Hi,
I dont know how to be precise in UV Editor with a texture mirror by a mirror modifier.
‘modified’ is check in the view menu (green)
How can I found the right ‘V Offset’ number (green : -0.4227) to have the exact opposite ?
Ouch, nobody knows ? 
On your UV editor, look on sidebar/view/display. Uncheck “pixel coordinates” (which are kinda useless for most purposes.) Now, select one of your bottom verts in the UV editor and look on sidebar/image/UV Vertex to see its position. I’m showing a V coordinate of 0.541. The default mirroring position is 0.5 (the center of the UV space). So select all UV coords and g y -0.41 to move the structure down to the midline of the UV space. After that, your mirrored UV will “connect”.
Alternately, you can set the mirror modifiers V Offset to 0.082 (0.541 - 0.5, times 2). Why times 2? Has to do with the transformation used by the mirror mod offset, which is designed to mirror about the upper margin at an offset of 1, or the bottom margin at an offset of -1. It is kind of confusing.
Thanx for youtr help.
In fact i dont understand where is my ‘activate’ UV vertex in the uvmaping.
It is Ok with only one UV vertex selected, but how can i move a group of UVvertex to 0.5 Y ?
(grr why gif animation doesn’t work ;(
Lots of ways. Place cursor (from sidebar), snap selection to cursor with offset in active element mode. Scale 0 to cursor in Y (with a single line selected). G y numeric (you can do the math in your head, or enter simple expressions, Blender can do the math for you.).
Thanx for your reply.
It is Ok for me to know where is the active vertex with 1 vertex or a align line of vertex, but where is the active vertex with a large selection of vertex ? It seems to be the ‘center of the geometry’…
If you box select, there isn’t necessarily an active vertex. You can nominate an active vertex by shift selecting an individual vertex. The active vertex has a different color.
But I guess UV editing can’t do “active element” pivot right now, so never mind on that technique.
