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Title: A Classification-Based Perspective on GAN Distributions

Paper content:
A fundamental, and still largely unanswered, question in the context of Generative Adversarial Networks (GANs) is whether GANs are actually able to capture the key characteristics of the datasets they are trained on.
The current approaches to examining this issue require significant human supervision, such as visual inspection of sampled images, and often offer only fairly limited scalability.
In this paper, we propose new techniques that employ classification-based perspective to evaluate synthetic GAN distributions and their capability to accurately reflect the essential properties of the training data.
These techniques require only minimal human supervision and can easily be scaled and adapted to evaluate a variety of state-of-the-art GANs on large, popular datasets.
They also indicate that GANs have significant problems in reproducing the more distributional properties of the training dataset.
In particular, the diversity of such synthetic data is orders of magnitude smaller than that of the original data.
Generative Adversarial Networks (GANs) BID6 have garnered a significant amount of attention due to their ability to learn generative models of multiple natural image datasets BID11 BID3 BID7 Zhu et al., 2017) .
Since their conception, a fundamental question regarding GANs is to what extent they truly learn the underlying data distribution.
This is a key issue for multiple reasons.
From a scientific perspective, understanding the capabilities of common GANs can shed light on what precisely the adversarial training setup allows the GAN to learn.
From an engineering standpoint, it is important to grasp the power and limitations of the GAN framework when applying it in concrete applications.
Due to the broad potential applicability of GANs, researchers have investigated this question in a variety of ways.When we evaluate the quality of a GAN, an obvious first check is to establish that the generated samples lie in the support of the true distribution.
In the case of images, this corresponds to checking if the generated samples look realistic.
Indeed, visual inspection of generated images is currently the most common way of assessing the quality of a given GAN.
Individual humans can performs this task quickly and reliably, and various GANs have achieved impressive results for generating realistic-looking images of faces and indoor scenes BID13 BID3 .Once
we have established that GANs produce realistic-looking images, the next concern is that the GAN might simply be memorizing the training dataset. While
this hypothesis cannot be ruled out entirely, there is evidence that GANs perform at least some non-trivial modeling of the unknown distribution. Previous
studies show that interpolations in the latent space of the generator produce novel and meaningful image variations BID11 , and that there is a clear disparity between generated samples and their nearest neighbors in the true dataset BID1 .Taken together
, these results provide evidence that GANs could constitute successful distribution learning algorithms, which motivates studying their distributions in more detail. The direct approach
is to compare the probability density assigned by the generator with estimates of the true distribution BID16 . However, in the context
of GANs and high-dimensional image distributions, this is complicated by two factors. First, GANs do not naturally
provide probability estimates for their samples. Second, estimating the probability
density of the true distribution is a challenging problem itself (the adversarial training framework specifically avoids this issue). Hence prior work has only investigated
the probability density of GANs on simple datasets such as MNIST BID16 .Since reliably computing probability densities
in high dimensions is challenging, we can instead study the behavior of GANs in low-dimensional problems such as two-dimensional Gaussian mixtures. Here, a common failure of GANs is mode collapse
, wherein the generator assigns a disproportionately large mass to a subset of modes from the true distribution BID5 . This raises concerns about a lack of diversity
in the synthetic GAN distributions, and recent work shows that the learned distributions of two common GANs indeed have (moderately) low support size for the CelebA dataset BID1 . However, the approach of BID1 heavily relies on
a human annotator in order to identify duplicates. Hence it does not easily scale to comparing many
variants of GANs or asking more fine-grained questions than collision statistics. Overall, our understanding of synthetic GAN distributions
remains blurry, largely due to the lack of versatile tools for a quantitative evaluation of GANs in realistic settings. The focus of this work is precisly to address this question
:Can we develop principled and quantitative approaches to study synthetic GAN distributions?To this end, we propose two new evaluation techniques for synthetic
GAN distributions. Our methods are inspired by the idea of comparing moments of distributions
, which is at the heart of many methods in classical statistics. Although simple moments of high-dimensional distributions are often not semantically
meaningful, we can extend this idea to distributions of realistic images by leveraging image statistics identified using convolutional neural networks. In particular, we train image classifiers in order to construct test functions corresponding
to semantically meaningful properties of the distributions. An important feature of our approach is that it requires only light human supervision and can
easily be scaled to evaluating many GANs and large synthetic datasets.Using our new evaluation techniques, we study five state-of-the-art GANs on the CelebA and LSUN datasets, arguably the two most common testbeds for advanced GANs. We find that most of the GANs significantly distort the relative frequency of even basic image
attributes, such as the hair style of a person or the type of room in an indoor scene. This clearly indicates a mismatch between the true and synthetic distributions. Moreover, we conduct
experiments to explore the diversity of GAN distributions. We use synthetic GAN
data to train image classifiers and find that these have significantly lower accuracy
than classifiers trained on the true data set. This points towards a lack of diversity in the GAN data, and again towards a discrepancy between the true
and synthetic distributions. In fact, our additional examinations show that the diversity in GANs is only comparable to a subset of the
true data that is 100× smaller.
In this paper, we put forth techniques for examining the ability of GANs to capture key characteristics of the training data, through the lens of classification.
Our tools are scalable, quantitative and automatic (no need for visual inspection of images).
They thus are capable of studying state-ofthe-art GANs on realistic, large-scale image datasets.
Further, they serve as a mean to perform a nuanced comparison of GANs and to identify their relative merits, including properties that cannot be discerned from mere visual inspection.We then use the developed techniques to perform empirical studies on popular GANs on the CelebA and LSUN datasets.
Our examination shows that mode collapse is indeed a prevalent issue for GANs.
Also, we observe that synthetic GAN-generated datasets have significantly reduced diversity, at least when examined from a classification perspective.
In fact, the diversity of such synthetic data is often few orders of magnitude smaller than that of the true data.
Furthermore, this gap in diversity does not seem to be bridged by simply producing much larger datasets by oversampling GANs.
Finally, we also notice that good perceptual quality of samples does not necessarily correlate -and might sometime even anti-correlate -with distribution diversity.
These findings suggest that we need to go beyond the visual inspection-based evaluations and look for more quantitative tools for assessing quality of GANs, such as the ones presented in this paper.
To assess GAN performance from the perspective of classification, we construct a set of classification tasks on the CelebA and LSUN datasets.
In the case of the LSUN dataset, images are annotated with scene category labels, which makes it straightforward to use this data for binary and multiclass classification.
On the other hand, each image in the CelebA dataset is labeled with 40 binary attributes.
As a result, a single image has multiple associated attribute labels.
Here, we construct classification tasks can by considering binary combinations of an attribute(s) (examples are shown in FIG1 ).
Attributes used in our experiments were chosen such that the resulting dataset was large, and classifiers trained on true data got high-accuracy so as to be good annotators for the synthetic data.
Details on datasets used in our classification tasks, such as training set size (N ), number of classes (C), and accuracy of the annotator, i.e., a classifier pre-trained on true data which is used to label the synthetic GAN-generated data, are provided in Table 2 .
Table 2 : Details of CelebA and LSUN subsets used for the studies in Section 3.3.
Here, we use a classifier trained on true data as an annotator that let's us infer label distribution for the synthetic, GAN-generated data.
N is the size of the training set and C is the number of classes in the true and synthetic datasets.
Annotator's accuracy refers to the accuracy of the classifier on a test set of true data.
For CelebA, we use a combination of attribute-wise binary classifiers as annotators due their higher accuracy compared to a single classifier trained jointly on all the four classes.

assistant: text: We propose new methods for evaluating and quantifying the quality of synthetic GAN distributions from the perspective of classification tasks