# Copyright 2021 The TensorFlow Authors. All Rights Reserved.
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# Licensed under the Apache License, Version 2.0 (the "License");
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"""Base box coder.

Box coders convert between coordinate frames, namely image-centric
(with (0,0) on the top left of image) and anchor-centric (with (0,0) being
defined by a specific anchor).

Users of a BoxCoder can call two methods:
 encode: which encodes a box with respect to a given anchor
  (or rather, a tensor of boxes wrt a corresponding tensor of anchors) and
 decode: which inverts this encoding with a decode operation.
In both cases, the arguments are assumed to be in 1-1 correspondence already;
it is not the job of a BoxCoder to perform matching.
"""
from abc import ABCMeta
from abc import abstractmethod
from abc import abstractproperty

import tensorflow as tf

# Box coder types.
FASTER_RCNN = 'faster_rcnn'
KEYPOINT = 'keypoint'
MEAN_STDDEV = 'mean_stddev'
SQUARE = 'square'


class BoxCoder(object):
  """Abstract base class for box coder."""
  __metaclass__ = ABCMeta

  @abstractproperty
  def code_size(self):
    """Return the size of each code.

    This number is a constant and should agree with the output of the `encode`
    op (e.g. if rel_codes is the output of self.encode(...), then it should have
    shape [N, code_size()]).  This abstractproperty should be overridden by
    implementations.

    Returns:
      an integer constant
    """
    pass

  def encode(self, boxes, anchors):
    """Encode a box list relative to an anchor collection.

    Args:
      boxes: BoxList holding N boxes to be encoded
      anchors: BoxList of N anchors

    Returns:
      a tensor representing N relative-encoded boxes
    """
    with tf.name_scope('Encode'):
      return self._encode(boxes, anchors)

  def decode(self, rel_codes, anchors):
    """Decode boxes that are encoded relative to an anchor collection.

    Args:
      rel_codes: a tensor representing N relative-encoded boxes
      anchors: BoxList of anchors

    Returns:
      boxlist: BoxList holding N boxes encoded in the ordinary way (i.e.,
        with corners y_min, x_min, y_max, x_max)
    """
    with tf.name_scope('Decode'):
      return self._decode(rel_codes, anchors)

  @abstractmethod
  def _encode(self, boxes, anchors):
    """Method to be overriden by implementations.

    Args:
      boxes: BoxList holding N boxes to be encoded
      anchors: BoxList of N anchors

    Returns:
      a tensor representing N relative-encoded boxes
    """
    pass

  @abstractmethod
  def _decode(self, rel_codes, anchors):
    """Method to be overriden by implementations.

    Args:
      rel_codes: a tensor representing N relative-encoded boxes
      anchors: BoxList of anchors

    Returns:
      boxlist: BoxList holding N boxes encoded in the ordinary way (i.e.,
        with corners y_min, x_min, y_max, x_max)
    """
    pass


def batch_decode(encoded_boxes, box_coder, anchors):
  """Decode a batch of encoded boxes.

  This op takes a batch of encoded bounding boxes and transforms
  them to a batch of bounding boxes specified by their corners in
  the order of [y_min, x_min, y_max, x_max].

  Args:
    encoded_boxes: a float32 tensor of shape [batch_size, num_anchors,
      code_size] representing the location of the objects.
    box_coder: a BoxCoder object.
    anchors: a BoxList of anchors used to encode `encoded_boxes`.

  Returns:
    decoded_boxes: a float32 tensor of shape [batch_size, num_anchors,
      coder_size] representing the corners of the objects in the order
      of [y_min, x_min, y_max, x_max].

  Raises:
    ValueError: if batch sizes of the inputs are inconsistent, or if
    the number of anchors inferred from encoded_boxes and anchors are
    inconsistent.
  """
  encoded_boxes.get_shape().assert_has_rank(3)
  if encoded_boxes.get_shape()[1].value != anchors.num_boxes_static():
    raise ValueError(
        'The number of anchors inferred from encoded_boxes'
        ' and anchors are inconsistent: shape[1] of encoded_boxes'
        ' %s should be equal to the number of anchors: %s.' %
        (encoded_boxes.get_shape()[1].value, anchors.num_boxes_static()))

  decoded_boxes = tf.stack([
      box_coder.decode(boxes, anchors).get()
      for boxes in tf.unstack(encoded_boxes)
  ])
  return decoded_boxes
