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Parts of object only visible in sunlight!?

I’ve got a weird behavior that I can’t make sense of. Little help?

So I made this generator (Substance Painter Shot):
01_Generator_Substance.jpg

Placed into my level, all is good WITHOUT a lightmap on the object:
5cb9f8846747f2fa6dd6cc375c45002b911bddf6.jpeg

As soon as I build lighting, the object turns dark, but some parts are still bright:
f7b8954b65d8500ca826ba40393672b3b29906b0.jpeg

It gets even weirder when I place the object into the starter content map “Advanced Lighting”. With the lightmap built, parts of the object turn black…
8deed80e6d9581dffc24a7f0eea3bef2a827a879.jpeg

…but strangely NOT on the sunlit side!
dfbc8e16b217e277296906dcba08ebff08afaa68.jpeg

Here are my lightmap UVs. Not an ideal layout, but I can’t see anything that explains that behavior:
06_LighmapUV.jpg
I also forced UE to generate a different UV layout by increasing the lightmap resolution, but nothing changed.

Lastly here’s my shader network for the material - it’s very simple:
45e26d299c5c7422cbd1d5d6a083a5e82e607e03.jpeg

I double-checked the face normals as well, couldn’t find a problem there.

Any help on this would be much appreciated!

Could it be that those are metallic and you don’t have any reflection probes?

What lightmap resolution are you using? Try looking at the model after you bake it with just a grey basecolor? The model doesn’t seem designed very well for having baked lighting, could you post a wireframe screen shot?

Thanks for looking, darthviper and Zac. Reflection probes are in place, I even tried a few more around the object to see if they make any difference (they didn’t). The whole object has metallic properties, but strangely only parts of it go dark.

I tried lightmap resolutions of 64, 256, 512 and 1024 - didn’t make a difference, except that more was black with the 64 lightmap if I remember correctly. I will try a bake on grey basecolor tomorrow, great idea!

Here’s a quick wireframe:
914f356151c12d77ae7494a97c7d12d04e50ef3d.jpeg

Yeah, meshes with lots of individual floating parts are not ideal, and that’s causing some of your issues (not sure if it’s the only problem) https://docs.unrealengine.com/latest/INT/Engine/Content/Types/StaticMeshes/LightmapUnwrapping/index.html

For something that is baked, you ideally want as few UV islands and floating parts as possible. I’m not saying it needs to be water tight, or have no floating details, but instead of having dozens of parts, try to keep it under a dozen.

OK, turns out that the UE4 automatic lightmap UV unwrapper indeed produced that error, although the unwrap seemed alright at first glance. What really threw me off was the fact that things looked fine in the “sunlight”. Changing the viewport to “Lightmap Density” was helpful in seeing that something is off - I should’ve thought of that earlier. I provided a second UV channel with a manual unwrap and all is now fine.

Thanks for the help!