code: # Copyright 2020 Google Research. All Rights Reserved.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  # ==============================================================================
  """Anchor definition."""
  import collections
  import numpy as np
  import tensorflow as tf
  
  import utils
  from object_detection import argmax_matcher
  from object_detection import box_list
  from object_detection import faster_rcnn_box_coder
  from object_detection import region_similarity_calculator
  from object_detection import target_assigner
  
  MAX_DETECTION_POINTS = 5000
  
  
  def decode_box_outputs(pred_boxes, anchor_boxes):
    """Transforms relative regression coordinates to absolute positions.
  
    Network predictions are normalized and relative to a given anchor; this
    reverses the transformation and outputs absolute coordinates for the input
    image.
  
    Args:
      pred_boxes: predicted box regression targets.
      anchor_boxes: anchors on all feature levels.
    Returns:
      outputs: bounding boxes.
    """
    anchor_boxes = tf.cast(anchor_boxes, pred_boxes.dtype)
    ycenter_a = (anchor_boxes[..., 0] + anchor_boxes[..., 2]) / 2
    xcenter_a = (anchor_boxes[..., 1] + anchor_boxes[..., 3]) / 2
    ha = anchor_boxes[..., 2] - anchor_boxes[..., 0]
    wa = anchor_boxes[..., 3] - anchor_boxes[..., 1]
    ty, tx, th, tw = tf.unstack(pred_boxes, num=4, axis=-1)
  
    w = tf.math.exp(tw) * wa
    h = tf.math.exp(th) * ha
    ycenter = ty * ha + ycenter_a
    xcenter = tx * wa + xcenter_a
    ymin = ycenter - h / 2.
    xmin = xcenter - w / 2.
    ymax = ycenter + h / 2.
    xmax = xcenter + w / 2.
    return tf.stack([ymin, xmin, ymax, xmax], axis=-1)
  
  
  class Anchors():
    """Multi-scale anchors class."""
  
    def __init__(self, min_level, max_level, num_scales, aspect_ratios,
                 anchor_scale, image_size):
      """Constructs multiscale anchors.
  
      Args:
        min_level: integer number of minimum level of the output feature pyramid.
        max_level: integer number of maximum level of the output feature pyramid.
        num_scales: integer number representing intermediate scales added
          on each level. For instances, num_scales=2 adds two additional
          anchor scales [2^0, 2^0.5] on each level.
        aspect_ratios: list of representing the aspect ratio anchors added
          on each level. For instances, aspect_ratios = [1.0, 2.0, 0..5]
          adds three anchors on each level.
        anchor_scale: float number representing the scale of size of the base
          anchor to the feature stride 2^level. Or a list, one value per layer.
        image_size: integer number or tuple of integer number of input image size.
      """
      self.min_level = min_level
      self.max_level = max_level
      self.num_scales = num_scales
      self.aspect_ratios = aspect_ratios
      if isinstance(anchor_scale, (list, tuple)):
        assert len(anchor_scale) == max_level - min_level + 1
        self.anchor_scales = anchor_scale
      else:
        self.anchor_scales = [anchor_scale] * (max_level - min_level + 1)
      self.image_size = utils.parse_image_size(image_size)
      self.feat_sizes = utils.get_feat_sizes(image_size, max_level)
      self.config = self._generate_configs()
      self.boxes = self._generate_boxes()
  
    def _generate_configs(self):
      """Generate configurations of anchor boxes."""
      anchor_configs = {}
      feat_sizes = self.feat_sizes
      for level in range(self.min_level, self.max_level + 1):
        anchor_configs[level] = []
        for scale_octave in range(self.num_scales):
          for aspect in self.aspect_ratios:
            anchor_configs[level].append(
                ((feat_sizes[0]['height'] / float(feat_sizes[level]['height']),
                  feat_sizes[0]['width'] / float(feat_sizes[level]['width'])),
                 scale_octave / float(self.num_scales), aspect,
                 self.anchor_scales[level - self.min_level]))
      return anchor_configs
  
    def _generate_boxes(self):
      """Generates multiscale anchor boxes."""
      boxes_all = []
      for _, configs in self.config.items():
        boxes_level = []
        for config in configs:
          stride, octave_scale, aspect, anchor_scale = config
          base_anchor_size_x = anchor_scale * stride[1] * 2**octave_scale
          base_anchor_size_y = anchor_scale * stride[0] * 2**octave_scale
          if isinstance(aspect, list):
            aspect_x, aspect_y = aspect
          else:
            aspect_x = np.sqrt(aspect)
            aspect_y = 1.0 / aspect_x
          anchor_size_x_2 = base_anchor_size_x * aspect_x / 2.0
          anchor_size_y_2 = base_anchor_size_y * aspect_y / 2.0
  
          x = np.arange(stride[1] / 2, self.image_size[1], stride[1])
          y = np.arange(stride[0] / 2, self.image_size[0], stride[0])
          xv, yv = np.meshgrid(x, y)
          xv = xv.reshape(-1)
          yv = yv.reshape(-1)
  
          boxes = np.vstack((yv - anchor_size_y_2, xv - anchor_size_x_2,
                             yv + anchor_size_y_2, xv + anchor_size_x_2))
          boxes = np.swapaxes(boxes, 0, 1)
          boxes_level.append(np.expand_dims(boxes, axis=1))
        # concat anchors on the same level to the reshape NxAx4
        boxes_level = np.concatenate(boxes_level, axis=1)
        boxes_all.append(boxes_level.reshape([-1, 4]))
  
      anchor_boxes = np.vstack(boxes_all)
      anchor_boxes = tf.convert_to_tensor(anchor_boxes, dtype=tf.float32)
      return anchor_boxes
  
    def get_anchors_per_location(self):
      return self.num_scales * len(self.aspect_ratios)
  
  
  class AnchorLabeler(object):
    """Labeler for multiscale anchor boxes."""
  
    def __init__(self, anchors, num_classes, match_threshold=0.5):
      """Constructs anchor labeler to assign labels to anchors.
  
      Args:
        anchors: an instance of class Anchors.
        num_classes: integer number representing number of classes in the dataset.
        match_threshold: float number between 0 and 1 representing the threshold
          to assign positive labels for anchors.
      """
      similarity_calc = region_similarity_calculator.IouSimilarity()
      matcher = argmax_matcher.ArgMaxMatcher(
          match_threshold,
          unmatched_threshold=match_threshold,
          negatives_lower_than_unmatched=True,
          force_match_for_each_row=True)
      box_coder = faster_rcnn_box_coder.FasterRcnnBoxCoder()
  
      self._target_assigner = target_assigner.TargetAssigner(
          similarity_calc, matcher, box_coder)
      self._anchors = anchors
      self._match_threshold = match_threshold
      self._num_classes = num_classes
  
    def _unpack_labels(self, labels):
      """Unpacks an array of labels into multiscales labels."""
      labels_unpacked = collections.OrderedDict()
      anchors = self._anchors
      count = 0
      for level in range(anchors.min_level, anchors.max_level + 1):
        feat_size = anchors.feat_sizes[level]
        steps = feat_size['height'] * feat_size[
            'width'] * anchors.get_anchors_per_location()
        indices = tf.range(count, count + steps)
        count += steps
        labels_unpacked[level] = tf.reshape(
            tf.gather(labels, indices),
            [feat_size['height'], feat_size['width'], -1])
      return labels_unpacked
  
    def label_anchors(self, gt_boxes, gt_labels):
      """Labels anchors with ground truth inputs.
  
      Args:
        gt_boxes: A float tensor with shape [N, 4] representing groundtruth boxes.
          For each row, it stores [y0, x0, y1, x1] for four corners of a box.
        gt_labels: A integer tensor with shape [N, 1] representing groundtruth
          classes.
      Returns:
        cls_targets_dict: ordered dictionary with keys
          [min_level, min_level+1, ..., max_level]. The values are tensor with
          shape [height_l, width_l, num_anchors]. The height_l and width_l
          represent the dimension of class logits at l-th level.
        box_targets_dict: ordered dictionary with keys
          [min_level, min_level+1, ..., max_level]. The values are tensor with
          shape [height_l, width_l, num_anchors * 4]. The height_l and
          width_l represent the dimension of bounding box regression output at
          l-th level.
        num_positives: scalar tensor storing number of positives in an image.
      """
      gt_box_list = box_list.BoxList(gt_boxes)
      anchor_box_list = box_list.BoxList(self._anchors.boxes)
  
      # cls_weights, box_weights are not used
      cls_targets, _, box_targets, _, matches = self._target_assigner.assign(
          anchor_box_list, gt_box_list, gt_labels)
  
      # class labels start from 1 and the background class = -1
      cls_targets -= 1
      cls_targets = tf.cast(cls_targets, tf.int32)
  
      # Unpack labels.
      cls_targets_dict = self._unpack_labels(cls_targets)
      box_targets_dict = self._unpack_labels(box_targets)
      num_positives = tf.reduce_sum(
          tf.cast(tf.not_equal(matches.match_results, -1), tf.float32))
  
      return cls_targets_dict, box_targets_dict, num_positives
  
apis: numpy.vstack tensorflow.gather tensorflow.stack tensorflow.unstack tensorflow.range numpy.concatenate numpy.swapaxes numpy.arange tensorflow.cast numpy.expand_dims tensorflow.convert_to_tensor tensorflow.not_equal numpy.sqrt tensorflow.math.exp numpy.meshgrid