python神经网络MobileNetV2模型的复现

什么是MobileNetV2模型

MobileNet它哥MobileNetV2也是很不错的呢

MobileNet模型是Google针对手机等嵌入式设备提出的一种轻量级的深层神经网络，其使用的核心思想便是depthwise separable convolution。

MobileNetV2是MobileNet的升级版，它具有两个特征点：

1、Inverted residuals，在ResNet50里我们认识到一个结构，bottleneck design结构，在3x3网络结构前利用1x1卷积降维，在3x3网络结构后，利用1x1卷积升维，相比直接使用3x3网络卷积效果更好，参数更少，先进行压缩，再进行扩张。而在MobileNetV2网络部分，其采用Inverted residuals结构，在3x3网络结构前利用1x1卷积升维，在3x3网络结构后，利用1x1卷积降维，先进行扩张，再进行压缩。

2、Linear bottlenecks，为了避免Relu对特征的破坏，在在3x3网络结构前利用1x1卷积升维，在3x3网络结构后，再利用1x1卷积降维后，不再进行Relu6层，直接进行残差网络的加法。

整体网络结构如下：（其中bottleneck进行的操作就是上述的创新操作）

MobileNetV2网络部分实现代码

#-------------------------------------------------------------#
#   MobileNetV2的网络部分
#-------------------------------------------------------------#
import math
import numpy as np
import tensorflow as tf
from tensorflow.keras import backend
from keras import backend as K
from keras.preprocessing import image
from keras.models import Model
from keras.layers.normalization import BatchNormalization
from keras.layers import Conv2D, Add, ZeroPadding2D, GlobalAveragePooling2D, Dropout, Dense
from keras.layers import MaxPooling2D,Activation,DepthwiseConv2D,Input,GlobalMaxPooling2D
from keras.applications import imagenet_utils
from keras.applications.imagenet_utils import decode_predictions
from keras.utils.data_utils import get_file


# TODO Change path to v1.1
BASE_WEIGHT_PATH = ('https://github.com/JonathanCMitchell/mobilenet_v2_keras/'
                  'releases/download/v1.1/')

# relu6！
def relu6(x):
  return K.relu(x, max_value=6)
  
# 用于计算padding的大小
def correct_pad(inputs, kernel_size):
  img_dim = 1
  input_size = backend.int_shape(inputs)[img_dim:(img_dim + 2)]

  if isinstance(kernel_size, int):
      kernel_size = (kernel_size, kernel_size)

  if input_size[0] is None:
      adjust = (1, 1)
  else:
      adjust = (1 - input_size[0] % 2, 1 - input_size[1] % 2)

  correct = (kernel_size[0] // 2, kernel_size[1] // 2)

  return ((correct[0] - adjust[0], correct[0]),
          (correct[1] - adjust[1], correct[1]))

# 使其结果可以被8整除，因为使用到了膨胀系数α
def _make_divisible(v, divisor, min_value=None):
  if min_value is None:
      min_value = divisor
  new_v = max(min_value, int(v + divisor / 2) // divisor * divisor)
  if new_v < 0.9 * v:
      new_v += divisor
  return new_v


def MobileNetV2(input_shape=[224,224,3],
              alpha=1.0,
              include_top=True,
              weights='imagenet',
              classes=1000):

  rows = input_shape[0]

  img_input = Input(shape=input_shape)

  # stem部分
  # 224,224,3 -> 112,112,32
  first_block_filters = _make_divisible(32 * alpha, 8)
  x = ZeroPadding2D(padding=correct_pad(img_input, 3),
                           name='Conv1_pad')(img_input)
  x = Conv2D(first_block_filters,
                    kernel_size=3,
                    strides=(2, 2),
                    padding='valid',
                    use_bias=False,
                    name='Conv1')(x)
  x = BatchNormalization(epsilon=1e-3,
                                momentum=0.999,
                                name='bn_Conv1')(x)
  x = Activation(relu6, name='Conv1_relu')(x)

  # 112,112,32 -> 112,112,16
  x = _inverted_res_block(x, filters=16, alpha=alpha, stride=1,
                          expansion=1, block_id=0)

  # 112,112,16 -> 56,56,24
  x = _inverted_res_block(x, filters=24, alpha=alpha, stride=2,
                          expansion=6, block_id=1)
  x = _inverted_res_block(x, filters=24, alpha=alpha, stride=1,
                          expansion=6, block_id=2)

  # 56,56,24 -> 28,28,32
  x = _inverted_res_block(x, filters=32, alpha=alpha, stride=2,
                          expansion=6, block_id=3)
  x = _inverted_res_block(x, filters=32, alpha=alpha, stride=1,
                          expansion=6, block_id=4)
  x = _inverted_res_block(x, filters=32, alpha=alpha, stride=1,
                          expansion=6, block_id=5)

  # 28,28,32 -> 14,14,64
  x = _inverted_res_block(x, filters=64, alpha=alpha, stride=2,
                          expansion=6, block_id=6)
  x = _inverted_res_block(x, filters=64, alpha=alpha, stride=1,
                          expansion=6, block_id=7)
  x = _inverted_res_block(x, filters=64, alpha=alpha, stride=1,
                          expansion=6, block_id=8)
  x = _inverted_res_block(x, filters=64, alpha=alpha, stride=1,
                          expansion=6, block_id=9)

  # 14,14,64 -> 14,14,96
  x = _inverted_res_block(x, filters=96, alpha=alpha, stride=1,
                          expansion=6, block_id=10)
  x = _inverted_res_block(x, filters=96, alpha=alpha, stride=1,
                          expansion=6, block_id=11)
  x = _inverted_res_block(x, filters=96, alpha=alpha, stride=1,
                          expansion=6, block_id=12)
  # 14,14,96 -> 7,7,160
  x = _inverted_res_block(x, filters=160, alpha=alpha, stride=2,
                          expansion=6, block_id=13)
  x = _inverted_res_block(x, filters=160, alpha=alpha, stride=1,
                          expansion=6, block_id=14)
  x = _inverted_res_block(x, filters=160, alpha=alpha, stride=1,
                          expansion=6, block_id=15)

  # 7,7,160 -> 7,7,320
  x = _inverted_res_block(x, filters=320, alpha=alpha, stride=1,
                          expansion=6, block_id=16)

  if alpha > 1.0:
      last_block_filters = _make_divisible(1280 * alpha, 8)
  else:
      last_block_filters = 1280

  # 7,7,320 -> 7,7,1280
  x = Conv2D(last_block_filters,
                    kernel_size=1,
                    use_bias=False,
                    name='Conv_1')(x)
  x = BatchNormalization(epsilon=1e-3,
                                momentum=0.999,
                                name='Conv_1_bn')(x)
  x = Activation(relu6, name='out_relu')(x)


  x = GlobalAveragePooling2D()(x)
  x = Dense(classes, activation='softmax',
                      use_bias=True, name='Logits')(x)

  inputs = img_input

  model = Model(inputs, x, name='mobilenetv2_%0.2f_%s' % (alpha, rows))

  # Load weights.
  if weights == 'imagenet':
      if include_top:
          model_name = ('mobilenet_v2_weights_tf_dim_ordering_tf_kernels_' +
                        str(alpha) + '_' + str(rows) + '.h5')
          weight_path = BASE_WEIGHT_PATH + model_name
          weights_path = get_file(
              model_name, weight_path, cache_subdir='models')
      else:
          model_name = ('mobilenet_v2_weights_tf_dim_ordering_tf_kernels_' +
                        str(alpha) + '_' + str(rows) + '_no_top' + '.h5')
          weight_path = BASE_WEIGHT_PATH + model_name
          weights_path = get_file(
              model_name, weight_path, cache_subdir='models')
      model.load_weights(weights_path)
  elif weights is not None:
      model.load_weights(weights)

  return model


def _inverted_res_block(inputs, expansion, stride, alpha, filters, block_id):
  in_channels = backend.int_shape(inputs)[-1]
  pointwise_conv_filters = int(filters * alpha)
  pointwise_filters = _make_divisible(pointwise_conv_filters, 8)
  x = inputs
  prefix = 'block_{}_'.format(block_id)

  # part1 数据扩张
  if block_id:
      # Expand
      x = Conv2D(expansion * in_channels,
                        kernel_size=1,
                        padding='same',
                        use_bias=False,
                        activation=None,
                        name=prefix + 'expand')(x)
      x = BatchNormalization(epsilon=1e-3,
                                    momentum=0.999,
                                    name=prefix + 'expand_BN')(x)
      x = Activation(relu6, name=prefix + 'expand_relu')(x)
  else:
      prefix = 'expanded_conv_'

  if stride == 2:
      x = ZeroPadding2D(padding=correct_pad(x, 3),
                               name=prefix + 'pad')(x)
  
  # part2 可分离卷积
  x = DepthwiseConv2D(kernel_size=3,
                             strides=stride,
                             activation=None,
                             use_bias=False,
                             padding='same' if stride == 1 else 'valid',
                             name=prefix + 'depthwise')(x)
  x = BatchNormalization(epsilon=1e-3,
                                momentum=0.999,
                                name=prefix + 'depthwise_BN')(x)

  x = Activation(relu6, name=prefix + 'depthwise_relu')(x)

  # part3压缩特征，而且不使用relu函数，保证特征不被破坏
  x = Conv2D(pointwise_filters,
                    kernel_size=1,
                    padding='same',
                    use_bias=False,
                    activation=None,
                    name=prefix + 'project')(x)

  x = BatchNormalization(epsilon=1e-3,
                                momentum=0.999,
                                name=prefix + 'project_BN')(x)

  if in_channels == pointwise_filters and stride == 1:
      return Add(name=prefix + 'add')([inputs, x])
  return x

图片预测

建立网络后，可以用以下的代码进行预测。

def preprocess_input(x):
  x /= 255.
  x -= 0.5
  x *= 2.
  return x

if __name__ == '__main__':
  model = MobileNetV2(input_shape=(224, 224, 3))
  model.summary()
  img_path = 'elephant.jpg'
  img = image.load_img(img_path, target_size=(224, 224))
  x = image.img_to_array(img)
  x = np.expand_dims(x, axis=0)
  x = preprocess_input(x)
  print('Input image shape:', x.shape)

  preds = model.predict(x)
  print(np.argmax(preds))
  print('Predicted:', decode_predictions(preds, 1))

预测所需的已经训练好的MobileNetV2模型会在运行时自动下载，下载后的模型位于C:\Users\Administrator.keras\models文件夹内。

可以修改MobileNetV2内不同的alpha值实现不同depth的MobileNetV2模型。可选的alpha值有：

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