REVIEW 4 major objections 5 minor 26 references
BagIt! An Adaptive Dual-Arm Manipulation of Fabric Bags for Object Bagging
T0 review · 4 major / 5 minor · reviewed 2026-08-04 · deepseek-v4-flash
Pith's one-line read Dual-arm bagging succeeds by tracking only the opening rim as an ellipse.
desk verdict A competent, honest dual-arm bagging system paper with a novel integrated formulation; the rim-only state and constant-perimeter assumptions are plausible for the tested bags but unverified as a general result. 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 Structure-of-Interest (SOI): the set of points along the bag's opening rim, treated as the entire bag state. The pipeline regularizes every planned subgoal into a standard ellipse (via the Regularization function) and enforces that the ellipse's perimeter stays close to the bag's initial rim perimeter (constraint C4). The local controller is an MPC-based shape servoing loop that uses a Broyden-updated deformation Jacobian to convert rim shape errors into dual-arm velocity commands.
What would settle it
Measure the true rim perimeter with vision during a bagging trial on a stretchy or slack bag; if the perimeter deviates beyond the constraint tolerance, the planned elliptical reference will diverge from the physical rim and the MPC will track a fictional shape, breaking the near-perfect success. A second check: test a bag whose rim cannot be approximated by an ellipse (e.g., a deeply creased or sagging opening) and observe whether planning and control fail.
Extended reading notes
Core claim
The central claim is that the bag opening rim, represented as a set of 3D points called the SOI, is an adequate state representation for the whole bag during a bagging task. The system does not rely on prior knowledge of bag material properties. It estimates the rim in real time using a Gaussian Mixture Model over raw point clouds, then solves an optimization to place a bagging ellipse around the object's bottom, ensuring the ellipse's perimeter matches the measured rim perimeter and contains the object's base vertices. Planned subgoals are regularized into ellipses with a perimeter-consistency constraint, and an MPC-based shape servoing controller, which approximates the deformation Jacobia
Load-bearing premise
The bag opening's perimeter stays roughly constant at its initially measured size throughout the entire bagging process, and every rim state can be faithfully approximated as an ellipse.
Editorial extensions
If this is right
- Deformable bag manipulation can be achieved with modest perception: only the rim needs to be tracked, not the full fabric surface.
- The two-phase strategy with an intermediate bagging SOI (g†) improves manipulation success compared to single-phase approaches.
- Constraint-aware planning that keeps perimeter consistent makes the planned elliptical rim shapes physically plausible and collision-free.
- MPC tracking of a time-varying deformation trajectory outperforms controllers that assume a static desired shape.
- The method transfers to different bag colors and to bundled/tilted objects, with success limited mainly by rim detection quality on solid-color bags.
Reading between the lines
- The rim-as-ellipse abstraction likely extends to other enveloping tasks such as covering, wrapping, or placing bags over objects, not just the bagging scenario demonstrated here.
- A natural testable extension is an adaptive perimeter estimator: if the system tracked rim perimeter changes in real time, it could handle stretchy or slack bags where the fixed-perimeter assumption breaks.
- The success with bundled objects suggests the SOI representation may work for arbitrary bottom cross-sections as long as the rim can be approximated by an ellipse; non-elliptical rim models could broaden applicability further.
- Because the system requires an initially open bag and a visible rim, the same feedback could be used to learn a bag-opening primitive, closing the gap to fully autonomous bagging from a closed bag.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a complete dual-arm robotic system for bagging objects into fabric bags. The bag's opening rim is treated as a Structure-of-Interest (SOI); a GMM is used to estimate the rim from raw point clouds, a constrained optimization generates a 'bagging SOI' ellipse around the object's bottom, CBiRRT plans a trajectory from the initial rim to this ellipse and then to a translated goal SOI, and an MPC-based shape-servoing controller with an online Broyden-estimated deformation Jacobian tracks the planned subgoals. The central claim is that the rim-only SOI is an adequate representation of the fabric state for the bagging task. The system is evaluated in 12 scenarios with different objects and bag types, with success rates and comparisons to several baselines.
Significance. If the central claim is correct, the paper offers a practical and reasonably general solution to a challenging deformable-object manipulation task, without requiring a physics model or material parameters. The paper's strengths are its real-hardware validation across multiple objects and bag types, the large simulation study for SOI generation, the inclusion of baseline planners/controllers, and the availability of a video. The reported 8/8 manipulation success in most scenarios is encouraging. However, the load-bearing constant-perimeter assumption is stated but not verified, and the statistical basis for the comparative success-rate claims is weak. These issues do not invalidate the demonstrated system but leave the generality of the rim-only representation insufficiently supported.
major comments (4)
- [III, Eqs. (7), (13), (16)] The constant-perimeter assumption is load-bearing but unverified. Section III states that "the size of the bag opening remains relatively unchanged during the bagging process." R is defined as a chord sum over the initial rim (Sec. V-A.2), not as fabric arc length. The bagging SOI optimization minimizes ||L-R||^2 (Eq. 7), every regularized subgoal enforces |R_x/R_y - 1| <= lambda_4 (Eq. 16), and the goal SOI is a translation of g-dagger (Eq. 13). If the physical rim sags, stretches, or is occluded, its observed chord perimeter changes even though the fabric length is constant; the MPC then tracks a planned ellipse that cannot match the physical rim. No online R_t measurement or adaptation is reported, so the central claim that rim-only SOI is adequate is not established beyond the specific bags tested. Please report R_t traces or add a re-estimation/re-planning mechanism.
- [VII-E, Tables I and II] The headline success rates are quoted as x/8 or x/10 without confidence intervals or repeated trials. With n=8, a single failure changes the rate by 12.5 percentage points; observed differences such as 8/8 vs 7/8 are not statistically meaningful. Planning success rates are also based on a single set of 10 runs. This weakens the comparative claim of "highest success rate." Please report repeated trials, confidence intervals, or a statistical test.
- [IV, Eq. (4)] The text says that a uniform distribution is incorporated into the GMM to handle outliers, but Eq. (4) sums only over n_x Gaussian components and has no uniform term. Either the equation is missing a uniform component or the implementation description is inaccurate. Since SOI extraction is the perception backbone of the whole pipeline, this inconsistency needs clarification.
- [VI-B, Eq. (18)] The deformation Jacobian is assumed to have full column rank. This is not verified or enforced. For a slack bag, some arm motions may produce little or no rim displacement, making J_t rank-deficient; the Broyden update (denominator u_t^T u_t) also becomes ill-conditioned when the arm velocity command is small. Please provide safeguards, a rank check, or experimental evidence that rank deficiency does not occur in the reported scenarios.
minor comments (5)
- [Eq. (4)] The equation should read "sum_j pi_j = 1" rather than "P pi_j = 1".
- [V-A.1, Eq. (5)] The x/y/z axes are computed from three randomly selected vertices of the object bottom. A near-collinear selection makes the cross products in Eq. (5) ill-conditioned. A PCA-based or otherwise robust frame estimation would avoid this numerical fragility.
- [VII-D] The note that g* is a simple translation of g-dagger and "may involve a more complex configuration" is important; please discuss this limitation in the conclusion as future work.
- [Notation] R is used both for the bag perimeter (Sec. V-A.2) and for the MPC control-weight matrix (Eq. 19). Using distinct symbols would avoid confusion.
- [Fig. 3] The flowchart is dense and the panel labels are small. Enlarging the figure or listing the steps in the caption would improve readability.
Circularity Check
Only minor tautological perimeter fit; central bagging success is externally validated.
-
fitted input called prediction
[Sec. VII-C (Experiments on Bagging SOI Generation), referring to Eq. (7)]
"The real perimeter of the bag’s rim is measured as R = 0.68m. The results show that effective bagging SOI g† can be generated for various objects and placements. In each scenario, g† meets the perimeter constraint R."
Equation (7) defines the bagging ellipse by minimizing ||L−R||^2, where R is the measured initial rim perimeter and L is the ellipse perimeter. Thus any solution g† has L ≈ R by construction. Reporting that 'g† meets the perimeter constraint R' as an experimental result is a restatement of the optimization objective, not an independent confirmation. This is a minor tautology in the evaluation of the SOI generator; it does not by itself establish the central bagging success, which is judged externally in Table II.
full rationale
The paper's central derivation chain is not circular. The bag state is defined as the rim points (Eq. 1); the bagging SOI is a least-squares ellipse fit to the measured rim perimeter (Eq. 7); the goal SOI is a translation of that ellipse (Eq. 13); planned subgoals are regularized ellipses with a perimeter-consistency constraint (Eq. 16); and the MPC tracks these references using an online Broyden estimate of the deformation Jacobian (Eq. 18). No uniqueness theorem is imported from the authors' prior work, and no prediction is equivalent to its input by construction. The only identifiable circularity is the evaluation in Sec. VII-C that g† satisfies the perimeter constraint R, which is guaranteed by the optimization objective ||L−R||^2. This is a minor, non-load-bearing tautology. The paper's overall bagging success and alignment error are measured externally (whether the bag envelops the object, and the distance between SOI center and object center), so the central claim has independent empirical content. The skeptic's concern about the constant-perimeter assumption is a correctness/fragility risk, not a circularity of the derivation.
Assumptions & free parameters
free parameters (9)
- lambda_1 (bagging ellipse envelopment) =
0.912
- lambda_2 (concentricity) =
0.007
- lambda_3 (orientation parallelism) =
0.9943
- lambda_4 (perimeter similarity in regularization) =
0.002
- lambda_5 (regularization concentricity) =
0.021
- lambda_d (bagging depth)
- epsilon (Broyden convergence rate)
- n_x (number of SOI points / GMM components)
- MPC weights Q, R and horizon T
assumptions (6)
- domain assumption All bottom vertices V of the target object are coplanar and known in advance
- domain assumption The size of the bag opening remains approximately constant during bagging
- domain assumption Every relevant bag rim state is well approximated by an ellipse
- domain assumption Deformation Jacobian J_t keeps full column rank throughout the task
- domain assumption Raw point cloud P_t is drawn from a GMM with n_x Gaussians plus outliers
- standard math Standard algorithmic facts: EM convergence, CBiRRT connectivity, PCA, Broyden rank-one update, farthest point sampling, ellipse perimeter formula
invented entities (1)
-
Structure-of-Interest (SOI) as the bag state
Cite this review
Pith. "Pith review of BagIt! An Adaptive Dual-Arm Manipulation of Fabric Bags for Object Bagging." pith.science (2026). https://pith.science/paper/G6FTATUR
@misc{pith2026250909484,
author = {Pith},
title = {Pith review of: BagIt! An Adaptive Dual-Arm Manipulation of Fabric Bags for Object Bagging},
year = {2026},
howpublished = {\url{https://pith.science/paper/G6FTATUR}},
note = {Machine review of arXiv:2509.09484}
}
read the original abstract
Bagging tasks, commonly found in industrial scenarios, are challenging considering deformable bags' complicated and unpredictable nature. This paper presents an automated bagging system from the proposed adaptive Structure-of-Interest (SOI) manipulation strategy for dual robot arms. The system dynamically adjusts its actions based on real-time visual feedback, removing the need for pre-existing knowledge of bag properties. Our framework incorporates Gaussian Mixture Models (GMM) for estimating SOI states, optimization techniques for SOI generation, motion planning via Constrained Bidirectional Rapidly-exploring Random Tree (CBiRRT), and dual-arm coordination using Model Predictive Control (MPC). Extensive experiments validate the capability of our system to perform precise and robust bagging across various objects, showcasing its adaptability. This work offers a new solution for robotic deformable object manipulation (DOM), particularly in automated bagging tasks. Video of this work is available at https://youtu.be/6JWjCOeTGiQ.
Figures
Figures from the paper (3 more)
Reference graph
Works this paper leans on
-
[1]
F. Gu, Y . Zhou, Z. Wang, S. Jiang, and B. He, “A survey on robotic manipulation of deformable objects: Recent advances, open challenges and new frontiers,”arXiv preprint arXiv:2312.10419, 2023
arXiv 2023
-
[2]
A joint learning of force feedback of robotic manipulation and textual cues for granular materials classification,
Z. Zhang, G. Chen, W. Chen, R. Jia, G. Chen, L. Zhang, J. Pan, and P. Zhou, “A joint learning of force feedback of robotic manipulation and textual cues for granular materials classification,”IEEE Robotics and Automation Letters, 2025
2025
-
[3]
Goats: Goal sampling adaptation for scooping with curriculum reinforcement learning,
Y . Niu, S. Jin, Z. Zhang, J. Zhu, D. Zhao, and L. Zhang, “Goats: Goal sampling adaptation for scooping with curriculum reinforcement learning,” in2023 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). IEEE, 2023, pp. 1023–1030
2023
-
[4]
Learning to estimate 3-d states of deformable linear objects from single-frame occluded point clouds,
K. Lvet al., “Learning to estimate 3-d states of deformable linear objects from single-frame occluded point clouds,” in2023 IEEE International Conference on Robotics and Automation, pp. 7119–7125
-
[5]
Llm-driven symbolic planning and hierarchical imitation learning for long-horizon deformable object assembly,
J. Qi, L. Lu, F. Wang, H.-Y . Lee, D. Navarro-Alarcon, Z. Zhang, and P. Zhou, “Llm-driven symbolic planning and hierarchical imitation learning for long-horizon deformable object assembly,”Robotics and Computer-Integrated Manufacturing, vol. 97, p. 103096, 2026
2026
-
[6]
One fling to goal: Environment-aware dynamics for goal- conditioned fabric flinging,
L. Yang, L. Yang, H. Sun, Z. Zhang, H. He, F. Wan, C. Song, and J. Pan, “One fling to goal: Environment-aware dynamics for goal- conditioned fabric flinging,”arXiv preprint arXiv:2406.14136, 2024
arXiv 2024
-
[7]
Fourier-based shape servoing: a new feedback method to actively deform soft objects into desired 2-d image contours,
D. Navarro-Alarcon and Y .-H. Liu, “Fourier-based shape servoing: a new feedback method to actively deform soft objects into desired 2-d image contours,”IEEE Transactions on Robotics, vol. 34, no. 1, pp. 272–279, 2017
2017
-
[8]
Unfolding an item of rectangular clothing using a single arm and an assistant instrument,
H. Yuba and K. Yamazaki, “Unfolding an item of rectangular clothing using a single arm and an assistant instrument,” in2014 IEEE/SICE International Symposium on System Integration, 2014, pp. 571–576
2014
Show all 26 references
-
[9]
Learning keypoints for robotic cloth manipulation using synthetic data,
T. Lips, V .-L. De Gusseme, and F. Wyffels, “Learning keypoints for robotic cloth manipulation using synthetic data,”IEEE Robotics and Automation Letters, vol. 9, no. 7, pp. 6528–6535, 2024
2024
-
[10]
Action-conditional implicit visual dynamics for de- formable object manipulation,
B. Shen, Z. Jiang, C. Choy, S. Savarese, L. J. Guibas, A. Anandkumar, and Y . Zhu, “Action-conditional implicit visual dynamics for de- formable object manipulation,”The International Journal of Robotics Research, vol. 43, no. 4, pp. 437–455, 2024
2024
-
[11]
Softgym: Benchmarking deep reinforcement learning for deformable object manipulation,
X. Lin, Y . Wang, J. Olkin, and D. Held, “Softgym: Benchmarking deep reinforcement learning for deformable object manipulation,” in Conference on Robot Learning. PMLR, 2021, pp. 432–448
2021
-
[12]
Visual-tactile learning of garment unfolding for robot- assisted dressing,
F. Zhanget al., “Visual-tactile learning of garment unfolding for robot- assisted dressing,”IEEE Robotics and Automation Letters, 2023
2023
-
[13]
Flingbot: The unreasonable effectiveness of dynamic manipulation for cloth unfolding,
H. Ha and S. Song, “Flingbot: The unreasonable effectiveness of dynamic manipulation for cloth unfolding,” inConference on Robot Learning. PMLR, 2022, pp. 24–33
2022
-
[14]
Shakingbot: dynamic manipula- tion for bagging,
N. Gu, Z. Zhang, R. He, and L. Yu, “Shakingbot: dynamic manipula- tion for bagging,”Robotica, vol. 42, no. 3, pp. 775–791, 2024
2024
-
[15]
Autobag: Learning to open plastic bags and insert objects,
L. Y . Chen, B. Shi, D. Seita, R. Cheng, T. Kollar, D. Held, and K. Goldberg, “Autobag: Learning to open plastic bags and insert objects,” in2023 IEEE International Conference on Robotics and Automation (ICRA), 2023, pp. 3918–3925
2023
-
[16]
Bimanual deformable bag manipulation using a structure-of-interest based neural dynamics model,
P. Zhou, P. Zheng, J. Qi, C. Li, H.-Y . Lee, Y . Pan, C. Yang, D. Navarro- Alarcon, and J. Pan, “Bimanual deformable bag manipulation using a structure-of-interest based neural dynamics model,”IEEE/ASME transactions on mechatronics, 2024
2024
-
[17]
Abramowitz and I
M. Abramowitz and I. A. Stegun,Handbook of mathematical functions with formulas, graphs, and mathematical tables. US Government printing office, 1948, vol. 55
1948
-
[18]
Pointasnl: Robust point clouds processing using nonlocal neural networks with adaptive sampling,
X. Yan, C. Zheng, Z. Li, S. Wang, and S. Cui, “Pointasnl: Robust point clouds processing using nonlocal neural networks with adaptive sampling,” inProceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition, 2020, pp. 5589–5598
2020
-
[19]
A coarse-to-fine framework for dual-arm manipulation of deformable linear objects with whole-body obstacle avoidance,
M. Yu, K. Lv, C. Wang, M. Tomizuka, and X. Li, “A coarse-to-fine framework for dual-arm manipulation of deformable linear objects with whole-body obstacle avoidance,” in2023 IEEE International Conference on Robotics and Automation, pp. 10 153–10 159
-
[20]
Adaptive shape servoing of elastic rods using parameterized regression features and auto-tuning motion controls,
J. Qi, G. Ran, B. Wang, J. Liu, W. Ma, P. Zhou, and D. Navarro- Alarcon, “Adaptive shape servoing of elastic rods using parameterized regression features and auto-tuning motion controls,”IEEE Robotics and Automation Letters, vol. 9, no. 2, pp. 1428–1435, 2023
2023
-
[21]
Contour moments based manipulation of compos- ite rigid-deformable objects with finite time model estimation and shape/position control,
J. Qiet al., “Contour moments based manipulation of compos- ite rigid-deformable objects with finite time model estimation and shape/position control,”IEEE/ASME transactions on mechatronics, vol. 27, no. 5, pp. 2985–2996, 2021
2021
-
[22]
A class of methods for solving nonlinear simultaneous equations,
C. G. Broyden, “A class of methods for solving nonlinear simultaneous equations,”Mathematics of computation, vol. 19, pp. 577–593, 1965
1965
-
[23]
Motion planning for a deformable linear object,
O. Roussel, M. Ta ¨ıx, and T. Bretl, “Motion planning for a deformable linear object,” inEuropean workshop on deformable object manipu- lation, 2014, pp. 153–158
2014
-
[24]
Tangent space rrt: A randomized planning algorithm on constraint manifolds,
C. Suh, T. T. Um, B. Kim, H. Noh, M. Kim, and F. C. Park, “Tangent space rrt: A randomized planning algorithm on constraint manifolds,” in2011 IEEE International Conference on Robotics and Automation. IEEE, 2011, pp. 4968–4973
2011
-
[25]
Image-based visual servoing control of robot manipulators using hybrid algorithm with feature constraints,
X. Ren, H. Li, and Y . Li, “Image-based visual servoing control of robot manipulators using hybrid algorithm with feature constraints,” IEEE Access, vol. 8, pp. 223 495–223 508, 2020
2020
-
[26]
A universal state-space approach to uncalibrated model-free visual servoing,
M. Hao and Z. Sun, “A universal state-space approach to uncalibrated model-free visual servoing,”IEEE/ASME Transactions on Mechatron- ics, vol. 17, no. 5, pp. 833–846, 2011
2011
Reviewed August 4, 2026 · model on record in the stance chip above.
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