I'm a final-year PhD student at the School of Informatics, University of Edinburgh, supervised by Iain Murray. I'm a member of the Centre for Doctoral Training in Data Science and co-funded by Microsoft Research.
I'm interested in probabilistic approaches to machine learning. Currently my work focuses on deep learning methods for density estimation and Bayesian inference. Previously, I studied Advanced Computing at Imperial College London and Electrical and Computer Engineering at the Aristotle University of Thessaloniki.
MSc by Research in Data Science, University of Edinburgh.
Grade 92%, Distinction. Won the MSc by Research in Data Science Class Prize.
MSc in Advanced Computing, Imperial College London.
Grade 90%, Distinction. Won the Corporate Partnership Programme Award for Academic Excellence and the Winton Capital Applied Computing MSc Project Prize.
MEng in Electrical and Computer Engineering, Aristotle University of Thessaloniki.
Grade 89.6%, Distinction.
Teaching assistant, University of Edinburgh.
I tutored (and sometimes marked) the following courses: Machine Learning & Pattern Recognition; Introductory Applied Machine Learning; Probabilistic Modelling & Reasoning; Informatics 2B - Algorithms, Data Structures & Learning; Introduction to Theoretical Computer Science.
Research assistant, Information Technologies Institute, Centre for Research & Technology Hellas.
I participated in the EU-funded project Adapt4EE and the Greek-funded project EnNoisis. Most of my work focused on automatic activity recognition in smart homes with ambient sensors and Kinect cameras. Quite a lot of machine learning and computer vision involved.
Research assistant, Aristotle University of Thessaloniki.
I participated in the EU-funded project AutoGPU, where I developed software for fast parallel low-level image processing on GPUs. I used to write a lot of CUDA back then.
Sequential Neural Likelihood is a fast and robust algorithm for inference in simulator models, which are models we can simulate but whose likelihood we can't compute. SNL works by trainining a Masked Autoregressive Flow on simulated data to learn the simulator model's intractable likelihood. By guiding simulations during training, we can reduce the simulation cost dramatically. SNL brings together ideas from likelihood-free inference and neural density estimation, and is a more robust alternative to related methods that learn the posterior directly.
Autoregressive models and normalizing flows are types of neural networks that achieve good performance in density estimation. We developed Masked Autoregressive Flow, a normalizing flow whose layers are autoregressive. Masked Autoregressive Flow is closely related to Inverse Autoregressive Flow and RealNVP, and performs well as a general-purpose density estimator.
For more information, see the paper and the code. There's also the oral presentation at NeurIPS 2017, and my interview with TWIMLAI. Since then, Masked Autoregressive Flow has become a standard part of TensorFlow probability.
Suppose we have a probabilistic model which we can simulate data from, but whose likelihood we can't evaluate. How can we do Bayesian inference in such a model? We propose learning the posterior with a Bayesian neural network trained on simulated data. By guiding future simulations, we can dramatically speed up the process.
In machine learning, many good models are large, expensive or intractable. Knowledge distillation refers to training a convenient model to match the performance of a good but cumbersome model. We apply knowledge distillation in: (a) model compression, where we compress large ensembles into small neural networks; (b) Bayesian inference, where we distil MCMC chains into closed-form predictive distributions; (c) intractable generative models, where we distil unnormalizable RBMs into tractable NADEs.
When represented as matrices, real-world data often have low-rank structure, whereas corruptions are often sparse. Based on this observation, several methods that aim to separate the low-rank from the sparse component have been developed. In this work, we extend existing matrix-based methods to tensors, and apply them to computer-vision problems.
For more information see my MSc thesis. This thesis won the Winton Capital Applied Computing MSc Project Prize.
Stochastic gradient descent is standard for training deep-learning models, because it scales well to large datasets. However, stochastic gradient descent has slower convergence rate compared to batch gradient descent. Semi-stochastic algorithms, such as S2GD and SAG, combine fast convergence with scalability. In this project, we compare semi-stochastic gradient descent to stochastic and batch gradient descent. We find that semi-stochastic gradient descent converges faster, but it doesn't necessarily lead to better generalization.
Convolution and correlation are fundamental low-level operations in image processing. In this project, we develop algorithms and software for their fast computation, based on (a) using the Fourier domain for large filters, and (b) parallelizing on GPUs. We developed the FLCC Library, a software tool that automatically determines which algorithm-device combination is the fastest for a given problem.
Sequential Neural Likelihood: Fast Likelihood-free Inference with Autoregressive Flows
G. Papamakarios, D. C. Sterratt, I. Murray
International Conference on Artificial Intelligence and Statistics, 2019
pdf bibtex code
Synthetic Ground Truth Data Generation for Automatic Trajectory-based ADL Detection
G. Papamakarios, D. Giakoumis, K. Votis, S. Segouli, D. Tzovaras, C. Karagiannidis
IEEE-EMBS International Conference on Biomedical and Health Informatics, 2014
Neural belief states for partially observed domains
P. Moreno, J. Humplik, G. Papamakarios, B. Á. Pires, L. Buesing, N. Heess, T. Weber
Reinforcement Learning under Partial Observability Workshop at Neural Information Processing Systems, 2018
A Tool to Monitor and Support Physical Exercise Interventions for MCI and AD Patients
G. Papamakarios, D. Giakoumis, M. Vasileiadis, K. Votis, D. Tzovaras, S. Segouli, C. Karagiannidis
Patient Rehabilitation Techniques Workshop at International Conference on Pervasive Computing Technologies for Healthcare, 2014
Human Computer Confluence in the Smart Home Paradigm: Detecting Human States and Behaviours for 24/7 Support of Mild-Cognitive Impairments
G. Papamakarios, D. Giakoumis, M. Vasileiadis, A. Drosou, D. Tzovaras
Human Computer Confluence: Transforming Human Experience Through Symbiotic Technologies, De Gruyter Open, 2016