def _off_faces(faces):
    """
    Return a string representing the faces in OFF format.

    Parameters
    ----------
    faces: numpy array of integers
        A 2D array containing 3 vertex indices per face. Dimension is (m, 3) for m faces.

    Returns
    -------
    string
        The OFF format string for the faces.
    """
    if faces.shape[0] == 0:
        return ''
    face_reps = ["3 %d %d %d" % (f[0], f[1], f[2]) for f in faces]         # vertex count and the 3 vertex indices defining the face, starting at 0
    return '\n'.join(face_reps) + '\n'