Need to change the topology while weight painting without breaking symmetry

Question

Here is the situation: I am weight painting on a symmetric mesh, and have put a lot of time into part of the rig. I discovered that there is bad topology in a certain section of the mesh that needs to be fixed by deleting vertices. However, if I delete any vertices, the vertex group symmetry option (Vertex Group X under the Tool options) completely breaks, and weight on the opposite side of the model will spray across random locations when I weight paint. This happens even if I edit the left and right sides of the mesh in exactly the same way. Symmetrizing the mesh with a function like Mesh > Symmetry would ruin the UVs.

What is there to do if you need to edit your mesh without breaking the symmetry of your weight painting?

Answer 1

If you're using weight painting symmetry, then your model is symmetrical, and presumably your bones are properly named with .l / .r, so you can use a mirror modifier instead (and thus, saving you some trouble when redoing some topo anyways.)

Delete half of your model. (Select midline vert, "Select side of active" operation, ctrl i inverse selection, x delete -> vertices.) Give it a mirror modifier. Move your mirror modifier above your armature modifier. Edit to your heart's content. Apply the mirror modifier when you're done if you want (or, leave it live.)

Alternately: paint the original mesh, and weight the edited version with a data transfer modifier targeting the original, on "nearest face interpolated" mode.