Hey everyone,
I am currently trying to bring to life a character that I am modeling. I already rigged it up and just started weight-painting it. To save some time, I enabled “X Mirror” in the weight painting options but it won’t work 
When I paint the character’s shoulder, the other one does not paint simultaneously. Attached is the file if you want to get a closer look. If you need additional informations just ask me 
Thanks
Character.blend (1.54 MB)
The issue there is that the mesh symmetrical center is slightly off in comparison to its origin :
See your mesh origin is exactly at the center , now check the Z axis (the blue line) that should then move through your mesh symmetrical axis :
And if i look at your mesh symmetrical axis, see if you zoom in enough :
The blue line isn’t going with the selected loop that is the mesh symmetrical axis, the mesh is then off its own origin location, this is what is puzzling the weight painting “x-mirror” option.
To fix , do the following :
In Pose mode, press A to select all your bones and press ALT+R , ALT+G and ALT+S to reset rotation/location/scale of those bones.
Go back to Object Mode, select only the model and go to Edit Mode, hold ALT and right click to select that central loop
Press SHIFT and S then Select “Cursor to Selected” to move the 3D Cursor into that selection
Go to Object mode, and in the Tool tab, click on the Origin button and select “Origin to 3D Cursor” to move the Origin to … the 3D Cursor location
Press N to open the properties panel , scroll down to the “3D Cursor tab” and look at the X value ( that is at -0.0152 )
Change that X value to 0
Now press A to select -both- your model and the armature and press SHIFT and S then “Selection to Cursor (Offset)” to move both model and armature to the new position of the 3D cursor, it will fix the slightly off center the model+armature were , that was the source of the x-mirror failing in weight paint mode
Press SHIFT+S then “3D Cursor to Center” to put the 3D Cursor back to the grid center where it was
Select your model (not the bones) and in the Tool tab similarly to before, press the Origin button and “Origin to 3D Cursor” to move it back where it was.
That’s all, now the x-mirror option of the weight painting will work.
Wow, I would never have been able to figure it all out by myself! Thank you very much!
