REVIEW 4 major objections 5 minor 1 cited by
Video2Reward: Generating Reward Function from Videos for Legged Robot Behavior Learning
T0 review · 4 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper proposes Video2Reward, which turns videos of target behaviors into keypoint-coordinate texts that an LLM converts into reward functions, and shows this beats text-only reward generation by over 37.6% in human normalized score on…
desk verdict A promising video-to-reward idea whose headline results are undercut by metric reuse and missing variance. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is a video-to-text transforming module combined with a video-assisted feedback loop. The module uniformly samples frames, runs pose estimation (RTMpose for humans, HRNet for animals) to obtain J keypoint coordinates per frame, and formats the resulting trajectories as coordinate text; this text is concatenated with environment code, task description, and reward-design rules into the LLM prompt. For feedback, the trained policy is rolled out in simulation, its 3D keypoint trajectories are projected to 2D along the direction of motion, segmented into two-period chunks using autocorrelation, and each segment is compared with the video trajectory by FastDTW; the per-joint similarity scores become textual feedback appended to the prompt for the next round. The reward generation loop samples K candidate reward functions per round, trains each with PPO, selects the best by the maximum training success metric, and repeats for N rounds, so the video acts both as the initial specification and as the persistent performance signal.
What would settle it
Run Video2Reward on a reference video that depicts a locomotion pattern the LLM has not seen in its text training, e.g., a dog performing a pace gait, and compare the learned robot behavior against the same video using an independent motion-capture similarity metric that does not rely on the projection used in the paper; if the video-conditioned reward does not yield lower distance to the reference than a text-only baseline across multiple seeds, the central claim fails.
Extended reading notes
Core claim
The central discovery is that the motion information in a video, once reduced to keypoint coordinate trajectories, is a sufficiently precise learning objective for an LLM to produce reward functions that drive reinforcement-learned policies to reproduce the demonstrated behavior. The paper shows this in two ways: quantitatively, the learned policies achieve higher maximum training success and lower DTW trajectory distance to the reference video than policies trained with Eureka, Eureka with a text description of the gait, or the expert-designed IsaacGym rewards; qualitatively, the robots exhibit the alternating footfalls of an amble or the extended hind-leg propulsion of a run as shown in the input videos. The authors also demonstrate through ablation that the video-assisted feedback loop — in which the robot's projected 3D keypoint trajectories are compared with the video's 2D keypoint trajectories using FastDTW, and per-joint similarity scores are returned as text to the LLM — is responsible for a substantial share of the gain. The claim is conditional on the simulator and the chosen LLM, but the mechanism is presented as domain-general for legged behavior learning.
Load-bearing premise
The load-bearing premise is that 2D keypoint trajectories extracted from a video and the 2D projections of a simulated robot's 3D keypoints measure the same thing about a behavior, so that FastDTW scores and the resulting text feedback are meaningful.
Editorial extensions
If this is right
- If the claim holds, switching the input video should change the learned gait without retraining the reward-design pipeline, enabling rapid re-purposing of one RL training setup across behaviors such as amble and run.
- Reward functions produced from video should reduce the workload of manually designing dense rewards for new legged platforms, since the same video-to-text conversion and feedback loop can be applied whenever a reference movement video exists.
- The video-assisted feedback is a stronger signal than text-only or reward-based feedback: even without feedback, video input beats text-only, and with feedback it surpasses expert-designed rewards on the tested tasks.
- The DTW-score improvements imply that trajectories of individual joints, not just task-level metrics, align more closely to the reference, which is what makes the learned behavior look natural.
Reading between the lines
- One extension the authors do not test is sensitivity to camera viewpoint: projecting the robot's 3D trajectories to 2D along its direction of motion is a critical freedom, and a substantial change in the reference video's camera angle could alter similarity scores for the same behavior, suggesting a need for camera-aware or multi-view projection.
- The same video-to-text and feedback loop could in principle apply to manipulation or whole-body tasks beyond legged locomotion, with the caveat that keypoint semantics must match between the video subject and the robot.
- A testable refinement would be to normalize temporal dynamics before FastDTW comparison, since raw coordinate trajectories may conflate speed variations with postural differences; isolating phase-normalized pose could give a cleaner behavioral similarity signal.
- As LLMs become increasingly multimodal, the explicit pose-extraction step might be replaced by direct video-conditioned reward generation, but the paper's evidence suggests that explicit keypoint text remains an effective and interpretable intermediate representation.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes Video2Reward, a method that uses videos depicting target behaviors to automatically generate reward functions for legged robot reinforcement learning. A video-to-text module extracts 2D keypoint trajectories from the video and feeds them, together with auxiliary textual context, to an LLM, which produces executable reward code. A video-assisted iterative refinement loop then compares the simulated robot's projected 3D keypoint trajectories with the video trajectories using FastDTW, converts the similarity scores into textual feedback, and prompts the LLM to refine the rewards. Experiments in IsaacGym on Anymal (amble and run) and Humanoid (run) tasks report that the method outperforms Eureka by 37.6% and 11.1% in human-normalized score, and also achieves lower DTW scores, indicating closer behavioral mimicry. The paper further presents ablations showing the contribution of video-assisted feedback and qualitative visualizations of learned gaits.
Significance. If the results are reliable, this work would be a meaningful step toward reducing manual reward engineering and enabling more controllable, video-specified behavior learning for legged robots. The proposed pipeline is straightforward and reproducible in principle, and the ablation study is a positive feature. However, the paper's central claims are weakened by two interconnected issues: the metric used to evaluate behavioral mimicry (DTW) is also the feedback signal used to optimize the reward via the LLM, and the trajectory comparison itself relies on an ill-defined 2D projection and an unspecified keypoint correspondence. The lack of multiple seeds and error bars further limits the strength of the quantitative conclusions. These concerns are specific and load-bearing; they affect both the headline performance numbers and the behavioral-mimicry claim, and they need to be addressed before the contribution can be considered established.
major comments (4)
- [Section 4.3, Algorithm 1, and Table 1] The DTW-based trajectory similarity is used both as the feedback signal d in Algorithm 1 (line 12) and as the evaluation metric for behavioral mimicry in Table 1. Since the LLM is explicitly prompted to reduce d during the refinement iterations, the lower DTW scores of the final policies are not independent evidence of closer behavioral mimicry; they may simply reflect optimization of the same objective. The authors should evaluate the learned policies with a held-out, independently defined similarity metric (for example, a learned pose-distance, a different projection, or a manual labeling of behavioral similarity) to support the claim that the method produces behaviors closer to the target videos.
- [Section 4.3] The projection of the simulated robot's 3D keypoint trajectories 'along the direction of motion' is not well-defined, and it is not matched to the camera geometry of the input video. The video keypoints are 2D projections under an unknown perspective camera, while the simulated keypoints are in 3D world coordinates; projecting them along an arbitrary axis can make the FastDTW similarity arbitrarily large or small. The paper also does not specify how the pixel-scale video coordinates are reconciled with the metric-scale simulation coordinates. The authors should specify the exact projection axis and normalization procedure, and show that the reported DTW scores and the feedback are robust to reasonable choices of projection direction.
- [Section 5.3 and 5.5] All experimental results are reported as single numbers without error bars, multiple random seeds, or statistical significance tests. Reinforcement learning in IsaacGym is stochastic with respect to initializations and sampling, so the headline claim of a 37.6% improvement over Eureka on the Anymal task is not supported without variance estimates. Please repeat the experiments with at least 3-5 seeds and report mean and standard deviation, and perform a significance test where appropriate.
- [Section 4.2 and Section 5.5 (Q1)] The paper compares keypoint trajectories extracted from the video with keypoints of the simulated robot, but it does not specify how the correspondence between, for example, a dog's nose, knees, and ankles in the video and the corresponding body parts of the Anymal robot is established. The HRNet and RTMpose keypoints follow a specific skeleton, while the simulated robot has its own kinematic structure. Without an explicit mapping, the per-keypoint DTW scores computed in Table 1 and the feedback used in Algorithm 1 are not well defined. The authors should provide the exact keypoint correspondence or use a common skeleton representation for both the video and the simulation.
minor comments (5)
- [Section 4.3 and Algorithm 1] The notation d is introduced as a scalar Fsim(T_hat, T), but earlier the paper computes per-joint similarity scores S_j; please clarify whether d is a scalar average or a vector/list and how it is formatted in the textual feedback.
- [Section 5.2] The 'Human' baseline is described only as a reward function designed by experts in IsaacGym; please specify which exact reward function is used and whether it is identical to the one used in the Eureka baseline for fair comparison.
- [Table 3 and Section 5.3] Human-normalized scores above 1.0 (e.g., 1.003 on Anymal and 1.180 on Humanoid) imply that the method surpasses the expert-designed reward; please comment on whether this is expected and whether the sparse-reward-based Hmts metric can be gamed by the policies.
- [Section 5.4] The video processing details, such as the frame-sampling interval and the normalization for moving cameras, are described only briefly and refer to supplementary material; please provide the essential details in the main text or ensure the supplementary material is included and accessible.
- [Section 5.5 and Conclusions] The paper claims to surpass 'state-of-the-art LLM-based reward generation methods' but only compares against Eureka; consider adding other baselines such as L2R or Text2Reward, or soften the claim to say that the method outperforms the evaluated baseline.
Circularity Check
The DTW-based behavioral-mimicry claim is partially circular: the same FastDTW score is fed back to the LLM as an optimization target and then reported as independent evidence of mimicry.
-
fitted input called prediction
[Algorithm 1 lines 11-14; Section 4.3 'Video-assisted reward function evaluation'; Section 5.3 'DTW score'; Table 1]
"Calculate the similarity between T and ˆT: d = Fsim( ˆT , T ) ... Taux := Taux + Rn_best + sn_best + d ... The Dynamic Time Warping (DTW) score is employed as a metric to measure the similarity between behavioral trajectories."
The same FastDTW function Fsim computes the d that is fed back to the LLM in line 14 and also produces the DTW scores reported in Table 1 as evidence that the learned behavior is closer to the target video. Because the LLM is explicitly prompted to reduce d and the best reward/policy is selected and refined through this feedback, Table 1 measures success at optimizing the method's own feedback signal rather than providing an independent test of behavioral mimicry. Without a held-out similarity metric, the conclusion that the learned behavior is 'closer to target behavior' is partly forced by construction.
full rationale
The core video-to-reward pipeline is not circular: the LLM could generate poor reward functions, and PPO training could fail, so the pipeline itself is not a tautology. However, the paper's headline behavioral-mimicry evidence is Table 1, and the DTW score used there is identical to the Fsim feedback signal injected into the LLM in Algorithm 1. This is a metric-reuse circularity: optimizing d and then reporting d as proof of success does not independently establish behavioral fidelity. The Hmts-based human normalized score is also used for selection in line 8, but since Eureka uses the same selection criterion, the +37.6% comparison is not invalidated by this reuse alone. The 2D projection 'along the direction of motion' raises a validity concern but is not a circularity per se. There is no load-bearing self-citation chain: the only same-author citation ([11]) is not invoked to justify the method. Overall, the partial circularity is concentrated in the DTW-based mimicry claim, warranting a score of 6 rather than higher; the central reward-generation result retains independent content.
Assumptions & free parameters
free parameters (3)
- Number of reward samples K and refinement iterations N =
K=16, N=5
- Frame sampling interval for keypoint extraction =
every 3 frames for humanoid, every 9 frames for Anymal
- Trajectory segmentation period =
two periods per segment, detected by autocorrelation
assumptions (4)
- domain assumption FastDTW similarity between projected 3D keypoint trajectories and 2D video keypoint trajectories is a valid measure of behavioral similarity.
- domain assumption The LLM can generate executable and effective reward functions from coordinate trajectory text plus environment code.
- domain assumption Pose estimation keypoints from human and animal videos correspond to the robot's relevant joints.
- standard math Autocorrelation period detection and FastDTW are mathematically well-defined algorithms.
Cite this review
Pith. "Pith review of Video2Reward: Generating Reward Function from Videos for Legged Robot Behavior Learning." pith.science (2026). https://pith.science/paper/PLSCFWQ7
@misc{pith2026241205515,
author = {Pith},
title = {Pith review of: Video2Reward: Generating Reward Function from Videos for Legged Robot Behavior Learning},
year = {2026},
howpublished = {\url{https://pith.science/paper/PLSCFWQ7}},
note = {Machine review of arXiv:2412.05515}
}
read the original abstract
Learning behavior in legged robots presents a significant challenge due to its inherent instability and complex constraints. Recent research has proposed the use of a large language model (LLM) to generate reward functions in reinforcement learning, thereby replacing the need for manually designed rewards by experts. However, this approach, which relies on textual descriptions to define learning objectives, fails to achieve controllable and precise behavior learning with clear directionality. In this paper, we introduce a new video2reward method, which directly generates reward functions from videos depicting the behaviors to be mimicked and learned. Specifically, we first process videos containing the target behaviors, converting the motion information of individuals in the videos into keypoint trajectories represented as coordinates through a video2text transforming module. These trajectories are then fed into an LLM to generate the reward function, which in turn is used to train the policy. To enhance the quality of the reward function, we develop a video-assisted iterative reward refinement scheme that visually assesses the learned behaviors and provides textual feedback to the LLM. This feedback guides the LLM to continually refine the reward function, ultimately facilitating more efficient behavior learning. Experimental results on tasks involving bipedal and quadrupedal robot motion control demonstrate that our method surpasses the performance of state-of-the-art LLM-based reward generation methods by over 37.6% in terms of human normalized score. More importantly, by switching video inputs, we find our method can rapidly learn diverse motion behaviors such as walking and running.
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