Normal Map Bug - Might be Related to Multiple UV Channels

I have a Mesh With two UV channels, one for a tiling texture, the other for the alpha mask. When I apply the tiling normal map, a shading bug forms at the UV seam for the alpha mask UV channel.

No normal map:

With normal map:

As you can see the mesh is shaded perfectly smooth with no normal map applied. Not sure whats causing the issue.



link here:

I have a hard time finding the spot with the issue. Does it happen in the 3D editor too?

Here’s a better shot to help find the areas where you can see it:

Those ‘lines’ are definitely along the wireframe for those areas, but are not viable in any single map channel. It’s almost as if there’s a mix of Normal map and surface normals in those areas (possibly relating to the way 2 UV channels are being used)

That’s useful, thanks! I’ll pass that along to our 3D team.

Hello, what software did you use for texturing, is it Substance Painter by any chance?

If you do, could you try to re export the texture making sure you unchecked this option in substance

The coins normal map was baked in marmoset. Interestingly, I tried importing the finalized scene to substance painter and tested the normal map. It displays the same shading error. This error is not present in either marmoset or blender.

Ok thanks for the precisions.
Did you try to play with the different Tangent basis options described in this tutrial?

No luck in fixing the seam in substance painter. The bakes from various tangent bases look identical. Its baking to a flat single quad.

As a second test, I downloaded this texture from sketchfab and tested in substance painter with my model. It resulted in the same seam as with my coin normal map.

Figured it out! When exporting an FBX from blender, you want Tangent Space unchecked Unfortunately this also ends up breaking the normals on the dragon and hands.