  We consider the problem of training a fully convolutional network to estimate
the relative 6D pose of a robot given a camera image, when the robot is
equipped with independent controllable LEDs placed in different parts of its
body. The training data is composed by few (or zero) images labeled with a
ground truth relative pose and many images labeled only with the true state
(\textsc{on} or \textsc{off}) of each of the peer LEDs. The former data is
expensive to acquire, requiring external infrastructure for tracking the two
robots; the latter is cheap as it can be acquired by two unsupervised robots
moving randomly and toggling their LEDs while sharing the true LED states via
radio. Training with the latter dataset on estimating the LEDs' state of the
peer robot (\emph{pretext task}) promotes learning the relative localization
task (\emph{end task}). Experiments on real-world data acquired by two
autonomous wheeled robots show that a model trained only on the pretext task
successfully learns to localize a peer robot on the image plane; fine-tuning
such model on the end task with few labeled images yields statistically
significant improvements in 6D relative pose estimation with respect to
baselines that do not use pretext-task pre-training, and alternative
approaches. Estimating the state of multiple independent LEDs promotes learning
to estimate relative heading. The approach works even when a large fraction of
training images do not include the peer robot and generalizes well to unseen
environments.
