目录

1.什么是pix2pix GAN

2.pix2pixGAN生成器的设计

3.pix2pixGAN判别器的设计

4.损失函数

5.代码实现 

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1.什么是pix2pix GAN

Pix2pixgan本质上是一个cgan，图片x作为此cGAN的条件，需要输入到G和D中。G的输入是x(x是需要转换的图片)，输出是生成的图片G(x)。D则需要分辨出（x，G(x)）和(x，y）

pix2pixGAN主要用于图像之间的转换，又称图像翻译。

2.pix2pixGAN生成器的设计

对于图像翻译任务来说，输入和输出之间会共享很多信息。比如轮廓信息是共享的。如何解决共享问题？需要我们从损失函数的设计当中去思考。

如果使用普通的卷积神经网络，那么会导致每一层都承载保存着所有的信息。这样神经网络很容易出错（容易丢失一些信息）

所以，我们使用UNet模型作为生成器

3.pix2pixGAN判别器的设计

D要输入成对的图像。这类似于cGAN,如果G(x)和x是对应的，对于生成器来说希望判别为1；

如果G（x）和x不是对应的，对于生成器来说希望判别器判别为0

pix2pixGAN中的D被论文中被实现为patch_D.所谓patch,是指无论生成的图片有多大，将其切分为多个固定大小的patch输入进D去判断。如上图所示。

这样设计的好处是：D的输入变小，计算量小，训练速度快

4.损失函数

D网络损失函数：输入真实的成对图像希望判定为1；输入生成图像与原图希望判定为0

G网络损失函数：输入生成图像与原图像希望判定为1

公式如下图所示： 

对于图像翻译任务而言，G的输入和输出之间其实共享了很多信息。因而为了保证输入图像和输出图像之间的相似度，还加入了L1loss,公式如下所示：

 所以，结合两个公式，总的损失函数为

5.代码实现 

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.utils import data
import torchvision #加载图片
from torchvision import transforms #图片变换
 
import numpy as np
import matplotlib.pyplot as plt #绘图
import os
import glob
from PIL import Image
 
imgs_path = glob.glob('base/*.jpg')
annos_path = glob.glob('base/*.png')
#预处理
transform = transforms.Compose([
    transforms.ToTensor(),
    transforms.Resize((256,256)),
    transforms.Normalize(mean=0.5,std=0.5
                )
    
])
 
#定义数据集
class CMP_dataset(data.Dataset):
    def __init__(self,imgs_path,annos_path):
        self.imgs_path =imgs_path
        self.annos_path = annos_path
    def __getitem__(self,index): 
        img_path = self.imgs_path[index]
        anno_path = self.annos_path[index]
        pil_img = Image.open(img_path) #读取数据
        pil_img = transform(pil_img)  #转换数据
        anno_img = Image.open(anno_path) #读取数据
        anno_img = anno_img.convert("RGB")
        pil_anno = transform(anno_img)  #转换数据
        return pil_anno,pil_img
    def __len__(self):
        return len(self.imgs_path)
 
#创建数据集
dataset = CMP_dataset(imgs_path,annos_path)
#将数据转化为dataloader的格式，方便迭代
BATCHSIZE = 32
dataloader = data.DataLoader(dataset,
                            batch_size = BATCHSIZE,
                            shuffle = True)
annos_batch,imgs_batch = next(iter(dataloader))
 
#定义下采样模块
class Downsample(nn.Module):
    def __init__(self,in_channels,out_channels):
        super(Downsample,self).__init__()
        self.conv_relu = nn.Sequential(
        nn.Conv2d(in_channels,out_channels,
                 kernel_size=3,
                 stride=2,
                 padding=1),
        nn.LeakyReLU(inplace=True))
        self.bn = nn.BatchNorm2d(out_channels)
    def forward(self,x,is_bn=True):
        x=self.conv_relu(x)
        if is_bn:
            x=self.bn(x)
        return x
 
 
#定义上采样模块
class Upsample(nn.Module):
    def __init__(self,in_channels,out_channels):
        super(Upsample,self).__init__()
        self.upconv_relu = nn.Sequential(
        nn.ConvTranspose2d(in_channels,out_channels,
                 kernel_size=3,
                 stride=2,
                 padding=1,
                 output_padding=1), #反卷积，变为原来的2倍
        nn.LeakyReLU(inplace=True))
        self.bn = nn.BatchNorm2d(out_channels)
    def forward(self,x,is_drop=False):
        x=self.upconv_relu(x)
        x=self.bn(x)
        if is_drop:
            x=F.dropout2d(x)
        return x
        
 
#定义生成器：包含6个下采样，5个上采样，一个输出层
class Generator(nn.Module):
    def __init__(self):
        super(Generator,self).__init__()
        self.down1 = Downsample(3,64)      #64,128,128
        self.down2 = Downsample(64,128)    #128,64,64
        self.down3 = Downsample(128,256)   #256,32,32
        self.down4 = Downsample(256,512)   #512,16,16
        self.down5 = Downsample(512,512)   #512,8,8
        self.down6 = Downsample(512,512)   #512,4,4
        
        self.up1 = Upsample(512,512)    #512,8,8
        self.up2 = Upsample(1024,512)   #512,16,16
        self.up3 = Upsample(1024,256)   #256,32,32
        self.up4 = Upsample(512,128)    #128,64,64
        self.up5 = Upsample(256,64)     #64,128,128
        
        self.last = nn.ConvTranspose2d(128,3,
                                      kernel_size=3,
                                      stride=2,
                                      padding=1,
                                      output_padding=1)  #3,256,256
        
    def forward(self,x):
        x1 = self.down1(x)
        x2 = self.down2(x1)
        x3 = self.down3(x2)
        x4 = self.down4(x3)
        x5 = self.down5(x4)
        x6 = self.down6(x5)
        
        x6 = self.up1(x6,is_drop=True)
        x6 = torch.cat([x6,x5],dim=1)
        
        x6 = self.up2(x6,is_drop=True)
        x6 = torch.cat([x6,x4],dim=1)
        
        x6 = self.up3(x6,is_drop=True)
        x6 = torch.cat([x6,x3],dim=1)
        
        x6 = self.up4(x6)
        x6 = torch.cat([x6,x2],dim=1)
        
        x6 = self.up5(x6)
        x6 = torch.cat([x6,x1],dim=1)
        
        
        x6 = torch.tanh(self.last(x6))
        return x6
 
#定义判别器  输入anno+img(生成或者真实)  concat
class Discriminator(nn.Module):
    def __init__(self):
        super(Discriminator,self).__init__()
        self.down1 = Downsample(6,64)
        self.down2 = Downsample(64,128)
        self.conv1 = nn.Conv2d(128,256,3)
        self.bn = nn.BatchNorm2d(256)
        self.last = nn.Conv2d(256,1,3)
    def forward(self,anno,img):
        x=torch.cat([anno,img],axis =1) 
        x=self.down1(x,is_bn=False)
        x=self.down2(x,is_bn=True)
        x=F.dropout2d(self.bn(F.leaky_relu(self.conv1(x))))
        x=torch.sigmoid(self.last(x))  #batch*1*60*60
        return x
 
device = "cuda" if torch.cuda.is_available() else'cpu'
gen = Generator().to(device)
dis = Discriminator().to(device)
d_optimizer = torch.optim.Adam(dis.parameters(),lr=1e-3,betas=(0.5,0.999))
g_optimizer = torch.optim.Adam(gen.parameters(),lr=1e-3,betas=(0.5,0.999))
#绘图
def generate_images(model,test_anno,test_real):
    prediction = model(test_anno).permute(0,2,3,1).detach().cpu().numpy()
    test_anno = test_anno.permute(0,2,3,1).cpu().numpy()
    test_real = test_real.permute(0,2,3,1).cpu().numpy()
    plt.figure(figsize = (10,10))
    display_list = [test_anno[0],test_real[0],prediction[0]]
    title = ['Input','Ground Truth','Output']
    for i in range(3):
        plt.subplot(1,3,i+1)
        plt.title(title[i])
        plt.imshow(display_list[i])
        plt.axis('off') #坐标系关掉
    plt.show()
 
test_imgs_path = glob.glob('extended/*.jpg')
test_annos_path = glob.glob('extended/*.png')
 
test_dataset = CMP_dataset(test_imgs_path,test_annos_path)
 
test_dataloader = torch.utils.data.DataLoader(
test_dataset,
batch_size=BATCHSIZE,)
 
#定义损失函数
#cgan 损失函数
loss_fn = torch.nn.BCELoss()
#L1 loss
 
 
annos_batch,imgs_batch = annos_batch.to(device),imgs_batch.to(device)
LAMBDA = 7  #L1损失的权重
 
D_loss = []#记录训练过程中判别器loss变化
G_loss = []#记录训练过程中生成器loss变化
 
#开始训练
for epoch in range(10):
    D_epoch_loss = 0
    G_epoch_loss = 0
    count = len(dataloader)
    for step,(annos,imgs) in enumerate(dataloader):
        imgs = imgs.to(device)
        annos = annos.to(device)
        #定义判别器的损失计算以及优化的过程
        d_optimizer.zero_grad()
        disc_real_output = dis(annos,imgs)#输入真实成对图片
        d_real_loss = loss_fn(disc_real_output,torch.ones_like(disc_real_output,
                                                             device=device))
        d_real_loss.backward()
        
        gen_output = gen(annos)
        disc_gen_output = dis(annos,gen_output.detach())
        d_fack_loss = loss_fn(disc_gen_output,torch.zeros_like(disc_gen_output,
                                                              device=device))
        d_fack_loss.backward()
        
        disc_loss = d_real_loss+d_fack_loss#判别器的损失计算
        d_optimizer.step()
        
        #定义生成器的损失计算以及优化的过程
        g_optimizer.zero_grad()
        disc_gen_out = dis(annos,gen_output)
        gen_loss_crossentropyloss = loss_fn(disc_gen_out,
                                            torch.ones_like(disc_gen_out,
                                                              device=device))
        gen_l1_loss = torch.mean(torch.abs(gen_output-imgs))
        gen_loss = gen_loss_crossentropyloss +LAMBDA*gen_l1_loss
        gen_loss.backward() #反向传播
        g_optimizer.step() #优化
        
        #累计每一个批次的loss
        with torch.no_grad():
            D_epoch_loss +=disc_loss.item()
            G_epoch_loss +=gen_loss.item()
    #求平均损失
    with torch.no_grad():
            D_epoch_loss /=count
            G_epoch_loss /=count
            D_loss.append(D_epoch_loss)
            G_loss.append(G_epoch_loss)
            #训练完一个Epoch,打印提示并绘制生成的图片
            print("Epoch:",epoch)
            generate_images(gen,annos_batch,imgs_batch)