Assumption: if I have a tangent space normal map that’s neutral (i.e. all RGBs = 127, 127, 255), it will not matter if UV direction is flipped or mirrored.
The character in the above turntable animation has a visible seam across the head.
The artist split the UVs right across the head:
So the UVs run in different X directions. It shouldn’t matter (and in UE4, Unity 5, Godot 3.1 it doesn’t), but Blender Cycles produces this very visible lighting/gloss difference.
That is going on here? Does Cycles’ “Normal Map” node immediately convert to world/object space?