【pytorch】孪生网络Siamese network入门教程

论文原文：《Learning a Similarity Metric Discriminatively, with Application to Face
Verification》
http://yann.lecun.com/exdb/publis/pdf/chopra-05.pdf

1. 数据集

数据采用的是AT&T人脸数据。共40个人，每个人有10张脸。数据集下载：AT&T

2. Imports

All the imports are defined here

import torchvision
import torchvision.datasets as dset
import torchvision.transforms as transforms
from torch.utils.data import DataLoader,Dataset
import matplotlib.pyplot as plt
import torchvision.utils
import numpy as np
import random
from PIL import Image
import torch
from torch.autograd import Variable
import PIL.ImageOps    
import torch.nn as nn
from torch import optim
import torch.nn.functional as F

3. Helper functions

Set of helper functions

def imshow(img,text=None,should_save=False):
    npimg = img.numpy()
    plt.axis("off")
    if text:
        plt.text(75, 8, text, style='italic',fontweight='bold',
            bbox={'facecolor':'white', 'alpha':0.8, 'pad':10})
    plt.imshow(np.transpose(npimg, (1, 2, 0)))
    plt.show()    

def show_plot(iteration,loss):
    plt.plot(iteration,loss)
    plt.show()

4. Configuration Class

A simple class to manage configuration

class Config():
    training_dir = "./data/faces/training/"
    testing_dir = "./data/faces/testing/"
    train_batch_size = 64
    train_number_epochs = 100

5. Custom Dataset Class

This dataset generates a pair of images. 0 for geniune pair and 1 for imposter pair

class SiameseNetworkDataset(Dataset):
    
    def __init__(self,imageFolderDataset,transform=None,should_invert=True):
        self.imageFolderDataset = imageFolderDataset    
        self.transform = transform
        self.should_invert = should_invert
        
    def __getitem__(self,index):
        img0_tuple = random.choice(self.imageFolderDataset.imgs)
        #we need to make sure approx 50% of images are in the same class
        should_get_same_class = random.randint(0,1) 
        if should_get_same_class:
            while True:
                #keep looping till the same class image is found
                img1_tuple = random.choice(self.imageFolderDataset.imgs) 
                if img0_tuple[1]==img1_tuple[1]:
                    break
        else:
            while True:
                #keep looping till a different class image is found
                
                img1_tuple = random.choice(self.imageFolderDataset.imgs) 
                if img0_tuple[1] !=img1_tuple[1]:
                    break

        img0 = Image.open(img0_tuple[0])
        img1 = Image.open(img1_tuple[0])
        img0 = img0.convert("L")
        img1 = img1.convert("L")
        
        if self.should_invert:
            img0 = PIL.ImageOps.invert(img0)
            img1 = PIL.ImageOps.invert(img1)

        if self.transform is not None:
            img0 = self.transform(img0)
            img1 = self.transform(img1)
        
        return img0, img1 , torch.from_numpy(np.array([int(img1_tuple[1]!=img0_tuple[1])],dtype=np.float32))
    
    def __len__(self):
        return len(self.imageFolderDataset.imgs)

6. Using Image Folder Dataset

folder_dataset = dset.ImageFolder(root=Config.training_dir)
siamese_dataset = SiameseNetworkDataset(imageFolderDataset=folder_dataset,
                                        transform=transforms.Compose([transforms.Resize((100,100)),
                                                                      transforms.ToTensor()
                                                                      ])
                                       ,should_invert=False)

7. Visualising some of the data

The top row and the bottom row of any column is one pair. The 0s and 1s correspond to the column of the image. 1 indiciates dissimilar, and 0 indicates similar.

vis_dataloader = DataLoader(siamese_dataset,
                        shuffle=True,
                        num_workers=8,
                        batch_size=8)
dataiter = iter(vis_dataloader)


example_batch = next(dataiter)
concatenated = torch.cat((example_batch[0],example_batch[1]),0)
imshow(torchvision.utils.make_grid(concatenated))
print(example_batch[2].numpy())

[[1.]
[1.]
[0.]
[0.]
[1.]
[1.]
[1.]
[1.]]

8. Neural Net Definition

We will use a standard convolutional neural network

class SiameseNetwork(nn.Module):
    def __init__(self):
        super(SiameseNetwork, self).__init__()
        self.cnn1 = nn.Sequential(
            nn.ReflectionPad2d(1),
            nn.Conv2d(1, 4, kernel_size=3),
            nn.ReLU(inplace=True),
            nn.BatchNorm2d(4),
            
            nn.ReflectionPad2d(1),
            nn.Conv2d(4, 8, kernel_size=3),
            nn.ReLU(inplace=True),
            nn.BatchNorm2d(8),


            nn.ReflectionPad2d(1),
            nn.Conv2d(8, 8, kernel_size=3),
            nn.ReLU(inplace=True),
            nn.BatchNorm2d(8),


        )

        self.fc1 = nn.Sequential(
            nn.Linear(8*100*100, 500),
            nn.ReLU(inplace=True),

            nn.Linear(500, 500),
            nn.ReLU(inplace=True),

            nn.Linear(500, 5))

    def forward_once(self, x):
        output = self.cnn1(x)
        output = output.view(output.size()[0], -1)
        output = self.fc1(output)
        return output

    def forward(self, input1, input2):
        output1 = self.forward_once(input1)
        output2 = self.forward_once(input2)
        return output1, output2

9. Contrastive Loss

class ContrastiveLoss(torch.nn.Module):
    """
    Contrastive loss function.
    Based on: http://yann.lecun.com/exdb/publis/pdf/hadsell-chopra-lecun-06.pdf
    """

    def __init__(self, margin=2.0):
        super(ContrastiveLoss, self).__init__()
        self.margin = margin

    def forward(self, output1, output2, label):
        euclidean_distance = F.pairwise_distance(output1, output2, keepdim = True)
        loss_contrastive = torch.mean((1-label) * torch.pow(euclidean_distance, 2) +
                                      (label) * torch.pow(torch.clamp(self.margin - euclidean_distance, min=0.0), 2))


        return loss_contrastive

10. Training Time!

train_dataloader = DataLoader(siamese_dataset,
                        shuffle=True,
                        num_workers=8,
                        batch_size=Config.train_batch_size)

net = SiameseNetwork().cuda()
criterion = ContrastiveLoss()
optimizer = optim.Adam(net.parameters(),lr = 0.0005 )

counter = []
loss_history = [] 
iteration_number= 0

for epoch in range(0,Config.train_number_epochs):
    for i, data in enumerate(train_dataloader,0):
        img0, img1 , label = data
        img0, img1 , label = img0.cuda(), img1.cuda() , label.cuda()
        optimizer.zero_grad()
        output1,output2 = net(img0,img1)
        loss_contrastive = criterion(output1,output2,label)
        loss_contrastive.backward()
        optimizer.step()
        if i %10 == 0 :
            print("Epoch number {}\n Current loss {}\n".format(epoch,loss_contrastive.item()))
            iteration_number +=10
            counter.append(iteration_number)
            loss_history.append(loss_contrastive.item())
show_plot(counter,loss_history)

参考

https://blog.csdn.net/yukai08008/article/details/108236724

https://www.cnblogs.com/king-lps/p/8342452.html

https://github.com/harveyslash/Facial-Similarity-with-Siamese-Networks-in-Pytorch/