Propsman pointed this one out to me yesterday: apparently Blender tangent-space normal maps run from a value of Z=-1 (no blue) to Z=1 (100% blue). This is not how X-Plane normal maps work; our normals go from Z=0 (no blue) to Z=1 (100% blue).
This difference is easy to miss because X-Plane has to renormalize the normal map as the last step of processing the normal map. This turns a big artifact into a small one. The general effect of using the Blender convention rather than X-Plane’s is that your normal map will look ‘less bumpy’ for fairly extreme amounts of bump.
To fix this, simply remap the colors of your blue channel in PhotoShop or some other image editing program. Basically you’ll want to set what was 50% blue to 0% blue, and keep 100% blue the same. This will extend the lighter half of the blue channel over the entire blue channel.
If you have any blue less than 50% in the image, um, that’s a normal that points backward, and X-Plane doesn’t support that.
Interesting. Thanks for the heads up. I however, never noticed this problem till you wrote this. Normally i would try one normal map, and test it in sim. If it didn't look right, i will try another setting and make a new one. Once i get the right look i save the settings to make the other maps i need.
Now that you have made this aware to me i may have to test a few more options to see what happens.