def whctrs(anchor):
    """
    Return width, height, x center, and y center for an anchor (window).
    """
    w = anchor[:, 2] - anchor[:, 0] + 1
    h = anchor[:, 3] - anchor[:, 1] + 1
    x_ctr = anchor[:, 0] + 0.5 * (w - 1)
    y_ctr = anchor[:, 1] + 0.5 * (h - 1)
    return w, h, x_ctr, y_ctr