I figured out by now, that it has to do with the distance to another surface (and since they are low on the ground… yeah, bad luck):
Looks to me, like its really the ambient occlusion going nuts, since the floor is white, it shouldnt cause any issues in terms of “light disappearing”.
Need to think about ways to work around this issue, since it is mesh based and not a material issue.
EDIT: I found a fix.
What is needed:
- A Box that is larger than the Bushes, but offset from the ground (in my case: 50cm)
This box is then put into the “Distance Field replacement Mesh” within the Settings of the problematic Bushes. (Unlike what one may expect, it doesnt replace the Mesh, but just works like a “cutout”. (Epic should rename this…))
Since the Box is offset from the ground, we do not have a MDF there anymore, and Lumen doesnt do weird things with the AO anymore.
Clearly visible, that the right bush is darker and has more AO than the left one, with the Box in the background:
How this looks in the Mesh Distance Field view then:
Is this a stupid fix?
Yes.
Does it Work?
Yes.
PS: completely disabling MDF for a mesh like this doesnt work, it looks weird since Lumen doesnt “see” it anymore, you lose all AO etc.