Pytorch 实现图片分类

CNN 网络适用于图片识别，卷积神经网络主要用于图片的处理识别。卷积神经网络，包括一下几部分，输入层、卷积层、池化层、全链接层和输出层。

使用 CIFAR-10 进行训练， CIFAR-10 中图片尺寸为 32 * 32。卷积层通过卷积核移动进行计算最终生成特征图。

通过池化层进行降维度

卷积网络结构从输入到输出， 3* 32*32 --> 10：

类型	Weight	BIAS
卷积(3, 12, 5)	(12, 3, 5, 5)	12
卷积(12, 12, 5)	(12, 12, 5, 5)	12
Norm	12	12
卷积(12, 24, 5)	(24, 12, 5, 5)	24
卷积(24 24, 5)	(24, 24, 5, 5)	24
Norm	24	24
Linear	(10, 2400)	10

训练分类模型

准备数据

from torchvision.datasets import CIFAR10
from torchvision.transforms import transforms
from torch.utils.data import DataLoader# Loading and normalizing the data.
# Define transformations for the training and test sets
transformations = transforms.Compose([transforms.ToTensor(),transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))
])# CIFAR10 dataset consists of 50K training images. We define the batch size of 10 to load 5,000 batches of images.
batch_size = 10
number_of_labels = 10 # Create an instance for training. 
# When we run this code for the first time, the CIFAR10 train dataset will be downloaded locally. 
train_set =CIFAR10(root="./data",train=True,transform=transformations,download=True)# Create a loader for the training set which will read the data within batch size and put into memory.
train_loader = DataLoader(train_set, batch_size=batch_size, shuffle=True, num_workers=0)
print("The number of images in a training set is: ", len(train_loader)*batch_size)# Create an instance for testing, note that train is set to False.
# When we run this code for the first time, the CIFAR10 test dataset will be downloaded locally. 
test_set = CIFAR10(root="./data", train=False, transform=transformations, download=True)# Create a loader for the test set which will read the data within batch size and put into memory. 
# Note that each shuffle is set to false for the test loader.
test_loader = DataLoader(test_set, batch_size=batch_size, shuffle=False, num_workers=0)
print("The number of images in a test set is: ", len(test_loader)*batch_size)print("The number of batches per epoch is: ", len(train_loader))
classes = ('plane', 'car', 'bird', 'cat', 'deer', 'dog', 'frog', 'horse', 'ship', 'truck')

创建网络

import torch
import torch.nn as nn
import torchvision
import torch.nn.functional as F# Define a convolution neural network
class Network(nn.Module):def __init__(self):super(Network, self).__init__()self.conv1 = nn.Conv2d(in_channels=3, out_channels=12, kernel_size=5, stride=1, padding=1)self.bn1 = nn.BatchNorm2d(12)self.conv2 = nn.Conv2d(in_channels=12, out_channels=12, kernel_size=5, stride=1, padding=1)self.bn2 = nn.BatchNorm2d(12)self.pool = nn.MaxPool2d(2,2)self.conv4 = nn.Conv2d(in_channels=12, out_channels=24, kernel_size=5, stride=1, padding=1)self.bn4 = nn.BatchNorm2d(24)self.conv5 = nn.Conv2d(in_channels=24, out_channels=24, kernel_size=5, stride=1, padding=1)self.bn5 = nn.BatchNorm2d(24)self.fc1 = nn.Linear(24*10*10, 10)def forward(self, input):output = F.relu(self.bn1(self.conv1(input)))      output = F.relu(self.bn2(self.conv2(output)))     output = self.pool(output)                        output = F.relu(self.bn4(self.conv4(output)))     output = F.relu(self.bn5(self.conv5(output)))     output = output.view(-1, 24*10*10)output = self.fc1(output)return output# Instantiate a neural network model 
model = Network()

定义损失函数

使用交叉熵函数作为损失函数，交叉熵分为两种

• 二分类交叉熵函数
  
• 多分类交叉熵函数
  

loss_fn = nn.CrossEntropyLoss()
optimizer = Adam(model.parameters(), lr=0.001, weight_decay=0.0001)

模型训练

from torch.autograd import Variable# Function to save the model
def saveModel():path = "./myFirstModel.pth"torch.save(model.state_dict(), path)# Function to test the model with the test dataset and print the accuracy for the test images
def testAccuracy():model.eval()accuracy = 0.0total = 0.0device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")with torch.no_grad():for data in test_loader:images, labels = data# run the model on the test set to predict labelsoutputs = model(images.to(device))# the label with the highest energy will be our prediction_, predicted = torch.max(outputs.data, 1)total += labels.size(0)accuracy += (predicted == labels.to(device)).sum().item()# compute the accuracy over all test imagesaccuracy = (100 * accuracy / total)return(accuracy)# Training function. We simply have to loop over our data iterator and feed the inputs to the network and optimize.
def train(num_epochs):best_accuracy = 0.0# Define your execution devicedevice = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")print("The model will be running on", device, "device")# Convert model parameters and buffers to CPU or Cudamodel.to(device)for epoch in range(num_epochs):  # loop over the dataset multiple timesrunning_loss = 0.0running_acc = 0.0for i, (images, labels) in enumerate(train_loader, 0):# get the inputsimages = Variable(images.to(device))labels = Variable(labels.to(device))# zero the parameter gradientsoptimizer.zero_grad()# predict classes using images from the training setoutputs = model(images)# compute the loss based on model output and real labelsloss = loss_fn(outputs, labels)# backpropagate the lossloss.backward()# adjust parameters based on the calculated gradientsoptimizer.step()# Let's print statistics for every 1,000 imagesrunning_loss += loss.item()     # extract the loss valueif i % 1000 == 999:    # print every 1000 (twice per epoch) print('[%d, %5d] loss: %.3f' %(epoch + 1, i + 1, running_loss / 1000))# zero the lossrunning_loss = 0.0# Compute and print the average accuracy fo this epoch when tested over all 10000 test imagesaccuracy = testAccuracy()print('For epoch', epoch+1,'the test accuracy over the whole test set is %d %%' % (accuracy))# we want to save the model if the accuracy is the bestif accuracy > best_accuracy:saveModel()best_accuracy = accuracy

测试模型

import matplotlib.pyplot as plt
import numpy as np# Function to show the images
def imageshow(img):img = img / 2 + 0.5     # unnormalizenpimg = img.numpy()plt.imshow(np.transpose(npimg, (1, 2, 0)))plt.show()# Function to test the model with a batch of images and show the labels predictions
def testBatch():# get batch of images from the test DataLoader  images, labels = next(iter(test_loader))# show all images as one image gridimageshow(torchvision.utils.make_grid(images))# Show the real labels on the screen print('Real labels: ', ' '.join('%5s' % classes[labels[j]] for j in range(batch_size)))# Let's see what if the model identifiers the  labels of those exampleoutputs = model(images)# We got the probability for every 10 labels. The highest (max) probability should be correct label_, predicted = torch.max(outputs, 1)# Let's show the predicted labels on the screen to compare with the real onesprint('Predicted: ', ' '.join('%5s' % classes[predicted[j]] for j in range(batch_size)))

执行模型

if __name__ == "__main__":# Let's build our modeltrain(5)print('Finished Training')# Test which classes performed welltestAccuracy()# Let's load the model we just created and test the accuracy per labelmodel = Network()path = "myFirstModel.pth"model.load_state_dict(torch.load(path))# Test with batch of imagestestBatch()

总结

pytorch 搭建一个 CNN 模型比较简单，5 轮训练之后，效果就可以达到 60%，10 张图片中预测对了 6 张。