Generative adversarial networks (GANs) have been greeted with real excitement since their creation back in 2014 by Ian Goodfellow and his research team. Yann LeCun, Facebook’s Director of AI Research went as far as describing GANs as “the most interesting idea in the last 10 years in ML.” With all this excitement, however, it can be easy to miss the subtle diversity of GANs; there are a number of different types of generative adversarial networks, each one working in slightly different ways and helping engineers to achieve slightly different results.
To give you a deeper insight on GANs, in this article we’ll look at three different generative adversarial networks: SRGANs, CycleGANs, and InfoGANs. We’ll explore how these different GANs work and how they can be used. This should give you a solid foundation to explore GANs in more depth and begin to apply them in your own experiments and projects.
This article is an excerpt from the book, Deep Learning with TensorFlow 2 and Keras, Second Edition by Antonio Gulli, Amita Kapoor, and Sujit Pal.
SRGAN – Super Resolution GANs
Remember seeing a crime-thriller where our hero asks the computer guy to magnify the faded image of the crime scene? With the zoom we are able to see the criminal’s face in detail, including the weapon used and anything engraved upon it! Well, SRGAN can perform similar magic.
Here a GAN is trained in such a way that it can generate a photorealistic high-resolution image when given a low-resolution image. The SRGAN architecture consists of three neural networks: a very deep generator network, a discriminator network, and a pretrained VGG-16 network.
How do SRGANs work?
SRGANs use the perceptual loss function (developed by Johnson et al, Perceptual Losses for Real-Time Style Transfer and Super-Resolution). The difference in the feature map activations in high layers of a VGG network between the network output part and the high-resolution part comprises the perceptual loss function. Besides perceptual loss, the authors further added content loss and an adversarial loss so that images generated look more natural and the finer details more artistic. The perceptual loss is defined as the weighted sum of content loss and adversarial loss:
lSR = lSR X+ 10−3×lSRGen
The first term on the right-hand side is the content loss, obtained using the feature maps generated by pretrained VGG 19. Mathematically it is the Euclidean distance between the feature map of the reconstructed image (that is the one generated by the generator) and the original high-resolution reference image.
The second term on the right-hand side is the adversarial loss. It is the standard generative loss term, designed to ensure that images generated by the generator are able to fool the discriminator. You can see in the following figure taken from the original paper that the image generated by SRGAN is much closer to the original high-resolution image:
Another noteworthy architecture is CycleGAN; proposed in 2017, it can perform the task of image translation. Once trained you can translate an image from one domain to another domain. For example, when trained on horse and zebra data set, if you give it an image with horses in the ground, the CycleGAN can convert the horses to zebra with the same background.
How does CycleGAN work?
Have you ever imagined how a scenery would look if Van Gogh or Manet had painted it? We have many sceneries, and many landscapes painted by Gogh/Manet, but we do not have any collection of input-output pairs. CycleGAN performs the image translation, that is, transfers an image given in one domain (scenery for example) to another domain (Van Gogh painting of the same scene, for instance) in the absence of training examples. CycleGAN’s ability to perform image translation in the absence of training pairs is what makes it unique.
To achieve image translation the authors of CycleGAN used a very simple and yet effective procedure. They made use of two GANs, the generator of each GAN performing the image translation from one domain to another.
To elaborate, let us say the input is X, then the generator of the first GAN performs a mapping G: X → Y, thus its output would be Y = G(X). The generator of the second GAN performs an inverse mapping F: Y → X, resulting in X = F(Y). Each discriminator is trained to distinguish between real images and synthesized images. The idea is shown as follows:
To train the combined GANs, the authors added beside the conventional GAN adversarial loss a forward cycle consistency loss (left figure) and a backward cycle consistency loss (right figure). This ensures that if an image X is given as input, then after the two translations F(G(X)) ~ X the obtained image is the same X (similarly the backward cycle consistency loss ensures the G(F(Y)) ~ Y).
Following are some of the successful image translations by CycleGAN:
Following are few more examples, you can see the translation of seasons (summer → winter), photo → painting and vice versa, horses → zebra:
The GAN architectures that we have considered up to now provide us with little or no control over the generated images. InfoGAN changes this; it provides control over various attributes of the images generated. The InfoGAN uses concepts from information theory such that the noise term is transformed into latent codes which provide predictable and systematic control over the output.
How does InfoGAN work?
The generator in InfoGAN takes two inputs the latent space Z and a latent code c, thus the output of generator is G(Z,c). The GAN is trained such that it maximizes the mutual information between the latent code c and the generated image G(Z,c). The following figure shows the architecture of InfoGAN:
The concatenated vector (Z,c) is fed to the Generator. Q(c|X) is also a neural network, combined with the generator it works to form a mapping between random noise Z and its latent code c_hat, it aims to estimate c given X. This is achieved by adding a regularization term to the objective function of conventional GAN:
minDmaxG VI(D,G) = VG(D,G) −λI(c;G(Z,c))
The term VG(D,G) is the loss function of conventional GAN, and the second term is the regularization term, where λ is a constant. Its value was set to 1 in the paper, and I(c;G(Z,c)) is the mutual information between the latent code c and the Generator generated image G(Z,c).
Below is the results of InfoGAN on the MNIST dataset:
That concludes our brief look at three different types of generative adversarial networks. You can find the book from which this article was taken on the Packt store or you can read the first chapter for free on the Packt subscription platform.