It doesn't make any sense to me why, on a model with averaged vertex normals (one smoothing group), you would get a seam down the middle of the model if you mirror your UVs.

The texels on either side of the seams should have the exact same normal in the normal map and as a result should have the exact same shading. Why then is there a seam?

I'm just trying to figure this out so I can explain it in a document I'm writing if anyone can offer any insight into this I would really appreciate it.

I don't know if this is your problem, but on my pc 3DS MAX likes to pronounce all the seams even though they're fine in other programs.

They shouldn't cause any problems if you bake them correctly. The problem arises when you delete one half, bake, then add the other half, because the vertex normals on half a mesh are different than on a full mesh.

Yeah I'm aware of the broken vertex normal issue if you delete half. I never do that.

I guess basically what I am asking is if mirrored seams are handled any different than non-mirrored ones. Is there anything special about them, or is it just that all seams will be some what visible and there is nothing you can do about it?

No, I just completed a little project where I mirrored seams, nothing out of the ordinary happened.

Mirror seams usually occur because the tangent basis isn't calculated properly to handle that case. The axes for the vertices in the tengent basis have to be twisted around along the seam, so each side will get its own proper "flow" in the tangent basis. Hard to explain. Need to make pictures.

Okay I think I had a false impression because of the some information the Wiki about how Unreal handles normal maps, but I think that's because it makes UV border edges hard by default which screwed up the relationship between the vertex normals and the normal map.

I guess there its safe to say there is nothing special about mirrored seams vs standard ones.

EDIT: Ah yeah Eric, I see what you mean. Its really a tangent basis issue not a normal map issue. Its amazing how many misconceptions there are floating around. I keep running into them and wondering what's real and whats not.

Thanks for the responses everyone.

Originally Posted by jocose

Its amazing how many misconceptions there are floating around. I keep running into them and wondering what's real and whats not.

That was my main reason for starting that wiki page, try to sort it all out. Not entirely there yet tho.