After digging deeper into the SDK and light system, this issue seems not directly caused by the day/night cycle itself, but by the fact that different light types are used at night for artificial lights.
In the SDK there appear to be at least two distinct light types:
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Scenery / Street / Advanced Lights → highly optimized, used for airports and aprons, do not cast shadows
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FX Lights (legacy / effect-based) → support shadow-related parameters (e.g. shadow softness, cast shadow flags), but are much more expensive
At night, airports rely almost exclusively on Scenery/Street/Advanced Lights, which explains why SimObjects (aircraft, GSX vehicles, GSE) suddenly stop casting shadows:
the objects themselves are shadow-capable, but there is no active shadow-casting light affecting them.
This also explains why:
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Daytime shadows (sun/moon = directional light) work correctly
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Shadow settings, shadow map size, contact shadows etc. have no effect at night
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Raytraced shadows partially help (different render path)
From the SDK documentation, only FX Lights explicitly reference shadow behavior, likely because shadow-casting local lights are intentionally restricted for performance reasons.
Personally my wish from Asobo (if my hypothesis even proves to be true) is to create a new light type:
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Street Light – very fast, no shadows (current behavior)
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Apron Light – like Street Light but limited shadow casting (short range, simplified shadows)
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FX / Heavy Light – full shadow casting, high cost
This would allow airport developers to selectively enable local night shadows at gates and aprons without forcing full FX lights everywhere.