CooHOI: Learning Cooperative Human-Object Interaction with Manipulated Object Dynamics

Authors: jiawei  gao
ziqin  wang
zeqi  xiao
jingbo  wang
tai  wang
jinkun  cao
xiaolin  hu
si  liu
jifeng  dai
jiangmiao  pang
Venue: NeurIPS_cc_2024_Conference
Published: 2024-05-11 18:33:42

Abstract: Enabling humanoid robots to clean rooms has long been a pursued dream within humanoid research communities. However, many tasks require multi-humanoid collaboration, such as carrying large and heavy furniture together. Given the scarcity of motion capture data on multi-humanoid collaboration and the efficiency challenges associated with multi-agent learning, these tasks cannot be straightforwardly addressed using training paradigms designed for single-agent scenarios. In this paper, we introduce **Coo**perative **H**uman-**O**bject **I**nteraction (**CooHOI**), a framework designed to tackle the challenge of multi-humanoid object transportation problem through a two-phase learning paradigm: individual skill learning and subsequent policy transfer. First, a single humanoid character learns to interact with objects through imitation learning from human motion priors. Then, the humanoid learns to collaborate with others by considering the shared dynamics of the manipulated object using centralized training and decentralized execution (CTDE) multi-agent RL algorithms. When one agent interacts with the object, resulting in specific object dynamics changes, the other agents learn to respond appropriately, thereby achieving implicit communication and coordination between teammates. Unlike previous approaches that relied on tracking-based methods for multi-humanoid HOI, CooHOI is inherently efficient, does not depend on motion capture data of multi-humanoid interactions, and can be seamlessly extended to include more participants and a wide range of object types.

Main paper: 
 introduction: 
 Imagine the scenario where you're moving home and seeking help from humanoid robots. However, certain items, like beds and sofas, are too large and heavy for a single robot to manage effectively. Despite significant advancements in learning and control of physics-based humanobject interactions [6, 43, 21, 37, 20, 45, 7] and humanoid robots performing complex operations [29, 19, 26, 2, 42, 41, 10, 9, 39] , the area of multiple humanoids collaboratively transporting objects-particularly when a single individual may find it challenging to handle heavy or long items-remains relatively under-investigated. Some previous efforts [44] tried to address these challenges using tracking-based methods, capturing interactions between two characters during tasks like box carrying, and training policies to mimic these actions. However, obtaining comprehensive motion data on interactions involving multiple participants is costly, and tracking-based methods struggle to adapt to scenarios with varying object sizes and an increased number of agents. Another straightforward idea might be to directly train a multi-agent policy from scratch, using an approach Figure 1 : Our framework empowers physically simulated characters to execute multi-agent humanobject interaction (HOI) tasks with naturalness and precision. similar to that for single agents. However, the expanded action sampling space significantly slows down the overall training process, preventing the policy from converging properly.Inspired by how humans learn cooperation skills, i.e. beginning with mastering tasks independently and then developing strategies for collaborative efforts in coordination tasks, we propose a twostage framework for training multi-agent cooperation strategies, named Cooperative Human-Object Interaction, CooHOI. In the initial stage, we train a single-agent object carrying policy utilizing the AMP framework [25, 7] . To ensure agents focus significantly on the dynamics of the objects they are tasked with transporting, we include the state and velocity information of the object's bounding box in the agent's observation space. The second stage aims to transfer these individual carrying skills into collaborative strategies. When two humans collaborate to carry a long object, they typically hold it at opposite ends. Moreover, when one individual takes action, it affects the dynamics of the object, enabling the other individual to perceive this change and adjust their actions accordingly for effective cooperation. Therefore, we adjust the observation for both agents to focus on the bounding boxes located at the ends of the long objects. This refinement aligns with the observation space used during the single-agent training phase, effectively leveraging the previously developed single carrying skills. Additionally, the inherent rigid body characteristic of long objects facilitates implicit communication between agents. This setup allows an agent to adeptly adjust to their teammate's actions by observing changes in the object's dynamics, thereby enhancing coordination and cooperation in carrying tasks. The overall framework is illustrated in Figure 2 .To validate the effectiveness of our framework, we conducted experiments where we trained control policies for two humanoid characters to carry various long objects, such as boxes and sofas. Our results demonstrate that our framework enables these characters to exhibit natural-looking behaviors while successfully completing cooperative tasks, utilizing only motion capture data from one single agent. We compared our approach against the baseline method of training from scratch and performed detailed ablation studies to evaluate the impact of our design decisions, also testing the limitations of our framework. In summary, our main contributions are as follows: 1)We have developed an efficient and robust framework, CooHOI, for training physically simulated characters in cooperative object transporting tasks, demonstrating significant effectiveness. 2)We have established that utilizing object dynamics for communication proves to be an effective strategy in learning cooperative object transporting tasks. 
related work: 
 Physics-based Human-Object Interaction Motion Synthesis. Synthesizing natural and physically plausible human-scene interactions, such as humanoid characters sitting on chairs, lying on beds, and carrying boxes, is crucial for advancements in character animation and robotics. Physics-based methods leverage physics simulators [18, 35] to control characters modeled as interconnected rigid bodies through joint torques and deep reinforcement learning methods. To facilitate the training process, some strategies employ tracking-based techniques [16, 23, 1, 38, 20, 44] , which rely on the availability of high-quality reference motions. This reliance often limits their application in humanscene interactions due to the scarcity of suitable data and affects their versatility across different scenarios. Recently, the Adversarial Motion Priors(AMP) framework [25] introduced the use of a discriminator to ensure that generated motions align with the distribution of reference motions. This approach has shown success in various downstream tasks [11, 24, 34] , including human-scene interactions [7, 21, 37] . Despite these advancements, there has been limited focus on synthesizing cooperative behaviors among multiple characters interacting with objects-a gap that our work aims to address.Multi-Character Control. While significant advancements have been made in synthesizing motions for single agents, the realm of multi-character animation remains relatively unexplored. Existing approaches predominantly rely on kinematic-based methods [36, 14, 32, 30, 5] and datasets of multicharacter interactions [13, 31, 4] . These methods, however, require high-quality interaction data and often fall short in ensuring physical plausibility or adequately handling multi-character cooperative interactions with objects. Physics-based techniques for character animation have primarily focused on aspects like crowd navigation [8, 27] , limiting themselves to behaviors such as pedestrian movement and collision avoidance. Although [44] showcases interactions among multiple characters and object manipulation, such as carrying long boxes, these tracking-based approaches necessitate motion capture data of human interactions or human-object interactions and struggle to scale to an arbitrary number of agents or different types of objects. In contrast, our framework requires only single agent motion capture data for multi-character object transporting tasks and can easily extend to different types of objects and different numbers of agents.Question: Does the paper discuss the limitations of the work performed by the authors?Answer: [Yes] Justification: We discussed the limitations of our methods at the last section of our main paper. We also test the influence of different factors in our work in ablation studies.Guidelines:• The answer NA means that the paper has no limitation while the answer No means that the paper has limitations, but those are not discussed in the paper. • The authors are encouraged to create a separate "Limitations" section in their paper.• The paper should point out any strong assumptions and how robust the results are to violations of these assumptions (e.g., independence assumptions, noiseless settings, model well-specification, asymptotic approximations only holding locally). The authors should reflect on how these assumptions might be violated in practice and what the implications would be. • The authors should reflect on the scope of the claims made, e.g., if the approach was only tested on a few datasets or with a few runs. In general, empirical results often depend on implicit assumptions, which should be articulated. • The authors should reflect on the factors that influence the performance of the approach.For example, a facial recognition algorithm may perform poorly when image resolution is low or images are taken in low lighting. Or a speech-to-text system might not be used reliably to provide closed captions for online lectures because it fails to handle technical jargon. • The authors should discuss the computational efficiency of the proposed algorithms and how they scale with dataset size. • If applicable, the authors should discuss possible limitations of their approach to address problems of privacy and fairness. • While the authors might fear that complete honesty about limitations might be used by reviewers as grounds for rejection, a worse outcome might be that reviewers discover limitations that aren't acknowledged in the paper. The authors should use their best judgment and recognize that individual actions in favor of transparency play an important role in developing norms that preserve the integrity of the community. Reviewers will be specifically instructed to not penalize honesty concerning limitations. 
methodology: 
 Figure 2 shows the overall framework of our approach. Our method utilizes motion data from individual agents and involves a two-stage learning process to create cooperative control policies. First, we train a policy for single-agent carrying tasks, using the dynamics of the manipulated object as feedback, as described in Section 3.2. Then, we use parallel training to develop cooperative strategies, with changes in the object's dynamics acting as a form of implicit communication. This is further explained in Section 3.3.! " # $ ! # $ " " ! $ # ! # #PolicyPhysics-based Character Control. We formulate physics-based character control as a goalconditioned reinforcement learning task. At each time step t, the agent samples an action from its policy π (a t | s t , g t ) based on the current state s t and the task-specific goal feature g t . When this action is applied to the character, the environment transitions to the next state s t+1 , and the agent receives a task reward r G (s t , g t , s t+1 ). To train control policies that enable characters to achieve high-level tasks in a natural and life-like manner, we adopt the AMP framework [25] . While this framework aims to optimize the expected cumulative task reward J(π), it introduces a discriminator to encourage the character to produce behaviors similar to those in the dataset by providing a style reward r S (s t , s t+1 ). The agent's reward r t at each time step t is defined by r t = w G r G (s t , g t , s t+1 ) + w S r S (s t , s t+1 ). More details can be found in our appendix and the original AMP paper [25] .Multi-Agent Reinforcement Learning. We formulate our cooperative task as a Partially Observable Markov Decision Process (POMDP) [15] . A POMDP with n agents is defined by {S, A 1 , • • • , A n , O 1 , • • • , O n ,(θ) = E a t 1 ,••• ,a t n ,s t Σ T t=0 γ t r t .In developing our approach for single-agent object manipulation, we introduce several advancements based on previous methods. We integrate the dynamics of the manipulated object into the observation space and introduce a reward function framework for object manipulation tasks. These enhancements allow the trained policy to easily adapt to multi-agent settings.Feature with Manipulated Object Dynamics as Feedback.For successful object carrying tasks, we emphasize the critical role of using the dynamics of the manipulated object as feedback. These dynamics are captured through the eight vertices of the object o's bounding box b ver t , its rotation angle b facing t , its velocity b v t and its angular velocity b w t , as described in Equation (1) .D t = concatenate(b ver t , b facing t , b v t , b w t ).(1) By incorporating these dynamics information into the observation, we establish a feedback mechanism that keeps agents continually informed about the outcomes of their actions. This also equips agents with the capability to react appropriately, whether engaged in single-agent tasks or collaborative multi-agent environments. Along with the state of the agent s t and the position of the target d pos t , we formulate the dynamics-aware task observation as:EQUATION3.2.2 Enriched Task Design facilitating Efficient Skill Transfer.To facilitate the transition from single-agent to multi-agent object carrying tasks, we decompose the carrying process into three sub-tasks: walking towards the objects, lifting the object from the ground, and carrying the object to its intended destination. Consequently, the reward system is structured into three components: r walk , r held and r target .To encourage the agents to choose the face of the object they will face while carrying, we introduce an additional goal feature called stand points x stand t . During training, these stand points are randomly allocated to positions directly in front of the various faces of the object at time step t. This strategy is designed to facilitate multi-agent cooperative training, helping the agents learn to avoid walking to the long side of the object where it is difficult to grasp and carry. We then define r walk as the distance between the agent and the stand point, as specified in Equation (3) .EQUATIONAdditionally, in scenarios where multiple agents are transporting an object, the object's considerable size often makes it difficult for agents to find an appropriate grip point for lifting. To address this, we introduce a novel concept called held points. Specifically, we select the geometric center h t of each end of the object as the held point and encourage the agents to interact at these points. The reward function r held , as defined in Equation ( 4), uses x hand t to represent the mean position of the agent's two hands. This design aids the transition from individual to collective task environments by encouraging agents to identify the held points at the geometric center of each end of the long object.EQUATIONWe then define r target to encourage agents to carry the object to the destination:r target = exp ∥x target t -x object t ∥ 2 .(5)Upon mastering single-agent tasks involving object carrying, it is crucial to effectively transfer and further enhance these abilities to boost the cooperative learning process. Initially, we facilitate the skill transfer by guiding each agent to lift and transport one end of a large object. Subsequently, we refine the control policy through the application of the Multi-Agent Proximal Policy Optimization (MAPPO) algorithm [40] , leveraging the dynamics of manipulated objects for feedback and implicit communication.Initially, we replicate the single-agent object-carrying policy for all agents and then fine-tune it within a cooperative framework. However, handling a long object poses a challenge due to its size, which complicates the agents' ability to identify suitable lifting points, thereby hindering the efficient application of their object-carrying skills. To address this issue, we encourage agents to observe the dynamics information-specifically, the state and velocity data of the bounding box at each end of the long object, as outlined in Section 3.2.1. This approach allows agents to simulate carrying a smaller box positioned at the long object's ends, using the dynamics of this "smaller box" as feedback, similar to the single-agent skill training process. Additionally, we incorporate previous methodologies, such as stand points and held points described in Section 3.2.2, to enhance the smooth transition of skills.Moreover, this design of dynamics as observation acts as implicit communication channel between agents. When an agent takes an action, the object dynamics will change, and because we use the manipulated object dynamics as feedback in single-agent training, the change in object dynamics will result in a corresponding change in strategy for the other agents. This method of implicit communication presents a straightforward yet effective approach for enhancing teamwork in multiagent settings. Furthermore, this framework proves to be adaptable with variations in the number of participating agents.The non-stationary environment of multi-agent reinforcement learning, coupled with sample efficiency issues, presents a significant challenge in training agents to collaborate effectively. During the cooperation training phase, we continue to use a reward function similar to that used in the singleagent training phase and employ the centralized training and decentralized execution (CTDE) [15] scheme for coordination training. During the training phase, we utilize the Multi-Agent Proximal Policy Optimization (MAPPO) algorithm [40] to develop cooperative strategies among agents. This approach involves updating the value function network V ϕ according to Equation ( 6), using the trajectories D that are accumulated and shared among all agents. More details can be found in our appendix and the original MAPPO paper [40] .EQUATIONIn addition, given the homogeneous roles of agents in the collaborative task of carrying long objects, we adopt the strategy of parameter sharing, a method proven to enhance performance across various cooperative tasks [3, 33, 40] . Specifically, we share the parameters of both the policy and value networks among all agents to improve the training of cooperative behaviors.Question: For each theoretical result, does the paper provide the full set of assumptions and a complete (and correct) proof?Question: Does the paper provide open access to the data and code, with sufficient instructions to faithfully reproduce the main experimental results, as described in supplemental material?Answer: [Yes] Justification: The motion capture data used in our work is a public dataset, and our code is provided in the supplementary materials.Guidelines:• The answer NA means that paper does not include experiments requiring code. • Providing as much information as possible in supplemental material (appended to the paper) is recommended, but including URLs to data and code is permitted. 
experiments: 
 We conducted extensive experiments to test the effectiveness and also the boundary of capabilities of our framework. The basic experiment setups are explained in Section 4.1 and in Appendix. We evaluate our framework on various object-carrying tasks in Section 4.2. To better understand the importance of different design decisions in our framework, we performed extensive ablation studies in Section 4.3. Since our method primarily focuses on interactions between characters and objects, we also provide extensive visual analysis and presentations to demonstrate our framework.Datasets and Initialization. Our primary source of motion data is the AMASS dataset [17] , which provides motions encoded in SMPL [22] parameters. We collected a total of four types of basic reference motion data, including 9 motions related to walking, 5 related to picking up, 4 related to carrying, and 5 related to putting down. To enhance the robustness of the carrying process, we randomly initialized the weight and size of the object, as well as the distance from the person to the destination. Specifically, for a single individual, the object's weight ranged from 5KG to 25KG, its size varied between 0.5 to 1.5 times its original scale, and the distances between the agent and the object, as well as between the object and the destination, ranged from 1 m to 20 m. To enhance the robustness of the carrying process, we randomly varied the weight, size, and of the object, as well as the distance from the person to the endpoint. Furthermore, considering that the process of multiple people carrying might involve situations where someone walks backward, we ensured that the single agent mastered the basic movements for multi-person collaboration by introducing additional motion data involving backward and side walking. More training details can be found in the appendix. Metrics. Following InterPhys [7] , we utilize Success Rate and Precision as the primary metrics. Specifically, a task is considered successful if the distance between the object and its destination is less than 0.2m. Precision measures the distance between the object and the target point across all completed tasks. Moreover, all evaluations are based on average results across 4096 environments and 4 random seeds.Comprehensive Evaluation on Box Carrying. We evaluate our policy in both single-agent and two-agent scenarios. Table 1 presents the performance statistics for the carrying task. Consistent with the settings described in Interphys [7] , we randomize the weight and distance as outlined in Section4.1. It is important to note that InterPhys [7] includes the weight information of objects as part of the observation, enabling it to achieve a broader weight range. To ensure result comparability, as shown in Line 2, we also incorporated this information into individual training. The results indicate that, with a similar success rate, we increased the transportation distance and improved transportation accuracy, even when using discrete motion data rather than the costly whole-body motion capture data used in InterPhys [7] . As shown in Line 3, although removing the weight information reduces the weight range the agent can handle, it represents a more realistic setting and provides comparable baseline results for future research. Lines 4-5 show the results in multi-agent scenarios. Without using our CooHOI framework and simply employing parallel training for multi-agent tasks, the training fails. More analysis and visualizations will be provided in the following sections.Extend to Diverse Daily Life Objects. Moreover, we are not satisfied with merely transporting simple boxes. Therefore, we have sampled nearly 40 common everyday objects from [6] , including three types of single-person objects-Table, Armchair, and HighStool-as well as one type of twoperson object, the Sofa, as illustrated in Figure 3 and Line 1 in Figure 4 . Additional visualizations of these objects can be found in appendix. Additionally, we present the results of quantitative experiments in Table 2 , which indicate that our policy can effectively handle most daily life objects with a high success rate. Due to the greater variability in the shape and weight of sofas, the accuracy in Line 4 shows a slight drop compared to the result in Line 4 of Table 1 . It is important to note that the significant variations in object shapes make it challenging to learn a unified object representation with a limited amount of data, especially since our input is merely a simple bounding box. Therefore, for single-person transport, different types of objects need to be trained separately from scratch, similar to how we handle boxes. For multi-person transport, objects like sofas, similar to boxes, require fine-tuning based on the weights obtained from single-person box transport.To evaluate and understand the importance of different design choices in CooHOI, we conduct a detailed analysis of scenarios involving single and multiple agents. This includes boundary analysis, which explores factors that lead to a decrease in the success rate during object carrying. Furthermore, to fully validate the scalability of our method, we also tested and included the results in a four-person scenario. To ensure the reliability of our results, our experiments continue to utilize the average of 4 random seeds as previously mentioned.Single-Agent-Based Box Carrying. We have demonstrated our framework's effectiveness in Table 1 . Moreover, we aim to further investigate the maximum potential of our approach. We examined the robustness of the single-agent policy from various perspectives, including object shape, scale, weight, and trajectory length, as shown in Figure 5 . First, we observed that a single agent can handle approximately 13 kg of weight. Continuously increasing the weight makes it difficult for the agent to lift the object. As mentioned in 4.2, since our policy input does not include object density, it is challenging for the agents to generalize across different weights. When the weight reaches 20 kg, the accuracy drops to just 30%. Additionally, due to the limited reach of human arms and the lack of dexterous hands in our agent, it is challenging for the agent to lift boxes that are either too large or too small. To validate this hypothesis, we restricted the object's width to 1x while scaling the length and height to 1.5x. As shown by the green circle in the second figure of Figure 5 , the success rate could reach 97.8%. Furthermore, our strategy is quite robust to distance variations and is generally unaffected by them.Multi-Agent-Based Box Carrying. We conducted upper-limit testing on the multi-agent policy using a similar evaluation method as the single-agent case. As shown in Figure 5 , the conclusions for two agents are similar to those for a single agent. The curves in the first figure demonstrate that with an increase in the number of agents, we can easily lift larger and heavier objects that a single person cannot handle, highlighting the necessity of having multiple agents. However, due to the increased complexity of coordinating two agents, boundary conditions have a more pronounced impact on the policy. For instance, objects that are excessively large or small can cause the policy to fail. As shown by the purple circle in the second figure of Figure 5 , we conducted a similar experiment to the single-agent scenario. By restricting the object's width to 1x and scaling the length and height to 1.5x, the success rate increased from 0 to 88.67%.Analysis of CooHOI Framework. To thoroughly investigate the contribution of CooHOI, we analyzed the results of each method mentioned in Section 3 separately, as shown by the training curves in Figure 8 . The first factor is the influence of the Stand Point, which refers to whether an extra point is introduced in front of the object to encourage the agent to walk toward it. During the experiments, we discovered that without this, the agent sometimes fails to approach the shortest edge of the object, resulting in a lower hold reward. This leads to an incomplete lift and the subsequent inability to carry the object. Dynamic Observation is the second factor, indicating whether we use dynamic information as observation for each agent. Without it, the observation for each agent is limited to the state information of the long object. We found that without the dynamics information, the agent just stands in front of the object, seemingly unsure of what to do. Reverse Walk indicates whether the training process includes motion data for walking backward and a reward function focused on learning this movement. We found that if the policy for a single agent is restricted to only forward movement, training with two agents then leads to a deadlock state. As shown in Figure 8 , the agents might be able to contact the box, but they cannot carry it to the destination. Initialization refers to whether the two-agent policy is initialized using the single-agent policy and then be fine-tuned. In our experiment, even with extended training duration, training the two-agent policy from scratch still failed to achieve successful carrying, as shown in Figure 8 . Based on the results above, the absence of any of the aforementioned methods causes our policy to fall into a locally suboptimal solution, preventing the completion of the transportation task. Moreover, you can also find more interesting visual examples of the above failure cases while extending agent numbers from one to two in the appendix. 
conclusion and limitation: 
 In this paper, we present Cooperative Human-Object Interaction (CooHOI), a framework designed to address cooperative object transporting tasks through a two-phase learning approach. By initially focusing on individual skill mastery via the Adversarial Motion Priors (AMP) method, followed by a strategic transition to multi-agent collaboration using Multi-Agent Proximal Policy Optimization (MAPPO), our approach facilitates a sophisticated interplay of shared dynamics and implicit communication among agents, resulting in an efficient and generalized performance.However, within the scope of this paper, our huamnoid characters lack dexterous hands, which limit their ability to manipulate slippery objects or perform more precise actions. We also utilize object bounding box information as the goal feature for our task, which limits our framework's capacity to generalize to a diverse range of object shapes. Additionally, our experiments are limited to humanoid characters in simulation, without any real-world deployment. In future research, we aim to incorporate dexterous hands to enable the manipulation of a wider variety of objects, as well as integrate active perception and navigational abilities to make our framework more generalizable.• Rotation of the root: 6 dimensions.• Linear and angular velocity of the root: 6 dimensions.• Position of local joints: 42 dimensions.• Rotations of local joints: 84 dimensions.• Linear and angular velocity of local joints: 84 dimensions.While the root height is measured in the global reference frame, all other components are defined in the frame local to the character. Rotations follow a 6-dimensional representation for continuity [46] . The simulated character aligns with [25, 24, 7, 21] , featuring 12 internally movable joints and a total of 28 degrees of freedom.The goal feature g comprises a 75-dimensional vector, including:• Position of the object: 3 dimensions.• Rotation of the object: 6 dimensions.• Dynamics of the object, which cover the bounding box position, along with linear and angular velocities: 33 dimensions. • Target location: 3 dimensions. • Target orientation: 6 dimensions.• Dimensions of the target's bounding box: 24 dimensions. These are measured in the frame local to the character. 
experimental setting/details: 
 Question: Does the paper specify all the training and test details (e.g., data splits, hyperparameters, how they were chosen, type of optimizer, etc.) necessary to understand the results?Answer: [Yes] Justification: We fully described the training and test details of our experiments.Guidelines:• The answer NA means that the paper does not include experiments.• The experimental setting should be presented in the core of the paper to a level of detail that is necessary to appreciate the results and make sense of them. • The full details can be provided either with the code, in appendix, or as supplemental material. 
experiment statistical significance: 
 Question: Does the paper report error bars suitably and correctly defined or other appropriate information about the statistical significance of the experiments?Answer: [Yes]Justification: We reported error bars and random seeds of our experiments in the paper.Guidelines:• The answer NA means that the paper does not include experiments.• The authors should answer "Yes" if the results are accompanied by error bars, confidence intervals, or statistical significance tests, at least for the experiments that support the main claims of the paper. • The factors of variability that the error bars are capturing should be clearly stated (for example, train/test split, initialization, random drawing of some parameter, or overall run with given experimental conditions).• The method for calculating the error bars should be explained (closed form formula, call to a library function, bootstrap, etc.) • The assumptions made should be given (e.g., Normally distributed errors). • It should be clear whether the error bar is the standard deviation or the standard error of the mean. • It is OK to report 1-sigma error bars, but one should state it. The authors should preferably report a 2-sigma error bar than state that they have a 96% CI, if the hypothesis of Normality of errors is not verified. • For asymmetric distributions, the authors should be careful not to show in tables or figures symmetric error bars that would yield results that are out of range (e.g. negative error rates). • If error bars are reported in tables or plots, The authors should explain in the text how they were calculated and reference the corresponding figures or tables in the text. 
experiments compute resources: 
 Question: For each experiment, does the paper provide sufficient information on the computer resources (type of compute workers, memory, time of execution) needed to reproduce the experiments?Answer: [Yes]Justification: We described type of computing resources, time of execution of our experiments.Guidelines:• The answer NA means that the paper does not include experiments.• The paper should indicate the type of compute workers CPU or GPU, internal cluster, or cloud provider, including relevant memory and storage. • The paper should provide the amount of compute required for each of the individual experimental runs as well as estimate the total compute. • The paper should disclose whether the full research project required more compute than the experiments reported in the paper (e.g., preliminary or failed experiments that didn't make it into the paper). 
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broader impacts: 
 Question: Does the paper discuss both potential positive societal impacts and negative societal impacts of the work performed?Answer: [No] Justification: We mainly focus on addressing cooperative object transporting tasks of humanoid characters, there may not be any societal impact of our work.Guidelines:• The answer NA means that there is no societal impact of the work performed.• If the authors answer NA or No, they should explain why their work has no societal impact or why the paper does not address societal impact.• Examples of negative societal impacts include potential malicious or unintended uses (e.g., disinformation, generating fake profiles, surveillance), fairness considerations (e.g., deployment of technologies that could make decisions that unfairly impact specific groups), privacy considerations, and security considerations. • The conference expects that many papers will be foundational research and not tied to particular applications, let alone deployments. However, if there is a direct path to any negative applications, the authors should point it out. For example, it is legitimate to point out that an improvement in the quality of generative models could be used to generate deepfakes for disinformation. On the other hand, it is not needed to point out that a generic algorithm for optimizing neural networks could enable people to train models that generate Deepfakes faster. • The authors should consider possible harms that could arise when the technology is being used as intended and functioning correctly, harms that could arise when the technology is being used as intended but gives incorrect results, and harms following from (intentional or unintentional) misuse of the technology. • If there are negative societal impacts, the authors could also discuss possible mitigation strategies (e.g., gated release of models, providing defenses in addition to attacks, mechanisms for monitoring misuse, mechanisms to monitor how a system learns from feedback over time, improving the efficiency and accessibility of ML). 
safeguards: 
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 Appendix: The agent's reward r t at each time step t is defined byEQUATIONFollow the formulation of the AMP framework [25] , the style reward r S is calculated according to the discriminator:EQUATIONAnd the discriminator is trained by the following objective: arg minEQUATIONThe task reward function r G is generally segmented into three components, as in Equation ( 10): 1) r G walk , which encourages the agent to approach the object intended for manipulation. 2) r G held , which encourages the agent to align the center of its hands with the center of the box. 3) r G target , which encourages the agent to transport the object to the specified destination.EQUATION)The walk reward r G walk is formulated as Equation (11) , where x standing t denotes the position of the standing point near the object,v * denotes the target velocity, and d * denotes the desired direction from root to the object.EQUATIONThe held reward r G held is formulated in Equation ( 12), where x hand t denotes the center of the agent's two hands and h t is the position of the object helding point.EQUATIONThe target reward r G target consist of two parts, r carry and r face , as described in Equation ( 13). r G target = 0.5 * r carry + 0.5 * r face .The face reward r face guides the agent to walk either forwards or backward. As shown in Equation ( 14), this is achieved by comparing the agent's velocity direction with its orientation relative to the endpoint's location, thereby cultivating the agent's proficiency in bidirectional locomotion.EQUATIONThe carry reward r carry , is designed to guarantee that the object is delivered to the precise location at a specific angle. As outlined in Eq. 15, we constrain the agent's movement direction, alongside the proximity to the end destination and the intended angle. wherer far t = exp -0.5 p * t -p root t 2 r near t = exp -10.0 x * t -x root t 2 r dir t = rot * • rot object t 2An episode ends either after reaching a predetermined duration or upon the activation of early termination (ET) conditions. During our experiments, we observed that lower object heights could lead to kicking actions, where the agent tend to kick the object to destination, significantly slowing down the training process. To address this, we assess the object's velocity and height to determine the presence of kicking phenomena. If the height of the object is lower than 0.3m and its velocity in x-y plane is greater than 1m/s, the kicking early termination (KET) condition is triggered. Experimental results show that this strategy significantly stabilize the training process.C.1 Training Details.Adopting the methodology of AMP [25] , we develop a low-level controller encompassing both policy and discriminator networks. The policy network is bifurcated into a critic and an actor-network, each initiating with a CNN layer and proceeding to two MLP layers configured with [1024, 1024, 512] units. The discriminator network is similarly structured, featuring two MLP layers with [1024, 1024, 512] units. We select PPO [28] as the primary reinforcement learning algorithm, coupled with the Adam optimizer [12] at a learning rate of 2e-5. The only difference between the multi-agent setting and the single-agent setting during training is whether a pre-trained weight is loaded. Our experiments are conducted on the IsaacGym simulator [18] using a single Nvidia GTX 3090Ti GPU. We run 4096 parallel environments across 15,000 epochs, which takes approximately 15 hours to complete.Following previous work [25, 7, 21] , we use the Isaac Gym simulator [18] . The simulation runs at 60Hz and the control policy runs at 30Hz.Besides, the hyperparameters we used in the training process is detailed below: Failure case visualization. Here, we conducted a visual analysis of the fail cases. First, for the case lacking a stand point, we can clearly see that the agent moves towards the nearest face, even though it is not the shortest edge, which leads to the agent's inability to carry the object. In the second image, in the absence of dynamic input, we observe that the agent stands still, unable even to squat. In the third image, which depicts the scenario without reverse walking, the agent is able to lift the box, but because it cannot learn the backward gait, the two agents end up pushing the box against each other, causing a deadlock. Ablation on performance of CooHOI under noisy scenarios. In CooHOI, all state information is provided using ground truth data from simulators. However, in real-world settings, input data is often noisy and prone to errors. To evaluate the robustness of our framework in such conditions, we introduce random noise into the observation space of our policy and assess its performance under observation noise, as shown in Table 4 . Justification: We clearly state our assumptions and contributions in the abstract and introduction. These claims match the experimental results shown in our paper.Guidelines:• The answer NA means that the abstract and introduction do not include the claims made in the paper. • The abstract and/or introduction should clearly state the claims made, including the contributions made in the paper and important assumptions and limitations. A No or NA answer to this question will not be perceived well by the reviewers. • The claims made should match theoretical and experimental results, and reflect how much the results can be expected to generalize to other settings. • It is fine to include aspirational goals as motivation as long as it is clear that these goals are not attained by the paper.Justification: We conform with the NeurIPS Code of Ethics.Guidelines:• The answer NA means that the authors have not reviewed the NeurIPS Code of Ethics.• If the authors answer No, they should explain the special circumstances that require a deviation from the Code of Ethics. • The authors should make sure to preserve anonymity (e.g., if there is a special consideration due to laws or regulations in their jurisdiction). 


Decision: This paper introduces CooHOI, a framework for learning cooperative human-object interactions in physics-based character animation. The method demonstrates the ability to learn multi-agent object transportation tasks using only single-agent motion capture data, with good generalization to objects of varying shapes and weights. All reviewers recommended acceptance, praising the paper's well-designed approach, comprehensive experiments, and valuable contribution to an under-studied area. The reviewers consistently highlighted the effectiveness of using manipulated object dynamics for implicit communication between agents and the framework's ability to transfer single-agent skills to multi-agent scenarios.

Initial concerns focused on the limited generalization to diverse object shapes, the use of simplified humanoid models without dexterous hands, and questions about the robustness of the system. The authors provided detailed responses, clarifying their approach to object diversity, explaining the rationale behind their design choices, and offering additional experiments demonstrating the framework's capabilities. These responses addressed many reviewer concerns, though some reservations remain about generalization and the limitations imposed by simplified hand models.

Despite these limitations, the consensus is that CooHOI represents a significant contribution to the field of physics-based character animation and multi-agent cooperation. The paper's thorough evaluations, clear presentation, and novel insights outweigh its current limitations. For the final version, I encourage the authors to incorporate their rebuttal explanations, particularly regarding object diversity and the rationale behind their design choices. Additionally, discussing potential future work on incorporating dexterous hands and improving generalization to more diverse objects would strengthen the paper further.

Reviews: review: paper_summary: paper_summary: This paper proposes a framework for multi-agent cooperative manipulation in the context of humanoids carrying and transporting large furniture. This task is decomposed into several steps. First, they train a single humanoid to learn how to hold and carry relatively small objects. The agent is trained with ground-truth object information and AMP for natural humanoid behavior. Next, they train a multi-agent policy initialized from the single-agent policy for transporting larger objects. The authors present several ablations and report high performance compared to baseline methods in simulation.
strength_weakness: strength_weakness: I’m curious about the robustness of the proposed system. If I understand correctly, the bounding box information is given as ground-truth parameters. However, in reality, these quantities are far from perfect.

The task is restricted to moving and transporting large furniture-type objects.
questions: questions: In Table 1, what does the “w/o CooHOI” mean? Does that mean training the entire policy from scratch? Is that policy trained with AMP and the same rewards? And what is the definition of CooHOI? What is included in CooHOI?

Regarding the robustness of the system: How would the policy perform in noisy situations? For example, when the object bounding boxes are not accurate (in real scenarios, object perception will have errors).

In Figure 6, can you elaborate more on why policies, except for CooHOI, fail to carry objects even when they can still hold objects? Specifically, what are the failure cases? Do these policies fail to maintain a stable holding motion, or do they fail to accurately move the objects to the target location?

In addition, r_carry is only defined in the supplementary material. I suggest including it in Section 3.2.2.

It would be better to plot the standard deviations in Figure 6, given that you evaluate four seeds.

What is the action space of the policy? Is it the target joint position for each joint?
limitations: limitations: The authors provide a comprehensive limitation section of their method.
paper_summary: paper_summary: In this paper, the authors introduce a novel framework, Cooperative Human-Object Interaction (CooHOI), aimed at tackling the problem of multi-agent object transportation. The framework consists of two phases: initially, a single agent learns to perform tasks, followed by multiple agents learning to collaborate through shared dynamics of manipulated objects. The utilization of shared object dynamics has proven effective in learning cooperative tasks. The authors conduct comprehensive experiments to validate the efficacy of their approach and explore its capability boundaries.
strength_weakness: strength_weakness: * It appears that the shapes and categories of the manipulated objects are not very diverse, potentially constraining the method's ability to generalize to novel objects. I recommend that the authors summarize the counts of shapes and categories used in the experiments.  Additionally, if the shapes of the manipulated objects are limited and the grasp actions are abstracted in the simulation, the authors are encouraged to compare CooHOI with some heuristic methods. An intuitive heuristic baseline could involve having two agents grasp each end of a long object, and then move at the same speed to the target position.
* While the authors provide detailed explanations in the appendix, it would enhance comprehension if they provide simple explaination when introducing a new concept in the main paper. Here are some examples:
  * In Section 3.1, the authors introduce the task-specific goal feature $g_t$ without explanation.
  * While it is understandable that the proposed style reward can help the method produce behaviors similar to those in the dataset, it would be better to provide a simple explanation of how to evaluate such similarity.
  * It would be beneficial to provide a brief introduction to the baseline InterPhys in one or a few sentences.
questions: questions: * While acknowledging the idea of utilizing the object’s dynamics as implicit communication between agents, such communication is not synchronous. Specifically, agents adjust their actions only after one agent changes the object dynamics, resulting in a delay between the change in object dynamics and the response actions taken. I wonder if this delay could lead to jitter of the manipulated object. If not, could the reviewers discuss why such jitter would not occur?
* The held point is described as the geometric center of each end of the object. However, in Figure 1, when cooperatively carrying a large box, the characters grasp the edges of the box instead of the center of its face. I am unsure if I misunderstood the definition of the held point, or if the current definition is inadequate, leading agents to choose other positions they find more suitable for completing the task.

* In the second row of Figure 4, I am curious about why the four agents fail to put down the box. The authors attribute this failure to the absence of dexterous hands and limited friction, however, similar limitations exist in two-agent manipulation tasks. Besides, the multi-agent put-down operation closely resembles the multi-agent pick-up operation (but in reverse), while the agents can successfully pick up the box, they struggle to put it down. Could the authors provide more discussion or visualizations for better understanding?
limitations: limitations: The authors have provided a detailed discussion of limitation and failure cases.
paper_summary: paper_summary: This work address the problem of multi-character collaboration for object transporting tasks. Different from previous works approaching the multi-character HOI task with tracking-based method, this work learns a physics-based multi-agent policy with reinforcement learning. Instead of directly training a multi-agent policy from scratch, the authors suggest using a two-phase learning approach: CooHOI uses a two-phase learning approach: individual skill acquisition through Adversarial Motion Priors (AMP) and then collaborative learning with Multi-Agent Proximal Policy Optimization (MAPPO). Agents learn to coordinate implicitly by responding to changes in object dynamics caused by others.
strength_weakness: strength_weakness: * This paper build a framework for multi-agent cooperation policy learning with AMP for single-agent policy pertaining and MAPPO for cooperation skill learning. The technical contribution to the community is minor.
* The method include few heuristic designs to simplify the problem, for example, the manually defined held point, and object bounding box observation, making it not easy to generalize and scale to more diverse objects.
* The current framework can be hardly generalise to objects with different size and different shape, and the human motion styles are quite limited.
        * Regarding generalization to objects weights and scales, are the results presented in Figure 5  the transferring results to different weights and scales or the per-object training results.
        * Regarding the motion styles, in the training dataset, limited motion sequences are involved with only walking, lifting up and putting down, would the proposed method build upon AMP be able to cover more diverse skills, and this could be helpful to generate more natural and diverse HOI motions.
* It is presented that when increasing the number of characters, the policy has problem in putting the object down due to the absence of dexterous hands and limited friction. Could the author elaborate more on the major bottlenecks encountered when dealing with a larger number of characters? Assuming that the friction parameter can be adjusted in the simulator and should not be a significant issue for motion generation tasks purely within the simulation, what specific contributions could dexterous hands make to this scenario? Additionally, while the introduction of dexterous hands would undoubtedly enhance the agent's capabilities, what other factors should be considered?
questions: questions: See more in the weakness section :-)
limitations: limitations: Yes, the authors addressed the limitation.
paper_summary: paper_summary: This paper works on learning multi-human cooperative object manipulation, specifically the collaborative carrying of objects.
This paper proposes a two-stage method to learn collaborative object carrying. In the first stage, the agent learns single-person object carrying from motion capture data and heuristic-based task rewards following the Adversarial Motion Priors (AMP) framework. In the second stage, the agent policy is fine-tuned using multi-agent reinforcement learning to learn multi-agent collaborative object carrying.
Experiments show that the trained policy can successfully control the agents to complete collaborative objects carrying tasks given objects of varying shapes and weights.
strength_weakness: strength_weakness: 1. This paper simulates an oversimplified humanoid without hand modeling considering the fact that it works on object manipulation problems, for which the hands play essential roles. The authors also recognize this issue leads to capacity limitations.

2. The agent observation features may not be sufficient for more complex manipulations. In the presented paper, the agent observation mainly consists of the self-motion and cropped object bounding box features. However, more complex collaborative tasks like carrying articulated objects and assembling a sofa require a global overview of the object instead of only a local observation. In addition, the agent also needs to observe the collaborators to avoid interference such as grasping at the same point.

3. The proposed method needs to train a separate policy for a category of objects with similar shapes (L246-247), which indicates a generalization problem.
questions: questions: 1. How the object weights can affect the humanoid in the simulation? Is there an upper bound for the joint force of the simulated humanoid? How does the humanoid apply forces to the object in the simulation?

2. Are the standing point and held point also part of the goal inputs? If not, during the training, the same motion sequence can have different rewards given differently sampled standing points? How are the standing points sampled given an arbitrary object mesh?
limitations: limitations: The author discussed the limitation of lacking dexterous hands, which limits the agent interaction capacity and is left as future work. 
Please also refer to the weakness discussion above.

Comments: title: To reviewer ehnN : Please respond to rebuttal
To reviewer kufk : Please respond to rebuttal
comment: Thanks for your insight and valuable feedback!
Thanks for your insights and valuable feedback!
Thanks a lot for the authors' detailed response and additional experiments and visualization. The additional experiment on the H-shape objects transporting with 4 agents demonstrates the necessity of carefully selected object types, while I agree that generalizing to challenging object shapes remains a very challenging task, and the author also gives insights on how to further approach it. 

Overall, I appreciate the efforts in this work to build a framework to tackle the multi-agent HOI problem, and I have also read other reviews as well as authors' responses, and I will maintain my original rating as weak accept.
Hi reviewer ehnN ,

Thank you for your initial review. Please kindly respond to the rebuttal posted by the authors.
Does the rebuttal answer your questions/concerns? If not, why?

Best,
AC
Hi reviewer kufk ,

Thank you for your initial review. Please kindly respond to the rebuttal posted by the authors.
Does the rebuttal answer your questions/concerns? If not, why?

Best,
AC
Thank you for the clarification provided in the rebuttal. I will maintain my score and continue to support the acceptance of this paper.
Thank you for your insights and suggestions. The H-shape box represents our effort to explore the boundaries of CooHOI’s generalization capability regarding the number of agents. In future work, we will continue to exploring its generalization capability across various objects and interaction skills, incorporating more experiments and settings involving dexterous hands.
I appreciate the additional explanation and visualization provided in the rebuttal. However, the H-shape box example does not solve the concerns about generalization because it needs a specially designed object shape for the 4-person cooperation to work.
Generalization capability to various objects and interaction skills remain a weakness of this work. Moreover, simulating a humanoid without dexterous hand modeling inherently limits the method to work for interactions requiring complex controls, as also raised by other reviewers.

Overall, I still appreciate the contributions of exploring learning cooperative human-object interaction, the design of object dynamics-based state observation and collaborative training framework, and extensive experiments. I believe this paper makes a valuable contribution and recommend accepting it.