To invert a normal map in Blender, you primarily need to invert the green channel of the normal map using an RGB Curves node.
Here's how to do it:
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Add an RGB Curves Node: Place an RGB Curves node between your Normal Map node and the Normal Map input of your Principled BSDF shader (or whichever shader you are using). Connect the color output of your normal map to the input of the RGB Curves node and the output of the RGB Curves node to the normal map input on your shader.
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Select the Green Channel: In the RGB Curves node, select the green channel curve.
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Invert the Green Channel: Click and drag the top-right point of the green channel curve to the bottom-right corner, and the bottom-left point to the top-left corner. This effectively reverses the values of the green channel. You're creating a diagonal line with a negative slope.
Essentially, this process flips the direction of the normal map's green channel, which often corresponds to the "up" or "down" direction of the surface details. This can be useful for correcting normal map orientation issues.
Why this works: Normal maps store directional information about the surface. Inverting the green channel flips that directional information, which in turn changes how light interacts with the surface, giving the appearance of inverted normals. If the normals look incorrect after inverting only the green channel, experiment with inverting the red channel as well, following the same procedure as outlined for the green channel.
