1,990 research outputs found
Detection of unusual fish trajectories from underwater videos
Fish behaviour analysis is a fundamental research area in marine ecology as it is helpful
for detecting environmental changes by observing unusual fish patterns or new fish
behaviours. The traditional way of analysing fish behaviour is by visual inspection
using human observers, which is very time consuming and also limits the amount of
data that can be processed. Therefore, there is a need for automatic algorithms to identify
fish behaviours by using computer vision and machine learning techniques. The
aim of this thesis is to help marine biologists with their work. We focus on behaviour
understanding and analysis of detected and tracked fish with unusual behaviour detection
approaches. Normal fish trajectories exhibit frequently observed behaviours while
unusual trajectories are outliers or rare trajectories.
This thesis proposes 3 approaches to detecting unusual trajectories: i) a filtering
mechanism for normal fish trajectories, ii) an unusual fish trajectory classification
method using clustered and labelled data and iii) an unusual fish trajectory classification
approach using a clustering based hierarchical decomposition.
The rule based trajectory filtering mechanism is proposed to remove normal fish
trajectories which potentially helps to increase the accuracy of the unusual fish behaviour
detection system. The aim is to reject normal fish trajectories as much as possible
while not rejecting unusual fish trajectories. The results show that this method
successfully filters out normal trajectories with a low false negative rate. This method
is useful to assist building a ground truth data set from a very large fish trajectory
repository, especially when the amount of normal fish trajectories greatly dominates
the unusual fish trajectories. Moreover, it successfully distinguishes true fish trajectories
from false fish trajectories which result from errors by the fish detection and
tracking algorithms.
A key contribution of this thesis is the proposed flat classifier, which uses an outlier
detection method based on cluster cardinalities and a distance function to detect unusual
fish trajectories. Clustered and labelled data are used to select feature sets which
perform best on a training set. To describe fish trajectories 10 groups of trajectory
descriptions are proposed which were not previously used for fish behaviour analysis.
The proposed flat classifier improved the performance of unusual fish detection
compared to the filtering approach.
The performance of the flat classifier is further improved by integrating it into a
hierarchical decomposition. This hierarchical decomposition method selects more specific
features for different trajectory clusters which is useful considering the trajectory
variety. Significantly improved results were obtained using this hierarchical decomposition
in comparison to the flat classifier. This hierarchical framework is also applied
to classification of more general imbalanced data sets which is a key current topic in
machine learning. The experiments showed that the proposed hierarchical decomposition
method is significantly better than the state of art classification methods, other
outlier detection methods and unusual trajectory detection methods. Furthermore, it is
successful at classifying imbalanced data sets even though the majority and minority
classes contain varieties, and classes overlap which is frequently seen in real-world
applications.
Finally, we explored the benefits of active learning in the context of the hierarchical
decomposition method, where active learning query strategies choose the most
informative training data. A substantial performance gain is possible by using less labelled
training data compared to learning from larger labelled data sets. Additionally,
active learning with feature selection is investigated. The results show that feature selection
has a positive effect on the performance of active learning. However, we show
that random selection can be as effective as popular active learning query strategies in
combination with active learning and feature selection, especially for imbalanced set
classification
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