For easy computations assume octagonal cells. Their vertices will have 135 degrees each. The edges of the portal will be visible because the left and right imag

For easy computations assume octagonal cells. Their vertices will have 135 degrees each. The edges of the portal will be visible because the left and right images do not match (since 3*135 ≠ 360). And a object (as long as it is modelled as an Euclidean 3D object, not a point) will not be able to go through such a vertex.