Unsupervised Image Classification

InfoGAN is a type of generative adversarial network that modifies the GAN objective to encourage it to learn interpretable and meaningful representations. This is done by maximizing the mutual information between a fixed small subset of the GAN’s noise variables and the observations. Formally, InfoGAN is defined as a minimax game with a variational regularization of mutual information and the hyperparameter $\lambda$: $$ \min\_{G, Q}\max\_{D}V\_{INFOGAN}\left(D, G, Q\right) = V\left(D, G\right) - \lambda{L}\_{I}\left(G, Q\right) $$ Where $Q$ is an auxiliary distribution that approximates the posterior $P\left(c\mid{x}\right)$ - the probability of the latent code $c$ given the data $x$ - and $L\_{I}$ is the variational lower bound of the mutual information between the latent code and the observations. In the practical implementation, there is another fully-connected layer to output parameters for the conditional distribution $Q$ (negligible computation ontop of regular GAN structures). Q is represented with a softmax non-linearity for a categorical latent code. For a continuous latent code, the authors assume a factored Gaussian.

SCAN automatically groups images into semantically meaningful clusters when ground-truth annotations are absent. SCAN is a two-step approach where feature learning and clustering are decoupled. First, a self-supervised task is employed to obtain semantically meaningful features. Second, the obtained features are used as a prior in a learnable clustering approach. Image source: Gansbeke et al.