Abstract
Robust model fitting plays a vital role in computer vision, and research into algorithms for robust fitting continues to be active. Arguably the most popular paradigm for robust fitting in computer vision is consensus maximisation, which strives to find the model parameters that maximise the number of inliers. Despite the significant developments in algorithms for consensus maximisation, there has been a lack of fundamental analysis of the problem in the computer vision literature. In particular, whether consensus maximisation is “tractable” remains a question that has not been rigorously dealt with, thus making it difficult to assess and compare the performance of proposed algorithms, relative to what is theoretically achievable. To shed light on these issues, we present several computational hardness results for consensus maximisation. Our results underline the fundamental intractability of the problem, and resolve several ambiguities existing in the literature.
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Notes
It may be the case that MAXCON is in class APX, i.e., that it could be approximated in polynomial time to some factor. However, we are not aware of any such algorithms.
Since RANSAC does not provide any approximation guarantees, it is not an “approximation scheme” by standard definition (Vazirani 2001).
A dicussion between Tat-Jun Chin and Komei Fukuda in Feb 2018 suggested that a direct reduction from MaxFS to MAXCON is itself infeasible, due to the intractability of bounding a hyperplane arrangement (Fukuda et al. 1997, Theorem 5.1).
If one was ever interested in only, say, robust affine registration of 2D point sets, then one could say that robust 2D affine registration is tractable, since the technique of Enqvist et al. (2012, 2015) can be used to construct an \(\mathcal {O}(N^7)\) algorithm, which is polynomial in the number of input correspondences N. However, robust fitting in general, where N and d can both vary, is not tractable by the reasons already alluded to above.
This can be ensured by infinitesimal data perturbations (Matoušek 1995).
We do not report the result of Algorithm 1 or 2 since they are too slow even for small o and d (both algorithms did finish in 2 hours for one data instance when \(d > 7\) and \(o > 7\)).
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This work was supported by ARC Grant DP160103490.
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Communicated by Yair Weiss.
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Chin, TJ., Cai, Z. & Neumann, F. Robust Fitting in Computer Vision: Easy or Hard?. Int J Comput Vis 128, 575–587 (2020). https://doi.org/10.1007/s11263-019-01207-y
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DOI: https://doi.org/10.1007/s11263-019-01207-y