  The segmentation of the retinal vasculature from eye fundus images represents
one of the most fundamental tasks in retinal image analysis. Over recent years,
increasingly complex approaches based on sophisticated Convolutional Neural
Network architectures have been slowly pushing performance on well-established
benchmark datasets. In this paper, we take a step back and analyze the real
need of such complexity. Specifically, we demonstrate that a minimalistic
version of a standard U-Net with several orders of magnitude less parameters,
carefully trained and rigorously evaluated, closely approximates the
performance of current best techniques. In addition, we propose a simple
extension, dubbed W-Net, which reaches outstanding performance on several
popular datasets, still using orders of magnitude less learnable weights than
any previously published approach. Furthermore, we provide the most
comprehensive cross-dataset performance analysis to date, involving up to 10
different databases. Our analysis demonstrates that the retinal vessel
segmentation problem is far from solved when considering test images that
differ substantially from the training data, and that this task represents an
ideal scenario for the exploration of domain adaptation techniques. In this
context, we experiment with a simple self-labeling strategy that allows us to
moderately enhance cross-dataset performance, indicating that there is still
much room for improvement in this area. Finally, we also test our approach on
the Artery/Vein segmentation problem, where we again achieve results
well-aligned with the state-of-the-art, at a fraction of the model complexity
in recent literature. All the code to reproduce the results in this paper is
released.
