def METHOD_NAME(self, bitmap, _box):
    h, w = bitmap.shape[:2]
    box = _box.copy()
    xmin = np.clip(np.floor(box[:, 0].min()).astype(np.int32), 0, w - 1)
    xmax = np.clip(np.ceil(box[:, 0].max()).astype(np.int32), 0, w - 1)
    ymin = np.clip(np.floor(box[:, 1].min()).astype(np.int32), 0, h - 1)
    ymax = np.clip(np.ceil(box[:, 1].max()).astype(np.int32), 0, h - 1)
    mask = np.zeros((ymax - ymin + 1, xmax - xmin + 1), dtype=np.uint8)
    box[:, 0] = box[:, 0] - xmin
    box[:, 1] = box[:, 1] - ymin
    cv2.fillPoly(mask, box.reshape(1, -1, 2).astype(np.int32), 1)
    return cv2.mean(bitmap[ymin:ymax + 1, xmin:xmax + 1], mask)[0]