Mirror modifier results in incorrect normals

by Dan   Last Updated October 09, 2019 16:15 PM

I'm trying to do something simple: Export an FBX of a model that uses the Mirror Modifier. However, no matter what I do, I find that the resulting FBX's normals seemed messed up on the mirrored portion of the mesh.

To show a simple example of this, I've taken a cylinder and divided it so that I just have 1/4 of the cylinder, then I've added a Mirror Modifer on X and Y. I've then unwrapped the mesh. This results in the following:

enter image description here

I export the FBX, applying modifiers on export. But now if I bring this FBX into another program, like Substance Painter, or Unity, the normals on the mirrorred surfaces appear reversed, or otherwise messed up. Here you can see the top of the cylinder with a glossy material applied. The seams are clearly visible, and the four different quarters of the top of the object reflect light at slightly different angles from one another:

enter image description here

So I'm wondering what I'm doing wrong. I've tried mirroring the UVs in the Mirror Modifier, but this doesn't appear to affect the normals at all. Is it, in fact, possible to get correct normals on an FBX when exporting a model containing a mirror modifier?

Tags : normals mirror

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