  We introduce a new architecture called a conditional invertible neural
network (cINN), and use it to address the task of diverse image-to-image
translation for natural images. This is not easily possible with existing INN
models due to some fundamental limitations. The cINN combines the purely
generative INN model with an unconstrained feed-forward network, which
efficiently preprocesses the conditioning image into maximally informative
features. All parameters of a cINN are jointly optimized with a stable, maximum
likelihood-based training procedure. Even though INN-based models have received
far less attention in the literature than GANs, they have been shown to have
some remarkable properties absent in GANs, e.g. apparent immunity to mode
collapse. We find that our cINNs leverage these properties for image-to-image
translation, demonstrated on day to night translation and image colorization.
Furthermore, we take advantage of our bidirectional cINN architecture to
explore and manipulate emergent properties of the latent space, such as
changing the image style in an intuitive way.
