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?

  • $\begingroup$ What happen if you import the .fbx into blender? Do you get the same results when shading? Because I tried to replicate your issue and it worked as expected. $\endgroup$ – Arthur Blaquart Oct 9 '19 at 15:38
  • $\begingroup$ It seems as though if I import the FBX back into Blender it looks okay. I wonder what Blender is doing that other programs aren't doing. $\endgroup$ – Dan Oct 9 '19 at 15:49
  • $\begingroup$ A bit more on this: It seems this is probably an issue with whether a given program uses OpenGL normals, or Direct3D normals. When bringing an FBX into Substance Painter, for example, it looks like there's an option to use OpenGL normals. If I choose that, then things look correct. So, maybe it's as simple as just using OpenGL normals. $\endgroup$ – Dan Oct 9 '19 at 16:48
  • $\begingroup$ maybe it's a stupid question, but why don't you apply the modifier? $\endgroup$ – Tareyes Oct 9 '19 at 20:41
  • $\begingroup$ It's preferable to leave modifiers unapplied, to make future edits easier. If you apply a mirror modifier, then want to make more changes, you need to make them manually on both (or all four, or all six) parts of your model. Applying modifiers is "destructive", making future work more difficult, so if at all possible it's best to leave modifiers unapplied. $\endgroup$ – Dan Oct 10 '19 at 15:33

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