目录

• 目标
• 框架
• 理论
• 数值实验
• 代码

Generative Adversarial Nets

这篇文章，引领了对抗学习的思想，更加可贵的是其中的理论证明，证明很少却直击要害.

GAN，译名生成对抗网络，目的就是训练一个网络来拟合数据的分布，以前的方法，类似高斯核，Parzen窗等都可以用来估计（虽然不是很熟).

GAN有俩个网络，一个是G(z)生成网络，和D(x)判别网络, 其中\(z\)服从一个随机分布，而\(x\)是原始数据, \(z\)服从一个随机分布，是很重要的一点，假设\(\hat{x}=G(x)\), 则:

\[p(\hat{x})=\int p(z)I(G(z)=\hat{x})\mathrm{d}z
\]

其中\(I\)表示指示函数，这意味着，网络\(G\)也是一个分布，而我们所希望的，就是这个分布能够尽可能取拟合原始数据\(x\)的分布.

GAN需要训练上面的俩个网络，D的输出是一个0~1的标量，其含义是输入的x是否为真实数据(真实为1）, 故其损失函数为(V(D,G)部分):

在实际操作中，固定网络G更新网络D，再固定网络D更新网络G，反复迭代:

至于为什么可以这么做，作者给出了精炼的证明.

上面的证明唯一令人困惑的点在于\(p_z \rightarrow p_g\)的变化，我一开始觉得这个是利用换元，但是从别的博客中看到，似乎是用了测度论的导数的知识，最后用到了变分的知识.

其中：

其证明思路是，当\(p_g=p_{data}\)的时候，\(C(G)=-\log 4\), 所以只需证明这个值为最小值，且仅再\(p_g=p_{data}\)的时候成立那么证明就结束了，为了证明这一点，作者凑了一个JSD， 而其正好满足我们要求(实际上只需KL散度即可Gibb不等式).

在MNIST数据集上做实验(代码是仿别人的写的), 我们的目标自然是给一个z, G能够给出一些数字.

用不带卷积层的网络:

带卷积层的网络，不过不论\(z\)怎么变，结果都一样，感觉有点怪，但是实际上，如果\(G\)一直生成的都是比方说是1, 那也的确能够骗过\(D\), 这个问题算是什么呢？有悖啊…

代码需要注意的一点是，用BCELoss, 但是更新G网络的时候，并不是传入fake_label, 而是real_label，因为G需要骗过D, 不知道该怎么说，应该明白的.

import torch
import torchvision
import torchvision.transforms as transforms
import torch.nn as nn
import numpy as np
import matplotlib.pyplot as plt

class Generator(nn.Module):
    def __init__(self, input_size):
        super(Generator, self).__init__()
        self.dense = nn.Sequential(
            nn.Linear(input_size, 256),
            nn.ReLU(),
            nn.Linear(256, 256),
            nn.ReLU(),
            nn.Linear(256, 784)
        )

    def forward(self, x):
        out = self.dense(x)
        return out

class Discriminator(nn.Module):
    def __init__(self):
        super(Discriminator, self).__init__()
        self.dense = nn.Sequential(
            nn.Linear(784, 256),
            nn.ReLU(),
            nn.Linear(256, 256),
            nn.ReLU(),
            nn.Linear(256, 1),
            nn.Sigmoid()
        )

    def forward(self, x):
        x = x.view(x.size(0), -1)
        out = self.dense(x)
        return out

class Train:
    def __init__(self, trainset, batch_size, z_size=100, criterion=nn.BCELoss(), lr=1e-3):
        self.generator = Generator(z_size)
        self.discriminator = Discriminator()
        self.opt1 = torch.optim.SGD(self.generator.parameters(), lr=lr, momentum=0.9)
        self.opt2 = torch.optim.SGD(self.discriminator.parameters(), lr=lr, momentum=0.9)
        self.trainset = trainset
        self.batch_size = batch_size
        self.real_label = torch.ones(batch_size)
        self.fake_label = torch.zeros(batch_size)
        self.criterion = criterion
        self.z_size = z_size

    def train(self, epoch_size, path):
        running_loss1 = 0.0
        running_loss2 = 0.0
        for epoch in range(epoch_size):
            for i, data in enumerate(self.trainset, 0):
                try:
                    real_img, _ = data

                    out1 = self.discriminator(real_img)
                    real_loss = self.criterion(out1, self.real_label)

                    z = torch.randn(self.batch_size, self.z_size)
                    fake_img = self.generator(z)
                    out2 = self.discriminator(fake_img)
                    fake_loss = self.criterion(out2, self.fake_label)

                    loss = real_loss + fake_loss
                    self.opt2.zero_grad()
                    loss.backward()
                    self.opt2.step()

                    z = torch.randn(self.batch_size, self.z_size)
                    fake_img = self.generator(z)
                    out2 = self.discriminator(fake_img)
                    fake_loss = self.criterion(out2, self.real_label) #real_label!!!!

                    self.opt1.zero_grad()
                    fake_loss.backward()
                    self.opt1.step()

                    running_loss1 += fake_loss
                    running_loss2 += real_loss
                    if i % 10 == 9:
                        print("[epoch:{}    loss1: {:.7f}   loss2: {:.7f}]".format(
                            epoch,
                            running_loss1 / 10,
                            running_loss2 / 10
                        ))
                        running_loss1 = 0.0
                        running_loss2 = 0.0
                except ValueError as err:
                    print(err)  #最后一批的数据可能不是batch_size
                    continue
        torch.save(self.generator.state_dict(), path)

    def loading(self, path):
        self.generator.load_state_dict(torch.load(path))
        self.generator.eval()



"""
加了点卷积
"""
import torch
import torchvision
import torchvision.transforms as transforms
import torch.nn as nn
import numpy as np
import matplotlib.pyplot as plt

class Generator(nn.Module):
    def __init__(self, input_size):
        super(Generator, self).__init__()
        self.dense = nn.Sequential(
            nn.Linear(input_size, 256),
            nn.ReLU(),
            nn.Linear(256, 256),
            nn.ReLU(),
            nn.Linear(256, 784)
        )

    def forward(self, x):
        out = self.dense(x)
        return out

class Discriminator(nn.Module):
    def __init__(self):
        super(Discriminator, self).__init__()
        self.conv = nn.Sequential(
            nn.Conv2d(1, 32, 5, 3, 2),  # 1x28x28 --> 32x10x10
            nn.ReLU(),
            nn.MaxPool2d(2, 2),  # 32 x 10 x 10 --> 32x5x5
            nn.Conv2d(32, 64, 3, 1, 1),  # 32x5x5-->32x5x5
            nn.ReLU()
        )
        self.dense = nn.Sequential(
            nn.Linear(1600, 512),
            nn.ReLU(),
            nn.Linear(512, 1),
            nn.Sigmoid()
        )

    def forward(self, x):
        x = x.view(x.size(0), 1, 28, 28)
        x = self.conv(x)
        x = x.view(x.size(0), -1)
        out = self.dense(x)
        return out

class Train:
    def __init__(self, trainset, batch_size, z_size=100, criterion=nn.BCELoss(), lr=1e-3):
        self.generator = Generator(z_size)
        self.discriminator = Discriminator()
        self.opt1 = torch.optim.SGD(self.generator.parameters(), lr=lr, momentum=0.9)
        self.opt2 = torch.optim.SGD(self.discriminator.parameters(), lr=lr, momentum=0.9)
        self.trainset = trainset
        self.batch_size = batch_size
        self.real_label = torch.ones(batch_size)
        self.fake_label = torch.zeros(batch_size)
        self.criterion = criterion
        self.z_size = z_size

    def train(self, epoch_size, path):
        running_loss1 = 0.0
        running_loss2 = 0.0
        for epoch in range(epoch_size):
            for i, data in enumerate(self.trainset, 0):
                try:
                    real_img, _ = data

                    out1 = self.discriminator(real_img)
                    real_loss = self.criterion(out1, self.real_label)

                    z = torch.randn(self.batch_size, self.z_size)
                    fake_img = self.generator(z)
                    out2 = self.discriminator(fake_img)
                    fake_loss = self.criterion(out2, self.fake_label)

                    loss = real_loss + fake_loss
                    self.opt2.zero_grad()
                    loss.backward()
                    self.opt2.step()

                    z = torch.randn(self.batch_size, self.z_size)
                    fake_img = self.generator(z)
                    out2 = self.discriminator(fake_img)
                    fake_loss = self.criterion(out2, self.real_label) #real_label!!!!

                    self.opt1.zero_grad()
                    fake_loss.backward()
                    self.opt1.step()

                    running_loss1 += fake_loss
                    running_loss2 += real_loss
                    if i % 10 == 9:
                        print("[epoch:{}    loss1: {:.7f}   loss2: {:.7f}]".format(
                            epoch,
                            running_loss1 / 10,
                            running_loss2 / 10
                        ))
                        running_loss1 = 0.0
                        running_loss2 = 0.0
                except ValueError as err:
                    print(err)  #最后一批的数据可能不是batch_size
                    continue
        torch.save(self.generator.state_dict(), path)

    def loading(self, path):
        self.generator.load_state_dict(torch.load(path))
        self.generator.eval()