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Title: Edge-based motion segmentation
Author: Smith, Paul Alexander
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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Motion segmentation is the process of dividing video frames into regions which have different motions, providing a cut-out of the moving objects. Such a segmentation is a necessary first stage in many video analysis applications, but providing an accurate, efficient motion segmentation still presents a challenge. This dissertation proposes a novel approach to motion segmentation, using the image edges in a frame. Using edges, a motion can be calculated for each object. Edges provide good motion information, and it is shown that a set of edges, labelled according to the object motion that they obey, is sufficient to completely determine the labelling of the whole frame, up to unresolvable ambiguities. The areas of the frame between edges are divided into regions, grouping together pixels of similar colour, and these regions can each be assigned to different motion layers by reference to the edges. The depth ordering of these layers can also be deduced. A Bayesian framework is presented, which determines the most likely region labelling and depth ordering, given edges labelled with their probability of obeying each of the object motions. An efficient implementation of this framework is presented, initially for segmenting two motions (foreground and background) using two frames. The ExpectationMaximisation algorithm is used to determine the two motions and calculate the label probability for each edge. The frame is then segmented into regions. The best motion labelling for these regions is determined using simulated annealing. Extensions of this simple implementation are then presented. It is demonstrated how, by tracking the edges into further frames, the statistics may be accumulated to provide an even more accurate and robust segmentation. This also allows a complete sequence to be segmented. It is then demonstrated that the framework can be extended to a larger number of motions. A new hierarchical method of initialising the Expectation-Maximisation algorithm is described, which also determines the best number of motions. These techniques have been extensively tested on thirty-four real sequences, covering a wide range of genres. The results demonstrate that the proposed edge-based approach is an accurate and efficient method of obtaining a motion segmentation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral