box_matcher.py

# Copyright 2021 The TensorFlow Authors. 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.


"""Box matcher implementation."""


import tensorflow as tf


class BoxMatcher:
  """Matcher based on highest value.

  This class computes matches from a similarity matrix. Each column is matched
  to a single row.

  To support object detection target assignment this class enables setting both
  positive_threshold (upper threshold) and negative_threshold (lower thresholds)
  defining three categories of similarity which define whether examples are
  positive, negative, or ignored, for example:
  (1) thresholds=[negative_threshold, positive_threshold], and
      indicators=[negative_value, ignore_value, positive_value]: The similarity
      metrics below negative_threshold will be assigned with negative_value,
      the metrics between negative_threshold and positive_threshold will be
      assigned ignore_value, and the metrics above positive_threshold will be
      assigned positive_value.
  (2) thresholds=[negative_threshold, positive_threshold], and
      indicators=[ignore_value, negative_value, positive_value]: The similarity
      metric below negative_threshold will be assigned with ignore_value,
      the metrics between negative_threshold and positive_threshold will be
      assigned negative_value, and the metrics above positive_threshold will be
      assigned positive_value.
  """

  def __init__(self, thresholds, indicators, force_match_for_each_col=False):
    """Construct BoxMatcher.

    Args:
      thresholds: A list of thresholds to classify boxes into
        different buckets. The list needs to be sorted, and will be prepended
        with -Inf and appended with +Inf.
      indicators: A list of values to assign for each bucket. len(`indicators`)
        must equal to len(`thresholds`) + 1.
      force_match_for_each_col: If True, ensures that each column is matched to
        at least one row (which is not guaranteed otherwise if the
        positive_threshold is high). Defaults to False. If True, all force
        matched row will be assigned to `indicators[-1]`.

    Raises:
      ValueError: If `threshold` not sorted,
        or len(indicators) != len(threshold) + 1
    """
    if not all([lo <= hi for (lo, hi) in zip(thresholds[:-1], thresholds[1:])]):
      raise ValueError('`threshold` must be sorted, got {}'.format(thresholds))
    self.indicators = indicators
    if len(indicators) != len(thresholds) + 1:
      raise ValueError('len(`indicators`) must be len(`thresholds`) + 1, got '
                       'indicators {}, thresholds {}'.format(
                           indicators, thresholds))
    thresholds = thresholds[:]
    thresholds.insert(0, -float('inf'))
    thresholds.append(float('inf'))
    self.thresholds = thresholds
    self._force_match_for_each_col = force_match_for_each_col

  def __call__(self, similarity_matrix):
    """Tries to match each column of the similarity matrix to a row.

    Args:
      similarity_matrix: A float tensor of shape [N, M] representing any
        similarity metric.

    Returns:
      A integer tensor of shape [N] with corresponding match indices for each
      of M columns, for positive match, the match result will be the
      corresponding row index, for negative match, the match will be
      `negative_value`, for ignored match, the match result will be
      `ignore_value`.
    """
    squeeze_result = False
    if len(similarity_matrix.shape) == 2:
      squeeze_result = True
      similarity_matrix = tf.expand_dims(similarity_matrix, axis=0)

    static_shape = similarity_matrix.shape.as_list()
    num_rows = static_shape[1] or tf.shape(similarity_matrix)[1]
    batch_size = static_shape[0] or tf.shape(similarity_matrix)[0]

    def _match_when_rows_are_empty():
      """Performs matching when the rows of similarity matrix are empty.

      When the rows are empty, all detections are false positives. So we return
      a tensor of -1's to indicate that the columns do not match to any rows.

      Returns:
        matches:  int32 tensor indicating the row each column matches to.
      """
      with tf.name_scope('empty_gt_boxes'):
        matches = tf.zeros([batch_size, num_rows], dtype=tf.int32)
        match_labels = -tf.ones([batch_size, num_rows], dtype=tf.int32)
        return matches, match_labels

    def _match_when_rows_are_non_empty():
      """Performs matching when the rows of similarity matrix are non empty.

      Returns:
        matches:  int32 tensor indicating the row each column matches to.
      """
      # Matches for each column
      with tf.name_scope('non_empty_gt_boxes'):
        matches = tf.argmax(similarity_matrix, axis=-1, output_type=tf.int32)

        # Get logical indices of ignored and unmatched columns as tf.int64
        matched_vals = tf.reduce_max(similarity_matrix, axis=-1)
        matched_indicators = tf.zeros([batch_size, num_rows], tf.int32)

        match_dtype = matched_vals.dtype
        for (ind, low, high) in zip(self.indicators, self.thresholds[:-1],
                                    self.thresholds[1:]):
          low_threshold = tf.cast(low, match_dtype)
          high_threshold = tf.cast(high, match_dtype)
          mask = tf.logical_and(
              tf.greater_equal(matched_vals, low_threshold),
              tf.less(matched_vals, high_threshold))
          matched_indicators = self._set_values_using_indicator(
              matched_indicators, mask, ind)

        if self._force_match_for_each_col:
          # [batch_size, M], for each col (groundtruth_box), find the best
          # matching row (anchor).
          force_match_column_ids = tf.argmax(
              input=similarity_matrix, axis=1, output_type=tf.int32)
          # [batch_size, M, N]
          force_match_column_indicators = tf.one_hot(
              force_match_column_ids, depth=num_rows)
          # [batch_size, N], for each row (anchor), find the largest column
          # index for groundtruth box
          force_match_row_ids = tf.argmax(
              input=force_match_column_indicators, axis=1, output_type=tf.int32)
          # [batch_size, N]
          force_match_column_mask = tf.cast(
              tf.reduce_max(force_match_column_indicators, axis=1),
              tf.bool)
          # [batch_size, N]
          final_matches = tf.where(force_match_column_mask, force_match_row_ids,
                                   matches)
          final_matched_indicators = tf.where(
              force_match_column_mask, self.indicators[-1] *
              tf.ones([batch_size, num_rows], dtype=tf.int32),
              matched_indicators)
          return final_matches, final_matched_indicators
        else:
          return matches, matched_indicators

    num_gt_boxes = similarity_matrix.shape.as_list()[-1] or tf.shape(
        similarity_matrix)[-1]
    result_match, result_matched_indicators = tf.cond(
        pred=tf.greater(num_gt_boxes, 0),
        true_fn=_match_when_rows_are_non_empty,
        false_fn=_match_when_rows_are_empty)

    if squeeze_result:
      result_match = tf.squeeze(result_match, axis=0)
      result_matched_indicators = tf.squeeze(result_matched_indicators, axis=0)

    return result_match, result_matched_indicators

  def _set_values_using_indicator(self, x, indicator, val):
    """Set the indicated fields of x to val.

    Args:
      x: tensor.
      indicator: boolean with same shape as x.
      val: scalar with value to set.

    Returns:
      modified tensor.
    """
    indicator = tf.cast(indicator, x.dtype)
    return tf.add(tf.multiply(x, 1 - indicator), val * indicator)