Local features for view matching across independently moving cameras.

PhD ThesisMoving platforms, such as wearable and robotic cameras, need to recognise the same place observed from different viewpoints in order to collaboratively reconstruct a 3D scene and to support augmented reality or autonomous navigation. However, matching views is challenging for independently moving cameras that directly interact with each other due to severe geometric and photometric differences, such as viewpoint, scale, and illumination changes, can considerably decrease the matching performance. This thesis proposes novel, compact, local features that can cope with with scale and viewpoint variations. We extract and describe an image patch at different scales of an image pyramid by comparing intensity values between learnt pixel pairs (binary test), and employ a cross-scale distance when matching these features. We capture, at multiple scales, the temporal changes of a 3D point, as observed in the image sequence of a camera, by tracking local binary descriptors. After validating the feature-point trajectories through 3D reconstruction, we reduce, for each scale, the sequence of binary features to a compact, fixed-length descriptor that identifies the most frequent and the most stable binary tests over time. We then propose XC-PR, a cross-camera place recognition approach that stores locally, for each uncalibrated camera, spatio-temporal descriptors, extracted at a single scale, in a tree that is selectively updated, as the camera moves. Cameras exchange descriptors selected from previous frames within an adaptive temporal window and with the highest number of local features corresponding to the descriptors. The other camera locally searches and matches the received descriptors to identify and geometrically validate a previously seen place. Experiments on different scenarios show the improved matching accuracy of the joint multi-scale extraction and temporal reduction through comparisons of different temporal reduction strategies, as well as the cross-camera matching strategy based on Bag of Binary Words, and the application to several binary descriptors. We also show that XC-PR achieves similar accuracy but faster, on average, than a baseline consisting of an incremental list of spatio-temporal descriptors. Moreover, XC-PR achieves similar accuracy of a frame-based Bag of Binary Words approach adapted to our approach, while avoiding to match features that cannot be informative, e.g. for 3D reconstruction

DART: Distribution Aware Retinal Transform for Event-based Cameras

We introduce a generic visual descriptor, termed as distribution aware retinal transform (DART), that encodes the structural context using log-polar grids for event cameras. The DART descriptor is applied to four different problems, namely object classification, tracking, detection and feature matching: (1) The DART features are directly employed as local descriptors in a bag-of-features classification framework and testing is carried out on four standard event-based object datasets (N-MNIST, MNIST-DVS, CIFAR10-DVS, NCaltech-101). (2) Extending the classification system, tracking is demonstrated using two key novelties: (i) For overcoming the low-sample problem for the one-shot learning of a binary classifier, statistical bootstrapping is leveraged with online learning; (ii) To achieve tracker robustness, the scale and rotation equivariance property of the DART descriptors is exploited for the one-shot learning. (3) To solve the long-term object tracking problem, an object detector is designed using the principle of cluster majority voting. The detection scheme is then combined with the tracker to result in a high intersection-over-union score with augmented ground truth annotations on the publicly available event camera dataset. (4) Finally, the event context encoded by DART greatly simplifies the feature correspondence problem, especially for spatio-temporal slices far apart in time, which has not been explicitly tackled in the event-based vision domain.Comment: 12 pages, revision submitted to TPAMI in Nov 201

Action Recognition in Videos: from Motion Capture Labs to the Web

This paper presents a survey of human action recognition approaches based on visual data recorded from a single video camera. We propose an organizing framework which puts in evidence the evolution of the area, with techniques moving from heavily constrained motion capture scenarios towards more challenging, realistic, "in the wild" videos. The proposed organization is based on the representation used as input for the recognition task, emphasizing the hypothesis assumed and thus, the constraints imposed on the type of video that each technique is able to address. Expliciting the hypothesis and constraints makes the framework particularly useful to select a method, given an application. Another advantage of the proposed organization is that it allows categorizing newest approaches seamlessly with traditional ones, while providing an insightful perspective of the evolution of the action recognition task up to now. That perspective is the basis for the discussion in the end of the paper, where we also present the main open issues in the area.Comment: Preprint submitted to CVIU, survey paper, 46 pages, 2 figures, 4 table

Early Recognition of Human Activities from First-Person Videos Using Onset Representations

In this paper, we propose a methodology for early recognition of human activities from videos taken with a first-person viewpoint. Early recognition, which is also known as activity prediction, is an ability to infer an ongoing activity at its early stage. We present an algorithm to perform recognition of activities targeted at the camera from streaming videos, making the system to predict intended activities of the interacting person and avoid harmful events before they actually happen. We introduce the novel concept of 'onset' that efficiently summarizes pre-activity observations, and design an approach to consider event history in addition to ongoing video observation for early first-person recognition of activities. We propose to represent onset using cascade histograms of time series gradients, and we describe a novel algorithmic setup to take advantage of onset for early recognition of activities. The experimental results clearly illustrate that the proposed concept of onset enables better/earlier recognition of human activities from first-person videos