Paper ID: | 1633 |
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Title: | Regularizing Trajectory Optimization with Denoising Autoencoders |

The paper addresses the problem of reducing the exploitation of inaccuracies of learned dynamics models by trajectory optimization algorithms in model-based Reinforcement Learning. For this, it proposes to add a regularizer to the optimization cost which writes as an estimation of the log probability (in a local window) of sampling the optimized trajectory from the distribution of known trajectories. The idea is to avoid trajectories deviating too much from the data used to learn the dynamics model, and hence avoid unreliable solutions. The authors propose to estimate the log probability term with a denoising autoencoder network. They provide multiple experiments comparing their method to other state-of-the-art approaches on known environments/datasets. The paper is really well organized and well written, which makes it easy to read and understand.Nonetheless, I found confusing/misleading to state in line 161 that exploration is out of the scope of the paper. Indeed, in my understanding, the motivation for the regularization proposed is to penalize exploration of unfamiliar parts of the state-control domain and in that sense it has everything to do with exploration. Similarly, you say in line 260 that your method is a good solution for safe exploration in real-world problems while system safety (i.e. preventing the system from entering a state in which it will break) is never really considered in the article. What is really considered is the reliability of the learned dynamics model along the proposed trajectory. Indeed, the method proposed penalizes exploration so that new trajectories don’t go too far from old known ones, and by iterating on this process of regularized optimization —> gathering data along optimized trajectories —> updating estimation of trajectories distributions —> re-optimizing new regularized trajectories, it seems that it is possible to obtain unsafe trajectories at some point if no other assumptions are made. Other clarity related minor remarks are listed below: - What are the gray shades in figure 3 ? Not in the caption nor text. - Line 192: as efficiently as - Line 192-193: it seems that you mis-switched open-loop and closed-loop Concerning the methodological part, the idea of regularizing the optimization using the density of real trajectories used to train the dynamics model is not new. As pointed in the paper itself (references [17, 20, 30, 31]), KL-divergence based regularizers have already been proposed in the model-based RL literature. Also, the use of local density estimators trained on past experience to penalize unreliable trajectories is a known technique in applied optimal control (see for example Gaussian Mixture Penalty for Trajectory Optimization Problems, 2018). I believe that further comments motivating the choice of a DAE could be useful for the reader considering that DAE only estimates the derivative of the log likelihood. Indeed, variational autoencoders or other more standards density estimation models like Gaussian Mixture models or Kernel density estimators seem like a more natural choice. One potential advantage of deep network models such as DAE or VAE could be that they are less sensitive to dimensionality curse than standard methods. Indeed, it is said in the article that high-dimensional control spaces are the focus of the study and that adversarial effects are more frequent in such context. However, the dimensions of the problems considered in the experimental part are not mentioned. I believe that this information is crucial to better understand the strengths of the proposed approach. Besides this, I must say that the experimental part of the paper is of great quality given the large number of experiments considered, the fairly complex control tasks tackled and a comparison which seems fair to some state-of-the-art techniques. I wonder however why the authors chose to compare the proposed method to CEM instead of a more standard trajectory optimization technique, such as dynamic programming. Also, concerning figure 4, we can see on the results of the Pusher, the Half-cheetah and the Ant that none of the returns have stabilized after 10 episodes and it seems natural to wonder what happens next. There is indeed a comment on that in line 230, saying that the proposed method does not reach the asymptotic performance of PETS, with more interesting results in figure 8 of the supplementary material showing that the proposed method regularizes exploration too much and does not beat PETS with noise injection after 50 episodes. Despite the honest comment on this limitation in the main document, I believe that the results from figure 8 are too interesting to be left to supplementary material. On the contrary, concerning footnote 1 in page 7, I think that the results of the verifications carried on the re-implementation of PETS should be added to the supplemental material, as they seem important for reproducibility. Despite the clarity of the paper and the great quality of the experiments presented therein, it appears to be a fairly straightforward application of existing techniques with new models (RNN, DAE). As such, I think it would be well-suited for publication in a more applied journal/conference or maybe in a NIPS workshop. -- I have read the authors response. Although they have addressed most of my concerns, I still think that experimental results are needed to justify their claims.

The authors begin with a survey of the related work which is a good representative of the current work in this area. I like their first process control experiment as it clearly lays out what is being achieved, with respect to conventional state of the art (i.e., not just the last few years' DRL papers). I would like to have better understood how well the proposed algorithms would fare by also looking at a baseline with the true models. This would tell us not just the improvements from below, but the upper envelopes in this setup. I find that the experiments have explored the factors of variations suitably, within the confines of the environment within which they are setup. One major source of model uncertainty, in the real world, is uncertainty in the environment (both epistemic and aleatory). It is not yet clear to me how this method could cope with that, and that would be a helpful experiment.

Originality: The denoising autoencoder regularized trajectory optimization is a novel method to deal with the adversarial effect with the learned dynamic model in the model-based RL framework. Instead of investigating new ways to learn the dynamic model, this paper tries to improve the performance and sample efficiency from the trajectory optimization regularization point of view, which is interesting and nice try. Quality: The quality of the paper is good. The only concern I have is about the experimental evaluations. The chosen tasks are relatively simple, the implementation of the more complex experiments are expected. Also, a comparison to more algorithms would be useful. The method of [Ha & Schmidhuber] is an important work in the field of model-based RL, the comparison to world model is encouraged to include. I understand that the problem set up in the current submission is different to world model. However, the dynamic model learning method should also affect the performance, which is not discussed in the paper. Thus, the model learning method in [Ha & Schmidhuber] (VAE-based model) or some other methods, is encouraged to try in the implementation of your model learning part. [Ha & Schmidhuber] David Ha and Jurgen Schmidhuber. World models. arXiv: 1803.10122, 2018. Clarity: The paper is well written and organized, and it is easy to understand. Significance: The applicability of model-based RL is on the premise that the dynamic of environment is learned accurately. However, we all know that the approximation of the environment is challenging especially in the complex real-world problem. This paper tries to mitigate this bottleneck problem of model-based RL methods from a different view, which is with great significance. It is nice to see more real-world applications of the proposed method. ****=======**** I have read the author response. I have to clarify that my concerns is not on dynamics models, but on the dynamic model learning method. Although most of my concerns are responded, I still think that more comparative discussion with other dynamic model learning approaches are necessary.