【源码解读】cycleGAN（二) ：训练

源码地址：https://github.com/aitorzip/PyTorch-CycleGAN

训练的代码见于train.py，首先定义好网络，两个生成器A2B, B2A和两个判别器A, B，以及对应的优化器（优化器的设置保证了只更新生成器或判别器，不会互相影响）

###### Definition of variables ######
# Networks
netG_A2B = Generator(opt.input_nc, opt.output_nc)
netG_B2A = Generator(opt.output_nc, opt.input_nc)
netD_A = Discriminator(opt.input_nc)
netD_B = Discriminator(opt.output_nc)

# Optimizers & LR schedulers
optimizer_G = torch.optim.Adam(itertools.chain(netG_A2B.parameters(), netG_B2A.parameters()),
                                lr=opt.lr, betas=(0.5, 0.999))
optimizer_D_A = torch.optim.Adam(netD_A.parameters(), lr=opt.lr, betas=(0.5, 0.999))
optimizer_D_B = torch.optim.Adam(netD_B.parameters(), lr=opt.lr, betas=(0.5, 0.999))

然后是数据

# Dataset loader
transforms_ = [ transforms.Resize(int(opt.size*1.12), Image.BICUBIC), 
                transforms.RandomCrop(opt.size), 
                transforms.RandomHorizontalFlip(),
                transforms.ToTensor(),
                transforms.Normalize((0.5,0.5,0.5), (0.5,0.5,0.5)) ]
dataloader = DataLoader(ImageDataset(opt.dataroot, transforms_=transforms_, unaligned=True), 
                        batch_size=opt.batchSize, shuffle=True, num_workers=opt.n_cpu)

接着就可以求取损失，反传梯度，更新网络，更新网络的时候首先更新生成器，然后分别更新两个判别器

生成器：损失函数=身份损失+对抗损失+循环一致损失

###### Generators A2B and B2A ######
        optimizer_G.zero_grad()

        # Identity loss
        # G_A2B(B) should equal B if real B is fed
        same_B = netG_A2B(real_B)
        loss_identity_B = criterion_identity(same_B, real_B)*5.0
        # G_B2A(A) should equal A if real A is fed
        same_A = netG_B2A(real_A)
        loss_identity_A = criterion_identity(same_A, real_A)*5.0

        # GAN loss
        fake_B = netG_A2B(real_A)
        pred_fake = netD_B(fake_B)
        loss_GAN_A2B = criterion_GAN(pred_fake, target_real)

        fake_A = netG_B2A(real_B)
        pred_fake = netD_A(fake_A)
        loss_GAN_B2A = criterion_GAN(pred_fake, target_real)

        # Cycle loss
        recovered_A = netG_B2A(fake_B)
        loss_cycle_ABA = criterion_cycle(recovered_A, real_A)*10.0

        recovered_B = netG_A2B(fake_A)
        loss_cycle_BAB = criterion_cycle(recovered_B, real_B)*10.0

        # Total loss
        loss_G = loss_identity_A + loss_identity_B + loss_GAN_A2B + loss_GAN_B2A + loss_cycle_ABA + loss_cycle_BAB
        loss_G.backward()
        
        optimizer_G.step()

判别器A  损失函数= 真实样本分类损失 + 虚假样本分类损失

###### Discriminator A ######
        optimizer_D_A.zero_grad()

        # Real loss
        pred_real = netD_A(real_A)
        loss_D_real = criterion_GAN(pred_real, target_real)

        # Fake loss
        fake_A = fake_A_buffer.push_and_pop(fake_A)
        pred_fake = netD_A(fake_A.detach())
        loss_D_fake = criterion_GAN(pred_fake, target_fake)

        # Total loss
        loss_D_A = (loss_D_real + loss_D_fake)*0.5
        loss_D_A.backward()

        optimizer_D_A.step()
        ###################################

判别器B 损失函数= 真实样本分类损失 + 虚假样本分类损失

###### Discriminator B ######
        optimizer_D_B.zero_grad()

        # Real loss
        pred_real = netD_B(real_B)
        loss_D_real = criterion_GAN(pred_real, target_real)
        
        # Fake loss
        fake_B = fake_B_buffer.push_and_pop(fake_B)
        pred_fake = netD_B(fake_B.detach())
        loss_D_fake = criterion_GAN(pred_fake, target_fake)

        # Total loss
        loss_D_B = (loss_D_real + loss_D_fake)*0.5
        loss_D_B.backward()

        optimizer_D_B.step()
        ###################################

可以注意到，判别器损失中，虚假样本fake_A，fake_B都采用detach()操作，脱离计算图，这样判别器的损失进行反向传播不会对整个网络计算梯度，避免了不必要的计算