Mac Aodha, O; (2014) Supervised Algorithm Selection for Flow and Other Computer Vision Problems. Doctoral thesis , UCL (University College London).

Abstract

Motion estimation is one of the core problems of computer vision. Given two or more frames from a video sequence, the goal is to find the temporal correspondence for one or more points from the sequence. For dense motion estimation, or optical flow, a dense correspondence field is sought between the pair of frames. A standard approach to optical flow involves constructing an energy function and then using some optimization scheme to find its minimum. These energy functions are hand designed to work well generally, with the intention that the global minimum corresponds to the ground truth temporal correspondence. As an alternative to these heuristic energy functions we aim to assess the quality of existing algorithms directly from training data. We show that the addition of an offline training phase can improve the quality of motion estimation. For optical flow, decisions such as which algorithm to use and when to trust its accuracy, can all be learned from training data. Generating ground truth optical flow data is a difficult and time consuming process. We propose the use of synthetic data for training and present a new dataset for optical flow evaluation and a tool for generating an unlimited quantity of ground truth correspondence data. We use this method for generating data to synthesize depth images for the problem of depth image super-resolution and show that it is superior to real data. We present results for optical flow confidence estimation with improved performance on a standard benchmark dataset. Using a similar feature representation, we extend this work to occlusion region detection and present state of the art results for challenging real scenes. Finally, given a set of different algorithms we treat optical flow estimation as the problem of choosing the best algorithm from this set for a given pixel. However, posing algorithm selection as a standard classification problem assumes that class labels are disjoint. For each training example it is assumed that there is only one class label that correctly describes it, and that all other labels are equally bad. To overcome this, we propose a novel example dependent cost-sensitive learning algorithm based on decision trees where each label is instead a vector representing a data point's affinity for each of the algorithms. We show that this new algorithm has improved accuracy compared to other classification baselines on several computer vision problems.

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