背景

猫狗识别是学习CNN中最有趣的一个应用，本次我在Kaggle上的“Dogs vs. Cats Redux”进行了一次尝试。考虑到训练速度和实验的简易性，我使用了比较简单的AlexNet，并且还用了对应ImageNet的预训练模型（https://github.com/Cadene/pretrained-models.pytorch）。
因此，下文中将按照数据处理、模型预处理、训练和测试结果几个部分展开，实验代码如下：

https://github.com/Yannnnnnnnnnnn/learnPyTorch/tree/master/catVSdog

---

数据处理

从Kaggle上下载完比赛数据后，首先要做两个处理：
1、将train数据中不同类别的数据放到不同的文件夹下，例如猫的数据放在“0”文件夹下，狗的数据放在“1”文件夹下，如下图所示：

2、完成train数据处理后，则需要进一步将其分成train数据和validation数据；本次实验时，我通过随机采样的方式，从train中选取了20%的数据用于验证，代码如下：

# -*- coding:utf-8 -*- 
# https://blog.csdn.net/mdjxy63/article/details/78946455
__author__ = 'xuy'

 
import os
import shutil
import random
 
root_dir = r'/home/yqs/Desktop/dogs-vs-cats-redux-kernels-edition/train/0'
output_dir = r'/home/yqs/Desktop/dogs-vs-cats-redux-kernels-edition/validation/0'

percentage = 0.2 

for root, dirs, files in os.walk(root_dir):
    number_of_files = len(os.listdir(root))
    ref_copy = int(round(percentage * number_of_files))#随机筛选20%的图片到新建的文件夹当中
    for i in range(ref_copy):
        chosen_one = random.choice(os.listdir(root))
        file_in_track = root
        file_to_copy = file_in_track + '/' + chosen_one
        if os.path.isfile(file_to_copy) == True:
            shutil.move(file_to_copy,output_dir)

---

模型预处理

模型预处理的主要内容读取ImageNet预训练的模型，然后对AlexNet网络最后一层进行修改，使得分类类别数量为2,代码如下：

import torch
import torch.nn as nn
from torch import optim
from torchvision.datasets import ImageFolder
from torchvision import transforms

# AlexNet网络结构
class MineAlexNet(nn.Module):
    def __init__(self, num_classes=2):
        super(MineAlexNet, self).__init__()
        
        self.features = nn.Sequential(
            nn.Conv2d(3, 64, kernel_size=11, stride=4, padding=2),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
            nn.Conv2d(64, 192, kernel_size=5, padding=2),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
            nn.Conv2d(192, 384, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(384, 256, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(256, 256, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
        )
        self.avgpool = nn.AdaptiveAvgPool2d((6, 6))
        self.classifier = nn.Sequential(
            nn.Dropout(),
            nn.Linear(256 * 6 * 6, 4096),
            nn.ReLU(inplace=True),
            nn.Dropout(),
            nn.Linear(4096, 4096),
            nn.ReLU(inplace=True),
            nn.Linear(4096, num_classes),
        )
        # softmax
        self.logsoftmax = nn.LogSoftmax(dim=1)

    def forward(self, x):
        x = self.features(x)
        x = self.avgpool(x)
        x = x.view(x.size(0), 256 * 6 * 6)
        x = self.classifier(x)
        x = self.logsoftmax(x)
        return x
    

# 读取提前下载的预训练模型
state_dict = torch.load('alexnetImageNet.pth')
alexNet = MineAlexNet(1000)
alexNet.load_state_dict(state_dict)

# 修改AlexNet网络结构的后两层
alexNet.classifier[6] = nn.Linear(4096, 2)

# 保存包含了预训练参数的，猫狗分类AlexNet模型
torch.save(alexNet.state_dict(), 'begin.pth')

---

训练

训练过程，目前我还没有明白炼丹的具体技巧，所以就随便训练了30epoch，代码如下：

import torch
import torch.nn as nn
from torch import optim
from torchvision.datasets import ImageFolder
from torchvision import transforms

# 数据预处理
# 非常重要，如果不处理，效果可能非常差！！！
normalize = transforms.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])
transform = transforms.Compose([
    transforms.Resize(size=(227,227)),
    transforms.RandomRotation(20),
    transforms.RandomHorizontalFlip(),
    transforms.ToTensor(), #将图片转换为Tensor,归一化至[0,1]
    normalize
])

# 从文件夹中读取训练数据
train_dataset = ImageFolder(r'/home/yqs/Desktop/dogs-vs-cats-redux-kernels-edition/train',transform=transform)
trainloader = torch.utils.data.DataLoader(train_dataset, batch_size=512, shuffle=True)

# 从文件夹中读取validation数据
validation_dataset = ImageFolder(r'/home/yqs/Desktop/dogs-vs-cats-redux-kernels-edition/validation',transform=transform)
validationloader = torch.utils.data.DataLoader(validation_dataset, batch_size=512, shuffle=True)

# AlexNet
class MineAlexNet(nn.Module):
    def __init__(self, num_classes=2):
        super(MineAlexNet, self).__init__()
        
        self.features = nn.Sequential(
            nn.Conv2d(3, 64, kernel_size=11, stride=4, padding=2),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
            nn.Conv2d(64, 192, kernel_size=5, padding=2),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
            nn.Conv2d(192, 384, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(384, 256, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(256, 256, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
        )
        self.avgpool = nn.AdaptiveAvgPool2d((6, 6))
        self.classifier = nn.Sequential(
            nn.Dropout(),
            nn.Linear(256 * 6 * 6, 4096),
            nn.ReLU(inplace=True),
            nn.Dropout(),
            nn.Linear(4096, 4096),
            nn.ReLU(inplace=True),
            nn.Linear(4096, num_classes),
        )

        self.logsoftmax = nn.LogSoftmax(dim=1)

    def forward(self, x):
        x = self.features(x)
        x = self.avgpool(x)
        x = x.view(x.size(0), 256 * 6 * 6)
        x = self.classifier(x)
        x = self.logsoftmax(x)
        return x
    

# 读取转换后的AlexNet模型
state_dict = torch.load('begin.pth')
alexNet = MineAlexNet(2)
alexNet.load_state_dict(state_dict)

# cuda
alexNet.cuda()



criterion = nn.CrossEntropyLoss()
optimizer = optim.SGD(alexNet.parameters(), lr=0.00005)

epochs = 30
train_losses, validation_losses = [], []

# 训练
for e in range(epochs):
    running_loss = 0
    for images,labels in trainloader:
        
        images = images.cuda()
        labels = labels.cuda()
       
        # TODO: Training pass
        optimizer.zero_grad()
        
        output = alexNet(images)
        loss = criterion(output, labels)
        loss.backward()
        optimizer.step()
        
        running_loss += loss.item()
    else:
        validation_loss = 0
        accuracy = 0
        
        # Turn off gradients for validation, saves memory and computations
        with torch.no_grad():
            for images, labels in validationloader:
                images = images.cuda()
                labels = labels.cuda()
                
                log_ps = alexNet(images)
                validation_loss += criterion(log_ps, labels)
                
                ps = torch.exp(log_ps)
                top_p, top_class = ps.topk(1, dim=1)
                equals = top_class == labels.view(*top_class.shape)
                accuracy += torch.mean(equals.type(torch.FloatTensor))
                
        train_losses.append(running_loss/len(trainloader))
        validation_losses.append(validation_loss/len(validationloader))

        torch.save(alexNet.state_dict(), str(e+1+37) +'.pth')

        print("Epoch: {}/{}.. ".format( e+1, epochs),
              "Training Loss: {:.3f}.. ".format(running_loss/len(trainloader)),
              "Test Loss: {:.3f}.. ".format(validation_loss/len(validationloader)),
              "Test Accuracy: {:.3f}".format(accuracy/len(validationloader)))
      
# 画一下精度图      
import matplotlib.pyplot as plt
plt.plot(train_losses, label='Training loss')
plt.plot(validation_losses, label='Validation loss')
plt.legend(frameon=False)

---

测试结果

最后我用test数据测试了一下训练的模型，在kaggle的得分如下，虽然结果不是特别好，但好歹也还行吧，排名大概在700左右。

预测的代码如下：

import torch
import torch.nn as nn
from torchvision import transforms

normalize = transforms.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])
transform = transforms.Compose([
    transforms.Resize(size=(227,227)),
    transforms.ToTensor(), 
    normalize
])


# AlexNet
class MineAlexNet(nn.Module):
    def __init__(self, num_classes=2):
        super(MineAlexNet, self).__init__()
        
        self.features = nn.Sequential(
            nn.Conv2d(3, 64, kernel_size=11, stride=4, padding=2),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
            nn.Conv2d(64, 192, kernel_size=5, padding=2),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
            nn.Conv2d(192, 384, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(384, 256, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(256, 256, kernel_size=3, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(kernel_size=3, stride=2),
        )
        self.avgpool = nn.AdaptiveAvgPool2d((6, 6))
        self.classifier = nn.Sequential(
            nn.Dropout(),
            nn.Linear(256 * 6 * 6, 4096),
            nn.ReLU(inplace=True),
            nn.Dropout(),
            nn.Linear(4096, 4096),
            nn.ReLU(inplace=True),
            nn.Linear(4096, num_classes),
        )

        self.logsoftmax = nn.LogSoftmax(dim=1)

    def forward(self, x):
        x = self.features(x)
        x = self.avgpool(x)
        x = x.view(x.size(0), 256 * 6 * 6)
        x = self.classifier(x)
        x = self.logsoftmax(x)
        return x
    

# load pretrained
state_dict = torch.load('30.pth')
alexNet = MineAlexNet(2)
alexNet.load_state_dict(state_dict)


from PIL import Image
import matplotlib.pyplot as plt
import os

f = open('/home/yqs/Desktop/dogs-vs-cats-redux-kernels-edition/submission.csv', 'w')
f.write("id,label \n")
path=r'/home/yqs/Desktop/dogs-vs-cats-redux-kernels-edition/test/'
for root, dirs, files in os.walk(path, topdown=False):
    for name in files:
     
        ## 打开图片并转成灰度
        image = Image.open(os.path.join(root, name))
        # 转成tensor
        tensor = transform(image)
        tensor = tensor.unsqueeze(0)
        inputdata = torch.autograd.Variable(tensor,requires_grad=False)
        outputdata = alexNet(inputdata)
        ps = torch.exp(outputdata)
        
        
        top_p, top_class = ps.topk(1, dim=1)
        print(top_p)
        print(top_class)

        
        
        filename,extension = os.path.splitext(name)
        # 输出结果
        prob = 0
        if top_class.detach().numpy()[0][0]==0:
            prob = 1 - top_p.detach().numpy()[0][0]
        else:
            prob = top_p.detach().numpy()[0][0]
        print(prob)
        f.write(filename+","+str(prob)+" \n")
f.close()

---

总结

应为已经有代码了，所以整体写的比较简单，主要目的还是鼓励自己。