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Title: Optimizing the Latent Space of Generative Networks

Paper content:
Generative Adversarial Networks (GANs) have achieved remarkable results in the task of generating realistic natural images.
In most applications, GAN models share two aspects in common.
On the one hand, GANs training involves solving a challenging saddle point optimization problem, interpreted as an adversarial game between a generator and a discriminator functions.
On the other hand, the generator and the discriminator are parametrized in terms of deep convolutional neural networks.
The goal of this paper is to disentangle the contribution of these two factors to the success of GANs.
In particular, we introduce Generative Latent Optimization (GLO), a framework to train deep convolutional generators without using discriminators, thus avoiding the instability of adversarial optimization problems.
Throughout a variety of experiments, we show that GLO enjoys many of the desirable properties of GANs: learning from large data, synthesizing visually-appealing samples, interpolating meaningfully between samples, and performing linear arithmetic with noise vectors.
Generative Adversarial Networks (GANs) BID15 are a powerful framework to learn generative models of natural images.
GANs learn these generative models by setting up an adversarial game between two learning machines.
On the one hand, a generator plays to transform noise vectors into fake samples, which resemble real samples drawn from a distribution of natural images.
On the other hand, a discriminator plays to distinguish between real and fake samples.
During training, the generator and the discriminator learn in turns.
First, the discriminator learns to assign high scores to real samples, and low scores to fake samples.
Then, the generator learns to increase the scores of fake samples, as to fool the discriminator.
After proper training, the generator is able to produce realistic natural images from noise vectors.Recently, GANs have been used to produce high-quality images resembling handwritten digits, human faces, and house interiors BID36 .
Furthermore, GANs exhibit three strong signs of generalization.
First, the generator translates linear interpolations in the noise space into semantic interpolations in the image space.
In other words, a linear interpolation in the noise space will generate a smooth interpolation of visually-appealing images.
Second, the generator allows linear arithmetic in the noise space.
Similarly to word embeddings BID31 , linear arithmetic indicates that the generator organizes the noise space to disentangle the nonlinear factors of variation of natural images into linear statistics.
Third, the generator is able to to synthesize new images that resemble those of the data distribution.
This allows for applications such as image in-painting BID18 and super-resolution BID26 .Despite
their success, training and evaluating GANs is notoriously difficult. The adversarial
optimization problem implemented by GANs is sensitive to random initialization, architectural choices, and hyper-parameter settings. In many cases,
a fair amount of human care is necessary to find the correct configuration to train a GAN in a particular dataset. It is common to
observe generators with similar architectures and hyper-parameters to exhibit dramatically different behaviors. Even when properly
trained, the resulting generator may synthesize samples that resemble only a few localized regions (or modes) of the data distribution BID14 . While several advances
have been made to stabilize the training of GANs BID37 , this task remains more art than science.The difficulty of training GANs is aggravated by the challenges in their evaluation: since evaluating the likelihood of a GAN with respect to the data is an intractable problem, the current gold standard to evaluate the quality of GANs is to eyeball the samples produced by the generator. The evaluation of discriminators
is also difficult, since their visual features do not always transfer well to supervised tasks BID12 BID13 . Finally, the application of GANs
to non-image data has been relatively limited.Research question To model natural images with GANs, the generator and discriminator are commonly parametrized as deep Convolutional Networks (convnets) BID24 . Therefore, it is reasonable to hypothesize
that the reasons for the success of GANs in modeling natural images come from two complementary sources: (A1) Leveraging the powerful inductive bias of deep convnets. (A2) The adversarial training protocol.This
work
attempts to disentangle the factors of success (A1) and (A2) in GAN models. Specifically, we propose and study one algorithm
that relies on (A1) and avoids (A2), but still obtains competitive results when compared to a GAN.
The experimental results presented in this work suggest that, in the image domain, we can recover many of the properties of GAN models by using convnets trained with simple reconstruction losses.
While this does not invalidate the promise of GANs as generic models of uncertainty or as methods for building generative models, our results suggest that, in order to more fully test the adversarial construction, research needs to move beyond images and convnets.
On the other hand, practitioners who care only about generating images for a particular application, and find that the parameterized discriminator does improve their results can use reconstruction losses in their model searches, alleviating some of the instability of GAN training.While the visual quality of the results are promising, especially on the CelebA dataset, they are not yet to the level of the results obtained by GANs on the LSUN bedrooms.
This suggest several research directions: one possibility, suggested by 3, is that being able to cover the entire dataset is too onerous a task if all that is required is to generate a few nice samples.
In that figure we see that GANs have trouble reconstructing randomly chosen images at the same level of fidelity as their generations.
However, GANs can produce good images after a single pass through the data with SGD.
In future work we hope to better understand the tension between these two observations.
There are many possibilities for improving the quality of GLO samples beyond understanding the effects of coverage.
For example other loss functions (e.g. a VGG metric, as in BID32 ), model architectures (here we stayed close to DCGAN for ease of comparison), and more sophisticated sampling methods after training the model all may improve the visual quality of the samples.There is also much work to be done in adding structure to the Z space.
Because the methods here keep track of the correspondence between samples and their representatives, and because the Z space is free, we hope to be able to organize the Z in interesting ways as we train.

assistant: text: Are GANs successful because of adversarial training or the use of ConvNets? We show a ConvNet generator trained with a simple reconstruction loss and learnable noise vectors leads many of the desirable properties of a  GAN.