I'm investigating software that uses the unnormalized, interpolated vertex normal for creating its tangent space, creating a scaled tangent space that depends on concavity/convexity of the mesh. It both bakes and reads based around this assumption, so it creates unnormalized normal maps.
These normal maps would be incompatible with a rendering engine that used a normalized tangent space-- the angle from the normal changes with the length of the tangent space's Z basis. Likewise, normal maps built around an orthonormal tangent space would be incompatible with engines using scaled tangent spaces. This means that even if orthonormal space would be preferable, that engines might not use them just to meet a common standard.
How do commonly used engines handle this? Do they use orthonormal space or not? I have investigated Blender and xNormal, and neither use orthonormal tangent spaces, but I'm interested in how more popular commercial software handles their normal maps.
Any information about specific, big name software is appreciated-- names like Maya are basically industry standards all on their own.