UV seams on mirrored objects.

Linfo

So, I work a lot with objects that are mirrored on the X axis in order to save texture space.
But my meshes have this very visible seam along the center. 

See example: 
https://i.imgur.com/OZRgIqfg.jpg

In this example, the mesh has a visible seam already and I haven't even baked yet.

So, I've tried using custom vertex normals.
I've also tried offsetting the mirrored normals but that only minimizes the issue.

So, is there a way to fix it or is SP2 unable to handle mirrored objects?

I'm using Blender by the way.

poopipe

There'll be something wonky with your bake settings, probably that you have average normals ticked.

Bear in mind that even if you resolve the normal seam issue you're still going to get crappy butterfly artefacts in the other maps - there will be a better way to lay out your uvs in the same space

Linfo

Yeah, after some research I've arrived to the conclusion that I simply cannot use symmetrical UVs anymore.
I'll have find other tricks to save on texture space.

Bek

Just because it's a UV seam doesn't mean it HAS to be a hard edge. Soften the edge and your seam goes away — but you'll have to be careful that nothing too unique gets textured there, or you'll have that very obvious butterfly mirroring.

And depending on the object you can mirror things not just on the centerlines, or use instances for repeating parts. You might even be able to have a mix of unique and tiling (or shared) parts depending on your situation/engine capabilities. With something very flexible like UE4 you have plenty of options for saving UV space, like covering up tiling texture repetitions with some vertex-blended textures, deferred decals, other shader shenanigans... You just need to assess the situation and think of some creative solutions.

Linfo

Yeah, my problem is that the project I'm working on is Unity 5, so my options when it comes to shader magic are extremely limited.

Still, I find it so weird that I can't get this to work.
I've been searching quite a bit on this subject and I've seen several people with the same issue, but not a single resolution.

I've tried applying the mirror modifier, then entered edit mode to make sure the center line was merged. 
I also verified it wasn't marked as sharp and applied weighted normals after the mirror modifier, and I can still see the seam on the low poly even before baking.

Larry

you can use the same uv's overlapping, but there could be some issues. 1st, do not trust your viewport on 3ds max, maya etc. Secondly, try to flip the normals on the mirrored side.Or try to flip the whole uv island in this particular case. And make sure these 2 sides are in the same smoothing group

Froyok

Try also changing the sampling quality in the shader parameter, it could affect the visibility of some seams in our viewport. This also means that in your target application the seam won't be visible.