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Title: Improving Sample Efficiency in Model-Free Reinforcement Learning from Images

Paper content:
Training an agent to solve control tasks directly from high-dimensional images with model-free reinforcement learning (RL) has proven difficult.
The agent needs to learn  a latent representation together with a control policy to perform the task.
Fitting a high-capacity encoder using a scarce reward signal is not only extremely sample inefficient, but also prone to suboptimal convergence.
Two ways to improve sample efficiency are to learn a good feature representation and use off-policy algorithms.
We dissect various approaches of learning good latent features, and conclude that the image reconstruction loss is the essential ingredient that enables efficient and stable representation learning in image-based RL.
Following these findings, we devise an off-policy actor-critic algorithm with an auxiliary decoder that trains end-to-end and matches state-of-the-art performance across both model-free and model-based algorithms on many challenging control tasks.
We release our code to encourage future research on image-based RL.
Cameras are a convenient and inexpensive way to acquire state information, especially in complex, unstructured environments, where effective control requires access to the proprioceptive state of the underlying dynamics.
Thus, having effective RL approaches that can utilize pixels as input would potentially enable solutions for a wide range of real world problems.
The challenge is to efficiently learn a mapping from pixels to an appropriate representation for control using only a sparse reward signal.
Although deep convolutional encoders can learn good representations (upon which a policy can be trained), they require large amounts of training data.
As existing reinforcement learning approaches already have poor sample complexity, this makes direct use of pixel-based inputs prohibitively slow.
For example, model-free methods on Atari (Bellemare et al., 2013) and DeepMind Control (DMC) (Tassa et al., 2018) take tens of millions of steps (Mnih et al., 2013; Barth-Maron et al., 2018) , which is impractical in many applications, especially robotics.
A natural solution is to add an auxiliary task with an unsupervised objective to improve sample efficiency.
The simplest option is an autoencoder with a pixel reconstruction objective.
Prior work has attempted to learn state representations from pixels with autoencoders, utilizing a two-step training procedure, where the representation is first trained via the autoencoder, and then either with a policy learned on top of the fixed representation (Lange & Riedmiller, 2010; Munk et al., 2016; Higgins et al., 2017b; Zhang et al., 2018; Nair et al., 2018) , or with planning (Mattner et al., 2012; Finn et al., 2015) .
This allows for additional stability in optimization by circumventing dueling training objectives but leads to suboptimal policies.
Other work utilizes end-to-end model-free learning with an auxiliary reconstruction signal in an on-policy manner (Jaderberg et al., 2017) .
We revisit the concept of adding an autoencoder to model-free RL approaches, but with a focus on off-policy algorithms.
We perform a sequence of careful experiments to understand why previous approaches did not work well.
We found that a pixel reconstruction loss is vital for learning a good representation, specifically when trained end-to-end.
Based on these findings, we propose a simple autoencoder-based off-policy method that can be trained end-to-end.
Our method is the first modelfree off-policy algorithm to successfully train simultaneously both the latent state representation and policy in a stable and sample-efficient manner.
(Tassa et al., 2018) used in our experiments.
Each task offers an unique set of challenges, including complex dynamics, sparse rewards, hard exploration, and more.
Refer to Appendix A for more information.
Of course, some recent state-of-the-art model-based RL methods (Hafner et al., 2018; Lee et al., 2019) have demonstrated superior sample efficiency to leading model-free approaches on pixel tasks from (Tassa et al., 2018) .
But we find that our model-free, off-policy, autoencoder-based approach is able to match their performance, closing the gap between model-based and model-free approaches in image-based RL, despite being a far simpler method that does not require a world model.
This paper makes three main contributions:
(i) a demonstration that adding a simple auxiliary reconstruction loss to a model-free off-policy RL algorithm achieves comparable results to state-of-the-art model-based methods on the suite of continuous control tasks from Tassa et al. (2018) ;
(ii) an understanding of the issues involved with combining autoencoders with model-free RL in the off-policy setting that guides our algorithm; and
(iii) an open-source PyTorch implementation of our simple method for researchers and practitioners to use as a strong baseline that may easily be built upon.
We have presented the first end-to-end, off-policy, model-free RL algorithm for pixel observations with only reconstruction loss as an auxiliary task.
It is competitive with state-of-the-art model-based methods, but much simpler, robust, and without requiring learning a dynamics model.
We show through ablations the superiority of end-to-end learning over previous methods that use a two-step training procedure with separated gradients, the necessity of a pixel reconstruction loss over reconstruction to lower-dimensional "correct" representations, and demonstrations of the representation power and generalization ability of our learned representation.
We find that deterministic models outperform β-VAEs (Higgins et al., 2017a) , likely due to the other introduced instabilities, such as bootstrapping, off-policy data, and end-to-end training with auxiliary losses.
We hypothesize that deterministic models that perform better even in stochastic environments should be chosen over stochastic ones with the potential to learn probability distributions, and argue that determinism has the benefit of added interpretability, through handling of simpler distributions.
In the Appendix we provide results across all experiments on the full suite of 6 tasks chosen from DMC (Appendix A), and the full set of hyperparameters used in Appendix B. There are also additional experiments autoencoder capacity (Appendix E), a look at optimality of the learned latent representation (Appendix H), importance of action repeat (Appendix I), and a set of benchmarks on learning from proprioceptive observation (Appendix J).
Finally, we opensource our codebase for the community to spur future research in image-based RL.

assistant: text: We design a simple and efficient model-free off-policy method for image-based reinforcement learning that matches the state-of-the-art model-based methods in sample efficiency