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(not really) Overlapping UV's.

UE4 complains that my mesh has overlapping lightmap UV's. Around 11-12%. But when I look at UV's they are fine. And I can't find any visual errors on the mesh after I build lighting. What could be causing this? alt text

Product Version: UE 4.20
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asked Oct 11 '18 at 05:55 PM in Using UE4

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Have you found an answer to this? I have the same problem and can't find any way to fix this.

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answered Jun 20 '19 at 09:37 PM

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avatar image Kligan Jun 20 '19 at 11:02 PM

I found a workaround.

If the mesh is too complex and has way too many UV shells on its lightmap, then UE4 will give you a warning about overlapping UVs. What I do in this case is separate the mesh into parts, make UV lightmaps for each part, and assemble the parts inside a blueprint to make the object whole again.

On a screenshot in my original post I tried to make a big section of rail tracks as a single mesh. The idea was to combine spline meshes into a single mesh to reduce draw calls, and then generate lightmap channel for it in UE4. Didn't work so well, as you can see - the amount of UV shells on the lightmap channel was too high. I then split the tracks into several sections and generated lightmap UVs for each of the sections, and the problem was gone.

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avatar image Dave600 Jun 23 '19 at 12:37 AM

That's quite strange. Is this an actual bug or a wanted limitation from Unreal? It appears to happen very randomly as I can subdivide a cube 6 times, so it consists of basically infinitely small faces in my 3d modeling program (blender) but UE4 still accepts the lightmap for it. A school-building I have modeled however has around 70% overlapping UV's even though they are perfectly fine. I guess I'll have to start taking my assets apart and making the pieces single objects. Thanks for the quick answer! That picture looks awesome btw. ;)

avatar image Kligan Jun 23 '19 at 02:20 AM

It's not about the complexity of geometry. It's about how much UV shells you have on your lightmap. The thing is, if there are too many UV shells, then even the highest lightmap texture resolution will not have enough pixel space between UV shells (you need to have at lest 4 pixels between shells, so at some point you won't be able to fit more shells even on 4k resolution lightmap). And that's when UE4 gives you this warning.

If you are making an entire building, I would imagine it has lots of elements, each with their own bunch of UV shells on the lightmap.

Do you mind showing me your school building? So I could tell you better what's wrong. Post a screenshot of it please. And another one with its lightmap UV channel.

avatar image Dave600 Jun 23 '19 at 03:18 PM

Interestingly it seems to work now. I have UV-unwrapped every object once more and played a bit around with the "margin" setting within blender to increase the space between UV shells and now it seems to be working. I tried that before, but I guess something went wrong last time.

I can send you the UE4 file if you want to have a look at it. My lighting still looks weird, if you have some time on your hands and nothing else to do, you can have a look at my model. Is there a way I can DM you the file? As the building I have modeled is a replica of my high school, I could get into trouble for publishing it online which I want to avoid ^^

avatar image Kligan Jun 23 '19 at 05:05 PM

Hm.. I'm not sure if you even can DM people here. Tell you what, send it to my e-mail:

I'll remove my e-mail address from this comment once I receive a message from you.

avatar image Dave600 Jun 23 '19 at 06:57 PM

Thanks, I've just sent you an email.

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