LeNet 诞生于 1994 年，是最早的卷积神经网络之一，并且推动了深度学习领域的发展。自从 1988 年开始，在许多次成功的迭代后，这项由 Yann LeCun 完成的开拓性成果被命名为 LeNet5。
LeNet-5 出自论文 Gradient-Based Learning Applied to Document Recognition，是一种用于手写体字符识别的非常高效的卷积神经网络。Lenet-5 是 Yann LeCun 提出的，对 MNIST 数据集的分识别准确度可达 99.2%。
1、LeNet-5 Architecture
The LeNet-5 architecture consists of two sets of convolutional and average pooling layers, followed by a flattening convolutional layer, then two fully-connected layers and finally a softmax classifier.
1.1 First Layer
The input for LeNet-5 is a 32×32 grayscale image which passes through the first convolutional layer with 6 feature maps or filters having size 5×5 and a stride of one. The image dimensions changes from 32x32x1 to 28x28x6.
1.2 Second Layer
Then the LeNet-5 applies average pooling layer or sub-sampling layer with a filter size 2×2 and a stride of two. The resulting image dimensions will be reduced to 14x14x6.
1.3 Third Layer
Next, there is a second convolutional layer with 16 feature maps having size 5×5 and a stride of 1. In this layer, only 10 out of 16 feature maps are connected to 6 feature maps of the previous layer as shown below.
The main reason is to break the symmetry in the network and keeps the number of connections within reasonable bounds. That’s why the number of training parameters in this layers are 1516 instead of 2400 and similarly, the number of connections are 151600 instead of 240000.
1.4 Fourth Layer
The fourth layer (S4) is again an average pooling layer with filter size 2×2 and a stride of 2. This layer is the same as the second layer (S2) except it has 16 feature maps so the output will be reduced to 5x5x16.
1.5 Fifth Layer
The fifth layer (C5) is a fully connected convolutional layer with 120 feature maps each of size 1×1. Each of the 120 units in C5 is connected to all the 400 nodes (5x5x16) in the fourth layer S4.
1.6 Sixth Layer
The sixth layer is a fully connected layer (F6) with 84 units.
1.7 Output Layer
Finally, there is a fully connected softmax output layer ŷ with 10 possible values corresponding to the digits from 0 to 9.
2、Summary of LeNet-5 Architecture
3、Implementation of LeNet-5 Using Tensorflow2.0
3.1 导入相关包
import tensorflow as tf
from tensorflow.keras import Sequential, layers, losses, optimizers, datasets
from tensorflow.keras.callbacks import TensorBoard
3.2 加载数据集并进行预处理
数据预处理函数：
def preprocess(x, y):
"""
预处理函数
"""
x = tf.cast(x, dtype=tf.float32) / 255
x = tf.expand_dims(x, axis=3)
y = tf.cast(y, dtype=tf.int32)
y = tf.one_hot(y, depth=10)
return x,y
加载手写数据集：
# 加载手写数据集
(x, y), (x_test, y_test) = datasets.mnist.load_data()
print(x.shape, y.shape, x_test.shape, y_test.shape)
输出：
(60000, 28, 28) (60000,) (10000, 28, 28) (10000,)
转化为Dataset数据集：
batchsz = 1000
# 转化为Dataset数据集
train_db = tf.data.Dataset.from_tensor_slices((x, y))
# 随机打散
train_db = train_db.shuffle(10000)
train_db = train_db.batch(batchsz)
# 数据预处理
train_db = train_db.map(preprocess)
test_db = tf.data.Dataset.from_tensor_slices((x_test, y_test))
test_db = test_db.batch(batchsz).map(preprocess)
sample = sample = next(iter(train_db))
print('batch:', sample[0].shape, sample[1].shape)
print('batch:', sample[0].shape, sample[1].shape)
输出：
batch: (1000, 28, 28, 1) (1000, 10)
batch: (1000, 28, 28, 1) (1000, 10)
3.3 创建网络
model = Sequential([
layers.Conv2D(6, kernel_size=5, strides=1, activation="relu"), # Conv Layer 1
layers.MaxPool2D(pool_size=2, strides=2), # Pooling Layer 2
layers.Conv2D(16, kernel_size=5, strides=1, activation="relu"), # Conv Layer 3
layers.MaxPool2D(pool_size=2, strides=2), # Pooling Layer 4
layers.Flatten(), # flatten 层，方便全连接处理
layers.Dense(120, activation="relu"), # Fully connected layer 1
layers.Dense(84, activation="relu"), # Fully connected layer 2
layers.Dense(10) # Fully connected layer
])
打印网络结构：
model.build(input_shape=(None, 28, 28, 1))
model.summary()
输出：
Model: "sequential"
_________________________________________________________________
Layer (type) Output Shape Param #
=================================================================
conv2d (Conv2D) multiple 156
_________________________________________________________________
max_pooling2d (MaxPooling2D) multiple 0
_________________________________________________________________
conv2d_1 (Conv2D) multiple 2416
_________________________________________________________________
max_pooling2d_1 (MaxPooling2 multiple 0
_________________________________________________________________
flatten (Flatten) multiple 0
_________________________________________________________________
dense (Dense) multiple 30840
_________________________________________________________________
dense_1 (Dense) multiple 10164
_________________________________________________________________
dense_2 (Dense) multiple 850
=================================================================
Total params: 44,426
Trainable params: 44,426
Non-trainable params: 0
_________________________________________________________________
3.4 模型训练与验证
模型装配：
model.compile(
optimizer=optimizers.Adam(lr=1e-3),
loss=tf.losses.CategoricalCrossentropy(from_logits=True),
metrics=['accuracy']
)
模型训练：
model.fit(
train_db,
epochs=5,
validation_data=test_db,
validation_freq=2
)
输出：
Train for 60 steps, validate for 10 steps
Epoch 1/5
60/60 [==============================] - 15s 253ms/step - loss: 1.0019 - accuracy: 0.7417
Epoch 2/5
60/60 [==============================] - 15s 246ms/step - loss: 0.2996 - accuracy: 0.9113 - val_loss: 0.2260 - val_accuracy: 0.9320
Epoch 3/5
60/60 [==============================] - 14s 232ms/step - loss: 0.2050 - accuracy: 0.9399
Epoch 4/5
60/60 [==============================] - 15s 243ms/step - loss: 0.1517 - accuracy: 0.9545 - val_loss: 0.1196 - val_accuracy: 0.9625
Epoch 5/5
60/60 [==============================] - 14s 228ms/step - loss: 0.1231 - accuracy: 0.9631
模型验证：
model.evaluate(test_db)
输出：
10/10 [==============================] - 1s 71ms/step - loss: 0.0971 - accuracy: 0.9701
转载自： https://engmrk.com/lenet-5-a-classic-cnn-architecture/
