NeurIPS 2020

EvolveGraph: Multi-Agent Trajectory Prediction with Dynamic Relational Reasoning

Review 1

Summary and Contributions: The authors propose a novel relational reasoning model that, unlike previous work, infers both the graph structure and updated node parameters at each time step (after a burn-in period). I think there are two contributions in this paper: 1. the graph structure of the system at hand should predicted dynamically rather than assumed fixed and given or fixed but learned 2. A specific method to estimate

Strengths: Predicting both updated node attributes and graph structure is a neat idea. Relational reasoning has received considerable attention in recent years at NeurIPS and I think this contribution is novel and significant. I appreciated how the authors managed to separate their contributions throughout the paper. In particular, as some of the baselines can be adapted to work dynamically (e.g. NRI can just refresh the inferred graph structure at every step after a burn-in period), separating the general idea of dynamically predicting the graph structure and the specific way of doing so was very useful.

Weaknesses: Inferring the graph structure might be especially beneficial for relatively large systems, where methods based on over-connected graphs might struggle to scale. I would love to see an experiment on a system with many nodes (see e.g. Learning to Simulate Complex Physics with Graph Networks). It would be really good to add some comments on what structure is actually predicted. Especially on large systems (e.g. Learning to Simulate Complex Physics with Graph Networks) one could start to make some serious analysis of the predictions themselves and how they are influenced by the context (e.g. an particle cloud representing an object becomes connected to another object upon contact).

Correctness: Yes

Clarity: Yes, some of the notation reinvents the wheel a bit and could rest on the previous work from e.g. Relational inductive biases, deep learning, and graph networks, but it's a minor thing.

Relation to Prior Work: Yes

Reproducibility: Yes

Additional Feedback: Very minor suggestion: I am not sure "computer vision" is the correct sub-field for this work. I think this will affect which poster session you are sent to, and therefore who comes to your poster. Take a quick look if there is a more appropriate sub-field if you have time. Maybe: "Algorithms -> Relational Learning; Deep Learning -> Recurrent Networks"? Thank you for sharing these cool ideas, I hope my suggestions help. ============ After Author Feedback =========== Thank you for taking the time to write a rebuttal, and for sharing these cool ideas. I stand by my initial assessment that this paper should be accepted. Best.

Review 2

Summary and Contributions: This paper proposes a framework for trajectory prediction for multiple interactive agents based on latent interaction graphs. This framework can be used exploit multiple modal data capture the multi-modality of future behaviours. They propose a double-stage training pipeline that improves training efficiency, and model performance.

Strengths: 1. From my understanding, their main contributions are two part: 1) their framework can exploit multimodal data for behaviours modelling 2) they propose a dynamic mechanism and a double stage training pipeline to improve performance. 2. The experiments are extensive. Their EvolveGraph outperforms other baselines.

Weaknesses: 1. Some parts of this paper are hard to understand, for example, Section 3 and 4. 2. There is no discussion on why their proposed dynamic mechanism and double stage training pipeline improve the performance. I would suggest to include some intuitive explanation and empirical evidence in introduction or it is hard to convince readers to accept the superiority of your model. For me, the novelty of introducing these two modules and integrating multimodal data is still limited for a top machine learning conference.

Correctness: The claims and method seem correct.

Clarity: Most parts are well-written but some parts are hard to understand.

Relation to Prior Work: They have discussed the previous work in related work and intro.

Reproducibility: Yes

Additional Feedback: I read the authors' response and appreciate the authors' efforts on improving this paper. Some of my concerns have been addressed. Integrating multi-modalities into graph learning could be an interesting contribution to this community. Thus, I upgraded my score to 6.

Review 3

Summary and Contributions: The authors propose a method for multi-agent trajectory prediction. In the prediction framework, it models interaction of multi-agents with a structure called EvolveGraph which could be dynamically updated. With a double-stage training pipeline, EvolveGraph firstly extracts interaction patterns, called static interaction graph, from the observed trajectories and context information. And then the static interaction graph could be evolved to reflect the dynamic relations of agents.

Strengths: Using graph to represent the interaction of multi-agents is one of the new trends in trajectory prediction area.

Weaknesses: The paper presents several critical issues. 1. Although double stage results are better than single stage results, their ADE/FDE are very close in table2, 3 and 4. Since double stage has dynamic interaction graph, it may has a strong advantage in relatively long time prediction. Dynamic interaction graph is the main innovation point of this work, it would be better EvolveGraph could be evaluated and compared in a long time prediction task. 2. It looks nothing new in the aspect of multi-modality.

Correctness: It seems correct.

Clarity: Well written and easy to follow.

Relation to Prior Work: No. Lack of discussion and compare with other graph based trajectory prediction methods.

Reproducibility: Yes

Additional Feedback:

Review 4

Summary and Contributions: This paper is an extension of the Neural Relational Inference framework for relational reasoning and trajectory prediction. It extends NRI in two key ways: first, relations are re-predicted periodically, leading to a dynamic relation graph. Second, the model ihs trained to predict a mixture of distributions, which allows for multimodal trajectory prediction. The approach is compared against NRI as well as other trajectory prediction methods to demonstrate its approved ability to recover known relations over NRI as well as its ability to predict future trajectories better than all compared methods.

Strengths: The work is very well-motivated and explained in a clear manner. It is sensible to extend NRI to dynamic relation graphs as well as to produce multi-modal predictions, and the empirical evaluation demonstrates that the approach is practically useful in addition to being conceptually interesting. Furthermore, the experiments are extensive - EvolveGraph is compared against not only all of the relevant NRI baselines but also a number of trajectory prediction models that include relational reasoning. A number of ablations are given in the supplementary material which demonstrate the impact of various modeling choices, which is very useful information for a practitioner.

Weaknesses: The ability of EvolveGraph to uncover known dynamic relations is not explored in as much detail as it could be. More specifically, the one synthetic experiment designed to evaluate this is somewhat simple, in that all relations change from "active" to "inactive" for all entities at the same moment in time, and this switch happens once. What happens when relations change at different times for different variables? What happens if the re-encoding gap is "out of sync" with the actual change in relations? How well does the model perform if relations change multiple times aperiodically? These questions are not explored here. There are a few modeling decisions which are made that are not explained or explored either. The ones that stick out to me: - The observation model has learned attentional coefficients that seem to be static across time. Do these contribute meaningfully to model performance? Also, doesn't the fact that these coefficients are static mean that they "pre-determine" the impact some variables have on others in a data-agnostic manner? - A different prediction mode is selected for each variable for every time step. What happens if modes are re-evaluated less often? How do the frequency of mode selection and relation re-prediction relative to each other impact final performance? - How many modes does the model predict, and how does performance vary as the number of predicted modes changes? Right now, it's difficult to understand if the performance improvements are primarily due to modeling multi-modality, modeling dynamic relations, or both. These criticisms are relatively minor, however; there is enough present in this work for it to be a worthwhile publication.

Correctness: Yes, the paper and methodology are correct.

Clarity: The paper is very easy to understand and explained in sufficient detail for the most part. The attention coefficients I mentioned previously are not explained in sufficient detail. Also, the loss equation used for training is not in the main paper but instead deferred to the supplementary, when it would be much clearer to include it in the appropriate section in the main paper.

Relation to Prior Work: The connection between this work and NRI is made very clearly, and other appropriate baselines are also used for experimentation.

Reproducibility: Yes

Additional Feedback: -Equations 8-10 are somewhat redundant with eqs. 5-7. These can be compressed to save space/make it clear that the same overall process is happening during both phases but using different inputs. This space would be better used to move the loss function equations into the main paper. -Unlike NRI, EvolveGraph was not trained as a VAE - specifically, no prior was enforced on the relations. What impact does using a prior here have on model performance? -------------------------------------------------------------------------------------------------------------- POST AUTHOR RESPONSE EDIT: Thanks for the response. I still think this paper is very interesting and should definitely be accepted.