one face's normal refuses to unify direction with all other face normals in same mesh

I’ve got a mesh with a few standalone disconnected faces floating freely. If I use the Make Normals Consistent command on the entire mesh (either Ctrl+N or Ctrl+Shift+N), the normal direction of one of the faces always wants to point in the opposite direction of the other faces.

If I select just that problematic face and use that same command on it to flip its normal, and then afterwards select all faces in the mesh and use the same command, the problematic face goes back to facing the wrong direction again.

Other things I’ve tried:

  • The Flip Normals command, on just the problematic face
  • The Set Normals From Faces command, on just the problematic face
  • Bridging the problematic face to one of the “good” faces. This works, until I remove the temporary bridging face.
  • Rotating the face so it faces the right way.
  • Scaling the face by -1 on one of the axes.

Basically, I can tell just the problematic face to have its normal pointing in the desired direction, but once I use Make Normals Consistent on the entire mesh, the problematic face goes back to misbehaving.

What gives? If the above isn’t clear, imagine the center face in this diagram always wants to point in the opposite direction of the other two faces when using Make Normals Consistent, no matter what I do:

I have had this issue before. That command is not perfect and sometimes just fails. The only thing I have got to work is go in and manually flip the offending faces with the W menu.

Not sure what you’re expecting that to do with disconnected faces. Make Normals Consistent does just that in a mesh, meaning that the normals of the faces that are connected are set to either point outside or inside of the mesh.

I can understand that logic, and I guess I can treat disconnected faces as special cases from now on and not worry about Make Normals Consistent acting weird around them.

At the same time, I’m curious about the logic behind the operation of it on these faces in particular. I don’t think it’s just random chaos: there’s something about that one face that makes Blender insist it’s facing one way, and there’s something about all the other faces that makes Blender insist they’re facing the other way. I wonder if that property (whatever it is) is user-accessible. That’s all. :confused:

There’s no inside or outside with the shown geometry. Blender checks connected geometry from three directions to determine the sides when making normals consistent, that’s how it’s able to often get it right when the geometry is non-manifold but has a curvature. A plane is non-manifold and has no curvature, so it has no information on which side is which. The direction is decided and it’s technically correct whichever way it faces, only you know which direction is suitable for you.