I know I’m late to the party but I wanted to add something here:
Again, there’s already a separate diffuse pre-baked lighting solution that works, and IBL diffuse does not look good. It’s not a good solution for large open world games with dynamic night/day cycles as it costs too much to recalculate the lighting all the time. So there’s no particular reason for it to exist. What, precisely, does IBL diffuse do that UE4 does not/does not plan to that you want so much?
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It is in fact a good solution for open world games. MGSV uses IBL probes to light the environments in a cheap enough way to be useful in last-gen consoles. They capture the cubemaps at different times of the day and then interpolate between them as time goes on in the game.
I tried to do this in CryEngine but you couldn’t blend between two cubemaps at runtime. It can be done in UE4 but you have to manually create all the texture assets which is a pain in the *** so obviously nobody does it.