen one sided:

Lumen two sided (GI partially destroyed and, huge dependence on screen traces):

RT shadows 2 sided, nanite mode 0:

RT shadows 2 sided, nanite mode 1 (ray tracing not tracing against full mesh over distance; look at the patches at the center of the ceiling. Working fine when going closer):

Project download: Dropbox

Thank you

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There’s a long standing bug with Nanite and two-sided. You need to either don’t use two-sided or disable Nanite on that mesh.

yeh. why would one do 2 sided on this concrete prop in the first place? it’s only ever visible from one side. makes no sense to use 2 sided.

to me, it only further proofs dude is just breaking things, to break things. maybe to get some “feedback” in. i smh tbh.

missing proper application. i’d use 2 sided only on foliage or thin fabric materials where the directionality matters.

the pt screenshot for sure looks f-ed tho. is that tracing using proxy mesh? shadow intersections? where they differ?

Thank you @Krzysztof.N ,

Understood. I hope it arrives in the future. In addition, it’s a pitty that RT shadows + nanite and + LODs (yes…) are also broken, with those distant patches. I know that this it not your field, but I have already reported it somewhere, but nobody cares.

EDIT: I have disabled nanite in that mesh, but I don’t notice any difference regarding PT nor RT. But it bypasses the error of lost Lumen GI when 2 sided mat.

@glitchered The original one it’s a marketplace asset so, sometimes, we need to adapt ourselves to what we get. I can’t remodel a hyper realistic 3D scan just to make it render correctly.

Unfortunately it’s quite usual that the active and innovative developers encounter tons of bug and/or challenges during our Unreal 5 projects; it’s not (obviously) intended. Only when making real projects, you discover how the things really work.

About the screenshots all info is in them, but that project it’s just straight fordware. Impossible to be more simple and standard. Anyone can also check the project by himself.

You do make a very good point there. ArchVis does love their white rooms and perfect mirrors, and the people I’ve talked to would probably love what you’ve figured out. The notion of reflecting unshadowed skylight with ‘painted-in’ DFAO is honestly very interesting to me, your results are pretty convincing.

To your point again, even a little bit of art direction goes a very long way. It’s very similar to the instructions epic gave in the old reflections enviroment setup page: Reflection Environment | Unreal Engine 4.27 Documentation

In a way, lumen doesn’t really change the rules on making good real-time reflections: non-mirror and non-planar surfaces produce far better results than anything perfectly smooth. and If you want genuinely perfect reflections, just use GPU lightmass and standalone reflections.

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I’m speaking from Archviz and I say that the solution is not viable in production projects. The absence of GI makes the mirror a stranger to the total photorealism of the rest of the scene. With this LightMode 3 solution I also noticed a certain phantom doubling of some objects reflected in a particular camera/mirror angle. Beyond this aspect of the “mirror” object, there remains the problem of noise on reflective surfaces, especially if in dim light.

Mirrors are just not possible at the moment. There are a million ways to somewhat improve it a bit but nothing that really solves the issue. Somehow smoothing out the surface cache would probably get us much closer to a usable result.

in 5.4.2 lighting mode 3 works to some degree if an artist can handle the required tweaks.

i’ll see how 5.5 handles it (in a couple hours, cool speed compile day). i’ve seen some code changes to the indirect rays. hmm…



I’m new here. Can I ask a few questions about Lumen Mesh Cards and the Yellow coverage area in the Lumen scene?

Since small slats with many quantities will be used in the interior space, in a few shots, they will occupy more screen space than in this example I have shared. I’m trying to get better lighting behavior out of these models when I work on the production. Is there any way to fix the yellow coverage area? I want to eliminate yellow at all in my scene with the selected objects. Some one in this forum have mention, Lumen tends to ignore smaller objects. Is there any way to tell lumen this value is smaller for my this scene instead picking a value for itself? Kindly note that FPS is not a priority here!!

Once the mesh gets a scale in one direction, the size has changed to a large one but still, no mesh card is generated for this model, why? To fix this what should I have to do except import scaled model?

In the Object edit window, the model has not scaled but in the viewport it does?

sorry for the basic questions, I would appreciate any help/advice on the topic.


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Question - in pc-VR I see glitches in the left eye only when using Lumen (+Nanite +RT translucency). See video. Any suggestions how to fix that besides not-using Lumen?
Edit: its a VSM issue
Edit2: the issue is easily fixed by either settings the the lights that affect the surface to ray traced shadows OR disabling one optimization of VSM by r.Shadow.Virtual.OnePassProjection=0

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that is a vsm issue not lumen.

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