Normals: Mirror Modifier vs. Mirror Object (by hand)

Hello there :eyebrowlift2:

actual situation is: i created a highpoly sculpt of a left shoe. I made a retopo-lowpoly, uv unwrapped it and baked the highpoly to the low poly. All fine so far. After being done with that i copied that shoe and mirrored it by hand (select object -> Object -> Mirror -> Global X). The result was a wrong shaded object (the baked normals did not work for the mirrored object) …

That way i came up with the old topic “Messed up normal-maps after mirroring geometry” …
I know there are plenty of threads with endless discussions about it and i don’t want to find the in-depth-reason behind it because i already found a solution to get that problem fixed (and that recognition will also lead to my question :smiley: … )

I can mirror the object by using the mirror-modifier. That creates a mirrored version of my left show with perfect lighting. But if i copy the object and mirror it by hand instead of using the mirror modifier (like i just mentioned above) i run into shading issues.

Now i wonder if anyone of you knows why it works using the mirror-modifier and why it does not work when i copy and mirror it by hand. I guess the mirror-modifier does some further things to fix tangent- or normal-information of the geometry but this is just assumption. Would be nice to know it in detail …

PS: For simplicity the image shows a box of respectively 6 faces with baked normals for soft edges. You can cleary see the differences between the mirrored object by the mirror-modifier and the one mirrored by hand …


Regards … Daniel