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I want to mirror object using it's face as a mirroring plane. For the example, I want to use the selected face on the cube as a mirror plane.

enter image description here

I created a custom transform orientation using that face, but is not the one which gets used by mirror modifier. I also positioned a 3D cursor at that face and then transformed origin to it, but then rotation is wrong (from local axis). I also see I can set mirror object, but my face is not an object. I added an empty, but it's orientation is not aligned to the face either.

I am out of ideas now. I think best way is to set local axis to that custom transform. I will be happy to do it anyway, because local axis are completely off and the object is hard to work with.

EDIT:

Result using the method suggested by MrZak and lemon:

enter image description here

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    $\begingroup$ You can separate that face into separate object and use that in the Mirror mod. You can align empty with the normal of that face using snapping and use empty with modifier. Also you could provide screenshot of the problem for clarifying. $\endgroup$
    – Mr Zak
    Commented Sep 22, 2016 at 13:42
  • $\begingroup$ @MrZak Thanks! I have a feeling it is now the end of it, I need to somehow attach that empty to the face so that it doesn't get lost when I rotate my object, am I right? I also added a picture, but it is not very exciting. $\endgroup$
    – Noidea
    Commented Sep 22, 2016 at 14:12
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    $\begingroup$ In complement to @MrZak answer, yes, you can parent the empty to your mesh so that it stays "stuck" to the wanted face. But 2 things : this face is not necessarily plane (that can be 2 tris) and the empty wont follow if you change the mesh (the face) in edit mode $\endgroup$
    – lemon
    Commented Sep 22, 2016 at 14:30
  • $\begingroup$ @lemon, thanks, this is important information. I added empty, in the snap tool I chose "Align rotation with the snapping target", which worked, but location doesn't snap. I chose "Face/Closest" in the settings. $\endgroup$
    – Noidea
    Commented Sep 22, 2016 at 14:34
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    $\begingroup$ @lemon ok, no, it works:) just the snapping circle is not displayed and the empty center has to stay on the surface in the viewport. $\endgroup$
    – Noidea
    Commented Sep 22, 2016 at 15:48

1 Answer 1

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I found a workflow for this task in Blender 2.79 that does not involve any snapping or creating empties:

  1. Make sure your "Transformation Orientation" is set to "Normal" (usually on the toolbar just below the 3D view, next to the translate/rotate/scale buttons). Also make sure your "Pivot Center" (the selector just to the left of these same buttons) is set to "Active Element."
  2. In 3D view object mode, select the object you want to mirror.
  3. Assuming you want to end up with the original and the mirrored copy, as shown in the question's screenshots, duplicate the object (e.g., Shift-D then return). You can skip this step to just flip the object through a face, not leaving behind the original.
  4. With the duplicated object still selected, enter Mesh Mode (Tab).
  5. Select the face through which you want to mirror.
  6. Now we want to select that whole mesh, leaving that face as the active face. Hitting "A" twice should accomplish this.
  7. Now it is time to mirror. We need to mirror in the local z-axis, set by the normal orientation of the active face. What works for me is to hit "Control-M" for mirror, followed by Space (don't know why that's needed). Then in the panel to the left, a bunch of Mirror options come up, and I click on Z.
  8. Note that the newly mirrored copy will have its normals pointing in the opposite directions of the original. If you want them pointing in the same directions, now is a good point at which to flip them. (For projects in which I am doing a lot of reflecting through faces, I bind the "Flip Normals" Mesh action to a key for this.)

That's it, you should now have your mesh reflected through the selected face; you can Tab back to object mode and repeat the process if you want to reflect again through another face.

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  • $\begingroup$ Just had occasion to use this process again in Blender 3.4.1. Essentially the same workflow still works well, although I found for step 6 that simply executing "Select All" (once) left the face selected in step 5 as the active face; and that executing "Interactive Mirror", hitting "Z", and then the Enter key worked well for step 7. $\endgroup$
    – Glen Whitney
    Commented Jan 20, 2023 at 16:18

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