//  [python]
def encode_boxes_lrtb(boxes, anchors, weights=None):
  """Encode boxes to targets on lrtb (=left,right,top,bottom) format.

  Args:
    boxes: a tensor whose last dimension is 4 representing the coordinates
      of boxes in ymin, xmin, ymax, xmax order.
    anchors: a tensor whose shape is the same as, or `broadcastable` to `boxes`,
      representing the coordinates of anchors in ymin, xmin, ymax, xmax order.
    weights: None or a list of four float numbers used to scale coordinates.

  Returns:
    encoded_boxes_lrtb: a tensor whose shape is the same as `boxes` representing
      the encoded box targets. The box targets encode the left, right, top,
      bottom distances from an anchor location to the four borders of the
      matched groundtruth bounding box.
    center_targets: centerness targets defined by the left, right, top, and
      bottom distance targets. The centerness is defined as the deviation of the
      anchor location from the groundtruth object center. Formally, centerness =
      sqrt(min(left, right)/max(left, right)*min(top, bottom)/max(top, bottom)).

  Raises:
    ValueError: If the last dimension of boxes is not 4.
  """
  if boxes.shape[-1] != 4:
    raise ValueError(
        'boxes.shape[-1] is {:d}, but must be 4.'.format(boxes.shape[-1]))

  with tf.name_scope('encode_boxes_lrtb'):
    boxes = tf.cast(boxes, dtype=anchors.dtype)
    ymin = boxes[..., 0:1]
    xmin = boxes[..., 1:2]
    ymax = boxes[..., 2:3]
    xmax = boxes[..., 3:4]
    # box_h = ymax - ymin + 1.0
    # box_w = xmax - xmin + 1.0
    box_h = ymax - ymin
    box_w = xmax - xmin

    anchor_ymin = anchors[..., 0:1]
    anchor_xmin = anchors[..., 1:2]
    anchor_ymax = anchors[..., 2:3]
    anchor_xmax = anchors[..., 3:4]
    # anchor_h = anchor_ymax - anchor_ymin + 1.0
    # anchor_w = anchor_xmax - anchor_xmin + 1.0
    anchor_h = anchor_ymax - anchor_ymin
    anchor_w = anchor_xmax - anchor_xmin
    anchor_yc = anchor_ymin + 0.5 * anchor_h
    anchor_xc = anchor_xmin + 0.5 * anchor_w

    box_h += EPSILON
    box_w += EPSILON
    anchor_h += EPSILON
    anchor_w += EPSILON

    left = (anchor_xc - xmin) / anchor_w
    right = (xmax - anchor_xc) / anchor_w
    top = (anchor_yc - ymin) / anchor_h
    bottom = (ymax - anchor_yc) / anchor_h

    # Create centerness target. {
    lrtb_targets = tf.concat([left, right, top, bottom], axis=-1)
    valid_match = tf.greater(tf.reduce_min(lrtb_targets, -1), 0.0)

    # Centerness score.
    left_right = tf.concat([left, right], axis=-1)

    left_right = tf.where(tf.stack([valid_match, valid_match], -1),
                          left_right, tf.zeros_like(left_right))
    top_bottom = tf.concat([top, bottom], axis=-1)
    top_bottom = tf.where(tf.stack([valid_match, valid_match], -1),
                          top_bottom, tf.zeros_like(top_bottom))
    center_targets = tf.sqrt(
        (tf.reduce_min(left_right, -1) /
         (tf.reduce_max(left_right, -1) + EPSILON)) *
        (tf.reduce_min(top_bottom, -1) /
         (tf.reduce_max(top_bottom, -1) + EPSILON)))
    center_targets = tf.where(valid_match,
                              center_targets,
                              tf.zeros_like(center_targets))
    if weights:
      left *= weights[0]
      right *= weights[1]
      top *= weights[2]
      bottom *= weights[3]

    encoded_boxes_lrtb = tf.concat(
        [left, right, top, bottom],
        axis=-1)

    return encoded_boxes_lrtb, center_targets