I don’t think that was the case. This is what I did. I started with this polysurface and trimmed it.
Which gave me this polysurface.
As you can see, I have given the “inside” of the model a different color. I do this for all my models, because that automatically shows me the direction of the normals. So I assume those were alright.
I then did Mirror twice, and Joined, which gave me that weird model in the first two images and the first video.
That worked! … But in a kind of weird way. When I explode this entire model:
I get this:
As you can see the normals flipped, and that weird indentation in the center of the model is no longer there. When I then do Join, everything seems fine again as I get this:
However, when I did Explode after the two Mirror, (but before joining the three parts), and then Join, it worked also and the model looked the way it was supposed to. Without the normals being flipped.
This makes no sense to me, since the only boolean I did was to connect the semi torus to the other part, and the normals all seemed to be pointing in the correct direction after that boolean union.
I don’t understand this. Why would this yield a different result if the “Direction” command on the complete model already shows that the normals are all pointing in the correct direction?
So I now know how to fix this when something like this happens again, and I would still like to have an explanation as to why this happened in the first place.