Normal Map symmetry issue

[Prophecies]

Hey,
I've never encountered this before, but here's my issue.
So I unwrapped half of a model to save texture space and baking the normal map on that half.
Once baked, I applied a symmetry modifier and got strange results. The picture below explains what I mean. Basically, when I unchecked "weld seams", the result came closer to what I wanted. However, the seams aren't welded, making it impossible to skin the model for a pose. Here is the result before and after the seam along the middle is welded. After welding, the normals along the middle seem flipped, but are fine along the rest of the model, it also seems to flatten out all the detail... I really have no idea what is going on here.
http://a.imageshack.us/img413/4756/griffonbug.jpg (http://img413.imageshack.us/i/griffonbug.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

Hopefully someone can help! :)

[ScoobyDoofus]

Well, one thing to try is you should probably have your model mirrored and welded before you bake. Take the mirrored half and move its UV coordinates over by 1 unit, then re-bake. This may resolve your issue.

Also, post up your flats/UV's if that doesn't solve it so we can troubleshoot further.
It tends to help if you have your mirror seam perfectly vertical in your UV's.(especially for Unreal Engine)

[Prophecies]

Here are my UVs. Basically I unwrapped one half of the body. But the normal map issue only occurs right on the front. The back seems fine. However, since I unwrapped half the body, I can't really make the mirror seam perfectly vertical like you said. The entire contour of the UV shell is the mirror seam.
http://a.imageshack.us/img194/4756/griffonbug.jpg (http://img194.imageshack.us/i/griffonbug.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

[ScoobyDoofus]

Well, try what I said about mirroring, merging and then moving the mirrored half in your UV coordinates, then rebake. I'm willing to bet that fixes your problem.

[Prophecies]

I don't understand why it works, but it worked. Thanks alot! I truly appreciate it! :)

[teaandcigarettes]

I don't understand why it works, but it worked. Thanks alot! I truly appreciate it! :)

It's because welding your geometry post bake, changes the vertex normals - technically you're no longer using the same model you did the bake with, hence some visual errors are bound to happen.

By moving your UV's by 1 unit, the mirrored part still shares the same, relative coordinates (be they in 0-1, 1-2, etc. space) - however, it's also ignored upon baking.

All in all, what this process that is that it allows you to make all the necessary mirroring before the bake giving you a chance to avoid editing your mesh and messing up the vertex normals. If you look around, you'll find some explanatory images; can't find any of them atm.

[mortalhuman]

Since it's finished, to get some more info jam packed into this topic, i could use some help as well:

I was excited to see this, only to find it wasn't the problems I face lately.

Every normal map I bake looks almost like that even without symmetry models. Take a rock for example, unwrapped all the way around itself then a hi poly baked to it - both the diffuse I baked and the normal maps are somewhat like a jar and a jar lid, slightly turned. It's almost closed, but there it is, offset, twisted, just a little bit.

The bakes otherwise are coming out flawless, but my normal maps are turned like that. No changes to geometry etc.

It's so slight that I almost feel like even if it wasn't slightly offset, it would still have a seam.

We use a really small scale, and it does disappear upon the first mip map coming in, but everything has them. I see seams in a lot of triple a games too - are they just a fact of life? Usually well hidden, sometimes you cant, pray for the mip map or preferred viewing angle? :P

[teaandcigarettes]

Mortal Human, could you post some images? Something tells me this might a typical hard-edges/split UV islands issue. Also, where do you bake your maps? Are you using a cage?

[Prophecies]

If this may help, have you flipped your green channel? Give that a shot :)

[mortalhuman]

@teaandcigarettes - definitely what it is - hard edges and on the UV islands. Happens anywhere I ever try to bake :P Yes, cages.

Prophecies: I will try it next time! :D