Generative adversarial network

Generative Adversarial Network illustration

Generative Adversarial Networks (GANs) are a class of artificial intelligence algorithms used in unsupervised learning, introduced by Ian Goodfellow and his colleagues in 2014. GANs are composed of two models: a generative model (G) that captures the data distribution, and a discriminative model (D) that estimates the probability that a sample came from the training data rather than G. The training procedure for G is to maximize the probability of D making a mistake. This framework corresponds to a minimax two-player game. In the space of artificial neural networks, GANs are used to generate new data instances that resemble the training data.

Overview

The core idea behind GANs is to have two networks contest with each other in a game. Given a training set, this technique learns to generate new data with the same statistics as the training set. For example, a GAN trained on photographs can generate new photographs that look at least superficially authentic to human observers, having many realistic characteristics. Though originally proposed as a form of generative model for unsupervised learning, GANs have also proven useful for semi-supervised learning, fully supervised learning, and reinforcement learning.

The generative network generates candidates while the discriminative network evaluates them. The generative network’s training objective is to increase the error rate of the discriminative network (i.e., "fool" the discriminator network by producing novel synthesized instances that appear to have come from the true data distribution).

Architecture

The architecture of a GAN involves two neural networks: the generator (G) and the discriminator (D). The generator creates samples intended to come from the same distribution as the training set. The discriminator examines samples to determine whether they are real or fake. The training involves adjusting the parameters of both networks until the discriminator cannot distinguish real data from fake data, meaning it guesses at random.

Generator

The generator model takes a random noise as input and generates samples as output. The generator's goal is to produce data that will be recognized by the discriminator as real.

Discriminator

The discriminator model takes samples from both the real data and the generated data and predicts whether these samples are real or fake. The goal of the discriminator is to correctly classify the samples as real or fake.

Training

Training a GAN involves presenting it with samples from the dataset until the generator produces a plausible imitation. Initially, the discriminator easily distinguishes between real and generated samples. However, as training progresses, the generator improves at producing realistic samples, and the discriminator's accuracy decreases. The process reaches equilibrium when the discriminator guesses at chance level, indicating the generator produces indistinguishable from real data.

Applications

GANs have a wide range of applications, including but not limited to:

• Image generation
• Photo realistic image synthesis
• Style transfer
• Image-to-image translation
• Super-resolution
• Drug discovery
• Data augmentation
• Voice generation
• Video generation

Challenges

Despite their potential, GANs face several challenges:

• Mode collapse: When the generator starts producing a limited variety of outputs.
• Training instability: GANs are notoriously difficult to train. This instability is due to the minimax nature of the training process.
• Evaluation of generated samples: It is challenging to evaluate the quality of the generated samples objectively.

Future Directions

Research in GANs continues to evolve, with efforts focused on making them more stable, efficient, and capable of generating high-quality outputs. Improvements in training methods, architecture designs, and loss functions are among the areas of active research.