How to stop freestyle from rendering overlapping vertices?

Question

I have this object that I created by mirroring one half of it (the mirror joint is highlighted):

When I render it normally there's no trace of the joint that's between mirrors:

But when I try to render it using Freestyle it renders the joint:

I know I could apply the modifier and then merge the vertices but I'd rather not do that because then I'd have to do it manually for all objects in the scene. I would also lose my ability to edit all of them at once.

How can I make the mirror joints appear seamless on a render?

Answer 1

You have interior faces there. Apart from the issue with Freestyle strokes, it is not a good idea because they make the mesh non-manifold and can lead to other problems as well.

Here is an object which has a mirror modifier. One part has an interior face at the mirror center. Below a comparison of the difference with and without interior face, this is the mesh:

And this is the Freestyle result:

But you can simply select the face in Edit Mode without applying the Mirror modifier, then hit X > Delete > Faces.

And this is the result of the Freestyle render - without applying the Mirror modifier: