def get_passive_el(passive_coord, centroids):
    """ Gets index of passive elements .
    
    Args:
        passive_coord (:obj:`tuple`): Region that the shape will not be changed.
        centroids (:obj:`numpy.array`): Coordinate (x,y) of the centroid of each element.

    Returns:
        Index of passive elements.
    """
    mask = (centroids[:, 0] >= passive_coord[0][0]) & (centroids[:, 0] <= passive_coord[0][1]) & (centroids[:, 1] >= passive_coord[1][0]) & (centroids[:, 1] <= passive_coord[1][1])
    return (mask > 0).nonzero()[0]