Lumen GI and PBR values

[GGavanozov]

We’re exploring an issue with Lumen GI in locations like tunnels and bigger darker interiors.

When entering interiors Lumen scene is noisy and updates very slow, in dark interiors there are a lot of artefact caused by the high lighting intensity, fog also appears to catch these values and looks brighter than the scene intensity. The slow update is more noticeable when entering the ‘’interior’’ with a fast moving camera. The options in the Post Processing Volume Lumen GI settings are not helping, also sky leaking isn’t an option as it adds high intensity and makes the local lights not visible, but also flattened the scene.

Lighting is set close to PBR values with average EV 12 (exterior). Sun intensity is 22K, skylight/HDRi reads from the sky atmosphere illuminance. Lumen is using default settings, no ray tracing. Geometry is split to pieces and Nanite. UE 5.5

There is a post processing volume in the tunnel just to reduce the bloom and lens flare. Fog is off in most of the videos but the one in the entrance to show case the GI leak in the volumetric fog.

Video: Tunnel_PBR_01 it’s a 180m long tunnel with moving camera.
Tunnel_PBR_01.mkv (25.4 MB)

Video: Tunnel_PBR_03 – same tunnel but faster moving camera where we can see the slow GI update.
Tunnel_PBR_03.mkv (17.4 MB)

Video: Tunnel_PBR_02 – it’s very long tunnel, end of the geometry is outside of the GDF range.
Tunnel_PBR_02.mkv (14.3 MB)

Video: Entrance_PBR_01a – 100 m long tunnel with a curve is going to same size room, high contrast on the entrance, them splotchy artefacts before the GI slowly update and fade to black, the noise is moving based on the camera position.
Enrance_PBR_01a.mkv (13.7 MB)

Enrance_Fog_PBR_02 – same interior but with the fog ON.
Enrance_Fog_PBR_02.mkv (11.8 MB)

We did try with very low (non PBR) values and GI looks stable no artifacts (or at least not visible), Sun intensity is set to 160 Lux, average EV 4.5 (exterior).

Video: Tunnel_nonPBR_01 – no artefacts.
Tunnel_nonPBR_01.mkv (12.9 MB)

How we can improve Lumen to cooperate with sun/sky PBR values, without these visual artefacts?

[ZacD]

Is there a reason you’re not using hardware ray tracing with lumen? With only software the best option might be just faking it and fading in a rec light at the entrance and exit when the player is in the tunnel.

[GGavanozov]

Hey ZacD - as faking and fading do you mean the lighting in the interiors? Like to gut out the exterior lighting when in and add bright light on the entrance?

[GGavanozov]

Also do you suggest using ray tracing will fix the problem?

[jblackwell]

Software RT is a partly deprecated path now, and I’m not sure if the updates to improve lumen’s functionality with physicalized values was fully implemented to that path.

What version of UE are you using? I’m pretty sure full physical light range parity was only supported inn either 5.5 or 5.6.

[GGavanozov]

Hey @jblackwell , thank you for the comment. It’s UE 5.54.

[GGavanozov]

Hey @ZacD @jblackwell, turning on raytracing cleaned up a bit the GI, but I still can see the issue.

RT_ON_02a.mkv (22.3 MB)

[GertyMoonMan]

This is basically a worst case scenario for Lumen. It’s causing a similar issue to using a very small, bright emissive. Your tunnel entrance acts as an extremely bright and small light source, which means it gets missed by the vast majority of probe traces into the environment, and the traces that do hit return huge values.

Because there’s an under-sampling of light happening, the tunnel is darker than it should be and then auto exposure is desperately trying to compensate which exacerbates the artifacts by highlighting the contrast between lit and unlit areas

Your best bet is to clamp exposure down to a level where the artifacts are less noticeable, and use other techniques to increase light levels - such as adding actual lights, increasing skylight leaking or introducing hidden emissives that still contribute to distance field lighting. Toggling on path tracing can be a helpful tool to figure out what it “should” look like.

[GGavanozov]

Thank you @GertyMoonMan , this is a very good suggestion, and can be a workflow, but it’s still kind of a off PBR hack. Higher exposure means local lights bright as the sun (intensity around 500000.0 lm), sky leak does the same thing, it flatten the lighting inside, so to bring back the contrast the lights should be with intensity closer and over the light (sky) leaking values. Also that will be inconsistent looking from outside the tunnel and opposite, and a bit harsh on the transitions, and we are back to the point why don’t use lower values instead of hacking the problems.
We are looking for more natural solution, with less help from Post Processing Volumes, animated lights, hidden emissive, etc.
Again, thank you for your help and useful comments, appreciate any more suggestions from anyone.