# 0. Introduction

In this tutorial, we generate images with generative adversarial network (GAN). It is a kind of generative model with deep neural network, and often applied to the image generation. The GAN technique is also applied to PaintsChainer a famous automatic coloring service.

In the tutorial, you will learn the following things:

1. The basic idea of generative model
2. The difference among GAN and other generative models
4. Implementation of DCGAN in Chainer

# 1. The basic idea of generative model

## 1.1 What is the Model?

In the field of science and engineering, we describe a system using mathematical concepts and language. The description is called as a mathematical model, and the process of developing a mathematical model is mathematical modeling. Especially, in the context of machine learning, we explain a target model by a map from an input to an output

\begin{align*} f: x \mapsto y \end{align*}

Therefore, the purpose of model training is obtaining the map from training data. In the case of unsupervised learning, we use the dataset of inputs as the training data, and create the model . In supervised learning, we use the dataset of inputs and their outputs . As a simple example, let's consider a supervised learning problem such as classifying images into dogs or cats. Then, the training dataset consist of input images and their labels .

## 1.2 What is the Generative Model?

When we think about the generative model, it models the probability distribution which generates the training data . The one of most simple ideas is that the generative model models the probability distribution with the map . We assign each and of as follows.

• : the training data
• : the likelihood of generating the training data

In the case, because we model the probability distribution explicitly, we can calculate the likelihood . In this situation, we can often optimize the parameters of to maximize the likelihood, and train the model with maximum likelihood estimation. So, there is an advantage that the training process is simple because you can just maximize the likelihood by optimizing the parameters. However, there is a disadvantage that we have to make a mechanism for sampling because we have only the way of calculating the likelihood.

In the first place, we often just want to sample according to the distribution in practice. The likelihood is used only for model training. In the case, we sometimes do not model the probability distribution directly, but other targets to facilitate sampling.

There are two approaches to model the probability distribution . The first case is to model the probability distributions and by introducing the latent variable . The VAE, which is described later, belongs to this category. Second, we introduce the latent variable and model the sample generator ]s = G(z)] which fits the distribution . The GAN belongs to this category. These models can generate the training data by generating the latent variable based on random numbers.

These generative models can be used for the following purposes:

• Assistance for creative activities (e.g. coloring of line drawings)
• Providing interfaces to people (e.g. generating natural sentences)
• Reduction of data creation cost (e.g. use as a simulator)

#### Note

When we talk about generative model, we sometimes explain it against discrimination
model in classification problem [1]. However, when we talk about generative
model in GAN, it is natural to define it as a model of probability distribution
that generates training data.


# 2. The difference among GAN and other generative models

As explained in the GAN tutorial in NIPS 2016 [2], the generative models can be classified into the categories as shown in the following figure:

cited from [1]

Besides GAN, other famous generative models are Fully visible belief networks (FVBNs) and Variational autoencoder (VAE).

## 2.1 Fully Visible Belief Networks (FVBNs)

FVBNs decomposes the probability distribution into one-dimensional probability distributions using the Bayes' theorem as shown in the following equation:

\begin{align*} p_{\mathrm{model}}({\bf s}) = \prod^{n}_{i=1}p_{\mathrm{model}} (s_i | s_1, \cdots, s_{i-1}) \end{align*}

Since the dimensions from , excluding the first dimension , are generated based on the dimensions previously generated, FVBNs can be said to be an autoregressive model. PixcelRNN and PixcelCNN, which are categorized as FVBNs, model one-dimensional distribution functions with Recurrent Neural Networks(RNN) and Convolutional Neural Networks(CNN), respectively. The advantage of FVBNs is that the model can learn with explicitly computable likelihood. The disadvantage is that the sampling cost can be expensive because each dimension can only be generated sequentially.

## 2.2 Variational Auto-Encoder (VAE)

The VAE models the probability distribution that generates the training data as follows:

\begin{align*} p_{\mathrm {model}}({\bf s}) = \int p_{\mathrm {model}}({\bf s}|{\bf z}) p_{\mathrm {model}}({\bf z}) d{\bf z} \end{align*}

is modeled by the two probability distributions and using the latent variable .

When training a model using the maximum likelihood estimation on , we should calculate or explicitly. However, because includes integral operator as shown in above equation, it is difficult to calculate and .

So, VAE approximates with . As a result, we can calculate the lower bound of the likelihood, and train the model by maximizing the lower bound of the likelihood. The advantage of VAE is that sampling is low cost, and you can estimate latent variables by . The disadvantage is that calculation of the probability distribution is difficult, and approximate values are used for training model.

# 3. Generarive Adversarial Networks (GAN)

## 3.1 What is the GAN?

Unlike FVBNs and VAE, GAN does not explicitly models the probability distribution that generates the training data. Instead, we model a generator . The generator samples from the latent variable . Apart from the generator , we create a discriminator which identified the samples from the generator and the true samples from training data. While training the discriminator , the generator is also trained so that th generated samples cannot be identified by the discriminator. The advantages of GAN are low sampling cost and state-of-the-art in image generation. The disadvantage is that we cannot calculate the probability distribution because we do not model any probability distribution, and we cannot infer the latent variable from a sample.

## 3.2 How the GAN works?

As explained above, GAN uses the two models, the generator and the discriminator. In other words, we set up two neural networks for GAN.

When training the networks, we should match the distribution of the samples generated from the true distribution with the distribution of the samples generated from the generator.

The generator learns the target distribution on the idea of Nash equilibrium [3] of game theory. In detail, while training the discriminator , the generator is also trained so that the discriminator cannot identify the generated samples.

As an intuitive example, the relationship between counterfeiters of banknotes and police is frequently used. The counterfeiters try to make counterfeit notes that are similar to real banknotes. The police try to distinguish real bank notes from counterfeit notes. It is supposed that the ability of the police gradually rises, so that real banknotes and counterfeit notes can be recognized well. Then, the counterfeiters will not be able to use counterfeit banknotes, so they will build more similar counterfeit banknotes to real. As the police improve the skill further, so that they can distinguish real and counterfeit notes... Eventually, the counterfeiter will be able to produce as similar banknote as genuine.

The training process is explained by mathematical expressions as follows. First, since the discriminator is a probability that the sample is generated from the true distribution, it can be expressed as follows:

\begin{align*} D({\bf s}) = \frac{p({\bf s})}{p({\bf s}) + p_{\mathrm{model}}({\bf s})} \end{align*}

Then, when we match the distributions of the samples generated from true distribution and the samples generated from the generator , it means that we should minimize the dissimilarity between the two distributions. It is common to use Jensen-Shannon Divergence to measure the dissimilarity between the distributions[4].

The of and can be written as follows by using :

\begin{align*} 2 D_{\mathrm{JS}} &=& D_{\mathrm{KL}}(p(x)||\bar{p}(x)) + D_{\mathrm{KL}}(p_{\mathrm{model}}(x)||\bar{p}(x)) \\ &=& \mathbb{E}_{p(x)} \log \frac{2p(x)}{p(x) + p_{\mathrm{model}}(x)} + \mathbb{E}_{p_{\mathrm{model}}} \log \frac{2p_{\mathrm{model}}(x)}{p(x) + p_{\mathrm{model}}(x)} \\ &=& \mathbb{E}_{p(x)} \log D(x) + \mathbb{E}_{p_{\mathrm{model}}} \log (1-D(x)) + \log 4 \end{align*}

The will be maximized by the discriminator and minimized by the generator , or . And the distribution generated by can match the true distribution .

\begin{align*} \min_{G} \max_{D} \mathbb{E}_{p(x)} \log D(x) + \mathbb{E}_{p_{\mathrm{model}}} \log (1-D(x)) \end{align*}

When training actually, the above min-max problem is solved by alternately updating the discriminator and the generator [5].

cited from [5]

## 3.3 What is DCGAN?

In this section, we will introduce the model called DCGAN(Deep Convolutional GAN) proposed by Radford et al.[6]. As shown below, it is a model using CNN(Convolutional Neural Network) as its name suggests.

cited from [6]

In addition, although GAN is known for its difficulty in learning, this paper introduces various techniques for successful learning:

1. Convert max-pooling layers to convolution layers
2. Convert fully connected layers to global average pooling layers in the discriminator
3. Use batch normalization layers in the generator and the discriminator
4. Use leaky ReLU activation functions in the discriminator

# 4. Implementation of DCGAN in Chainer

There is an example of DCGAN in the official repository of Chainer, so we will explain how to implement DCGAN based on this: chainer/examples/dcgan

## 4.1 Define the generator model

First, let's define a network for the generator.

When we make a network in Chainer, we should follow some rules:

1. Define a network class which inherits chainer.Chain.
2. Make chainer.links 's instances in the init_scope(): of constructor __init__.
3. Concatenate chainer.links 's instances with chainer.functions to make whole network.

If you are not familiar with constructing a new network, you can read this tutorial.

As we can see from the constructor __init__, the Generator uses the deconvolution layer chainer.links.Deconvolution2D and the batch normalization chainer.links.BatchNormalization. In _call__, each layer is concatenated by chainer.functions.relu except the last layer.

Because the first argument of L.Deconvolution is the channel size of input and the second is the channel size of output, we can find that each layer halve the channel size. When we construct Generator with ch=1024, the network is same with the image above.

#### Note

Be careful when you concatenate a fully connected layer's output and
a convolutional layer's input. As we can see the 1st line of  __call__ ,
the output and input have to be concatenated with reshaping by
chainer.functions.reshape.


## 4.2 Define the discriminator model

In addtion, let's define a network for the discriminator.

The Discriminator network is almost same with the transposed network of the Generator. However, there are minor different points:

1. Use chainer.functions.leaky_relu as activation functions
2. Deeper than Generator
3. Add some noise when concatenating layers

## 4.3 Prepare dataset and iterator

Let's retrieve the CIFAR-10 dataset by using Chainer's dataset utility function chainer.datasets.get_cifar10. CIFAR-10 is a set of small natural images. Each example is an RGB color image of size 32x32. In the original images, each component of pixels is represented by one-byte unsigned integer. This function scales the components to floating point values in the interval [0, scale].

## 4.7 Start training

We can run the exemple as follows.

$pwd /root2chainer/chainer/examples/dcgan$ python train_dcgan.py --gpu 0
GPU: 0
# Minibatch-size: 50
# n_hidden: 100
# epoch: 1000

epoch       iteration   gen/loss    dis/loss  ................]  0.01%
0           100         1.2292      1.76914
total [..................................................]  0.02%
this epoch [#########.........................................] 19.00%
190 iter, 0 epoch / 1000 epochs
10.121 iters/sec. Estimated time to finish: 1 day, 3:26:26.372445.


The results will be saved in the director /root2chainer/chainer/examples/dcgan/result/. The image is generated by the generator trained with 1000 epochs, and the GIF image on the top of this page shows generated images at the each 10 epoch.