def compare_xforms(lta_list, norm_threshold=15):
    """
    Computes a normalized displacement between two affine transforms as the
    maximum overall displacement of the midpoints of the faces of a cube, when
    each transform is applied to the cube.
    This combines displacement resulting from scaling, translation and rotation.

    Although the norm is in mm, in a scaling context, it is not necessarily
    equivalent to that distance in translation.

    We choose a default threshold of 15mm as a rough heuristic.
    Normalized displacement above 20mm showed clear signs of distortion, while
    "good" BBR refinements were frequently below 10mm displaced from the rigid
    transform.
    The 10-20mm range was more ambiguous, and 15mm chosen as a compromise.
    This is open to revisiting in either direction.

    See discussion in
    `GitHub issue #681`_ <https://github.com/poldracklab/fmriprep/issues/681>`_
    and the `underlying implementation
    <https://github.com/nipy/nipype/blob/56b7c81eedeeae884ba47c80096a5f66bd9f8116/nipype/algorithms/rapidart.py#L108-L159>`_.

    Parameters
    ----------

      lta_list : :obj:`list` or :obj:`tuple` of :obj:`str`
          the two given affines in LTA format
      norm_threshold : :obj:`float`
          the upper bound limit to the normalized displacement caused by the
          second transform relative to the first (default: `15`)

    """
    from niworkflows.interfaces.surf import load_transform
    from nipype.algorithms.rapidart import _calc_norm_affine
    bbr_affine = load_transform(lta_list[0])
    fallback_affine = load_transform(lta_list[1])
    norm, _ = _calc_norm_affine([fallback_affine, bbr_affine],
        use_differences=True)
    return norm[1] > norm_threshold
