Henry Medeiros

Distributed Object Tracking Using a Cluster-Based Kalman Filter in Wireless Camera Networks

Local data aggregation is an effective means to save sensor node energy and prolong the lifespan of wireless sensor networks. However, when a sensor network is used to track moving objects, the task of local data aggregation in the network presents a new set of challenges, such as the necessity to estimate, usually in real time, the constantly changing state of the target based on information acquired by the nodes at different time instants. To address these issues, we propose a distributed object tracking system which employs a cluster-based Kalman filter in a network of wireless cameras. When a target is detected, cameras that can observe the same target interact with one another to form a cluster and elect a cluster head. Local measurements of the target acquired by members of the cluster are sent to the cluster head, which then estimates the target position via Kalman filtering and periodically transmits this information to a base station. The underlying clustering protocol allows the current state and uncertainty of the target position to be easily handed off among clusters as the object is being tracked. This allows Kalman filter-based object tracking to be carried out in a distributed manner. An extended Kalman filter is necessary since measurements acquired by the cameras are related to the actual position of the target by nonlinear transformations. In addition, in order to take into consideration the time uncertainty in the measurements acquired by the different cameras, it is necessary to introduce nonlinearity in the system dynamics. Our object tracking protocol requires the transmission of significantly fewer messages than a centralized tracker that naively transmits all of the local measurements to the base station. It is also more accurate than a decentralized tracker that employs linear interpolation for local data aggregation. Besides, the protocol is able to perform real-time estimation because our implementation takes into consideration the sparsity of the matrices involved in the problem. The experimental results show that our distributed object tracking protocol is able to achieve tracking accuracy comparable to the centralized tracking method, while requiring a significantly smaller number of message transmissions in the network.

A Light-Weight Event-Driven Protocol for Sensor Clustering in Wireless Camera Networks

We propose a light-weight event-driven protocol for wireless camera networks to allow for formation and propagation of clusters of cameras for the purpose of collaborative processing during object tracking. Cluster formation is triggered by the detection of objects with specific features. Our protocol allows for simultaneous formation and propagation of multiple clusters. Cameras being directional devices, more than one cluster may track a single object since groups of cameras outside each others communication range may see the same object. Entry into a cluster and cluster membership maintenance require a sensor node to confirm the presence of features of the object being tracked. Each cluster elects its own leader among the cameras that observe the same target. When a cluster leader loses track of an object, it assigns the leadership role to another cluster member. To avoid high communication overhead among cluster members, single-hop clusters are formed, i.e., every member of a cluster is within the communication range of the cluster head. We have implemented a simple version of this protocol on a test-bed and provide an experimental evaluation.

A parallel histogram-based particle filter for object tracking on SIMD-based smart cameras

We present a parallel implementation of a histogram-based particle filter for object tracking on smart cameras based on SIMD processors. We specifically focus on parallel computation of the particle weights and parallel construction of the feature histograms since these are the major bottlenecks in standard implementations of histogram-based particle filters. The proposed algorithm can be applied with any histogram-based feature sets-we show in detail how the parallel particle filter can employ simple color histograms as well as more complex histograms of oriented gradients (HOG). The algorithm was successfully implemented on an SIMD processor and performs robust object tracking at up to 30 frames per second-a performance difficult to achieve even on a modern desktop computer.

A parallel color-based particle filter for object tracking

Porting well known computer vision algorithms to low power, high performance computing devices such as SIMD linear processor arrays can be a challenging task. One especially useful such algorithm is the color-based particle filter, which has been applied successfully by many research groups to the problem of tracking non-rigid objects. In this paper, we propose an implementation of the color-based particle filter suitable for SIMD processors. The main focus of our work is on the parallel computation of the particle weights. This step is the major bottleneck of standard implementations of the color-based particle filter since it requires the knowledge of the histograms of the regions surrounding each hypothesized target position. We expect this approach to perform faster in an SIMD processor than an implementation in a standard desktop computer even running at much lower clock speeds.

Wireless Medical Sensor Networks: Design Requirements and Enabling Technologies

This article analyzes wireless communication protocols that could be used in healthcare environments (e.g., hospitals and small clinics) to transfer real-time medical information obtained from noninvasive sensors. For this purpose the features of the three currently most widely used protocols-namely, Bluetooth(®) (IEEE 802.15.1), ZigBee (IEEE 802.15.4), and Wi-Fi (IEEE 802.11)-are evaluated and compared. The important features under consideration include data bandwidth, frequency band, maximum transmission distance, encryption and authentication methods, power consumption, and current applications. In addition, an overview of network requirements with respect to medical sensor features, patient safety and patient data privacy, quality of service, and interoperability between other sensors is briefly presented. Sensor power consumption is also discussed because it is considered one of the main obstacles for wider adoption of wireless networks in medical applications. The outcome of this assessment will be a useful tool in the hands of biomedical engineering researchers. It will provide parameters to select the most effective combination of protocols to implement a specific wireless network of noninvasive medical sensors to monitor patients remotely in the hospital or at home.

Cluster-Based Distributed Face Tracking in Camera Networks

In this paper, we present a distributed multicamera face tracking system suitable for large wired camera networks. Unlike previous multicamera face tracking systems, our system does not require a central server to coordinate the entire tracking effort. Instead, an efficient camera clustering protocol is used to dynamically form groups of cameras for in-network tracking of individual faces. The clustering protocol includes cluster propagation mechanisms that allow the computational load of face tracking to be transferred to different cameras as the target objects move. Furthermore, the dynamic election of cluster leaders provides robustness against system failures. Our experimental results show that our cluster-based distributed face tracker is capable of accurately tracking multiple faces in real-time. The overall performance of the distributed system is comparable to that of a centralized face tracker, while presenting the advantages of scalability and robustness.

Online distributed calibration of a large network of wireless cameras using dynamic clustering

We present a cluster-based distributed algorithm for calibrating large networks of wireless cameras. Due to the complex nature of sensing modality of a camera sensor, the work presented here differs significantly from the previous localization methods. Our system does not require any beacon nodes; it only utilizes object features of moving objects in the scene extracted from image sequences. The algorithm is fully distributed, and the localization estimates can be improved as more object features are acquired in the network. We show simulations of our system using a graphical simulator we developed specifically for wireless camera sensor networks. Early results indicate that our system is capable of localizing a large network of cameras in an energy-efficient way.

Lightweight Data Compression in Wireless Sensor Networks Using Huffman Coding

This paper presents a lightweight data compression method for wireless sensor networks monitoring environmental parameters with low resolution sensors. Instead of attempting to devise novel ad hoc algorithms, we show that, given general knowledge of the parameters that must be monitored, it is possible to efficiently employ conventional Huffman coding to represent the same parameter when measured at different locations and time periods. When the data collected by the sensor nodes consists of integer measurements, the Huffman dictionary computed using statistics inferred from public datasets often approaches the entropy of the data. Results using temperature and relative humidity measurements show that even when the proposed method does not approach the theoretical limit, it outperforms popular compression mechanisms designed specifically for wireless sensor networks.

Vision-Based Self-contained Target Following Robot Using Bayesian Data Fusion

Several visual following robots have been proposed in recent years. However, many require the use of several, expensive sensors and often the majority of the image processing and other calculations are performed off-board. This paper proposes a simple and cost effective, yet robust visual following robot capable of tracking a general object with limited restrictions on target characteristics. To detect the objects, tracking-learning-detection (TLD) is used within a Bayesian framework to filter and fuse the measurements. A time-of-flight (ToF) depth camera is used to refine the distance estimates at short ranges. The algorithms are executed in real-time (approximately 30 fps) in a Jetson TK1 embedded computer. Experiments were conducted with different target objects to validate the system in scenarios including occlusions and various illumination conditions as well as to show how the data fusion between TLD and the ToF camera improves the distance estimation.

Measuring and modeling apple trees using time-of-flight data for automation of dormant pruning applications

Dormant pruning is one of the most expensive, labor-intensive, but, unavoidable procedure in the field of horticulture to ensure quality crop production. During winter, skilled farmers remove certain branches that are connected directly with the trunk of a tree carefully using a set of predefined rules. In order to reduce this dependence on a large manpower, our goal is to automate this pruning process by building 3D models of dormant apple trees, which eventually would be fed to an intelligent robotic system. In this paper, we present a semicircle fitting based robust 3D reconstruction scheme for modeling the trunk and primary branches of apple trees. The method involves estimating the diameter-error, creating semicircle fit model of the tree from a single depth image, and reconstructing the final 3D model of the tree by aligning a sequence of depth images. Analysis of the qualitative as well as the quantitative evaluations of our algorithm on five different dormant apple trees from our dataset under various indoor and outdoor environments demonstrate the effectiveness of the proposed framework for automatic 3D reconstruction. The results show that on an average, the proposed schemes provide a performance of 89.4% for correctly estimating the diameters of the primary branches with a tolerance of 5 mm and 100%c for correctly identifying the branches.