Paper:
Model-free deep reinforcement learning approaches have shown superhuman performance in simulated environments (e.g., Atari games, Go, etc). During training, these approaches often implicitly construct a latent space that contains key information for decision making. In this paper, we learn a forward model on this latent space and apply it to model-based planning in a miniature Real-time Strategy game with incomplete information (MiniRTS [Tian et al. (2017)]). We first show that the latent space constructed from existing actor-critic models contains relevant information of the game, and design training procedure to learn forward models. We also show that our learned forward model can predict meaningful future state and is usable for latent space Monte-Carlo Tree Search (MCTS), in terms of win rates against rule-based agents.

Review:
Summary: This paper proposes to use the latent representations learned by a model-free RL agent to learn a transition model for use in model-based RL (specifically MCTS). The paper introduces a strong model-free baseline (win rate ~80% in the MiniRTS environment) and shows that the latent space learned by this baseline does include relevant game information. They use the latent state representation to learn a model for planning, which performs slightly better than a random baseline (win rate ~25%).
Pros:
- Improvement of the model-free method from previous work by incorporating information about previously observed states, demonstrating the importance of memory.
- Interesting evaluation of which input features are important for the model-free algorithm, such as base HP ratio and the amount of resources available.
Cons:
- The model-based approach is disappointing compared to the model-free approach.
Quality and Clarity:
The paper in general is well-written and easy to follow and seems technically correct, though I found some of the figures and definitions confusing, specifically:
- The terms for different forward models are not defined (e.g. MatchPi, MatchA, etc.). I can infer what they mean based on Figure 1 but it would be helpful to readers to define them explicitly.
- In Figure 3b, it is not clear to me what the difference between the red and blue curves is.
- In Figure 4, it would be helpful to label which color corresponds to the agent and which to the rule-based AI.
- The caption in Figure 8 is malformatted.
- In Figure 7, the baseline of \hat{h_t}=h_{t-2} seems strange---I would find it more useful for Figure 7 to compare to the performance if the model were not used (i.e. if \hat{h_t}=h_t) to see how much performance suffers as a result of model error.
Originality:
I am unfamiliar with the MiniRTS environment, but given that it is only published in this year's NIPS (and that I couldn't find any other papers about it on Google Scholar) it seems that this is the first paper to compare model-free and model-based approaches in this domain. However, the model-free approach does not seem particularly novel in that it is just an extension of that from Tian et al. (2017) plus some additional features. The idea of learning a model based on the features from a model-free agent seems novel but lacks significance in that the results are not very compelling (see below).
Significance:
I feel the paper overstates the results in saying that the learned forward model is usable in MCTS. The implication in the abstract and introduction (at least as I interpreted it) is that the learned model would outperform a model-free method, but upon reading the rest of the paper I was disappointed to learn that in reality it drastically underperforms. The baseline used in the paper is a random baseline, which seems a bit unfair---a good baseline is usually an algorithm that is an obvious first choice, such as the model-free approach.