Surface Foundry / FIELD NOTE
Recognise the symptom
You import a textured object, but dents seem to rise outward and a highlight moves in an unexpected direction. Before turning up normal strength, make a simple test: one plane, one material and a light that can move around it. Keep the original maps intact.
Tangent-space normal maps describe a direction using image channels. Common OpenGL and DirectX conventions differ in the sign of the Y component, often described as the green-channel convention. Select the map matching your application's expected convention. When both NormalGL and NormalDX are supplied, they are alternatives; do not stack them.
Check the basic setup
The normal texture must be read as data and use the same UV layout as the surface. Route it through the appropriate normal-map node. The Blender normal-map documentation covers non-colour input and matching UV coordinates.
Move the light from left to right. Compare the shading with a visible groove in the colour map. If one convention behaves sensibly and the other inverts the relief, the comparison gives you a useful diagnosis. If both look wrong, inspect UV seams, mirrored islands and the mesh's shading before altering the image.
Change one thing
If your tool has an explicit green-channel inversion setting, use it only when the convention requires it. Inverting every channel is a different operation. Preserve an untouched map and label any converted copy with the target convention.
Oxidised metal is a useful test surface because its reflections make directional errors easier to inspect. Start with a moderate normal strength, then compare a close crop with the normal map disabled. Remember that a normal map changes the light response rather than the silhouette. Use geometry or an appropriate displacement workflow when the edge itself needs relief.