简介:Alex在2012年提出的Alex网络模型结构将深度学习神经网络推上了热潮,并获得了当年的图像识别大赛冠军,使得CNN网络成为图像分类的核心算法模型.
论文链接:ImageNet Classification with Deep Convolutional Neural Networks(2012)
网络结构:AlexNet模型一共有8层,5层卷积层,3层全连接层,在每一个卷积层之后包含Relu激活函数、局部响应归一化、降采样处理,使用2个GPU加速.
- conv1层:
输入:227×227×3,原始彩色图像大小是224×224×3,经过预处理后统一处理为227×227×3的规格
卷积核尺寸:11×11
卷积核个数:96个,2个GPU加速,每个GPU上48个
步长:4
输出:55×55×96 (计算过程:(227-11)/4+1=55 )
激活函数:Relu
降采样处理(pooling):kernel_size=3,stride=2,output=27×27×96(计算过程: (55-3)/2+1=27 )
归一化处理(LRN):local_size=5,output=27×27×96
- conv2层:
输入:27×27×96
填充边界(pad):2
卷积核尺寸:5×5
卷积核个数:256个,2个GPU加速,每个GPU上128个
步长:1
输出:27×27×256 (计算过程:(31-5)/1+1=27 )
激活函数:Relu
降采样处理(pooling):kernel_size=3,stride=2,output=13×13×256(计算过程: (27-3)/2+1=13 )
归一化处理(LRN):local_size=5,output=13×13×256
- conv3层:
输入:13×13×256
填充边界(pad):1
卷积核尺寸:3×3
卷积核个数:384个,2个GPU加速,每个GPU上192个
步长:1
输出:13×13×384 (计算过程:(15-3)/1+1=13 )
激活函数:Relu
降采样处理(pooling):无
归一化处理(LRN):无
- conv4层:
输入:13×13×384
填充边界(pad):1
卷积核尺寸:3×3
卷积核个数:384个,2个GPU加速,每个GPU上192个
步长:1
输出:13×13×384 (计算过程:(15-3)/1+1=13 )
激活函数:Relu
降采样处理(pooling):无
归一化处理(LRN):无
- conv5层:
输入:13×13×384
填充边界(pad):1
卷积核尺寸:3×3
卷积核个数:256个,2个GPU加速,每个GPU上128个
步长:1
输出:13×13×256 (计算过程:(15-3)/1+1=13 )
激活函数:Relu
降采样处理(pooling):kernel_size=3,stride=2,output=6×6×256(计算过程: (13-3)/2+1=6 )
归一化处理(LRN):无
- fc6层:
输入:6×6×256
输出:4096
激活函数:Relu
dropout:0.5
- fc7层:
输入:4096
输出:4096
激活函数:Relu
dropout:0.5
- fc8层:
输入:4096
输出:1000
通过结构可以看出,网络每次都提取出重要的特征,最后通过这些特征确定该图像的类别,抽象之后的结构如下:
创新点:
- Droupout随机权重裁剪:
- 在模型训练过程中,以一定概率随机舍弃某些隐含层节点的权值(认为是0),在更新权值时,不更新与该节点相连的权值.
- 但是被dropout的节点权值得保留下来(只是暂时的不更新),下个样本输入时继续使用该节点权值.
- 减少高激活神经元的数量,提高训练速度,抑制过拟合.
- LRN局部归一化:
- 公式:
- 提升较大响应,抑制较小响应
- 后来发现效果不明显,所以很少使用.
- 公式:
- MaxPool:
- 避免特征被平均池化模糊,提升特征鲁棒性.
- Relu激活函数:
- 训练中收敛速度更快.
tensorflow代码实现:
1 from __future__ import print_function 2 from tensorflow.examples.tutorials.mnist import input_data 3 import tensorflow as tf 4 import numpy as np 5 6 mnist = input_data.read_data_sets('tmp/data/', one_hot=True) 7 # MNIST数据集相关常数 8 input_node = 784 9 output_node = 10 10 image_size = 28 11 num_channel = 1 12 # 第一层卷积层的尺寸和深度 13 conv1_deep = 64 14 conv1_size = 3 15 16 # 第二层卷积层的尺寸和深度 17 conv2_deep = 128 18 conv2_size = 3 19 20 # 第三层卷积层的尺寸和深度 21 conv3_deep = 256 22 conv3_size = 3 23 24 # 全连接层的节点个数 25 fc_size = 1024 26 27 # 配置神经网络参数 28 batch_size = 64 29 learning_rate_base = 0.01 30 learning_rate_decay = 0.99 31 training_steps = 10000 32 moving_average_decay = 0.99 33 34 x = tf.placeholder(tf.float32, [None,input_node], name = 'x-input') 35 y_ = tf.placeholder(tf.float32, [None, output_node], name = 'y-input') 36 37 # 定义权重W和偏置b 38 weights = { 39 'conv1_weights': tf.Variable(tf.truncated_normal([conv1_size,conv1_size,num_channel,conv1_deep], stddev=0.1)), 40 'conv2_weights': tf.Variable(tf.truncated_normal([conv2_size,conv2_size,conv1_deep,conv2_deep], stddev=0.1)), 41 'conv3_weights': tf.Variable(tf.truncated_normal([conv3_size,conv3_size,conv2_deep,conv3_deep], stddev=0.1)), 42 'fc2_weights': tf.Variable(tf.truncated_normal([fc_size,fc_size], stddev=0.1)), 43 'out_weights': tf.Variable(tf.truncated_normal([fc_size,output_node], stddev=0.1)) 44 } 45 biases = { 46 'conv1_bias': tf.Variable(tf.constant(0.0, shape=[conv1_deep])), 47 'conv2_bias': tf.Variable(tf.constant(0.0, shape=[conv2_deep])), 48 'conv3_bias': tf.Variable(tf.constant(0.0, shape=[conv3_deep])), 49 'fc1_bias': tf.Variable(tf.constant(0.1, shape=[fc_size])), 50 'fc2_bias': tf.Variable(tf.constant(0.1, shape=[fc_size])), 51 'out_bias': tf.Variable(tf.constant(0.1, shape=[output_node])) 52 } 53 # 定义前向传播model 54 def cnn(input_tensor): 55 x_reshape = tf.reshape(input_tensor,shape=[-1,image_size,image_size,num_channel]) 56 57 conv1 = tf.nn.conv2d(x_reshape,weights['conv1_weights'],strides=[1,1,1,1],padding='SAME') 58 relu1 = tf.nn.relu(tf.nn.bias_add(conv1,biases['conv1_bias'])) 59 pool1 = tf.nn.max_pool(relu1,ksize=[1,2,2,1],strides=[1,2,2,1],padding='SAME') 60 norm1 = tf.nn.lrn(pool1,4,bias=1.0,alpha=0.001/9.0,beta=0.75,name='norm1') 61 norm1 = tf.nn.dropout(norm1,0.8) 62 63 conv2 = tf.nn.conv2d(norm1,weights['conv2_weights'],strides=[1,1,1,1],padding='SAME') 64 relu2 = tf.nn.relu(tf.nn.bias_add(conv2,biases['conv2_bias'])) 65 pool2 = tf.nn.max_pool(relu2,ksize=[1,2,2,1],strides=[1,2,2,1],padding='SAME') 66 norm2 = tf.nn.lrn(pool2,4,bias=1.0,alpha=0.001/9.0,beta=0.75,name='norm2') 67 norm2 = tf.nn.dropout(norm2,0.8) 68 69 conv3 = tf.nn.conv2d(norm2,weights['conv3_weights'],strides=[1,1,1,1],padding='SAME') 70 relu3 = tf.nn.relu(tf.nn.bias_add(conv3,biases['conv3_bias'])) 71 pool3 = tf.nn.max_pool(relu3,ksize=[1,2,2,1],strides=[1,2,2,1],padding='SAME') 72 norm3 = tf.nn.lrn(pool3,4,bias=1.0,alpha=0.001/9.0,beta=0.75,name='norm3') 73 norm3 = tf.nn.dropout(norm3,0.8) 74 #print(norm3.shape) 75 76 norm3_shape = norm3.get_shape().as_list() 77 nodes = norm3_shape[1] * norm3_shape[2] * norm3_shape[3] 78 reshaped = tf.reshape(norm3,[-1,nodes]) 79 #print(reshaped) 80 81 fc1_weightst = tf.Variable(tf.truncated_normal([nodes,fc_size], stddev=0.1)) 82 fc1 = tf.nn.relu(tf.matmul(reshaped,fc1_weightst) + biases['fc1_bias']) 83 84 fc2 = tf.nn.relu(tf.matmul(fc1,weights['fc2_weights']) + biases['fc2_bias']) 85 86 logit = tf.matmul(fc2,weights['out_weights']) + biases['out_bias'] 87 return logit 88 89 # 计算前向传播的结果 90 y = cnn(x) 91 92 global_step = tf.Variable(0, trainable=False) 93 # 初始化滑动平均类，给定训练轮数的变量可以加快训练早期的变量更新速度 94 variable_averages = tf.train.ExponentialMovingAverage(moving_average_decay, global_step) 95 # 对所有在神经网络上的参数使用滑动平均，辅助参数不使用（比如global_step） 96 variables_averages_op = variable_averages.apply(tf.trainable_variables()) 97 98 cross_entropy = tf.nn.sparse_softmax_cross_entropy_with_logits(logits=y, labels=tf.argmax(y_,1)) 99 loss = tf.reduce_mean(cross_entropy) 100 101 # 设置指数衰减学习率 102 learning_rate = tf.train.exponential_decay( 103 learning_rate_base,global_step,mnist.train.num_examples/batch_size,learning_rate_decay) 104 105 # 开始优化损失函数 106 optimizer = tf.train.GradientDescentOptimizer(learning_rate).minimize(loss) 107 correct_prediction = tf.equal(tf.argmax(y, 1), tf.argmax(y_, 1)) 108 # 使用tf.reduce_mean模型将前面的boolean值转换为float值，再求平均值，该值就是准确率 109 accurary = tf.reduce_mean(tf.cast(correct_prediction, tf.float32)) 110 111 112 with tf.Session() as sess: 113 # 初始化所有变量 114 tf.global_variables_initializer().run() 115 # 定义测试集 116 test_feed = { 117 x: mnist.test.images, 118 y_: mnist.test.labels 119 } 120 121 for i in range(training_steps): 122 xs, ys = mnist.train.next_batch(batch_size) 123 sess.run(optimizer,feed_dict={x:xs,y_:ys}) 124 if i%1000 == 0: 125 acc = sess.run(accurary,feed_dict={x:xs,y_:ys}) 126 print("After %d training step(s), train accuracy using average model is %g " % (i, acc)) 127 test_acc = sess.run(accurary, feed_dict=test_feed) 128 print("After %d training step(s), test accuracy using average model is %g " % (training_steps, test_acc))
运行结果:
