In Edit mode, how do I select the mirror of my current selection?

Question

I've got a mesh that I plan on rigging for animation. It's a complete figure, and it has perfect symmetry. While creating vertex groups, in Edit mode, I am going through the painstaking process of selecting every face that should belong to that group, for instance, upper_leg_R. When I want to select all the faces to establish upper_leg_L, I should be able to just select the opposite of upper_leg_R, but I can't figure out how to do that. When I select "Select Mirror", I get a yellow error message at the bottom that reads "0 faces mirrored, 63 failed".

I don't want to modify the mesh at all; i.e., I don't want to add or delete any part of it. I just want to select the symmetrical opposite of my current selection. How do I do this?

Answer 1

You can delete the half of your symmetric figure analog with the select box or with the booleon modifier if you have very small meshes. Do not delete the vertexs in the middle. Use a mirror modifier. When you apply the modifier then you have again your symmetric figure. All vertex Groups of the "not deleted" side include now the vertex of the other side. So all mesh.L-Groups are deletet but the mesh.R-Groups include all mesh.L-vertexes.

If there existe different Groups on the left and the right side and you want to keep both of them, you have to do it twice. Just copy the figure and hide one of them. On one figure you delete the left side and on the hidden figure you delete the right side. Make both visible and merge them by distance.

If you do not have a clear middle-line to merge (like you would have with the mirror) Apply a cube and cut it in the middle. Fill the hole. Add a Boolean Modifier (different) to the figure and use the cube as an object. After applying you can delete the cube and now you have a straight middle-line which you can use for merging.