Georgios Petkos

Sensing Trending Topics in Twitter

Online social and news media generate rich and timely information about real-world events of all kinds. However, the huge amount of data available, along with the breadth of the user base, requires a substantial effort of information filtering to successfully drill down to relevant topics and events. Trending topic detection is therefore a fundamental building block to monitor and summarize information originating from social sources. There are a wide variety of methods and variables and they greatly affect the quality of results. We compare six topic detection methods on three Twitter datasets related to major events, which differ in their time scale and topic churn rate. We observe how the nature of the event considered, the volume of activity over time, the sampling procedure and the pre-processing of the data all greatly affect the quality of detected topics, which also depends on the type of detection method used. We find that standard natural language processing techniques can perform well for social streams on very focused topics, but novel techniques designed to mine the temporal distribution of concepts are needed to handle more heterogeneous streams containing multiple stories evolving in parallel. One of the novel topic detection methods we propose, based on -grams cooccurrence and topic ranking, consistently achieves the best performance across all these conditions, thus being more reliable than other state-of-the-art techniques.

Social event detection using multimodal clustering and integrating supervisory signals

A large variety of features can be extracted from raw multimedia items. Moreover, in many contexts, like in the case of multimedia uploaded by users of social media platforms, items may be linked to metadata that can be very useful for a variety of analysis tasks. Nevertheless, such features are typically heterogeneous and are difficult to combine in a unified representation that would be suitable for analysis. In this paper, we discuss the problem of clustering collections of multimedia items with the purpose of detecting social events. In order to achieve this, a novel multimodal clustering algorithm is proposed. The proposed method uses a known clustering in the currently examined domain, in order to supervise the multimodal fusion and clustering procedure. It is tested on the MediaEval social event detection challenge data and is compared to a multimodal spectral clustering approach that uses early fusion. By taking advantage of the explicit supervisory signal, it achieves superior clustering accuracy and additionally requires the specification of a much smaller number of parameters. Moreover, the proposed approach has wider scope; it is not only applicable to the task of social event detection, but to other multimodal clustering problems as well.

Multimodal Graph-based Event Detection and Summarization in Social Media Streams

The paper describes a multimodal graph-based system for addressing the Yahoo-Flickr Event Summarization Challenge of ACM Multimedia 2015. The objective is to automatically uncover structure within a collection of 100 million photos/videos in the form of detecting and identifying events, and summarizing them succinctly for consumer consumption. The presented system uses a sliding window over the stream of multimedia items to build and maintain a multimodal same-event image graph and applies a graph clustering algorithm to detect events. In addition, it makes use of a graph-based diversity-oriented ranking approach and a versatile event retrieval mechanism to access summarized instances of the events of interest. A demo of the system is online at http://mklab.iti.gr/acmmm2015-gc/.

Graph-based multimodal clustering for social multimedia

Real world datasets often consist of data expressed through multiple modalities. Clustering such datasets is in most cases a challenging task as the involved modalities are often heterogeneous. In this paper we propose a graph-based multimodal clustering approach. The proposed approach utilizes an example relevant clustering in order to learn a model of the “same cluster” relationship between a pair of items. This model is subsequently used in order to organize the items of the collection to be clustered in a graph, where the nodes represent the items and a link between a pair of nodes exists if the model predicted that the corresponding pair of items belong to the same cluster. Eventually, a graph clustering algorithm is applied on the graph in order to produce the final clustering. The proposed approach is applied on two problems that are typically treated using clustering techniques; in particular, it is applied on the problem of detecting social events and to the problem of discovering different landmark views in collections of social multimedia.

Multimodal Event Detection and Summarization in Large Scale Image Collections

This paper describes a multimodal graph-based approach to address the problem of event detection and summarization in large scale image collections. A first version of our system was presented in the Yahoo-Flickr Event Summarization Challenge of ACM Multimedia 2015 [6]. The objective of the approach is to automatically detect events within millions of photos and summarizing them efficiently for user consumption. The presented approach uses a moving time window over the collection of multimedia items to build a same-event image graph and applies graph clustering to detect events. In addition, it makes use of a graph-based diversity-oriented ranking algorithm to summarize instances of the detected events. A demo of the system is online at: http://mklab.iti.gr/acmmm2015-gc/.

Perceived Versus Actual Predictability of Personal Information in Social Networks

This paper looks at the problem of privacy in the context of Online Social Networks (OSNs). In particular, it examines the predictability of different types of personal information based on OSN data and compares it to the perceptions of users about the disclosure of their information. To this end, a real life dataset is composed. This consists of the Facebook data (images, posts and likes) of 170 people along with their replies to a survey that addresses both their personal information, as well as their perceptions about the sensitivity and the predictability of different types of information. Importantly, we evaluate several learning techniques for the prediction of user attributes based on their OSN data. Our analysis shows that the perceptions of users with respect to the disclosure of specific types of information are often incorrect. For instance, it appears that the predictability of their political beliefs and employment status is higher than they tend to believe. Interestingly, it also appears that information that is characterized by users as more sensitive, is actually more easily predictable than users think, and vice versa (i.e. information that is characterized as relatively less sensitive is less easily predictable than users might have thought).