329 research outputs found

    Mobile Device Background Sensors: Authentication vs Privacy

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    The increasing number of mobile devices in recent years has caused the collection of a large amount of personal information that needs to be protected. To this aim, behavioural biometrics has become very popular. But, what is the discriminative power of mobile behavioural biometrics in real scenarios? With the success of Deep Learning (DL), architectures based on Convolutional Neural Networks (CNNs) and Recurrent Neural Networks (RNNs), such as Long Short-Term Memory (LSTM), have shown improvements compared to traditional machine learning methods. However, these DL architectures still have limitations that need to be addressed. In response, new DL architectures like Transformers have emerged. The question is, can these new Transformers outperform previous biometric approaches? To answers to these questions, this thesis focuses on behavioural biometric authentication with data acquired from mobile background sensors (i.e., accelerometers and gyroscopes). In addition, to the best of our knowledge, this is the first thesis that explores and proposes novel behavioural biometric systems based on Transformers, achieving state-of-the-art results in gait, swipe, and keystroke biometrics. The adoption of biometrics requires a balance between security and privacy. Biometric modalities provide a unique and inherently personal approach for authentication. Nevertheless, biometrics also give rise to concerns regarding the invasion of personal privacy. According to the General Data Protection Regulation (GDPR) introduced by the European Union, personal data such as biometric data are sensitive and must be used and protected properly. This thesis analyses the impact of sensitive data in the performance of biometric systems and proposes a novel unsupervised privacy-preserving approach. The research conducted in this thesis makes significant contributions, including: i) a comprehensive review of the privacy vulnerabilities of mobile device sensors, covering metrics for quantifying privacy in relation to sensitive data, along with protection methods for safeguarding sensitive information; ii) an analysis of authentication systems for behavioural biometrics on mobile devices (i.e., gait, swipe, and keystroke), being the first thesis that explores the potential of Transformers for behavioural biometrics, introducing novel architectures that outperform the state of the art; and iii) a novel privacy-preserving approach for mobile biometric gait verification using unsupervised learning techniques, ensuring the protection of sensitive data during the verification process

    Machine Learning Approaches for Semantic Segmentation on Partly-Annotated Medical Images

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    Semantic segmentation of medical images plays a crucial role in assisting medical practitioners in providing accurate and swift diagnoses; nevertheless, deep neural networks require extensive labelled data to learn and generalise appropriately. This is a major issue in medical imagery because most of the datasets are not fully annotated. Training models with partly-annotated datasets generate plenty of predictions that belong to correct unannotated areas that are categorised as false positives; as a result, standard segmentation metrics and objective functions do not work correctly, affecting the overall performance of the models. In this thesis, the semantic segmentation of partly-annotated medical datasets is extensively and thoroughly studied. The general objective is to improve the segmentation results of medical images via innovative supervised and semi-supervised approaches. The main contributions of this work are the following. Firstly, a new metric, specifically designed for this kind of dataset, can provide a reliable score to partly-annotated datasets with positive expert feedback in their generated predictions by exploiting all the confusion matrix values except the false positives. Secondly, an innovative approach to generating better pseudo-labels when applying co-training with the disagreement selection strategy. This method expands the pixels in disagreement utilising the combined predictions as a guide. Thirdly, original attention mechanisms based on disagreement are designed for two cases: intra-model and inter-model. These attention modules leverage the disagreement between layers (from the same or different model instances) to enhance the overall learning process and generalisation of the models. Lastly, innovative deep supervision methods improve the segmentation results by training neural networks one subnetwork at a time following the order of the supervision branches. The methods are thoroughly evaluated on several histopathological datasets showing significant improvements

    Robust Out-of-Distribution Detection in Deep Classifiers

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    Over the past decade, deep learning has gone from a fringe discipline of computer science to a major driver of innovation across a large number of industries. The deployment of such rapidly developing technology in safety-critical applications necessitates the careful study and mitigation of potential failure modes. Indeed, many deep learning models are overconfident in their predictions, are unable to flag out-of-distribution examples that are clearly unrelated to the task they were trained on and are vulnerable to adversarial vulnerabilities, where a small change in the input leads to a large change in the model’s prediction. In this dissertation, we study the relation between these issues in deep learning based vision classifiers. First, we benchmark various methods that have been proposed to enable deep learning meth- ods to detect out-of-distribution examples and we show that a classifier’s predictive confidence is well-suited for this task, if the classifier has had access to a large and diverse out-distribution at train time. We theoretically investigate how different out-of-distribution detection methods are related and show that several seemingly different approaches are actually modeling the same core quantities. In the second part we study the adversarial robustness of a classifier’s confidence on out- of-distribution data. Concretely, we show that several previous techniques for adversarial robustness can be combined to create a model that inherits each method’s strength while sig- nificantly reducing their respective drawbacks. In addition, we demonstrate that the enforce- ment of adversarially robust low confidence on out-of-distribution data enhances the inherent interpretability of the model by imbuing the classifier with certain generative properties that can be used to query the model for counterfactual explanations for its decisions. In the third part of this dissertation we will study the problem of issuing mathematically provable certificates for the adversarial robustness of a model’s confidence on out-of-distribution data. We develop two different approaches to this problem and show that they have comple- mentary strength and weaknesses. The first method is easy to train, puts no restrictions on the architecture that our classifier can use and provably ensures that the classifier will have low confidence on data very far away. However, it only provides guarantees for very specific types of adversarial perturbations and only for data that is very easy to distinguish from the in-distribution. The second approach works for more commonly studied sets of adversarial perturbations and on much more challenging out-distribution data, but puts heavy restrictions on the architecture that can be used and thus the achievable accuracy. It also does not guar- antee low confidence on asymptotically far away data. In the final chapter of this dissertation we show how ideas from both of these techniques can be combined in a way that preserves all of their strengths while inheriting none of their weaknesses. Thus, this thesis outlines how to develop high-performing classifiers that provably know when they do not know

    Siamese Cross-Domain Tracker Design for Seamless Tracking of Targets in RGB and Thermal Videos

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    Multimodal (RGB and thermal) applications are swiftly gaining importance in the computer vision community with advancements in self-driving cars, robotics, Internet of Things, and surveillance applications. Both the modalities have complementary performance depending on illumination constraints. Hence, a judicious combination of both modalities will result in robust RGBT systems capable of all-day all-weather applications. Several studies have been proposed in the literature for integrating the multimodal sensor data for object tracking applications. Most of the proposed networks try to delineate the information into modality-specific and modality shared features and attempt to exploit the modality shared features in enhancing the modality specific information. In this work, we propose a novel perspective to this problem using a Siamese inspired network architecture. We design a custom Siamese cross-domain tracker architecture and fuse it with a mean shift tracker to drastically reduce the computational complexity. We also propose a constant false alarm rate inspired coasting architecture to cater for real-time track loss scenarios. The proposed method presents a complete and robust solution for object tracking across domains with seamless track handover for all-day all-weather operation. The algorithm is successfully implemented on a Jetson-Nano, the smallest graphics processing unit (GPU) board offered by NVIDIA Corporation

    OddAssist - An eSports betting recommendation system

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    It is globally accepted that sports betting has been around for as long as the sport itself. Back in the 1st century, circuses hosted chariot races and fans would bet on who they thought would emerge victorious. With the evolution of technology, sports evolved and, mainly, the bookmakers evolved. Due to the mass digitization, these houses are now available online, from anywhere, which makes this market inherently more tempting. In fact, this transition has propelled the sports betting industry into a multi-billion-dollar industry that can rival the sports industry. Similarly, younger generations are increasingly attached to the digital world, including electronic sports – eSports. In fact, young men are more likely to follow eSports than traditional sports. Counter-Strike: Global Offensive, the videogame on which this dissertation focuses, is one of the pillars of this industry and during 2022, 15 million dollars were distributed in tournament prizes and there was a peak of 2 million concurrent viewers. This factor, combined with the digitization of bookmakers, make the eSports betting market extremely appealing for exploring machine learning techniques, since young people who follow this type of sports also find it easy to bet online. In this dissertation, a betting recommendation system is proposed, implemented, tested, and validated, which considers the match history of each team, the odds of several bookmakers and the general feeling of fans in a discussion forum. The individual machine learning models achieved great results by themselves. More specifically, the match history model managed an accuracy of 66.66% with an expected calibration error of 2.10% and the bookmaker odds model, with an accuracy of 65.05% and a calibration error of 2.53%. Combining the models through stacking increased the accuracy to 67.62% but worsened the expected calibration error to 5.19%. On the other hand, merging the datasets and training a new, stronger model on that data improved the accuracy to 66.81% and had an expected calibration error of 2.67%. The solution is thoroughly tested in a betting simulation encapsulating 2500 matches. The system’s final odd is compared with the odds of the bookmakers and the expected long-term return is computed. A bet is made depending on whether it is above a certain threshold. This strategy called positive expected value betting was used at multiple thresholds and the results were compared. While the stacking solution did not perform in a betting environment, the match history model prevailed with profits form 8% to 90%; the odds model had profits ranging from 13% to 211%; and the dataset merging solution profited from 11% to 77%, all depending on the minimum expected value thresholds. Therefore, from this work resulted several machine learning approaches capable of profiting from Counter Strike: Global Offensive bets long-term.É globalmente aceite que as apostas desportivas existem há tanto tempo quanto o próprio desporto. Mesmo no primeiro século, os circos hospedavam corridas de carruagens e os fãs apostavam em quem achavam que sairia vitorioso, semelhante às corridas de cavalo de agora. Com a evolução da tecnologia, os desportos foram evoluindo e, principalmente, evoluíram as casas de apostas. Devido à onda de digitalização em massa, estas casas passaram a estar disponíveis online, a partir de qualquer sítio, o que torna este mercado inerentemente mais tentador. De facto, esta transição propulsionou a indústria das apostas desportivas para uma indústria multibilionária que agora pode mesmo ser comparada à indústria dos desportos. De forma semelhante, gerações mais novas estão cada vez mais ligadas ao digital, incluindo desportos digitais – eSports. Counter-Strike: Global Offensive, o videojogo sobre o qual esta dissertação incide, é um dos grandes impulsionadores desta indústria e durante 2022, 15 milhões de dólares foram distribuídos em prémios de torneios e houve um pico de espectadores concorrentes de 2 milhões. Embora esta realidade não seja tão pronunciada em Portugal, em vários países, jovens adultos do sexo masculino, têm mais probabilidade de acompanharem eSports que desportos tradicionais. Este fator, aliado à digitalização das casas de apostas, tornam o mercado de apostas em eSports muito apelativo para a exploração técnicas de aprendizagem automática, uma vez que os jovens que acompanham este tipo de desportos têm facilidade em apostar online. Nesta dissertação é proposto, implementado, testado e validado um sistema de recomendação de apostas que considera o histórico de resultados de cada equipa, as cotas de várias casas de apostas e o sentimento geral dos fãs num fórum de discussão – HLTV. Deste modo, foram inicialmente desenvolvidos 3 sistemas de aprendizagem automática. Para avaliar os sistemas criados, foi considerado o período de outubro de 2020 até março de 2023, o que corresponde a 2500 partidas. Porém, sendo o período de testes tão extenso, existe muita variação na competitividade das equipas. Deste modo, para evitar que os modelos ficassem obsoletos durante este período de teste, estes foram re-treinados no mínimo uma vez por mês durante a duração do período de testes. O primeiro sistema de aprendizagem automática incide sobre a previsão a partir de resultados anteriores, ou seja, o histórico de jogos entre as equipas. A melhor solução foi incorporar os jogadores na previsão, juntamente com o ranking da equipa e dando mais peso aos jogos mais recentes. Esta abordagem, utilizando regressão logística teve uma taxa de acerto de 66.66% com um erro expectável de calibração de 2.10%. O segundo sistema compila as cotas das várias casas de apostas e faz previsões com base em padrões das suas variações. Neste caso, incorporar as casas de aposta tendo atingido uma taxa de acerto de 65.88% utilizando regressão logística, porém, era um modelo pior calibrado que o modelo que utilizava a média das cotas utilizando gradient boosting machine, que exibiu uma taxa de acerto de 65.06%, mas melhores métricas de calibração, com um erro expectável de 2.53%. O terceiro sistema, baseia-se no sentimento dos fãs no fórum HLTV. Primeiramente, é utilizado o GPT 3.5 para extrair o sentimento de cada comentário, com uma taxa geral de acerto de 84.28%. No entanto, considerando apenas os comentários classificados como conclusivos, a taxa de acerto é de 91.46%. Depois de classificados, os comentários são depois passados a um modelo support vector machine que incorpora o comentador e a sua taxa de acerto nas partidas anteriores. Esta solução apenas previu corretamente 59.26% dos casos com um erro esperado de calibração de 3.22%. De modo a agregar as previsões destes 3 modelos, foram testadas duas abordagens. Primeiramente, foi testado treinar um novo modelo a partir das previsões dos restantes (stacking), obtendo uma taxa de acerto de 67.62%, mas com um erro de calibração esperado de 5.19%. Na segunda abordagem, por outro lado, são agregados os dados utilizados no treino dos 3 modelos individuais, e é treinado um novo modelo com base nesse conjunto de dados mais complexo. Esta abordagem, recorrendo a support vector machine, obteve uma taxa de acerto mais baixa, 66.81% mas um erro esperado de calibração mais baixo, 2.67%. Por fim, as abordagens são postas à prova através de um simulador de apostas, onde sistema cada faz uma previsão e a compara com a cota oferecia pelas casas de apostas. A simulação é feita para vários patamares de retorno mínimo esperado, onde os sistemas apenas apostam caso a taxa esperada de retorno da cota seja superior à do patamar. Esta cota final é depois comparada com as cotas das casas de apostas e, caso exista uma casa com uma cota superior, uma aposta é feita. Esta estratégia denomina-se de apostas de valor esperado positivo, ou seja, apostas cuja cota é demasiado elevada face à probabilidade de se concretizar e que geram lucros a longo termo. Nesta simulação, os melhores resultados, para uma taxa de mínima de 5% foram os modelos criados a partir das cotas das casas de apostas, com lucros entre os 13% e os 211%; o dos dados históricos que lucrou entre 8% e 90%; e por fim, o modelo composto, com lucros entre os 11% e os 77%. Assim, deste trabalho resultaram diversos sistemas baseados em machine learning capazes de obter lucro a longo-termo a apostar em Counter Strike: Global Offensive

    Lifelong Learning in the Clinical Open World

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    Despite mounting evidence that data drift causes deep learning models to deteriorate over time, the majority of medical imaging research is developed for - and evaluated on - static close-world environments. There have been exciting advances in the automatic detection and segmentation of diagnostically-relevant findings. Yet the few studies that attempt to validate their performance in actual clinics are met with disappointing results and little utility as perceived by healthcare professionals. This is largely due to the many factors that introduce shifts in medical image data distribution, from changes in the acquisition practices to naturally occurring variations in the patient population and disease manifestation. If we truly wish to leverage deep learning technologies to alleviate the workload of clinicians and drive forward the democratization of health care, we must move away from close-world assumptions and start designing systems for the dynamic open world. This entails, first, the establishment of reliable quality assurance mechanisms with methods from the fields of uncertainty estimation, out-of-distribution detection, and domain-aware prediction appraisal. Part I of the thesis summarizes my contributions to this area. I first propose two approaches that identify outliers by monitoring a self-supervised objective or by quantifying the distance to training samples in a low-dimensional latent space. I then explore how to maximize the diversity among members of a deep ensemble for improved calibration and robustness; and present a lightweight method to detect low-quality lung lesion segmentation masks using domain knowledge. Of course, detecting failures is only the first step. We ideally want to train models that are reliable in the open world for a large portion of the data. Out-of-distribution generalization and domain adaptation may increase robustness, but only to a certain extent. As time goes on, models can only maintain acceptable performance if they continue learning with newly acquired cases that reflect changes in the data distribution. The goal of continual learning is to adapt to changes in the environment without forgetting previous knowledge. One practical strategy to approach this is expansion, whereby multiple parametrizations of the model are trained and the most appropriate one is selected during inference. In the second part of the thesis, I present two expansion-based methods that do not rely on information regarding when or how the data distribution changes. Even when appropriate mechanisms are in place to fail safely and accumulate knowledge over time, this will only translate to clinical usage insofar as the regulatory framework allows it. Current regulations in the USA and European Union only authorize locked systems that do not learn post-deployment. Fortunately, regulatory bodies are noting the need for a modern lifecycle regulatory approach. I review these efforts, along with other practical aspects of developing systems that learn through their lifecycle, in the third part of the thesis. We are finally at a stage where healthcare professionals and regulators are embracing deep learning. The number of commercially available diagnostic radiology systems is also quickly rising. This opens up our chance - and responsibility - to show that these systems can be safe and effective throughout their lifespan

    Evaluation of Developments in PET Methodology

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    AI for time-resolved imaging: from fluorescence lifetime to single-pixel time of flight

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    Time-resolved imaging is a field of optics which measures the arrival time of light on the camera. This thesis looks at two time-resolved imaging modalities: fluorescence lifetime imaging and time-of-flight measurement for depth imaging and ranging. Both of these applications require temporal accuracy on the order of pico- or nanosecond (10−12 − 10−9s) scales. This demands special camera technology and optics that can sample light-intensity extremely quickly, much faster than an ordinary video camera. However, such detectors can be very expensive compared to regular cameras while offering lower image quality. Further, information of interest is often hidden (encoded) in the raw temporal data. Therefore, computational imaging algorithms are used to enhance, analyse and extract information from time-resolved images. "A picture is worth a thousand words". This describes a fundamental blessing and curse of image analysis: images contain extreme amounts of data. Consequently, it is very difficult to design algorithms that encompass all the possible pixel permutations and combinations that can encode this information. Fortunately, the rise of AI and machine learning (ML) allow us to instead create algorithms in a data-driven way. This thesis demonstrates the application of ML to time-resolved imaging tasks, ranging from parameter estimation in noisy data and decoding of overlapping information, through super-resolution, to inferring 3D information from 1D (temporal) data
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