  Deep learning approaches in image processing predominantly resort to
supervised learning. A majority of methods for image denoising are no exception
to this rule and hence demand pairs of noisy and corresponding clean images.
Only recently has there been the emergence of methods such as Noise2Void, where
a deep neural network learns to denoise solely from noisy images. However, when
clean images that do not directly correspond to any of the noisy images are
actually available, there is room for improvement as these clean images contain
useful information that fully unsupervised methods do not exploit. In this
paper, we propose a method for image denoising in this setting. First, we use a
flow-based generative model to learn a prior from clean images. We then use it
to train a denoising network without the need for any clean targets. We
demonstrate the efficacy of our method through extensive experiments and
comparisons.
