def similarity_align(v_src, v_target, v_transform=None):
    """find a similarity transformation that best maps v_src to vs_target
    (points are in correspondence by index).

    Returns transformed v_transform if given, else transformed v_src
    """
    xform, scale = cv2.estimateAffine3D(v_src, v_target, force_rotation=True)
    src = v_src if v_transform is None else v_transform
    aligned = src.dot(xform[:, :3].T) * scale + xform[:, 3]
    return aligned