As the doc says: ’ Lumen Global Illumination replaces Screen Space Global Illumination (SSGI) and Distance Field Ambient Occlusion (DFAO). Lumen Reflections replace Unreal Engine 4’s Screen Space Reflections.’
I understand that Lumen could be compatible with SSAO then. But I’m seeing if I activate Lumen as GI method, the AO buffer turns totally white. Is it a bug?
Are there any kind of AO we can use in combination with Lumen GI?
Yes and no, I think. SSGI is bouncing light around the scene via the depth buffer, but I’m not sure if it’s actually in charge of measuring occlusion. Ambient occlusion is global illumination technically, but in unreal it generally refers to different things. Honestly the terminology is kind of jumbled IMHO.
Lumen is very scalable, so how and whether or not it replaces a given method can vary. Generally though, it is designed to be physically realistic, and since ambient occlusion is an aspect of lighting behavior that can’t really be abstracted from everything else, it would be non-physical to have AO as a separate control. TL;DR: AO is a good-looking cheat for lighting, and since lumen is trying to mimic how light actually behaves, you aren’t given an option to control is seperately.
DFAO is a sort of component of lumen, as using SDFs to propagate light around a scene. Lumen supercedes all rendering methods, basically.