Towards Principled Methods for Training Generative Adversarial Networks

总结

本文主要是通过数学的方法分析现有GAN训练中存在的问题
先从KL散度入手——散度不对称对于两种不同的损失（真实分布存在而生成分布不存在；真实分布不存在而生成分布存在）给予了两种截然不同的惩罚
然后分析了训练过程不稳定的原因
- 当生成器的损失函数是$E_{x \sim P_r}[logD(x)] + E_{x \sim P_g}[log(1 - D(x))]$=$ 2JS(P_r||P_g) - 2log2$时，在分类器最优时，此损失函数是一个常数(低维流形理论JS散度一直为常数$log2$)，根本没有梯度，所以随着存在梯度消失的问题
- 当生成器的损失函数是$-logD$ = $KL(P_g||P_r) - 2JS(P_r||P_g)$时有两个问题。第一个是两个散度的优化方向矛盾，一个是让两个分布尽量近一个是尽量远，导致训练方向不稳定；另外KL的散度加大了对于$P_r = 0 且 P_g = 1$的惩罚，导致模型坍塌
提出解决方案——给两个分布加噪音，使两个低维流形维度升高，产生重叠，从而使JS散度不为常数，之后慢慢将噪声去除
提出Wasserstein距离，但是没有过多讨论，留到下一篇Wasserstein GAN再说

Abstract

this paper is proposed to make theoretical steps towards fully understanding the training dynamics of GANs

three sections:

the problem at hand
studying and proving rigorously the problems including instability and saturation
examines a practical and theoretically grounded direction towards solving these problems

Introduction

this paper aims to change unstable behaviour of GANs by providing a solid understanding of these issues, and create principled research direction towards adressing them

KL divergence

traditional approaches is to minimize:

$KL(P_r||P_g) = \int_XP_r(x)log\frac {P_r(x)} {P_g(x)}dx$

this divergence is not symmetrical between $P_r$ and $P_g$:

if $P_r(x) > P_g(x)$ that means generator can not generate real data. KL grows quickly to infinity
if $P_r(x) < p_g(x)$ that means generator output a image that doesn’t look real. KL goes to 0

if we would minimize $KL(P_g||P_r)$ instead, meaning that this cost function would pay a high cost for generating not plausibly looking pictures

optimization gets massively unstable

several questions:

Why do updates get worse as the discriminator gets better? Both in the original and the new cost function.
Why is GAN training massively unstable?
Is the new cost function following a similar divergence to the JSD? If so, what are its
properties?
Is there a way to avoid some of these issues?

The fundamental contributions of this paper are the answer to all these questions

Source of instability

the discriminator will have cost at most $2log2 - 2JSD(P_r||P_g)$, in practice, if we just train D till convergence, its error will go to 0

if the supports of $P_r$ and $P_g$ are disjoint or lie in low dimensional manifolds, there is always a perfect discriminator between them

the perfect discriminator theorems

we will first explain the case where $P_r$ and $P_g$ have disjoint supports

Theorem 2.1:

in the next theorem, we take away the disjoint assumption

we can safely assume in practice that any two manifolds never perfectly align

We now state our perfect discrimination result for the case of two manifolds

if these divergencies are always maxed out attempting to minimize them by gradient descent isn’t really possible

the consequences, and the problems of each cost function

If the two distributions we care about have supports that are disjoint or lie on low dimensional manifolds, the optimal discriminator will be perfect and its gradient will be zero almost everywhere