text: """
  Copyright 2017-2018 cgratie (https://github.com/cgratie/)
  
  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.
  """
  
  
  import keras
  from keras.utils import get_file
  
  from . import retinanet
  from . import Backbone
  from utils.image import preprocess_image
  import configure
  
  
  class VGGBackbone(Backbone):
      """ Describes backbone information and provides utility functions.
      """
  
      def retinanet(self, *args, **kwargs):
          """ Returns a retinanet model using the correct backbone.
          """
          return vgg_retinanet(*args, backbone=self.backbone, **kwargs)
  
      def download_imagenet(self):
          """ Downloads ImageNet weights and returns path to weights file.
          Weights can be downloaded at https://github.com/fizyr/keras-models/releases .
          """
          if self.backbone == 'vgg16':
              resource = keras.applications.vgg16.vgg16.WEIGHTS_PATH_NO_TOP
              checksum = '6d6bbae143d832006294945121d1f1fc'
          elif self.backbone == 'vgg19':
              resource = keras.applications.vgg19.vgg19.WEIGHTS_PATH_NO_TOP
              checksum = '253f8cb515780f3b799900260a226db6'
          else:
              raise ValueError("Backbone '{}' not recognized.".format(self.backbone))
  
          return get_file(
              '{}_weights_tf_dim_ordering_tf_kernels_notop.h5'.format(self.backbone),
              resource,
              cache_subdir='models',
              file_hash=checksum
          )
  
      def fsaf(self, num_classes, modifier):
          """
          Returns a retinanet model using the correct backbone.
          """
          return vgg_fsaf(num_classes=num_classes, backbone=self.backbone, modifier=modifier)
  
      def validate(self):
          """ Checks whether the backbone string is correct.
          """
          allowed_backbones = ['vgg16', 'vgg19']
  
          if self.backbone not in allowed_backbones:
              raise ValueError('Backbone (\'{}\') not in allowed backbones ({}).'.format(self.backbone, allowed_backbones))
  
      def preprocess_image(self, inputs):
          """ Takes as input an image and prepares it for being passed through the network.
          """
          return preprocess_image(inputs, mode='caffe')
  
  
  def vgg_retinanet(num_classes, backbone='vgg16', inputs=None, modifier=None, **kwargs):
      """ Constructs a retinanet model using a vgg backbone.
  
      Args
          num_classes: Number of classes to predict.
          backbone: Which backbone to use (one of ('vgg16', 'vgg19')).
          inputs: The inputs to the network (defaults to a Tensor of shape (None, None, 3)).
          modifier: A function handler which can modify the backbone before using it in retinanet (this can be used to freeze backbone layers for example).
  
      Returns
          RetinaNet model with a VGG backbone.
      """
      # choose default input
      if inputs is None:
          inputs = keras.layers.Input(shape=(None, None, 3))
  
      # create the vgg backbone
      if backbone == 'vgg16':
          vgg = keras.applications.VGG16(input_tensor=inputs, include_top=False, weights=None)
      elif backbone == 'vgg19':
          vgg = keras.applications.VGG19(input_tensor=inputs, include_top=False, weights=None)
      else:
          raise ValueError("Backbone '{}' not recognized.".format(backbone))
  
      if modifier:
          vgg = modifier(vgg)
  
      # create the full model
      layer_names = ["block3_pool", "block4_pool", "block5_pool"]
      layer_outputs = [vgg.get_layer(name).output for name in layer_names]
      return retinanet.retinanet(inputs=inputs, num_classes=num_classes, backbone_layers=layer_outputs, **kwargs)
  
  
  def vgg_fsaf(num_classes, backbone='vgg16', inputs=None, modifier=None, **kwargs):
      """
      Constructs a retinanet model using a resnet backbone.
  
      Args
          num_classes: Number of classes to predict.
          backbone: Which backbone to use (one of ('vgg16', 'vgg19')).
          inputs: The inputs to the network (defaults to a Tensor of shape (None, None, 3)).
          modifier: A function handler which can modify the backbone before using it in retinanet (this can be used to freeze backbone layers for example).
  
      Returns
          FSAF model with a VGG backbone.
      """
      image_input = keras.layers.Input(shape=(None, None, 3))
      gt_boxes_input = keras.layers.Input(shape=(configure.MAX_NUM_GT_BOXES, 5))
      feature_shapes_input = keras.layers.Input((5, 2), dtype='int32')
  
      # create the resnet backbone
      if backbone == 'vgg16':
          vgg = keras.applications.VGG16(input_tensor=inputs, include_top=False, weights=None)
      elif backbone == 'vgg19':
          vgg = keras.applications.VGG19(input_tensor=inputs, include_top=False, weights=None)
      else:
          raise ValueError("Backbone '{}' not recognized.".format(backbone))
  
      # invoke modifier if given
      if modifier:
          vgg = modifier(vgg)
  
      layer_names = ["block3_pool", "block4_pool", "block5_pool"]
      layer_outputs = [vgg.get_layer(name).output for name in layer_names]
      # create the full model
      return retinanet.fsaf(inputs=[image_input, gt_boxes_input, feature_shapes_input],
                            num_classes=num_classes,
                            backbone_layers=layer_outputs)