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I have an existing mesh model with bones, UVs, animation etc. and I want to change the mesh slightly without causing too much destructive change.

enter image description here

The above shows the head with the -x and x direction labeled. The edge pointed by the green arrow is done by a knife cut because I want to cut new geometry to fill in a face tattoo. I also turned on "Enable X symmetry in the X direction" at the top right corner.

When I use the knife cut on the -X side, I expect the cut to mirror on the X side, but it didn't. Then I selected the new vertices and go to Mesh > Symmetrize, the new vertices is mirror to the -X side. However, they are on top of the existing geometry. In below image, on the -X side, I move the vertex at the side of the -x cheek to show the vertex is actually separate from the cheek's edge.

Question: How do I properly do the knife cut that can mirror -x/x? I will be happy is there is a way to merge the floating vertex to the existing edge on the -x side, while maintaining symmetry.

I am using Blender 4.0

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The knife cut won't be symmetrized automatically with the X symmetry option on, this tool will only move or remove the vertices that are moved or removed on the opposite side. What you can do though is select the new edge and Mesh > Symmetrize and it will be mirrored:

enter image description here

Also, the symmetry will work as long as you keep the other side symmetrical, if you begin to change it unsymmetrically, you are breaking the symmetrey and you won't be able to do any symmetrical operation:

enter image description here

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  • $\begingroup$ That's what I did "Mesh > Symmetrize and it will be mirrored", however, as seen from my screenshot, although it generate the new vertex at the correct X location, however, it is separate from the existing edge. I want it to be along the edge, same as the -X side where I cut the edge. Any advice how can it be done? Thanks. $\endgroup$
    – Jake
    Commented Apr 2 at 14:57
  • $\begingroup$ It's because your mesh is not really symmetrize, you could symmetrize the whole thing but you said that it would be too destructive $\endgroup$
    – moonboots
    Commented Apr 2 at 17:16
  • $\begingroup$ Thanks for your confirmation that Mesh > Symmetrize will not work. I think I don't full understand what is considered symmetry by Blender then. Anyway, I gave in and used a mirror modifier on the entire mesh. The surprising good news is that the vertex weights and groups are not destroyed. I not exactly sure why rig, weights, groups are preserved because I definitely cut the mesh into half using Auto Mirror addon tool. Good for me. $\endgroup$
    – Jake
    Commented Apr 2 at 22:05
  • $\begingroup$ The symmetry will work as long as you keep the other side symmetrical, if you begin to change it unsymmetrically, you are breaking the symmetrey and you won't be able to do any symmetrical operation. As for the Mirror modifier, it won't destroy your material if it was made of an image texture for example and if your UVs were already overlapping $\endgroup$
    – moonboots
    Commented Apr 3 at 5:53

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