Don't mirror normal maps!

[Christhiana]

I just wanted to share something I found out today that cost me a lot of extra work... Hopefully I can keep someone from making the same mistake.

I made a simple straight walkway in blender with a woodplank texture. To save UV space, I split the walkway in to halves and unwrapped them on top of eachother. To make the texture repeat a little less obvious I mirrored one of the sides on the UVmap. I baked the texture with AO and also a normal and specular map for use with SL material (the original woodplank texture came with normal and specular maps). After upload to SL I discovered the specular reflections worked the opposite way on the two halves of the walkway. In retrospect this is all very logical, I just never thought of it. I had to adjust a lot of walkways and LODs...

..the strange thing is that I get the same effect whether I mirror on X or Y axis while the woodplank pattern only runs along the x axis...

[RipleyVonD]

I thought this was a bug that was fixed a while back. Its possible to use Normal maps on mirrored UV's but it depends on if a 3d rendering engine supports it or not.

[Christhiana]

I used the same texture as in my other thread about baking normal mapos only this time it was on a straight walkway instead of a rounded corner. I mirrored one half of the walkway on the UV map on the x and Y axis and overlapped the UV's to save texture space. I understand why it would work the opposite way if i only mirrored on the x axis (the walkway runs along the Y axis) but not mirroring on the x axis, only on the Y also produced the same result. If that's the bug you're reffering to, I guess it's not fixed...

[RipleyVonD]

It seems like thats the bug, it can be confusing without seeing this kind of glitch with an image. But basicaly it happens when building one half of a mesh model, copy and mirror it to make the opposite symetrical half and use the same normal map.