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Title: Improving MMD-GAN Training with Repulsive Loss Function

Paper content:
Generative adversarial nets (GANs) are widely used to learn the data sampling process and their performance may heavily depend on the loss functions, given a limited computational budget.
This study revisits MMD-GAN that uses the maximum mean discrepancy (MMD) as the loss function for GAN and makes two contributions.
First, we argue that the existing MMD loss function may discourage the learning of fine details in data as it attempts to contract the discriminator outputs of real data.
To address this issue, we propose a repulsive loss function to actively learn the difference among the real data by simply rearranging the terms in MMD.
Second, inspired by the hinge loss, we propose a bounded Gaussian kernel to stabilize the training of MMD-GAN with the repulsive loss function.
The proposed methods are applied to the unsupervised image generation tasks on CIFAR-10, STL-10, CelebA, and LSUN bedroom datasets.
Results show that the repulsive loss function significantly improves over the MMD loss at no additional computational cost and outperforms other representative loss functions.
The proposed methods achieve an FID score of 16.21 on the CIFAR-10 dataset using a single DCGAN network and spectral normalization.
Generative adversarial nets (GANs) BID7 ) are a branch of generative models that learns to mimic the real data generating process.
GANs have been intensively studied in recent years, with a variety of successful applications (Karras et al. (2018) ; Li et al. (2017b) ; Lai et al. (2017) ; Zhu et al. (2017) ; BID13 ).
The idea of GANs is to jointly train a generator network that attempts to produce artificial samples, and a discriminator network or critic that distinguishes the generated samples from the real ones.
Compared to maximum likelihood based methods, GANs tend to produce samples with sharper and more vivid details but require more efforts to train.Recent studies on improving GAN training have mainly focused on designing loss functions, network architectures and training procedures.
The loss function, or simply loss, defines quantitatively the difference of discriminator outputs between real and generated samples.
The gradients of loss functions are used to train the generator and discriminator.
This study focuses on a loss function called maximum mean discrepancy (MMD), which is well known as the distance metric between two probability distributions and widely applied in kernel two-sample test BID8 ).
Theoretically, MMD reaches its global minimum zero if and only if the two distributions are equal.
Thus, MMD has been applied to compare the generated samples to real ones directly (Li et al. (2015) ; BID5 ) and extended as the loss function to the GAN framework recently (Unterthiner et al. (2018) ; Li et al. (2017a) ; ).In
this paper, we interpret the optimization of MMD loss by the discriminator as a combination of attraction and repulsion processes, similar to that of linear discriminant analysis. We
argue that the existing MMD loss may discourage the learning of fine details in data, as the discriminator attempts to minimize the within-group variance of its outputs for the real data. To
address this issue, we propose a repulsive loss for the discriminator that explicitly explores the differences among real data. The
proposed loss achieved significant improvements over the MMD loss on image generation tasks of four benchmark datasets, without incurring any additional computational cost. Furthermore
, a bounded Gaussian kernel is proposed to stabilize the training of discriminator. As such, using
a single kernel in MMD-GAN is sufficient, in contrast to a linear combination of kernels used in Li et al. (2017a) and . By using a single
kernel, the computational cost of the MMD loss can potentially be reduced in a variety of applications.The paper is organized as follows. Section 2 reviews
the GANs trained using the MMD loss (MMD-GAN) . We propose the repulsive
loss for discriminator in Section 3, introduce two practical techniques to stabilize the training process in Section 4, and present the results of extensive experiments in Section 5. In the last section, we
discuss the connections between our model and existing work.
This study extends the previous work on MMD-GAN (Li et al. (2017a) ) with two contributions.
First, we interpreted the optimization of MMD loss as a combination of attraction and repulsion processes, and proposed a repulsive loss for the discriminator that actively learns the difference among real data.
Second, we proposed a bounded Gaussian RBF (RBF-B) kernel to address the saturation issue.
Empirically, we observed that the repulsive loss may result in unstable training, due to factors including initialization (Appendix A.2), learning rate ( FIG7 and Lipschitz constraints on the discriminator (Appendix C.3).
The RBF-B kernel managed to stabilize the MMD-GAN training in many cases.
Tuning the hyper-parameters in RBF-B kernel or using other regularization methods may further improve our results.The theoretical advantages of MMD-GAN require the discriminator to be injective.
The proposed repulsive loss (Eq. 4) attempts to realize this by explicitly maximizing the pair-wise distances among the real samples.
Li et al. (2017a) achieved the injection property by using the discriminator as the encoder and an auxiliary network as the decoder to reconstruct the real and generated samples, which is more computationally extensive than our proposed approach.
On the other hand, ; imposed a Lipschitz constraint on the discriminator in MMD-GAN via gradient penalty, which may not necessarily promote an injective discriminator.The idea of repulsion on real sample scores is in line with existing studies.
It has been widely accepted that the quality of generated samples can be significantly improved by integrating labels (Odena et al. (2017); Miyato & Koyama (2018) ; Zhou et al. (2018) ) or even pseudo-labels generated by k-means method BID9 ) in the training of discriminator.
The reason may be that the labels help concentrate the data from the same class and separate those from different classes.
Using a pre-trained classifier may also help produce vivid image samples BID14 ) as the learned representations of the real samples in the hidden layers of the classifier tend to be well separated/organized and may produce more meaningful gradients to the generator.At last, we note that the proposed repulsive loss is orthogonal to the GAN studies on designing network structures and training procedures, and thus may be combined with a variety of novel techniques.
For example, the ResNet architecture BID11 ) has been reported to outperform the plain DCGAN used in our experiments on image generation tasks (Miyato et al. (2018) ; BID10 ) and self-attention module may further improve the results (Zhang et al. (2018) ).
On the other hand, Karras et al. (2018) proposed to progressively grows the size of both discriminator and generator and achieved the state-of-the-art performance on unsupervised training of GANs on the CIFAR-10 dataset.
Future work may explore these directions.
This section shows that constant discriminator output DISPLAYFORM0 may have no discrimination power.
First, we make the following assumptions:Assumption
3.
1. D is a multilayer perceptron where each layer l can be factorized into an affine transform and an element-wise activation function f l .
2. Each activation function f l ∈ C 0 ; furthermore, f l has a finite number of discontinuities and f l ∈ C 06
. 3. Input data to D is continuous and its support S is compact in R d with non-zero measure in each dimension and d > 1 7 .Based on Assumption 3, we have the following proposition:Proposition
2. If ∀x ∈ S, D(x) = c, where c is constant, then there always exists distortion δx such that x + δx ∈ S and D(x + δx) = c.

assistant: text: Rearranging the terms in maximum mean discrepancy yields a much better loss function for the discriminator of generative adversarial nets