Enlighten real time GI: strange behaviour of directional light indirect multiplier

[fickle spire]

I have a project that uses HDRP with physically based sky, directional light with parameters that represent the sun, and Enlighten for real time global illumination.

I’ve noticed a strange behaviour with the Indirect Multiplier of the directional light, together with Enlighten real time GI.
The documentation of the directional light states that the value is used when realtime or baked GI are active.
If I leave the Indirect Multiplier to the default value of 1, when I place the sky well below the horizon, the sky becomes entirely black (as it should be) but the scene continues to be lit by what appears to be the indirect lighting from Enlighten.

If I set the Indirect Multiplier to 0, the scene becomes pitch black as it should be. When I then place the sun above the horizon (still with indirect multiplier=0), Enlighten continues to provide indirect lighting (albeit with slightly reduced intensity). And in some respect it looks ever so slightly less flat than when indirect multiplier is set to 1.

Am I missing something?

[fickle spire]

To add another piece of info.
With indirect multiplier set to 1 and sun below the horizon, as said I get pitch black sky and scene still lit by indirect lighting. If I disable real time global illumination and rebuild lights, the entire scene goes entirely dark as it should.
Bottom line is that this almost seem like a bug to me. Enlighten is applying global illumination based on the direction of the light but doesn’t take into account the actual intensity of the light that is affected by its position in the sky and the effect of the atmosphere. Instead it seems to just consider the nominal intensity (which obviously doesn’t change when the sun is below the horizon)

[fresh lagoon]

In my limited experience Enlighten realtime GI's light accumulation isn't exactly accurate, for example some faint indirect lighting from a directional light will linger behind even if the source becomes occluded
Perhaps you're also using automatic exposure that boosts brightness equally as lighting conditions become darker?

[fickle spire]

In general I have auto exposure enabled, but I disabled it for this test.
The scene at night is totally lit, almost as if there are artificial area lights positioned in every window of the interior scene

[fresh lagoon]

By disabled you mean set to fixed exposure?

[fickle spire]

Yes, exactly

[fickle spire]

I've taken some screenshots to show in a clearer way what the issue is. In order they are:

1. Indirect multiplier = 0, day scene
2. Indirect multiplier = 1, day scene
3. Indirect multiplier = 1, night scene
4. Indirect multiplier = 0, night scene

As you can see, I get visually correct results both day and night, only with indirect multiplier = 0

[fresh lagoon]

Quite strange! I don't know enough about Enlighten to give more suggestions

[fickle spire]

Thanks for taking the time to reply. It seems that enlighten is working correctly by itself and responding to the indirect scaling set in Rendering->Lighting. Instead the indirect lighting associated with the directional light is applying a general ambient light whose color is constant to 6500K (which is the color of the directional light) and doesn’t interact with the atmosphere or with the occlusion of the sun.
This behaviour kicks in only when Enlighten is active.