Zhejun Zhang

I am a PhD student advised by Prof. Luc Van Gool at the Computer Vision Lab (CVL) of ETH Zurich. My project is funded by Toyota Research on Automated Cars in Europe (TRACE, TRACE Zurich). Before joining CVL I received my BSc in ITET from TU Munich and my MSc in ITET from ETH Zurich. As master thesis topic I worked on vision-based control and considered that as my research interest since then.

My current research interests include imitation learning, reinforcement learning, world models, neural rendering and their applications to autonomous driving. In particular, I’m trying to tackle the problem of autonomous driving from a data-driven perspective by combining end-to-end policy learning and neural simulation.

selected publications

TrafficBots: Towards World Models for Autonomous Driving Simulation and Motion Prediction

Zhejun Zhang, Alexander Liniger, Dengxin Dai, Fisher Yu, and Luc Van Gool

In International Conference on Robotics and Automation (ICRA) 2023

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Data-driven simulation has become a favorable way to train and test autonomous driving algorithms. The idea of replacing the actual environment with a learned simulator has also been explored in model-based reinforcement learning in the context of world models. In this work, we show data-driven traffic simulation can be formulated as a world model. We present TrafficBots, a multi-agent policy built upon motion prediction and end-to-end driving, and based on TrafficBots we obtain a world model tailored for the planning module of autonomous vehicles. Existing data-driven traffic simulators are lacking configurability and scalability. To generate configurable behaviors, for each agent we introduce a destination as navigational information, and a time-invariant latent personality that specifies the behavioral style. To improve the scalability, we present a new scheme of positional encoding for angles, allowing all agents to share the same vectorized context and the use of an architecture based on dot-product attention. As a result, we can simulate all traffic participants seen in dense urban scenarios. Experiments on the Waymo open motion dataset show TrafficBots can simulate realistic multi-agent behaviors and achieve good performance on the motion prediction task.
@inproceedings{zhang2023trafficbots, title = {TrafficBots: Towards World Models for Autonomous Driving Simulation and Motion Prediction}, booktitle = {International Conference on Robotics and Automation (ICRA)}, author = {Zhang, Zhejun and Liniger, Alexander and Dai, Dengxin and Yu, Fisher and Van Gool, Luc}, year = {2023}, }
A Multiplicative Value Function for Safe and Efficient Reinforcement Learning

Nick Bührer, Zhejun Zhang, Alexander Liniger, Fisher Yu, and Luc Van Gool

Under Review 2023

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An emerging field of sequential decision problems is safe Reinforcement Learning (RL), where the objective is to maximize the reward while obeying safety constraints. Being able to handle constraints is essential for deploying RL agents in real-world environments, where constraint violations can harm the agent and the environment. To this end, we propose a safe model-free RL algorithm with a novel multiplicative value function consisting of a safety critic and a reward critic. The safety critic predicts the probability of constraint violation and discounts the reward critic that only estimates constraint-free returns. By splitting responsibilities, we facilitate the learning task leading to increased sample efficiency. We integrate our approach into two popular RL algorithms, Proximal Policy Optimization and Soft Actor-Critic, and evaluate our method in four safety-focused environments, including classical RL benchmarks augmented with safety constraints and robot navigation tasks with images and raw Lidar scans as observations. Finally, we make the zero-shot sim-to-real transfer where a differential drive robot has to navigate through a cluttered room. Our code can be found at https://github.com/nikeke19/Safe-Mult-RL.
@article{buehrer2023multiplicative, title = {A Multiplicative Value Function for Safe and Efficient Reinforcement Learning}, journal = {Under Review}, author = {Bührer, Nick and Zhang, Zhejun and Liniger, Alexander and Yu, Fisher and Van Gool, Luc}, year = {2023}, }
End-to-End Urban Driving by Imitating a Reinforcement Learning Coach

Zhejun Zhang, Alexander Liniger, Dengxin Dai, Fisher Yu, and Luc Van Gool

In Proceedings of the IEEE/CVF International Conference on Computer Vision (ICCV) 2021

Abs arXiv Bib Code Website

End-to-end approaches to autonomous driving commonly rely on expert demonstrations. Although humans are good drivers, they are not good coaches for end-to-end algorithms that demand dense on-policy supervision. On the contrary, automated experts that leverage privileged information can efficiently generate large scale on-policy and off-policy demonstrations. However, existing automated experts for urban driving make heavy use of hand-crafted rules and perform suboptimally even on driving simulators, where ground-truth information is available. To address these issues, we train a reinforcement learning expert that maps bird’s-eye view images to continuous low-level actions. While setting a new performance upper-bound on CARLA, our expert is also a better coach that provides informative supervision signals for imitation learning agents to learn from. Supervised by our reinforcement learning coach, a baseline end-to-end agent with monocular camera-input achieves expert-level performance. Our end-to-end agent achieves a 78% success rate while generalizing to a new town and new weather on the NoCrash-dense benchmark and state-of-the-art performance on the challenging public routes of the CARLA LeaderBoard.
@inproceedings{zhang2021roach, title = {End-to-End Urban Driving by Imitating a Reinforcement Learning Coach}, booktitle = {Proceedings of the IEEE/CVF International Conference on Computer Vision (ICCV)}, author = {Zhang, Zhejun and Liniger, Alexander and Dai, Dengxin and Yu, Fisher and Van Gool, Luc}, year = {2021}, }