GANs? Are these aliens or something?
Aside from a data scientist, very few; I mean very few people know what a GAN is. From those very few, a minute amount know the concept behind it.
Because when you go to your nearest library to open a book on deep learning, you see math. Not the one chocolate chip cookie plus one chocolate chip cookie equals how many cookies but one containg sigmoids, dropouts and dense functions which might be giving you nightmares in the day!
A GAN or let’s be more open now that you have your attention here stands for Generative Adversarial Network. And we are going to explain about that in a simple, lucid language.
What is a GAN?
Yann LeCun wrote an answer on Quora where he said that “Adversarial Training is the coolest thing since sliced bread”. No offense intended to Prof. LeCun but he probably forgot stuff like Hot Chili sauce or spices or the coming of Torch that has affected the deep learning community.
When you google “GAN in simple english”, you come across a Wikipedia page about a language spoken in China. We are not going to talk about that, of course. But I might be able to explain to you in simple terms about what is a GAN.
Suppose you have a picture of yourself; you sipping a Virgin Bloody Mary at a pool wearing Ray-Ban glasses. You brag about it to your friends after the vacation gets over, adding a garnishing that this was Bali or Goa( I love both!). One of your eagle-eyed friend who’s probably watching a lot of Sherlock however makes it clear that the pool and the Mary were photoshopped. When such stuff happens with neural networks, it’s called a discriminative model. A discriminative model determines whether a picture is natural or has been created artificially. In this context, being natural is whether it belongs to the dataset of the network.
Coming back to the main idea, another friend of yours has got your back. He helps you produce another similar photo of yourself, this time you are drinking tequila shots. This friend of yours is a generative network. A generative network tries to create a natural image similar to the original data. For people who teetotal like me, get this straight: a generative network is North Korea(See this article from the Telegraph) and the discriminative model is the FBI.
Deeper into the deep learning aspect
The first thing a discriminator does is to look for the obvious stuff that separates a real image from a fake one. By training, the discriminator learns more and more and so does the generator. In fact, the generator takes advantage of what the discriminator learns in order to learn for itself. This continues for millions of iterations. The dicriminator learns more and more subtle ways to differentiate while a generator learns new moves to create a fake one.
With enough training, the generator finally makes fakes seem so real that even we humans are not able to distinguish them.
Implementing the GAN( fun ends here; hell starts here)
Let’s get a little serious now(you had been warned!). The generator(G) and discriminant(D) are feed-forward neural networks. The generator takes an input of vector of random numbers(z) and transforms it into a data it intends to copy. The discriminator takes a set of data as input and determines the probability whether it is real or not. Both functions are alternatively optimised on batches of real and generated data until the GAN will slowly converge to producing realistic data; almost as realistic as modelling it.
GANs are getting hot for image generation using convolutional neural networks. A DCGAN or a deep convolutional generative adversarial network is one great GAN used for image generation. You can read the paper in the link to know more about them.