def convert_voxels_original(voxels, nz_limit):
    """
    Convert the voxels gained after cropped
    to voxels before crooped
    (i.e. position at cropped image -> position at original image).
    """
    dim = voxels.shape[1]
    new_voxels = voxels.copy()
    for axis in range(dim):
        idx_min = nz_limit[axis][0]
        new_voxels[:, axis] += idx_min
    return new_voxels