[UDK] Strange seam along symmetry line

Otixa · Nov 2012

Hello everyone!
I'm having a very strange issue with a seam appearing on one of my models inside of UDK. It's not there when rendered in Marmoset or Xoliul in Max.

Below are some screenshots. Note that on the second screenshot, there is no normal map applied to the model! I thought it was the normal map at first, but now I have no idea what's going on.

The seam does not appear when the model is unlit.

Can anyone please help me with this? It's been driving me nuts for 3 days now

Santewi · Nov 2012

Is there a reason for having that edge there in the first place? If you can remove it, it will probably fix your problem.

If you can't remove the edge, can you post your UV layout? Also, whats your lightmap resolution?

Otixa · Nov 2012

Thanks for the quick reply, Santewi.
The lightmap res is 512x512. The bottom screenshot does not have lightmaps baked into it, which is also part of the oddness.
Here's the UV layout, the yellow line selected is the seam.
I can't remove it because it's my symmetry line.

Quack! · Nov 2012

Do you have a second set of uv's for the lightmaps?

blankslatejoe · Nov 2012

Hm...do you mirror the UVs at that edge? I may be outdated here, but mirrored edges used to give seams in the specularity/normal regardless of whatever you did to the lightmap itself.

Otixa · Nov 2012

@Quack!:
Yes, I have a properly set up secondary UV channel for the lightmaps.
@blankslatejoe:
Yes, the UVs are mirrored around that edge on channel 0. Is there any way around that?

I've tried importing the mesh with "Single Smooth Group Single Tangent" checked, which made the seem less visible, but created other shading issues.

blankslatejoe · Nov 2012

Well, it used to be that the solution was 'dont mirror UVs where you don't want a seam'...I'd like to think they'd have fixed it over the years, but I'm doubtful. You'd have to do some digging---I'm sure there are threads on here about others tackling the problem.

Otixa · Nov 2012

There's some progress.
I dug up some old thread where someone suggested rotating the normal map so that the faces with the lighting issue are vertical in texture space, so I did that and it improved the look a lot. But the issue is still there, to a lesser degree.

I'm determined to find out exactly why this is happening and how to avoid it, other than not mirroring in visible places.

Quack! · Nov 2012

I am assuming then that you have your uv's as seamless as possible for the lightmap uv's?

Otixa · Nov 2012

Yes, I tried to keep seems to a minimum and only place them in 90 degree angles and places where the light would generally create a seam(if that makes sense).

I've redone the UVs now and rebaked, while offsetting the mirrored UVs by 1 in UV space.
It seems to be giving me nice results so far, but I haven't fully tested it.

I'll report my findings later.

Otixa · Nov 2012

Mesh is cursed. What do?

Update: Ok, I gave up and just redid the UVs so that there's no seam there.
Would have been nice to find out what causes it to behave so strangely, but I would rather not have to deal with this anymore.

Thank you all for the help!

bugo · Nov 2012

There's no finding for it, UDK is that way, instead of doing pixel shader specular like any other engine, it creates a world specular and uses UV channels for light direction. So if you want this without a seam, you will have to redo the uvs there, unfortunately. UDK won't change it I believe as this increases performance. I also find it quite annoying, but you get used to it.

[UDK] Strange seam along symmetry line

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