Paper:
It would be desirable for a reinforcement learning (RL) based agent to learn behaviour by merely watching a demonstration. However, defining rewards that facilitate this goal within the RL paradigm remains a challenge. Here we address this problem with Siamese networks, trained to compute distances between observed behaviours and the agent's behaviours. Given a desired motion such Siamese networks can be used to provide a reward signal to an RL agent via the distance between the desired motion and the agent's motion. We experiment with an RNNbased comparator model that can compute distances in space and time between motion clips while training an RL policy to minimize this distance. Through experimentation, we have had also found that the inclusion of multi-task data and an additional image encoding loss helps enforce the temporal consistency. These two components appear to balance reward for matching a specific instance of a behaviour versus that behaviour in general. Furthermore, we focus here on a particularly challenging form of this problem where only a single demonstration is provided for a given task -the one-shot learning setting. We demonstrate our approach on humanoid agents in both 2D with 10 degrees of freedom (DoF) and 3D with 38 DoF.

Review:
This paper presents visual imitation with reinforcement learning (VIRL), an algorithm for learning to imitate expert trajectories based solely on visual observations, and without access to the expert’s actions.  The algorithm is similar in form to GAIL and its extensions, learning a reward function which captures the similarity between an observed behavior and the expert's demonstrations, while simultaneously using reinforcement learning to find a policy maximizing this reward, such that the learned policy will replicate the demonstrated behavior as well as possible.  A key feature of this method is that the learned reward function is defined by a learned distance metric, which evaluates the similarity between the agent's current trajectory, and the nearest demonstrated expert trajectory.
The network describing the distance metric is recurrent, such that the distance is defined between trajectories rather than individual states.  The distance function network is trained via a negative sampling approach, where expert trajectories are randomly reordered to produce examples that dissimilar to the expert trajectories.   The distance network also defines a variational autoencoder, and the reconstruction of the target trajectories is treated as an auxiliary task to help train better representations of the trajectory space.
While previous work has considered the problem of visual imitation learning, the approach taken here is novel in its architecture and loss function, and significantly outperforms the baselines in terms of the similarity between the resulting behavior and the expert behavior.
The clarity of the technical presentation could be improved, however.  In particular, it would be helpful for the reader if the definitions of the negative sampling loss and the autoencoder losses were given before the combined loss, and if we saw the form of the loss for both positive and negative sequence pairs.  Equation 4 could also be made explicit, with the full summation term included.