have a look at this:
- that’s your reflected skylight. that’s correct behavior
- things are dark here because the reflection shows the interior, but the interior is no longer affected by the skylight (because the probe is blocking the skylight inside)
- things are not dark here because the interior is affected by the IBL lighting
in UE4’s PBR a full-white metal with 0 roughness is a perfect mirror, so the reflection matches 100% what the reflection probe has captured.
in PBR a metal is an object that -only- reflects its environment (and is not affected by direct or indirect lighting, except to reflect the actual light), while a non-metal is affected by its lighting -and- can reflect on top of that.
in vanilla UE4 without IBL it works the same way, that’s just how PBR works. here’s a sphere with full-white color, metallic set to 1 and roughness set to 0:
I guess the confusing part for you is why the reflected interior is so dark, when the actual interior has some ambient lighting applied (from the IBL).
you should understand that the reflection probes are captured -without- the IBL lighting applied (otherwise re-capturing them would stack up the lighting, so every time you would recapture you would get a brighter result). therefore the -applied- IBL lighting does not appear on reflections. since IBL is made specifically to block the exterior skylight on interiors covered by reflection probes, the interiors’ ambient light will always be black.
for interiors the correct workflow would be to add actual lights, or alternatively to reduce the reflection probe’s IBL contribution to allow some of the skylight to affect the interiors.
hope that clears it up
[edit]
on second thought, I suppose for metals I could add the blurred reflection on top of the regular reflections which would fake them reflecting the IBL-affected interiors. as long as I modulate it as to not break the energy conservation law it would still be physically correct and the end result would be reflecting the interior IBL lighting.
I’ll give it a try eventually, but I don’t have time atm. I guess I can do it when 4.24 is out and I work on porting it over