Robust motion segmentation with subspace constraints
Creators: name: Ji, Pan
Publisher: name: The Australian National University
Year: 2016
Subjects: Motion segmentation
subspace clustering
feature tracking

description: Motion segmentation is an important task in computer vision with many applications such as dynamic scene understanding and multi-body structure from motion. When the point correspondences across frames are given, motion segmentation can be addressed as a subspace clustering problem under an affine camera model. In the first two parts of this thesis, we target the general subspace clustering problem and propose two novel methods, namely Efficient Dense Subspace Clustering (EDSC) and the Robust Shape Interaction Matrix (RSIM) method. Instead of following the standard compressive sensing approach, in EDSC we formulate subspace clustering as a Frobenius norm minimization problem, which inherently yields denser connections between data points. While in the noise-free case we rely on the self-expressiveness of the observations, in the presence of noise we recover a clean dictionary to represent the data. Our formulation lets us solve the subspace clustering problem efficiently. More specifically, for outlier-free observations, the solution can be obtained in closed-form, and in the presence of outliers, we solve the problem by performing a series of linear operations. Furthermore, we show that our Frobenius norm formulation shares the same solution as the popular nuclear norm minimization approach when the data is free of any noise. In RSIM, we revisit the Shape Interaction Matrix (SIM) method, one of the earliest approaches for motion segmentation (or subspace clustering), and reveal its connections to several recent subspace clustering methods. We derive a simple, yet effective algorithm to robustify the SIM method and make it applicable to real-world scenarios where the data is corrupted by noise. We validate the proposed method by intuitive examples and justify it with the matrix perturbation theory. Moreover, we show that RSIM can be extended to handle missing data with a Grassmannian gradient descent method. The above subspace clustering methods work well for motion segmentation, yet they require that point trajectories across frames are known {\it a priori}. However, finding point correspondences is in itself a challenging task. Existing approaches tackle the correspondence estimation and motion segmentation problems separately. In the third part of this thesis, given a set of feature points detected in each frame of the sequence, we develop an approach which simultaneously performs motion segmentation and finds point correspondences across the frames. We formulate this problem in terms of Partial Permutation Matrices (PPMs) and aim to match feature descriptors while simultaneously encouraging point trajectories to satisfy subspace constraints. This lets us handle outliers in both point locations and feature appearance. The resulting optimization problem is solved via the Alternating Direction Method of Multipliers (ADMM), where each subproblem has an efficient solution. In particular, we show that most of the subproblems can be solved in closed-form, and one binary assignment subproblem can be solved by the Hungarian algorithm. Obtaining reliable feature tracks in a frame-by-frame manner is desirable in applications such as online motion segmentation. In the final part of the thesis, we introduce a novel multi-body feature tracker that exploits a multi-body rigidity assumption to improve tracking robustness under a general perspective camera model. A conventional approach to addressing this problem would consist of alternating between solving two subtasks: motion segmentation and feature tracking under rigidity constraints for each segment. This approach, however, requires knowing the number of motions, as well as assigning points to motion groups, which is typically sensitive to motion estimates. By contrast, we introduce a segmentation-free solution to multi-body feature tracking that bypasses the motion assignment step and reduces to solving a series of subproblems with closed-form solutions. In summary, in this thesis, we exploit the powerful subspace constraints and develop robust motion segmentation methods in different challenging scenarios where the trajectories are either given as input, or unknown beforehand. We also present a general robust multi-body feature tracker which can be used as the first step of motion segmentation to get reliable trajectories.