神经网络学习小记录25——MobileNetV2模型的复现详解

学习前言

MobileNet它哥MobileNetV2也是很不错的呢。

什么是MobileNetV2模型

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：

	Top-1	Top-5	10-5	Size	Stem
MobileNetV2(alpha=0.35)	39.914	17.568	15.422	1.7M	0.4M
MobileNetV2(alpha=0.50)	34.806	13.938	11.976	2.0M	0.7M
MobileNetV2(alpha=0.75)	30.468	10.824	9.188	2.7M	1.4M
MobileNetV2(alpha=1.0)	28.664	9.858	8.322	3.5M	2.3M
MobileNetV2(alpha=1.3)	25.320	7.878	6.728	5.4M	3.8M