【入門】Pytorch實現簡單的圖片分類器
阿新 • • 發佈:2021-04-30
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文章目錄
前言
本文很適合一些想要入門機器視覺的小夥伴。本實驗推薦在Juypter Notebook上執行。
我們將按次序做如下步驟:
- 使用torchvision載入並且歸一化CIFAR10的訓練和測試資料集
- 定義一個卷積神經網路
- 定義一個損失函式
- 在訓練集上訓練模型
- 在測試集上測試模型
匯入庫
使用torchvision載入並歸一化資料集。
import torch
import torchvision
import torchvision.transforms as transforms
資料歸一化
torchvision資料集的輸出範圍是[0,1],我們將他們歸一化至[-1,1]
tranform = transforms.Compose([transform.ToTensor(),transform.Normalize((0.5,0.5,0.5),(0.5,0.5,0.5))])
trainset = torchvision.datasets.CIFAR10(root='./data',train = True,download=True,transform = transform)
trainloader = torch.utils.data.DataLoader(trainset, batch_size=4,shuffle=True, num_workers=2)
testset = torchvision.datasets.CIFAR10(root='./data', train=False,download=True, transform=transform)
testloader = torch.utils.data.DataLoader(testset, batch_size=4,shuffle=False, num_workers=2)
classes = ('plane' , 'car', 'bird', 'cat', 'deer', 'dog', 'frog', 'horse', 'ship', 'truck')
下載完成會有如下輸出:
檢視訓練集
# 展示其中的一些訓練圖片
import matplotlib.pyplot as plt
import numpy as np
# functions to show an image
def imshow(img):
img = img / 2 + 0.5 # unnormalize
npimg = img.numpy()
plt.imshow(np.transpose(npimg, (1, 2, 0)))
plt.show()
# get some random training images
dataiter = iter(trainloader)
images, labels = dataiter.next()
# show images
imshow(torchvision.utils.make_grid(images))
# print labels
print(' '.join('%5s' % classes[labels[j]] for j in range(4)))
torchvision.utils.make_grid(images):將多張圖片拼成一張圖片,在展示資料時很有用。
圖片展示:
構造網路
定義一個卷積神經網路
import torch.nn as nn
import torch.nn.functional as F
class Net(nn.Module):
def __init__(self):
super(Net, self).__init__()
self.conv1 = nn.Conv2d(3,6,5)
self.pool = nn.MaxPool2d(2,2)
self.conv2 = nn.Conv2d(6, 16, 5)
self.fc1 = nn.Linear(16 * 5 * 5, 120)
self.fc2 = nn.Linear(120, 84)
self.fc3 = nn.Linear(84, 10)
def forward(self, x):
x = self.pool(F.relu(self.conv1(x)))
x = self.pool(F.relu(self.conv2(x)))
x = x.view(-1, 16 * 5 * 5)
x = F.relu(self.fc1(x))
x = F.relu(self.fc2(x))
x = self.fc3(x)
return x
net = Net()
nn.Conv2d(3,6,5)表示輸入的圖片是3個通道(in_channel),輸出的是6個通道(out_channel),卷積核大小kernel.size等於5
nn.Conv2d(6, 16, 5)同理。
想檢視網路結構的話,可以輸出net康康。
print(net)
定義損失函式和優化器
使用分類交叉熵Cross-Enteopy作損失函式
使用動量SGD做優化器
import torch.optim as optim
criterion = nn.CrossEntropyLoss()
optimizer = optim.SGD(net.parameters(), lr=0.001, momentum=0.9)
開始訓練
for epoch in range(2): # loop over the dataset multiple times
running_loss = 0.0
for i, data in enumerate(trainloader, 0):
# get the inputs
inputs, labels = data
# zero the parameter gradients
optimizer.zero_grad()
# forward + backward + optimize
outputs = net(inputs)
loss = criterion(outputs, labels)
loss.backward()
optimizer.step()
# print statistics
running_loss += loss.item()
if i % 2000 == 1999: # print every 2000 mini-batches
print('[%d, %5d] loss: %.3f' %
(epoch + 1, i + 1, running_loss / 2000))
running_loss = 0.0
print('Finished Training')
訓練兩個epoch之後的效果:
檢視分類效果
correct = 0
total = 0
with torch.no_grad():
for data in testloader:
images, labels = data
outputs = net(images)
_, predicted = torch.max(outputs.data, 1)
total += labels.size(0)
correct += (predicted == labels).sum().item()
print('Accuracy of the network on the 10000 test images: %d %%' % (
100 * correct / total))
檢視每個類別的準確度
class_correct = list(0. for i in range(10))
class_total = list(0. for i in range(10))
with torch.no_grad():
for data in testloader:
images, labels = data
outputs = net(images)
_, predicted = torch.max(outputs, 1)
c = (predicted == labels).squeeze()
for i in range(4):
label = labels[i]
class_correct[label] += c[i].item()
class_total[label] += 1
for i in range(10):
print('Accuracy of %5s : %2d %%' % (classes[i], 100 * class_correct[i] / class_total[i]))
結果:
使用你的GPU訓練
首先我們定義GPU裝置為第一個可見的cuda裝置
device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
# Assume that we are on a CUDA machine, then this should print a CUDA device:
print(device)
net.to(device)
你必須在每個步驟向GPU傳送輸入和目標
inputs, labels = inputs.to(device), labels.to(device)
優化
- 在你的GPU上訓練分類器
- 更改網路結構,嘗試增加你的網路寬度,看看會得到怎樣的提升。
如果本文對你有幫助的話請繼續關注,點贊,收藏~ 後續我將繼續更新優化部分的內容,以及一些深度學習的東西。