text: # Copyright 2020-2021 Huawei Technologies Co., Ltd
  #
  # 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.
  # ============================================================================
  """FasterRcnn feature pyramid network."""
  
  import numpy as np
  import mindspore.nn as nn
  from mindspore.ops import operations as P
  from mindspore.common.tensor import Tensor
  from mindspore.common import dtype as mstype
  from mindspore.common.initializer import initializer
  
  
  def bias_init_zeros(shape):
      """Bias init method."""
      return Tensor(np.array(np.zeros(shape).astype(np.float32)))
  
  
  def _conv(in_channels, out_channels, kernel_size=3, stride=1, padding=0, pad_mode='pad'):
      """Conv2D wrapper."""
      shape = (out_channels, in_channels, kernel_size, kernel_size)
      weights = initializer("XavierUniform", shape=shape, dtype=mstype.float32).to_tensor()
      shape_bias = (out_channels,)
      biass = bias_init_zeros(shape_bias)
      return nn.Conv2d(in_channels, out_channels,
                       kernel_size=kernel_size, stride=stride, padding=padding,
                       pad_mode=pad_mode, weight_init=weights, has_bias=True, bias_init=biass)
  
  
  class FeatPyramidNeck(nn.Cell):
      """
      Feature pyramid network cell, usually uses as network neck.
  
      Applies the convolution on multiple, input feature maps
      and output feature map with same channel size. if required num of
      output larger then num of inputs, add extra maxpooling for further
      downsampling;
  
      Args:
          in_channels (tuple) - Channel size of input feature maps.
          out_channels (int) - Channel size output.
          num_outs (int) - Num of output features.
  
      Returns:
          Tuple, with tensors of same channel size.
  
      Examples:
          neck = FeatPyramidNeck([100,200,300], 50, 4)
          input_data = (normal(0,0.1,(1,c,1280//(4*2**i), 768//(4*2**i)),
                        dtype=np.float32) \
                        for i, c in enumerate(config.fpn_in_channels))
          x = neck(input_data)
      """
  
      def __init__(self,
                   in_channels,
                   out_channels,
                   num_outs):
          super(FeatPyramidNeck, self).__init__()
          self.num_outs = num_outs
          self.in_channels = in_channels
          self.fpn_layer = len(self.in_channels)
  
          assert not self.num_outs < len(in_channels)
  
          self.lateral_convs_list_ = []
          self.fpn_convs_ = []
  
          for _, channel in enumerate(in_channels):
              l_conv = _conv(channel, out_channels, kernel_size=1, stride=1, padding=0, pad_mode='valid')
              fpn_conv = _conv(out_channels, out_channels, kernel_size=3, stride=1, padding=0, pad_mode='same')
              self.lateral_convs_list_.append(l_conv)
              self.fpn_convs_.append(fpn_conv)
          self.lateral_convs_list = nn.layer.CellList(self.lateral_convs_list_)
          self.fpn_convs_list = nn.layer.CellList(self.fpn_convs_)
          self.interpolate1 = P.ResizeNearestNeighbor((48, 80))
          self.interpolate2 = P.ResizeNearestNeighbor((96, 160))
          self.interpolate3 = P.ResizeNearestNeighbor((192, 320))
          self.maxpool = P.MaxPool(kernel_size=1, strides=2, pad_mode="same")
  
      def construct(self, inputs):
          x = ()
          for i in range(self.fpn_layer):
              x += (self.lateral_convs_list[i](inputs[i]),)
  
          y = (x[3],)
          y = y + (x[2] + self.interpolate1(y[self.fpn_layer - 4]),)
          y = y + (x[1] + self.interpolate2(y[self.fpn_layer - 3]),)
          y = y + (x[0] + self.interpolate3(y[self.fpn_layer - 2]),)
  
          z = ()
          for i in range(self.fpn_layer - 1, -1, -1):
              z = z + (y[i],)
  
          outs = ()
          for i in range(self.fpn_layer):
              outs = outs + (self.fpn_convs_list[i](z[i]),)
  
          for i in range(self.num_outs - self.fpn_layer):
              outs = outs + (self.maxpool(outs[3]),)
          return outs
  