OpenGL vs DirectX Normal Maps: Check the Green Channel
For the tangent-space maps exported by Texel, OpenGL and DirectX describe opposite signs for the Y component in the green channel. Switching between them changes that direction. It does not add detail, change resolution or convert the map into geometry.
Understand what the format switch changes
Texel labels the options OpenGL (+Y) and DirectX (−Y). The same height estimate produces both versions; the green channel is inverted. Red and blue retain their values. This is different from flipping the image vertically, which also moves features to different positions.
Choose the convention your shader and import settings expect, not the operating system you happen to use. An importer can flip a channel after the file is loaded. Consequently, a filename or an engine name alone cannot tell you which version will render correctly in a particular project.
Check the destination’s interpretation
Blender’s current Normal Map node documentation identifies OpenGL as its default convention and provides a DirectX convention option. For tangent-space input, use a Normal Map node, a matching UV map and a Non-Color image. Older Blender versions may present different controls, so check the documentation for the installed release.
In Unity, import an existing RGB normal texture with Texture Type set to Normal map. The importer’s Flip Green Channel option reverses Y. Create from Grayscale is a separate conversion path; do not enable it simply because you are importing an already generated normal map.
Unreal’s Texture Asset Editor also exposes Flip Green Channel. Inspect the asset’s current settings before changing the source file. Use the material’s normal input and the appropriate texture interpretation rather than connecting this image as surface color.
Use a small, directional test
Pick a simple asymmetric feature with an obvious intended slope. A noisy stone image is harder to diagnose because many highlights can look plausible. View the material under one movable light and keep camera, strength and lighting fixed while comparing the two conventions.
Look for whether the feature responds to light from the expected direction, especially along the texture’s vertical axis. A green-channel mismatch reverses that axis; it does not reverse every surface direction. If both axes seem wrong, investigate the source height, UV orientation and other material settings too.
Apply the correction in one place
Either export the convention you need or let the importer perform the green-channel correction. Doing both cancels the intended change. Keep the unmodified export until the material is confirmed, and write down which correction is active.
After finding the right orientation, adjust strength for the intended viewing scale. More saturated colors or stronger shading do not prove a better map. Painted marks and photographic shadows may still create false relief because Texel estimates height from image brightness.
Record more than the file format
Save a note beside the PNG with its convention, source image, strength and any importer flip. A descriptive filename can help, but verify the actual settings when handing files to another artist. Recheck the material after a shader or import-preset change.
A normal map changes the response to lighting; it does not unwrap a mesh, bake a high-poly model or change the silhouette. Correct green-channel orientation solves one interpretation problem. It does not establish compatibility with every shader or guarantee that the material looks right in your scene.
Sources and further reading
Try the workflow with your own image: Create and compare normal maps →