How to detect if a mesh has split UV island instead of single island, using Python?

Question

So, Lets understand the problem or the issue I am trying to resolve. Lets say I have two mesh in the viewport as in the image. First mesh is a square with 2 loop cuts or 3 faces. And second is square with 4 faces. So I unwrap, and get UV of 1st as a perfect square and second one with 2 islands which is 1 face and other is 3 faced island. But lets say I somehow misplaced one face from 1st mesh UV island and it got shifted like in the image.: Is there a way I can detect that my mesh has that UV face splitted or have 2 island instead of one? But second UV mesh is correct so it doesn't get selected with the script because it has seams to mark it having split faces. As it can be bit troublesome if the no. of UV islands get high and gets time consuming. The result I am kind of wanting to get is like this , so it selects the UV islands which shouldn't have been splitted.Thank you.

Answer 1

You can use a loop's link_loop_radial_next to get the equivalent loop on the neighboring face. Assuming manifold geometry - each edge builds exactly 2 faces - or at least assuming no edge builds more than 2 faces, if there's another face, and so another radial loop - it goes in the opposite direction, and so matching this loop's start with other loop's end, and this loop's end with other loop's start will reveal if the UVs were moved:

import bpy, bmesh
from bpy import context as C
tolerance = 0
bpy.ops.mesh.select_all(action='DESELECT')
me = C.object.data
bm = bmesh.from_edit_mesh(me)  # assuming Edit Mode
uvl = bm.loops.layers.uv['UVMap']
for face in bm.faces:
    for loop in face.loops:
        neighbour_loop = loop.link_loop_radial_next
        if loop is neighbour_loop or loop.edge.seam:
            # edge.is_boundary = True, there's no neighboring face
            continue
        loop1_start_uv = loop[uvl].uv
        loop1_end_uv = loop.link_loop_next[uvl].uv
        loop2_start_uv = neighbour_loop[uvl].uv
        loop2_end_uv = neighbour_loop.link_loop_next[uvl].uv
        vert1_uv_distance = (loop1_start_uv - loop2_end_uv).length
        vert2_uv_distance = (loop1_end_uv - loop2_start_uv).length
        if  vert1_uv_distance > tolerance or vert2_uv_distance > tolerance:
            # edge is between connected faces, but is too far away in UV
            loop.edge.select = True

The result of running the script on Susanne:

Edit: added the or loop.edge.seam check and a seam on X=0 (keep in mind that's not the seam originally used to unwrap Susanne):