Possible to turn off Emissive Material for Lumen GI?

The emissive Material seems to be very expensive.
Is there a way, to still use an emissive material (to visualize the glow effect) but somehow exclude it from the Lumen GI calculation?

Instead, I would just use a simple light source.
Seems to be cleaner and much cheaper.

Thanks for any thoughts, appreciate it!

Lumen still picks up pixels from the screen, so really you can’t (this is a problem they’ve mentioned). Though, looking at the video in this post, it looks like it can be excluded from the lumen scene, but it still gets picked up by the screen.

However, you may able to “fake” emissive through a post process applied before tone mapping; just mask the emissive parts and set their color value to something bright. I don’t have UE5 installed right now, so I can’t test it.

Also, I thought emissive materials had no additional cost?

thx. I thought so too. For some reason, when I use the emissive material, the FPS drops by at least 60%. uff…

This thread has a custom node that can exclude the emissive from effecting lumen.

1 Like

the emissive feature in Lumen is def problematic in a practical sense. the smaller the emitting object is, the more noisy the gi around it gets. Sure turn up gi quality and the noise goes away but then u lose perf. if you have a flashing emissive material. lumen is too slow to update so it ends up looking swimmy. not to mention that other than obj size and emissive amount/color, there is no control over fall off.

its possible Epic is working to improve some of this :crossed_fingers:

def gonna try a post material solution

C

There is already a material based fix linked in my post above that can disable the emission from effecting lumen.

Ya gonna try it, but still hoping that using emissive in lumen will improve rather than just shutting it off.

Anyone get this working? seems to do nothing for me no matter how i arrange the inputs.
yeah we using Main

Yes it works fine for me. Make sure your inputs have the same name as the custom code and that you’re passing different values into each input. One input will output to the rastered image and the other will output to the lumen scene.

I demonstrated it working on this post:

@BananableOffense am I missing something? these are my results:





image

this really just needs to be a feature.

Your material setup looks fine - What you are seeing there is the screen trace. This is why it achieves the desired result on the faces that aren’t visible to the camera, but not on those that are in screen space. I didn’t notice it in my use case before but in a new test I do see it. It can be removed with the command:

r.Lumen.ScreenProbeGather.ScreenTraces 0

Changing this will probably have some other visual or performance implications.
Screen Trace On:


Screen Trace Off:

Maybe there’s another way to achieve this in material without disabling traces altogether - but I agree that it should just be a feature.

1 Like

It doesn’t work for me.

A user in another thread discovered you can achieve a similar result by unchecking the box “Affect Dynamic Indirect Lighting” in the mesh details panel without needing the custom node. Not sure if there are other differences between the methods (besides applying at the per mesh level instead of per pixel level), but it still leaves the screen trace visible so just like before it seems that needs to be disabled to get the desired result in most cases.

I can’t really point you in a specific direction with your comment because it most definitely works and my last post’s photo proves as much (in 5.0.2 at least). Try this checkbox, or double check your material and then either way disable screen traces.

It’s a bit hacky so I expect it could introduce some other issues elsewhere though, so hopefully it’s addressed in another way later.

1 Like

This might be one of these dirty Hacks, but i just discovered that setting the Blend Mode of a Emissive Material to Translucent (Opacity can stay at “1” aka default) makes Lumen completely ignore the emissive & reflective Part of the Material … might be exactly what you guys & girls are looking for :wink:

Maybe… until they make Lumen compatible with emissive translucent surfaces :innocent:

Now that would be a migrane and a half. Is that even optically possible irl? Can something glow and be translucent, or would that be some kind of blackbody problem?

Wait, given that glow sticks are a thing I believe I answered my own question. Still, to have something simultaneously emit light, receive GI, sort, and shadow correctly would probably be pushing the bounds of what’s even conceivable in real time. I’ll wait for 5.2 to introduce it :laughing:

1 Like

Thank you for all of your research on the issue @BananableOffense, knowing that the r.Lumen.ScreenProbeGather.ScreenTraces is creating the additional light in the scene even if the mesh is not in the lumen scene is really helpful.
As you said, having a per material solution to solve this without having to deactivate the traces for the whole scene would probably be the best way to solve this.

It’s actually a feature. You can use Ray Tracting Quality Replace Node for this: Hardware Ray Tracing Tips and Tricks in Unreal Engine | Unreal Engine 5.0 Documentation

1 Like

I couldn’t agree more with this comment. If this is easily possible (it might not be) this would be a massive add to Lumen (especially for non-physical rendering looks). For increased credibility, I would have killed for this when I worked at Riot as an art director.

1 Like

In the recent Fortnite focused tech blog something stuck out to me:

“Some problems were easier to solve. Lumen uses screen traces to add detail where software ray tracing is lacking, but grass is so detailed that these screen traces were introducing significant aliasing. A simple workaround to skip screen trace hits on foliage was enough to solve the artifacts.”

This seems to suggest it is indeed possible to skip scene traces for only certain elements.

Perhaps @Daniel_Wright could enlighten us on if this is something we can easily apply elsewhere when screen traces are unwanted?

1 Like