Two conventions differ in one component
Tangent-space conventions commonly called OpenGL and DirectX disagree about the vertical normal component. The flip switch changes the green channel direction while preserving red and blue. The correct choice depends on the engine and import settings, not the computer operating system.
Use a recognizable test shape
A simple raised circle or bevel makes inversion easier to see than a noisy photograph. Rotate the light vertically and check whether the expected high side receives light. If the result is inverted only along that direction, switch the green channel and repeat the test.
Avoid accidental color transforms
A normal map encodes vectors rather than display colors. Gamma or color-management transforms can bend those vectors and alter shading. Use the engine’s normal-map import mode or a non-color/data texture setting. Preserve the map at a reasonable resolution and avoid repeated JPEG compression.
Compare two exports without changing other settings
Generate a normal map with the green-channel switch off, then generate another with it on using the identical source and strength. Keep resolution, material, UV coordinates and light position unchanged. The second export reverses the vertical normal component; it does not flip the height image or mirror the object.
Use an asymmetrical height test, such as a ramp that rises from the lower edge to the upper edge. A rotationally symmetric noisy texture can hide the difference. If neither version behaves correctly, inspect the material connection and texture interpretation before repeatedly modifying the source image.
Do not apply the correction twice
Some importers already expose their own green-channel inversion option. Flipping during generation and again during import cancels the change. Record which application performs the conversion, and leave the other switch unchanged. Also check the object’s UV orientation and tangent basis when seams remain visible: a green-channel choice cannot repair mirrored UV mistakes, missing tangents or a normal map connected as an ordinary color texture.