code: # --------------------------------------------------------
  # Faster R-CNN
  # Copyright (c) 2015 Microsoft
  # Licensed under The MIT License [see LICENSE for details]
  # Written by Ross Girshick, Sean Bell and Xinlei Chen
  # --------------------------------------------------------
  from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  
  import numpy as np
  import numpy.random as npr
  from fcn.config import cfg
  from utils.bbox_transform import bbox_transform
  from utils.cython_bbox import bbox_overlaps
  
  def proposal_target_layer(rpn_rois, rpn_scores, gt_boxes, poses, _num_classes):
    """
    Assign object detection proposals to ground-truth targets. Produces proposal
    classification labels and bounding-box regression targets.
    """
  
    # Proposal ROIs (0, x1, y1, x2, y2) coming from RPN
    # (i.e., rpn.proposal_layer.ProposalLayer), or any other source
    all_rois = rpn_rois
    all_scores = rpn_scores
  
    # Include ground-truth boxes in the set of candidate rois
    if cfg.TRAIN.USE_GT:
      zeros = np.zeros((gt_boxes.shape[0], 1), dtype=gt_boxes.dtype)
      all_rois = np.vstack(
        (all_rois, np.hstack((zeros, gt_boxes[:, :-1])))
      )
      # not sure if it a wise appending, but anyway i am not using it
      all_scores = np.vstack((all_scores, zeros))
  
    num_images = 1
    rois_per_image = cfg.TRAIN.BATCH_SIZE / num_images
    fg_rois_per_image = np.round(cfg.TRAIN.FG_FRACTION * rois_per_image)
  
    # Sample rois with classification labels and bounding box regression
    # targets
    labels, rois, roi_scores, bbox_targets, bbox_inside_weights, poses_target, poses_weight = _sample_rois(
      all_rois, all_scores, gt_boxes, poses, fg_rois_per_image,
      rois_per_image, _num_classes)
  
    rois = rois.reshape(-1, 5)
    roi_scores = roi_scores.reshape(-1)
    labels = labels.reshape(-1, 1)
    bbox_targets = bbox_targets.reshape(-1, _num_classes * 4)
    bbox_inside_weights = bbox_inside_weights.reshape(-1, _num_classes * 4)
    bbox_outside_weights = np.array(bbox_inside_weights > 0).astype(np.float32)
  
    return rois, roi_scores, labels, bbox_targets, bbox_inside_weights, bbox_outside_weights, poses_target, poses_weight
  
  
  def _get_bbox_regression_labels(bbox_target_data, num_classes):
    """Bounding-box regression targets (bbox_target_data) are stored in a
    compact form N x (class, tx, ty, tw, th)
  
    This function expands those targets into the 4-of-4*K representation used
    by the network (i.e. only one class has non-zero targets).
  
    Returns:
        bbox_target (ndarray): N x 4K blob of regression targets
        bbox_inside_weights (ndarray): N x 4K blob of loss weights
    """
  
    clss = bbox_target_data[:, 0]
    bbox_targets = np.zeros((clss.size, 4 * num_classes), dtype=np.float32)
    bbox_inside_weights = np.zeros(bbox_targets.shape, dtype=np.float32)
    inds = np.where(clss > 0)[0]
    for ind in inds:
      cls = clss[ind]
      start = int(4 * cls)
      end = start + 4
      bbox_targets[ind, start:end] = bbox_target_data[ind, 1:]
      bbox_inside_weights[ind, start:end] = cfg.TRAIN.BBOX_INSIDE_WEIGHTS
    return bbox_targets, bbox_inside_weights
  
  
  def _compute_targets(ex_rois, gt_rois, labels):
    """Compute bounding-box regression targets for an image."""
  
    assert ex_rois.shape[0] == gt_rois.shape[0]
    assert ex_rois.shape[1] == 4
    assert gt_rois.shape[1] == 4
  
    targets = bbox_transform(ex_rois, gt_rois)
    if cfg.TRAIN.BBOX_NORMALIZE_TARGETS_PRECOMPUTED:
      # Optionally normalize targets by a precomputed mean and stdev
      targets = ((targets - np.array(cfg.TRAIN.BBOX_NORMALIZE_MEANS))
                 / np.array(cfg.TRAIN.BBOX_NORMALIZE_STDS))
    return np.hstack(
      (labels[:, np.newaxis], targets)).astype(np.float32, copy=False)
  
  
  def _compute_pose_targets(quaternions, labels, num_classes):
    """Compute pose regression targets for an image."""
  
    num = quaternions.shape[0]
    poses_target = np.zeros((num, 4 * num_classes), dtype=np.float32)
    poses_weight = np.zeros((num, 4 * num_classes), dtype=np.float32)
  
    for i in xrange(num):
      cls = labels[i]
      if cls > 0:
        start = int(4 * cls)
        end = start + 4
        poses_target[i, start:end] = quaternions[i, :]
        poses_weight[i, start:end] = 1.0
  
    return poses_target, poses_weight
  
  
  def _sample_rois(all_rois, all_scores, gt_boxes, poses, fg_rois_per_image, rois_per_image, num_classes):
    """Generate a random sample of RoIs comprising foreground and background
    examples.
    """
    # overlaps: (rois x gt_boxes)
    overlaps = bbox_overlaps(
      np.ascontiguousarray(all_rois[:, 1:5], dtype=np.float),
      np.ascontiguousarray(gt_boxes[:, :4], dtype=np.float))
    gt_assignment = overlaps.argmax(axis=1)
    max_overlaps = overlaps.max(axis=1)
    labels = gt_boxes[gt_assignment, 4]
    quaternions = poses[gt_assignment, 6:10]
  
    # Select foreground RoIs as those with >= FG_THRESH overlap
    fg_inds = np.where(max_overlaps >= cfg.TRAIN.FG_THRESH)[0]
    # Guard against the case when an image has fewer than fg_rois_per_image
    # Select background RoIs as those within [BG_THRESH_LO, BG_THRESH_HI)
    bg_inds = np.where((max_overlaps < cfg.TRAIN.BG_THRESH_HI) &
                       (max_overlaps >= cfg.TRAIN.BG_THRESH_LO))[0]
  
    # Small modification to the original version where we ensure a fixed number of regions are sampled
    if fg_inds.size > 0 and bg_inds.size > 0:
      fg_rois_per_image = min(fg_rois_per_image, fg_inds.size)
      fg_inds = npr.choice(fg_inds, size=int(fg_rois_per_image), replace=False)
      bg_rois_per_image = rois_per_image - fg_rois_per_image
      to_replace = bg_inds.size < bg_rois_per_image
      bg_inds = npr.choice(bg_inds, size=int(bg_rois_per_image), replace=to_replace)
    elif fg_inds.size > 0:
      to_replace = fg_inds.size < rois_per_image
      fg_inds = npr.choice(fg_inds, size=int(rois_per_image), replace=to_replace)
      fg_rois_per_image = rois_per_image
    elif bg_inds.size > 0:
      to_replace = bg_inds.size < rois_per_image
      bg_inds = npr.choice(bg_inds, size=int(rois_per_image), replace=to_replace)
      fg_rois_per_image = 0
    else:
      import pdb
      pdb.set_trace()
  
    # The indices that we're selecting (both fg and bg)
    keep_inds = np.append(fg_inds, bg_inds)
    # Select sampled values from various arrays:
    labels = labels[keep_inds]
    # Clamp labels for the background RoIs to 0
    labels[int(fg_rois_per_image):] = 0
    rois = all_rois[keep_inds]
    roi_scores = all_scores[keep_inds]
  
    # pose regression targets and weights
    poses_target, poses_weight = _compute_pose_targets(quaternions[keep_inds], labels, num_classes)
  
    bbox_target_data = _compute_targets(
      rois[:, 1:5], gt_boxes[gt_assignment[keep_inds], :4], labels)
  
    bbox_targets, bbox_inside_weights = \
      _get_bbox_regression_labels(bbox_target_data, num_classes)
  
    return labels, rois, roi_scores, bbox_targets, bbox_inside_weights, poses_target, poses_weight
  
apis: numpy.vstack numpy.append numpy.zeros numpy.ascontiguousarray numpy.hstack numpy.round numpy.where numpy.array