def rect_areas(rects):
    """Return an np.array of the areas of the rectangles

        :param rects: a list of rectangles, each specified as (x, y, w, h)
        :type rects: list
        :return: an numpy array of corresponding areas
        :rtype: :class:`numpy.ndarray`
        """
    w = rects[:, (2)]
    h = rects[:, (3)]
    return w * h
