After some extensive testing, tweaking geometry, lightmaps, and just experimenting with stuff to figure out how to correct this issue, here’s what I discovered.
I have this mesh of a magazine with curled cover:
Unreal doesn’t like it at all. Baked light for it gets all sorts of screwed.
However, if I split it into two pieces - the cover, and the block of pages, it fixes the lighting issue.
The problem is - now I have one more draw call for a small prop. Which is not ideal.
I tried to combine two pieces in maya, without merging vertices, but that only led to the same result as before, with one whole mesh.
I tried to combine two pieces in unreal with Merge Actor Tool, and, weirdly enough, that gives different results. There’s no more shadow bleeding, but there’s no occlusion either.
Here’s what I mean:
As you can see, two separate pieces look as we would expect - with all the shadows and occlusion in the right places.
Merged prop is missing occlusion. And I noticed that at some angles it still gets a bit of shadow bleeding.
And a single mesh, exported to unreal, is just awful.
This behavior is weird to me. I’m don’t have technical knowledge to explain what’s happening.