def offset_triangulation_vertices(triangles, offset):
    """
    Offset the index of each vertex in a triangulation. The triangles list is modified in-place.

    :param triangles: n by 3 list of indexed triangles which will be edited.
    :param offset: Offset for each vertex index.
    """
    for triangle in triangles:
        for j in range(3):
            triangle[j] += offset
    return triangles