Paper:
Understanding physical phenomena is a key component of human intelligence and enables physical interaction with previously unseen environments. In this paper, we study how an artificial agent can autonomously acquire this intuition through interaction with the environment. We created a synthetic block stacking environment with physics simulation in which the agent can learn a policy end-to-end through trial and error. Thereby, we bypass to explicitly model physical knowledge within the policy. We are specifically interested in tasks that require the agent to reach a given goal state that may be different for every new trial. To this end, we propose a deep reinforcement learning framework that learns policies for stacking tasks which are parametrized by a target structure -departing from conventional approaches based on simulation and planning. We validated the model on a toy example navigating in a grid world with different target positions and in a block stacking task with different target structures of the final tower. In contrast to prior work, our policies show better generalization across different goals.

Review:
Summary: This paper proposes to use deep Q-learning to learn how to reconstruct a given tower of blocks, where DQN is also parameterized by the desired goal state in addition to the current observed state.
Pros:
- Impressive results on a difficult block-stacking task.
Cons:
- The idea of parameterizing an RL algorithm by goals is not particularly novel.
Quality and Clarity:
The paper is extremely well-written, easy to follow, and largely technically correct, though I am somewhat concerned about how the results were obtained as it does not seem like the vanilla DQN agent could do so well, even on the 2-block scenes. Even just including stable scenes, I estimated based on Figure 5 that there must be about 70 different configurations that are stable (and this is likely an underestimate). So, if each of these scenes occurs equally often and the vanilla DQN agent does not receive any information about the target goal and just acts based on an "average" policy, I would expect it to only achieve success about 1/70th of the time. Am I missing something here?
Another thing that was unclear to me is how the rotation of the blocks is chosen: is the agent given the next block with the correct rotation, or can it also choose to rotate the block? In the text it is implied that the only actions are {left, right, down}, which seems to simplify the task immensely. It would be interesting to include results where the agent additionally has to choose from actions of {rotate left by 90 degrees, rotate right by 90 degrees}.
Also: are the scenes used during testing separate from those used during training? If not, it's not obvious that the agent isn't just learning to memorize the solution (which somewhat defeats the idea behind parameterizing the Q-network with new goals every time).
Originality and Significance:
The block-stacking task is very cool and is more complex than many other physics-based RL tasks in the literature, which often involve just stacking square blocks in a single tower. I think it is a useful contribution to introduce this task and the GDQN agent as a baseline. However, the notion of parameterizing the policy by the goal state is not particularly novel. While it is true that many RL papers do train to optimize just a single reward function for a single goal, it is also very straightforward to modify the state space to include a goal and indeed [1-4] are just a few examples of recent papers that have done this. In general, any time there is a procedurally generated environment (e.g. Sokoban, as in [5]) the goal necessarily is included as part of the state space---so the idea of GDQN isn't really that new.
[1] Oh, J., Singh, S., Lee, H., & Kohli, P. (2017). Zero-Shot Task Generalization with Multi-Task Deep Reinforcement Learning. arXiv Preprint arXiv:1706.05064.
[2] Dosovitskiy, A., & Koltun, V. (2017). Learning to act by predicting the future. Proceedings of the 5th International Conference on Learning Representations (ICLR 2017).
[3] Hamrick, J. B., Ballard, A. J., Pascanu, R., Vinyals, O., Heess, N., & Battaglia, P. W. (2017). Metacontrol for adaptive imagination-based optimization. Proceedings of the 5th International Conference on Learning Representations (ICLR 2017).
[4] Pascanu, R., Li, Y., Vinyals, O., Heess, N., Buesing, L., Racanière, S., … Battaglia, P. (2017). Learning model-based planning from scratch. arXiv Preprint arXiv: 1707.06170. Retrieved from https://arxiv.org/abs/1707.06170
[5] Weber, T., Racanière, S., Reichert, D. P., Buesing, L., Guez, A., Rezende, D. J., … Wierstra, D. (2017). Imagination-Augmented Agents for Deep Reinforcement Learning. arXiv Preprint arXiv: 1707.06203. Retrieved from http://arxiv.org/abs/1707.06203