Paper:
Coupled Generative Adversarial Networks
We propose coupled generative adversarial network (CoGAN) for learning a joint distribution of multi-domain images. In contrast to the existing approaches, which require tuples of corresponding images in different domains in the training set, CoGAN can learn a joint distribution without any tuple of corresponding images. It can learn a joint distribution with just samples drawn from the marginal distributions. This is achieved by enforcing a weight-sharing constraint that limits the network capacity and favors a joint distribution solution over a product of marginal distributions one. We apply CoGAN to several joint distribution learning tasks, including learning a joint distribution of color and depth images, and learning a joint distribution of face images with different attributes. For each task it successfully learns the joint distribution without any tuple of corresponding images. We also demonstrate its applications to domain adaptation and image transformation.

Review:
The paper proposes a method for learning generative models of pairs of corresponding images belonging to two different domains (e.g. the RGB image of a scene and the corresponding depth image). The method is based on two adversarial networks with partially shared weights. The generative networks share the weights that map the noise to an intermediate code but have separate weights that map from the intermediate code to each image type. This induces the model to generate pairs of corresponding images even when it is never trained with corresponding image pairs. The authors evaluate the proposed method with several image datasets, and also provide a demonstration of how it can be applied to domain adaptation and cross-domain transformation problems. 
I am in two minds about the paper. On the one hand the idea seems very interesting and powerful as it does not seem to rely on pairs of corresponding training images. The authors make a good effort of demonstrating and analyzing the capabilities of the approach in several ways (I especially like the unsupervised domain adaptation results). Visually the results look promising. (Although from the mostly qualitative evaluation it is not clear to what extent the models are overfitting to the training data, and e.g. for the RGB-D experiments I am finding it very hard to say anything more than that the generated pairs look superficially plausible.)  But on the other hand, it seems to me that the task is not well defined. Since the the model is never presented with corresponding image pairs there is actually nothing in the training data that establishes what “corresponding” means. The only pressure for the network to establish a sensible correspondence between images in the two domains comes from the particular weight sharing constraint which allows each network only limited capacity to map from the shared intermediate layer to the two different types of images (the evaluation in the paper uses networks that use only one or two non-shared layers). This may be appropriate, and work well, for domain pairs that differ mostly in terms of low-level features (e.g. faces with blonde / non-blonde hair, or RGB and D images, as in the paper). But it makes me wonder how easy it would be to impose just the appropriate capacity constraint for domain pairs where the correspondence is at a more abstract level and/or more stochastic (e.g. images and text).