def sample_rois(rois, fg_rois_per_image, rois_per_image, num_classes, cfg,
                labels=None, overlaps=None, bbox_targets=None, gt_boxes=None):
    """
    generate random sample of ROIs comprising foreground and background examples
    :param rois: all_rois [n, 4]; e2e: [n, 5] with batch_index
    :param fg_rois_per_image: foreground roi number
    :param rois_per_image: total roi number
    :param num_classes: number of classes
    :param labels: maybe precomputed
    :param overlaps: maybe precomputed (max_overlaps)
    :param bbox_targets: maybe precomputed
    :param gt_boxes: optional for e2e [n, 5] (x1, y1, x2, y2, cls)
    :return: (labels, rois, bbox_targets, bbox_weights)
    """
    if labels is None:
        overlaps = bbox_overlaps(rois[:, 1:].astype(np.float), gt_boxes[:, :4].astype(np.float))
        gt_assignment = overlaps.argmax(axis=1)
        overlaps = overlaps.max(axis=1)
        labels = gt_boxes[gt_assignment, 4]

    # foreground RoI with FG_THRESH overlap
    fg_indexes = np.where(overlaps >= cfg.TRAIN.FG_THRESH)[0]
    # guard against the case when an image has fewer than fg_rois_per_image foreground RoIs
    fg_rois_per_this_image = np.minimum(fg_rois_per_image, fg_indexes.size)
    # Sample foreground regions without replacement
    if len(fg_indexes) > fg_rois_per_this_image:
        fg_indexes = npr.choice(fg_indexes, size=fg_rois_per_this_image, replace=False)

    # Select background RoIs as those within [BG_THRESH_LO, BG_THRESH_HI)
    bg_indexes = np.where((overlaps < cfg.TRAIN.BG_THRESH_HI) & (overlaps >= cfg.TRAIN.BG_THRESH_LO))[0]
    # Compute number of background RoIs to take from this image (guarding against there being fewer than desired)
    bg_rois_per_this_image = rois_per_image - fg_rois_per_this_image
    bg_rois_per_this_image = np.minimum(bg_rois_per_this_image, bg_indexes.size)
    # Sample foreground regions without replacement
    if len(bg_indexes) > bg_rois_per_this_image:
        bg_indexes = npr.choice(bg_indexes, size=bg_rois_per_this_image, replace=False)

    # indexes selected
    keep_indexes = np.append(fg_indexes, bg_indexes)

    # pad more to ensure a fixed minibatch size
    while keep_indexes.shape[0] < rois_per_image:
        gap = np.minimum(len(rois), rois_per_image - keep_indexes.shape[0])
        gap_indexes = npr.choice(range(len(rois)), size=gap, replace=False)
        keep_indexes = np.append(keep_indexes, gap_indexes)

    # select labels
    labels = labels[keep_indexes]
    # set labels of bg_rois to be 0
    labels[fg_rois_per_this_image:] = 0
    rois = rois[keep_indexes]

    # load or compute bbox_target
    if bbox_targets is not None:
        bbox_target_data = bbox_targets[keep_indexes, :]
    else:
        targets = bbox_transform(rois[:, 1:], gt_boxes[gt_assignment[keep_indexes], :4])
        if cfg.TRAIN.BBOX_NORMALIZATION_PRECOMPUTED:
            targets = ((targets - np.array(cfg.TRAIN.BBOX_MEANS))
                       / np.array(cfg.TRAIN.BBOX_STDS))
        bbox_target_data = np.hstack((labels[:, np.newaxis], targets))

    bbox_targets, bbox_weights = \
        expand_bbox_regression_targets(bbox_target_data, num_classes, cfg)

    return rois, labels, bbox_targets, bbox_weights

    generate random sample of ROIs comprising foreground and background examples
    :param rois: all_rois [n, 4]; e2e: [n, 5] with batch_index
    :param fg_rois_per_image: foreground roi number
    :param rois_per_image: total roi number
    :param num_classes: number of classes
    :param labels: maybe precomputed
    :param overlaps: maybe precomputed (max_overlaps)
    :param bbox_targets: maybe precomputed
    :param gt_boxes: optional for e2e [n, 5] (x1, y1, x2, y2, cls)
    :return: (labels, rois, bbox_targets, bbox_weights)
    
faster_rcnn/core/rcnn.py