def bbox_pred(boxes, box_deltas):
    """
    Transform the set of class-agnostic boxes into class-specific boxes
    by applying the predicted offsets (box_deltas)
    :param boxes: !important [N 4]
    :param box_deltas: [N, 4 * num_classes]
    :return: [N 4 * num_classes]
    """
    if boxes.shape[0] == 0:
        return np.zeros((0, box_deltas.shape[1]))

    # eltwise
    boxes = boxes.astype(np.float32, copy=False)
    widths = boxes[:, 2] - boxes[:, 0]
    heights = boxes[:, 3] - boxes[:, 1]
    ctr_x = boxes[:, 0] + 0.5 * widths
    ctr_y = boxes[:, 1] + 0.5 * heights

    # slice
    dx = box_deltas[:, 0::4]
    dy = box_deltas[:, 1::4]
    dw = box_deltas[:, 2::4]
    dh = box_deltas[:, 3::4]

    # eltwise
    pred_ctr_x = dx * widths[:, None] + ctr_x[:, None]
    pred_ctr_y = dy * heights[:, None] + ctr_y[:, None]
    pred_w = np.exp(dw) * widths[:, None]
    pred_h = np.exp(dh) * heights[:, None]

    x1 = pred_ctr_x - 0.5 * pred_w
    y1 = pred_ctr_y - 0.5 * pred_h
    x2 = pred_ctr_x + 0.5 * pred_w
    y2 = pred_ctr_y + 0.5 * pred_h

    pred_boxes = np.hstack((x1, y1, x2, y2))

    return pred_boxes

    Transform the set of class-agnostic boxes into class-specific boxes
    by applying the predicted offsets (box_deltas)
    :param boxes: !important [N 4]
    :param box_deltas: [N, 4 * num_classes]
    :return: [N 4 * num_classes]
    
tests/unittest/ops/test_proposal.py