REVIEW 3 major objections 6 minor 37 references
FMT$^{x}$: An Efficient and Asymptotically Optimal Extension of the Fast Marching Tree for Dynamic Replanning
T0 review · 3 major / 6 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read The paper claims that a modified Fast Marching Tree, FMTx, can repair its tree after obstacle changes and still match FMT*'s asymptotic optimality.
desk verdict FMTX is a plausible, well-tested dynamic replanner, but the asymptotic optimality proof is not rigorous; the stress-test counterexample misfires, yet the proof gap remains real. 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 object is the cost-ordered min-heap open set together with the modified expansion condition $c(x)>c(z)+\mathrm{Cost}(z,x)$ for a neighbor $x$ reached from an expanding node $z$. The condition acts as a certificate: when it is true, $x$'s current cost is provably not the best available, so it is worth the more expensive search for the optimal parent $y_{\min}$ among $x$'s open neighbors. The dynamic-update subroutines (AddObstacles, RemoveObstacles, QueueNeighbors) feed this queue by pruning, resetting, and requeueing only the affected portions of the tree. In the proof, an equivalence lemma shows that the cost condition coincides with unvisited-ness in obstacle-free runs, and a superset lemma bounds FMTx's path cost by FMT*'s path cost in each static phase.
What would settle it
Run FMTx and a from-scratch FMT* on the same fixed set of $n$ samples in a static environment immediately after each obstacle insertion and removal, using identical connection radii; if across many trials FMTx's converged path cost ever exceeds FMT*'s, Lemma 2's superset claim is false. A cleaner version is to construct a small graph where a disappearing obstacle restores a shortcut whose endpoint was pruned and never requeued, then compare final path costs.
Extended reading notes
Core claim
The central discovery is that FMT*'s single-pass limitation is not essential to its wavefront structure. In obstacle-free space, the modified rule $c(x)>c(z)+\mathrm{Cost}(z,x)$ selects exactly the same neighbor set as the original unvisited-set rule, so FMTx behaves like FMT* when nothing changes. When an obstacle appears or disappears, FMTx prunes the descendant subtree of any blocked edge, resets those costs to infinity, requeues affected neighbors, and lets the cost-ordered expansion rewire them. The paper argues that this update set is a superset of FMT*'s operations, so the converged path cost on a static phase is no larger than FMT*'s, and then transfers FMT*'s asymptotic optimality in probability to FMTx. The paper also reports simulations in which FMTx beats RRTX on median replanning time, especially as obstacle count, sample count, and neighborhood radius grow.
Load-bearing premise
Lemma 2 rests on the claim that FMTx's update operations form a superset of standard FMT*'s, so starting from an inherited, pruned tree with revisited nodes cannot yield a worse path than a fresh FMT* run.
Editorial extensions
If this is right
- On any static interval, FMTx converges in probability to an optimal path as the sample count grows, so a robot can reuse its tree across obstacle changes without sacrificing FMT*'s optimality guarantee.
- Only the portions of the tree invalidated by an obstacle change are pruned and requeued; unaffected branches keep their costs and parents.
- Because FMTx retains FMT*'s lazy collision checking, expensive edge validation is deferred until a promising parent is chosen, which pays off in kinodynamic planning.
- The reported comparison predicts that FMTx's speed advantage over RRTX grows with sample count, obstacle density, and neighborhood radius, and is largest for high-dimensional dynamic models.
- FMTx remains a persistent-sample, batch planner rather than an anytime planner; the paper identifies adding new samples continuously as future work.
Reading between the lines
- A consequence not stated in the paper is that the same cost-based re-evaluation could probably handle goal changes by treating the goal entering or leaving the tree as a cost perturbation, with no change to the algorithm's structure.
- Because Lemma 2 compares converged path costs rather than repair time, a testable prediction is that FMTx's advantage over RRTX depends on the size of the invalidated region and may shrink when an obstacle change disrupts a large part of the tree.
- The superset argument suggests a stronger, also unproven property: repeated repairs should make node costs monotonically non-increasing across consecutive environmental updates, which could be checked empirically on a fixed sample set.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes FMTX, an extension of the Fast Marching Tree (FMT*) algorithm for dynamic replanning. The main algorithmic changes are a cost-based re-evaluation condition in the expansion step and incremental repair routines (AddObstacles, RemoveObstacles, QueueNeighbors) that update the tree when obstacles appear or disappear. The central theoretical claim is that, during any static interval between environmental changes, FMTX recovers a path whose cost is no larger than a from-scratch FMT* run on the same graph, and hence inherits FMT*'s asymptotic optimality (Lemma 2, Corollary 3). The paper reports simulations in 2D geometric and kinodynamic settings, comparing median replanning times against RRTX and reporting lower times in most configurations.
Significance. If the main claim were established, FMTX would be a valuable contribution: an asymptotically optimal dynamic replanner that retains FMT*'s lazy, batch-oriented expansion while repairing only locally invalidated parts of the tree. The experimental study is a strength: it covers multiple sample sizes, obstacle counts, neighborhood radius multipliers, and three kinodynamic models, with 30 trials per condition and publicly available source code. However, the asymptotic-optimality proof is load-bearing for the paper's stated contribution, and the proof of Lemma 2 is not valid; the algorithm as specified can fail to improve an inherited path even when a strictly better path becomes available after an obstacle removal. The experimental results may still be useful as evidence that the heuristic repair strategy is fast, but the paper's central theoretical claim is not supported.
major comments (3)
- [Section 5.3, Lemma 2] The superset argument in the proof of Lemma 2 is invalid because FMTX does not start from the same state as a from-scratch FMT* run after an environmental change. In the algorithm as specified, Algorithm 4 (RemoveObstacles) considers only edges in the current tree edge set E, and Algorithm 5 (QueueNeighbors) inserts only finite-cost neighbors y of an endpoint u with y not already in V_open; it never reinserts the endpoint u itself. Consequently, a closed endpoint of a newly unblocked edge is never expanded again, and a non-tree edge that becomes collision-free is not even considered. A concrete Euclidean counterexample is: G=(0,0), R=(10,0), A=(5,0), B=(5,4), with an obstacle configuration that initially forces the tree G-B-R and leaves A unvisited. After the obstacle is removed, the optimal path G-A-R has cost 10 while the inherited path G-B-R has cost about 12.8. RemoveObstacles does not place G-A in E+ because that edge is not in the current tree, and there is no open node from which A can be rediscovered. Thus FMTX returns a path of cost 12.8, violating the inequality c_FMTx,n(G_n^k) <= c_FMT*,n(G_n^k) and invalidating Corollary 3.
- [Section 5.2] The manuscript states that the probability of a rewiring event near obstacles is 'expected to be negligible' without providing a formal bound. This is not a proof, and the subsequent appeal to Theorem 4.1 of FMT* does not fill the gap: asymptotic optimality of the final path cost in the from-scratch algorithm does not by itself bound the probability of persistent suboptimal connections in a repaired tree that starts from an inherited, pruned state. Because Lemma 2 relies on the claim that FMTX performs every update that FMT* would perform, this missing bound is load-bearing. The paper needs either a rigorous probabilistic argument that such rewiring events are negligible, or a different proof strategy for the static-segment path-quality claim.
- [Section 5.1, Lemma 1] The proof of Lemma 1 invokes 'Invariant 1' and 'Invariant 2' of FMT* without stating them, and the case analysis assumes that open nodes' costs always satisfy a Bellman optimality condition. In the modified FMT*Expand, the cost-based condition can also trigger on open nodes with finite cost, causing priority updates and rewiring, so the equivalence claimed in Lemma 1 is not obviously maintained once such updates occur. The lemma is stated only for obstacle-free environments, but the proof should make the scope explicit, and the paper should explain how the invariants are re-established after UpdateObstacles before they are used in the dynamic-setting argument.
minor comments (6)
- [Section 5.2] Replace the phrase 'expected to be negligible' either with a formal probability bound or with an explicit statement that this is an unproven heuristic assumption.
- [Table 1] Several table entries lack whitespace between numbers, e.g., '103.32 1.60204.86 3.68'; the formatting should be corrected for readability.
- [Throughout] The notation is inconsistent: the title and abstract use FMT$^{x}$, while the body mostly uses FMTX; unify the notation.
- [Algorithm 7] The while loop condition 'c(V_open.top()) < c(v_robot) or v_robot in V_open' is not explained; clarify what happens when v_robot is not in V_open and has infinite cost, and why the expansion terminates.
- [Section 6.1] The claim that a minimal localized update costs O((log n)^2) depends on the assumption that each Near query returns O(log n) neighbors in expectation; this should be stated explicitly as an expected-case bound.
- [Section 7.1.3] The kinodynamic description introduces Near+ and Near- as distinct neighbor queries, but the pseudocode in Algorithms 5 and 7 only refers to Near; align the pseudocode with the kinodynamic definitions.
Circularity Check
No significant circularity: the paper imports FMT* as an external baseline and its derivations do not reduce to their own inputs.
full rationale
The paper's central claim is that FMTX inherits FMT*'s asymptotic optimality during static intervals. That claim rests on two lemmas. Lemma 1 is a genuine equivalence proof between the cost-based neighbor-selection criterion and FMT*'s unvisited-set criterion; it invokes FMT* invariants from Janson et al. (2015), which is an external, independently established result, and the equivalence is not assumed as the conclusion. Lemma 2 attempts to show that FMTX's update operations form a superset of FMT*'s, so its path cost cannot be worse; this is a claimed property of the algorithm's execution, not a redefinition of the target quantity, and it is argued from the algorithm's stated rules rather than from the conclusion being proved. The known weakness of Lemma 2 is that the superset assertion appears false after obstacle removals because closed endpoints of newly unblocked edges are never reinserted into V_open; that is a soundness gap in the proof, not a circularity. Section 5.2's statement that rewiring events are 'expected to be negligible' is an unformalized heuristic, but it is not presented as a theorem that defines the result into existence. The experimental section compares FMTX against RRTX using externally defined metrics and does not fit parameters to produce the claimed optimality guarantee. There are no self-citations, no fitted quantity renamed as a prediction, and no imported uniqueness theorem used to force a choice. Accordingly, no circular step can be exhibited, and the appropriate score is 0.
Assumptions & free parameters
free parameters (1)
- Neighborhood radius multiplier C =
C in {1.0, 1.5, 2.0} (experimental choices)
assumptions (4)
- domain assumption FMT* is asymptotically optimal (Theorem 4.1 in Janson et al. 2015)
- domain assumption FMT* Invariants 1 and 2 hold in the modified FMTX expansion
- domain assumption Sampling and connection-radius assumptions for asymptotic optimality
- standard math Cost obeys the triangle inequality on the r_n-disk graph
Cite this review
Pith. "Pith review of FMT$^{x}$: An Efficient and Asymptotically Optimal Extension of the Fast Marching Tree for Dynamic Replanning." pith.science (2026). https://pith.science/paper/CLORXR3N
@misc{pith2026250908521,
author = {Pith},
title = {Pith review of: FMT$^x$: An Efficient and Asymptotically Optimal Extension of the Fast Marching Tree for Dynamic Replanning},
year = {2026},
howpublished = {\url{https://pith.science/paper/CLORXR3N}},
note = {Machine review of arXiv:2509.08521}
}
abstract
Path planning in dynamic environments remains a core challenge in robotics, especially as autonomous systems are deployed in unpredictable spaces such as warehouses and public roads. While algorithms like Fast Marching Tree (FMT$^{*}$) offer asymptotically optimal solutions in static settings, their single-pass design prevents path revisions which are essential for real-time adaptation. On the other hand, full replanning is often too computationally expensive. This paper introduces FMT$^{x}$, an extension of the Fast Marching Tree algorithm that enables efficient and consistent replanning in dynamic environments. We revisit the neighbor selection rule of FMT$^{*}$ and demonstrate that a minimal change overcomes its single-pass limitation, enabling the algorithm to update cost-to-come values upon discovering better connections without sacrificing asymptotic optimality or computational efficiency. By maintaining a cost-ordered priority queue and applying a selective update condition that uses an expanding neighbor to identify and trigger the re-evaluation of any node with a potentially suboptimal path, FMT$^{x}$ ensures that suboptimal routes are efficiently repaired as the environment evolves. This targeted strategy preserves the inherent efficiency of FMT$^{*}$ while enabling robust adaptation to changes in obstacle configuration. FMT$^{x}$ is proven to recover an asymptotically optimal solution after environmental changes. Experimental results demonstrate that FMT$^{x}$ outperforms the influential replanner RRT$^{x}$, reacting more swiftly to dynamic events with lower computational overhead and thus offering a more effective solution for real-time robotic navigation in unpredictable worlds.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[1]
Arslan O and Tsiotras P (2013) Use of relaxation methods in sampling-based algorithms for optimal motion planning. 2013 IEEE International Conference on Robotics and Automation : 2421--2428 ://api.semanticscholar.org/CorpusID:14282741
work page 2013
-
[2]
Arslan O and Tsiotras P (2015) Dynamic programming guided exploration for sampling-based motion planning algorithms. 2015 IEEE International Conference on Robotics and Automation (ICRA) : 4819--4826 ://api.semanticscholar.org/CorpusID:12488088
work page 2015
-
[3]
Bohlin R and Kavraki LE (2000) Path planning using lazy prm. Proceedings 2000 ICRA. Millennium Conference. International Conference on Robotics and Automation. Symposia Proceedings (Cat. No.00CH37065) 1: 521--528 vol.1. ://api.semanticscholar.org/CorpusID:206541645
work page 2000
-
[4]
Numerische Mathematik 1(1): 269--271
Dijkstra EW (1959) A note on two problems in connexion with graphs. Numerische Mathematik 1(1): 269--271. doi:10.1007/BF01386390
-
[5]
Proceedings 2006 IEEE International Conference on Robotics and Automation, 2006
Ferguson D, Kalra N and Stentz A (2006) Replanning with rrts. Proceedings 2006 IEEE International Conference on Robotics and Automation, 2006. ICRA 2006. : 1243--1248 ://api.semanticscholar.org/CorpusID:12375387
work page 2006
-
[6]
The International Journal of Robotics Research 39: 543 -- 567
Gammell JD, Barfoot TD and Srinivasa SS (2017 a ) Batch informed trees (bit*): Informed asymptotically optimal anytime search. The International Journal of Robotics Research 39: 543 -- 567. ://api.semanticscholar.org/CorpusID:700325
work page 2017
-
[7]
IEEE Transactions on Robotics 34: 966--984
Gammell JD, Barfoot TD and Srinivasa SS (2017 b ) Informed sampling for asymptotically optimal path planning. IEEE Transactions on Robotics 34: 966--984. ://api.semanticscholar.org/CorpusID:3241650
work page 2017
-
[8]
Gammell JD, Srinivasa SS and Barfoot TD (2014 a ) Batch informed trees (bit*): Sampling-based optimal planning via the heuristically guided search of implicit random geometric graphs. 2015 IEEE International Conference on Robotics and Automation (ICRA) : 3067--3074 ://api.semanticscholar.org/CorpusID:15776115
work page 2014
Show all 37 references
-
[9]
2014 IEEE/RSJ International Conference on Intelligent Robots and Systems : 2997--3004 ://api.semanticscholar.org/CorpusID:12233239
Gammell JD, Srinivasa SS and Barfoot TD (2014 b ) Informed rrt*: Optimal sampling-based path planning focused via direct sampling of an admissible ellipsoidal heuristic. 2014 IEEE/RSJ International Conference on Intelligent Robots and Systems : 2997--3004 ://api.semanticschola...
2014
-
[10]
Gammell JD and Strub MP (2020) Asymptotically optimal sampling-based motion planning methods. Annu. Rev. Control. Robotics Auton. Syst. 4: 295--318. ://api.semanticscholar.org/CorpusID:231592912
2020
-
[11]
IEEE transactions on Systems Science and Cybernetics 4(2): 100--107
Hart PE, Nilsson NJ and Raphael B (1968) A formal basis for the heuristic determination of minimum cost paths. IEEE transactions on Systems Science and Cybernetics 4(2): 100--107
1968
-
[12]
2015 IEEE International Conference on Robotics and Automation (ICRA) : 2951--2957 ://api.semanticscholar.org/CorpusID:14989369
Hauser KK (2015) Lazy collision checking in asymptotically-optimal motion planning. 2015 IEEE International Conference on Robotics and Automation (ICRA) : 2951--2957 ://api.semanticscholar.org/CorpusID:14989369
2015
-
[13]
In: Proceedings of the International Symposium on Robotics Research (ISRR)
Janson L and Pavone M (2013) Fast marching trees: A fast marching sampling-based method for optimal motion planning in many dimensions. In: Proceedings of the International Symposium on Robotics Research (ISRR)
2013
-
[14]
The International journal of robotics research 34(7): 883--921
Janson L, Schmerling E, Clark A and Pavone M (2015) Fast marching tree: A fast marching sampling-based method for optimal motion planning in many dimensions. The International journal of robotics research 34(7): 883--921
2015
-
[15]
49th IEEE Conference on Decision and Control (CDC) : 7681--7687 ://api.semanticscholar.org/CorpusID:1555207
Karaman S and Frazzoli E (2010) Optimal kinodynamic motion planning using incremental sampling-based methods. 49th IEEE Conference on Decision and Control (CDC) : 7681--7687 ://api.semanticscholar.org/CorpusID:1555207
2010
-
[16]
The international journal of robotics research 30(7): 846--894
Karaman S and Frazzoli E (2011) Sampling-based algorithms for optimal motion planning. The international journal of robotics research 30(7): 846--894
2011
-
[17]
IEEE transactions on Robotics and Automation 12(4): 566--580
Kavraki LE, Svestka P, Latombe JC and Overmars MH (1996) Probabilistic roadmaps for path planning in high-dimensional configuration spaces. IEEE transactions on Robotics and Automation 12(4): 566--580
1996
-
[18]
a nder J, Hermann A, R \
Klemm S, Oberl \"a nder J, Hermann A, R \"o nnau A, Schamm T, Z \"o llner JM and Dillmann R (2015) Rrt*-connect: Faster, asymptotically optimal motion planning. 2015 IEEE International Conference on Robotics and Biomimetics (ROBIO) : 1670--1677 ://api.semanticscholar.org/Corpu...
2015
-
[19]
In: AAAI/IAAI
Koenig S and Likhachev M (2002) D*lite. In: AAAI/IAAI. ://api.semanticscholar.org/CorpusID:208940224
2002
-
[20]
IEEE Transactions on Robotics 21: 354--363
Koenig S and Likhachev M (2005) Fast replanning for navigation in unknown terrain. IEEE Transactions on Robotics 21: 354--363. ://api.semanticscholar.org/CorpusID:15664344
2005
-
[21]
Koenig S, Likhachev M and Furcy D (2004) Lifelong planning a. Artif. Intell. 155: 93--146. ://api.semanticscholar.org/CorpusID:7828197
2004
-
[22]
In: Proceedings of the fifth international joint conference on Autonomous agents and multiagent systems
Koenig, Sven and Likhachev, Maxim (2006) Real-time adaptive a. In: Proceedings of the fifth international joint conference on Autonomous agents and multiagent systems. pp. 281--288
2006
-
[23]
Proceedings 2000 ICRA
Kuffner JJ and LaValle SM (2000) Rrt-connect: An efficient approach to single-query path planning. Proceedings 2000 ICRA. Millennium Conference. IEEE International Conference on Robotics and Automation. Symposia Proceedings (Cat. No.00CH37065) 2: 995--1001 vol.2. ://api.semant...
2000
-
[24]
The annual research report ://api.semanticscholar.org/CorpusID:14744621
LaValle SM (1998) Rapidly-exploring random trees : a new tool for path planning. The annual research report ://api.semanticscholar.org/CorpusID:14744621
1998
-
[25]
The international journal of robotics research 20(5): 378--400
LaValle SM and Kuffner Jr JJ (2001) Randomized kinodynamic planning. The international journal of robotics research 20(5): 378--400
2001
-
[26]
The International Journal of Robotics Research 35(5): 528--564
Li Y, Littlefield Z and Bekris KE (2016) Asymptotically optimal sampling-based kinodynamic planning. The International Journal of Robotics Research 35(5): 528--564
2016
-
[27]
a ki J and H \
Naderi K, Rajam \"a ki J and H \"a m \"a l \"a inen P (2015) Rt-rrt*: a real-time path planning algorithm based on rrt*. Proceedings of the 8th ACM SIGGRAPH Conference on Motion in Games ://api.semanticscholar.org/CorpusID:6223542
2015
-
[28]
ArXiv abs/2309.13119
Orthey A, Chamzas C and Kavraki LE (2023) Sampling-based motion planning: A comparative review. ArXiv abs/2309.13119. ://api.semanticscholar.org/CorpusID:262459959
2023 arXiv
-
[29]
The International Journal of Robotics Research 35(7): 797--822
Otte M and Frazzoli E (2016) Rrtx: Asymptotically optimal single-query sampling-based motion planning with quick replanning. The International Journal of Robotics Research 35(7): 797--822
2016
-
[30]
2015 54th IEEE Conference on Decision and Control (CDC) : 2574--2581 ://api.semanticscholar.org/CorpusID:2267285
Schmerling E, Janson L and Pavone M (2014) Optimal sampling-based motion planning under differential constraints: The drift case with linear affine dynamics. 2015 54th IEEE Conference on Decision and Control (CDC) : 2574--2581 ://api.semanticscholar.org/CorpusID:2267285
2014
-
[31]
2023 IEEE International Conference on Robotics and Automation (ICRA) : 7837--7843 ://api.semanticscholar.org/CorpusID:259339259
Silveira J, Cabral KM, Givigi SN and Marshall JA (2023) Real-time fast marching tree for mobile robot motion planning in dynamic environments. 2023 IEEE International Conference on Robotics and Automation (ICRA) : 7837--7843 ://api.semanticscholar.org/CorpusID:259339259
2023
-
[32]
2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) : 2072--2078 ://api.semanticscholar.org/CorpusID:5808032
Starek JA, G \'o mez JV, Schmerling E, Janson L, Moreno LE and Pavone M (2015) An asymptotically-optimal sampling-based algorithm for bi-directional motion planning. 2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) : 2072--2078 ://api.semanticsch...
2015
-
[33]
://api.semanticscholar.org/CorpusID:14389316
Stentz A (1994) The d* algorithm for real-time planning of optimal traverses. ://api.semanticscholar.org/CorpusID:14389316
1994
-
[34]
2020 IEEE International Conference on Robotics and Automation (ICRA) : 3191--3198 ://api.semanticscholar.org/CorpusID:211133126
Strub MP and Gammell JD (2020 a ) Adaptively informed trees (ait*): Fast asymptotically optimal path planning through adaptive heuristics. 2020 IEEE International Conference on Robotics and Automation (ICRA) : 3191--3198 ://api.semanticscholar.org/CorpusID:211133126
2020
-
[35]
2020 IEEE International Conference on Robotics and Automation (ICRA) : 130--136 ://api.semanticscholar.org/CorpusID:211132476
Strub MP and Gammell JD (2020 b ) Advanced bit* (abit*): Sampling-based planning with advanced graph-search techniques. 2020 IEEE International Conference on Robotics and Automation (ICRA) : 130--136 ://api.semanticscholar.org/CorpusID:211132476
2020
-
[36]
The International Journal of Robotics Research 41: 390 -- 417
Strub MP and Gammell JD (2021) Adaptively informed trees (ait*) and effort informed trees (eit*): Asymmetric bidirectional sampling-based path planning. The International Journal of Robotics Research 41: 390 -- 417. ://api.semanticscholar.org/CorpusID:250221473
2021
-
[37]
ArXiv abs/1205.5088
Webb DJ and van den Berg JP (2012) Kinodynamic rrt*: Optimal motion planning for systems with linear differential constraints. ArXiv abs/1205.5088. ://api.semanticscholar.org/CorpusID:3170122
2012 arXiv
Reviewed August 15, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.