Working on computer vision and machine learning
Since 2016 I am a research scientist in LinkMedia team of Inria Rennes-Bretagne Atlantique, carrying out research on computer vision and machine learning, and teaching Deep Learning for Vision. I enjoy working on learning visual representations from data, with as little supervision as possible. My recent work is focusing on exploring the manifold structure of data and, apart from image retrieval, using it for unsupervised, semi-supervised and few-shot learning. I am also working on adversarial examples and on investigating the sparsity of convolutional activations, applied to spatial matching and unsupervised object discovery.
In 2015 I have been at the Laboratory of Algebraic and Geometric Algorithms (ΕρΓΑ) of National and Kapodistrian University of Athens (NKUA), where I have worked on large-scale clustering and nearest neighbor search with Ioannis Emiris. An achievement of this period has been Web-Scale Image Clustering.
Between 2001 and 2015 I have been at the Image, Video and Multimedia Systems Laboratory (IVML) of the National Technical University of Athens (NTUA). In the latter part of this period, since 2008, I have been leading the Image and Video Analysis (IVA) team. We have worked on a diverse set of problems including local feature and salient region detection, spatial and spatiotemporal visual representation and matching, object recognition and tracking, action recognition in video, scene classification, image/video indexing, retrieval and summarization.
A large body of this work has focused on local feature/descriptor representations. In this context, we have delivered repeatable feature detection, extremely fast spatial matching, geometry indexing, multi-view and single-view feature selection, approximate nearest neighbor search for large scale clustering and vocabulary construction, and mining 3d scenes from millions of images. Examples of this work are Hough Pyramid Matching (HPM), the Aggregated Selective Match Kernel (ASMK), and Locally Optimized Product Quantization (LOPQ). In 2017, using a CNN image representation, Yahoo! Research has chosen LOPQ to index and provide a similar image search functionality on its entire Flickr collection. A flagship product of this period has been our unique application Visual Image Retrieval and Localization (VIRaL).
In the preceding period before 2008, I have worked on problems related to object detection and image understanding. This includes regions-of-interest for generic object detection, semantic segmentation, integrated segmentation and region labeling, spatio-temporal object segmentation and tracking, image classification, as well as the use of visual context and prior knowledge.
While at NTUA, I have had the opportunity to collaborate with a large number of researchers across Europe by participating in Networks of Excellence Muscle and K-Space and Integrated Projects like aceMedia and WeKnowIt. I have developed a number of activities including being the Program Chair or General Chair of workshops and conferences in the field of multimedia like WIAMIS, CBMI, MMM and CIVR. For several years I have been teaching Signals and Systems and Image and Video Analysis.
Between 1996 and 2001 I have been working on my Ph.D. at NTUA with Stefanos Kollias, studying visual representations for video sequence analysis. I have investigated accurate spatio-temporal segmentation by fusing color, motion and depth, as well as a region-based representation for retrieval and summarization in the form of key-frames. I have designed an affine-invariant representation of object contours for shape-based object classification and retrieval. Finally, I have studied temporal segmentation and parsing of broadcast news based on human face detection.
In 1993-1994 I completed a M.Sc. with Distinction in Communications and Signal Processing at Imperial College, University of London. As part of my Masters thesis, I have worked on communications and investigated the capacity of a cellular CDMA system with Athanassios Manikas.
Between 1998 and 1993 I have studied Electrical and Computer Engineering at NTUA. As part of my Diploma thesis, I have worked on analog electronics and developed a hardware implementation of a fuzzy logic processor with Yannis Tsividis. Before that, my first exposure to computer vision and machine learning has been the study of an invariant representation for optical character recognition with Anastasios Delopoulos, leading to a conference publication before my Diploma.