SOLVED: The output of a Normal Map brick is NOT a tangent-space normal (which is what I thought). It’s the normal in camera-space after it’s been applied to the object. And by the way, camera space uses a left-handed axis system with the camera’s line of sight being the positive Z axis, so normals for ‘forward’-facing faces will have negative values.
I’ve run into a curious problem while trying to combine normal maps with DS4.5 Shader Mixer. I’ve managed to reduce the problem to something (hopefully) quite straightforward. I’ll start with the bit that works fine.
The four brick network shown here simply takes a normal map, splits it into it’s components and recombines them.
I’d expect this to work, and it does.