Abstract:
Systems and methods for receiving video images from a plurality of video cameras having respective fields of view that cover a geographical region. At a first time, first video images of an object may be output, where the images are captured by a first video camera selected from the plurality of video cameras. Location indications may be received, which specify a geographical location of the object in the geographical region and which are determined independently of the video images. At a second time subsequent to the first time, a second video camera from the plurality may be selected based on the location indications. The output may be switched to the second video images of the object, which are captured by the selected second video camera.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to video surveillance, and particularly to methods and systems for performing hand-off between video cameras. 
     BACKGROUND OF THE DISCLOSURE 
     Video surveillance systems are deployed and operated in various applications, such as airport security, crime prevention and access control. In a typical video surveillance application, multiple video cameras acquire video footage, which is viewed and/or recorded at a monitoring center. 
     Mobile communication networks deploy various techniques for measuring the geographical locations of wireless communication terminals. Such techniques are used, for example, for providing Location Based Services (LBS) and emergency services in cellular networks. Some location tracking techniques are based on passive probing of network events generated by the wireless terminals. Other techniques are active, i.e., proactively request the network or the terminal to provide location information. 
     SUMMARY OF THE DISCLOSURE 
     An embodiment that is described herein provides a method, including: 
     receiving video images from a plurality of video cameras having respective fields of view that cover a geographical region; 
     at a first time, outputting first video images of an object, which are captured by a first video camera selected from the plurality; 
     receiving location indications, which specify a geographical location of the object in the geographical region and which are determined independently of the video images; 
     at a second time subsequent to the first time, selecting based on the location indications a second video camera from the plurality, different from the first video camera; and 
     switching to output second video images of the object, which are captured by the selected second video camera. 
     In some embodiments, the object includes an individual moving through the geographical region. Additionally or alternatively, the object is associated with a wireless communication terminal that communicates with a communication network, and receiving the location indications includes receiving location measurements of the wireless communication terminal from the communication network. In some embodiments, selection of the second video camera is performed by a switch in the communication network. In a disclosed embodiment, outputting the first and second video images includes displaying the first and second video images to an operator. 
     In an embodiment, selecting the second video camera includes querying a predefined mapping of geographical locations to image locations in the fields of views of the video cameras. In another embodiment, selecting the second video camera includes calculating, based on the location indications, respective first and second image locations of the object in first and second fields of view of the first and second video cameras, and selecting the second camera based on the first and second image locations. In yet another embodiment, selecting the second video camera further includes controlling the second video camera so as to modify a respective field of view of the second video camera responsively to the location indications. 
     There is additionally provided, in accordance with an embodiment that is described herein, a system, including: 
     a first interface, which is configured to receive video images from a plurality of video cameras having respective fields of view that cover a geographical region; 
     a second interface, which is configured to receive location indications, which specify a geographical location of an object in the geographical region and which are determined independently of the video images; and 
     a processor, which is configured to output, at a first time, first video images of the object, which are captured by a first video camera selected from the plurality, to select from the plurality based on the location indications, at a second time subsequent to the first time, a second video camera different from the first video camera, and to switch to output second video images of the object, which are captured by the selected second video camera. 
     The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial, schematic illustration of a video surveillance system, in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a block diagram that schematically illustrates a video surveillance system, in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a diagram that schematically illustrates an operator display in a video surveillance system, in accordance with an embodiment of the present disclosure; and 
         FIG. 4  is a flow chart that schematically illustrates a surveillance method, in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Video surveillance systems typically collect video images from multiple video cameras and present the images to an operator. The operator may observe objects (e.g., individuals) of interest in the images and take appropriate actions. In a typical video surveillance system, each camera has a limited field-of-view, and the multiple cameras collectively cover a certain region of interest. 
     In many practical scenarios, an individual of interest is in motion, and in particular moves in and out of the fields-of-view of different cameras. The operator, on the other hand, is concerned with tracking the individual in question as seamlessly as possible. Tracking an individual in motion therefore requires hand-off between cameras, i.e., modifying the selection of the camera whose video is presented to the operator in an attempt to track the individual of interest without interruption. 
     Embodiments that are described hereinbelow provide improved methods and systems for automatic hand-off between video cameras. In some embodiments, a correlation processor receives location indications, which indicate the geographical location of the individual of interest. The correlation processor decides when to perform camera hand-off, and to which camera, based on the estimated location of the individual as reflected by the location indications. 
     The location indications used for triggering camera hand-off are determined independently of the video images, i.e., do not rely on image processing in any way. For example, the individual may carry a cellular phone or other wireless terminal, which communicates with a cellular network that also measures the terminal&#39;s location. Location measurements of the terminal, provided by the cellular network, may serve as location indications of the terminal&#39;s user for the purpose of camera hand-off. 
     Since the disclosed techniques use location indications that are independent of the video images, they are able to perform camera hand-off reliably under difficult visual conditions, e.g., when the camera&#39;s field-of-view is obstructed, when video quality is poor or when the fields-of-view of neighboring cameras do not overlap. 
     The embodiments described herein mainly address video surveillance of individuals that carry wireless communication terminals, and performing camera hand-off based on location measurements of the terminals. Nevertheless, the methods and systems described herein can also be used with various other types of tracked objects and location indications, such as in tracking objects that are fitted with Radio Frequency Identification (RFID) tags, or Automatic Vehicle Location (AVL) or Automatic Person Location (APL) transponders. 
     System Description 
       FIG. 1  is a pictorial, schematic illustration of a video surveillance system  20 , in accordance with an embodiment that is described herein. System  20  tracks individuals of interest within a certain geographical region  22 , which may comprise a city, city center, neighborhood, airport terminal or any other suitable area. Systems of this sort may be operated, for example, by law enforcement or other Government agencies, such as in airport security systems, crime prevention systems or anti-terrorism systems. 
     System  20  tracks individuals  24  using a video subsystem  36 , which comprises multiple video cameras. In the present example, subsystem  36  comprises five video cameras  32 A . . .  32 E, although any other number of cameras may be used. Each camera has a certain field-of-view, which covers a particular sector in area  22 . The cameras capture video images of their respective sectors and send the images to subsystem  36 . The cameras may have fixed fields-of-view, or they may comprise cameras whose fields-of-view are adjustable such as Pan-Tilt-Zoom (PTZ) cameras. 
     At least some of individuals  24  communicate with a mobile communication network  40  by operating wireless communication terminals  28 . (Individuals  24  are therefore sometimes referred to herein as users. The two terms are used interchangeably.) Terminals  28  may comprise, for example, cellular phones, wireless-enabled computers or Personal Digital Assistants (PDAs), or any other suitable communication or computing device having wireless communication capabilities. Communication network  40  and terminals  28  may conform to any suitable communication standard or protocol, such as Long Term Evolution (LTE), Universal Mobile Telecommunication System (UMTS), CDMA2000 or other third generation (3G) cellular standard, Global System for Mobile communication (GSM) or Integrated Digital Enhanced Network (IDEN). Alternatively, the network and terminals may conform to the IEEE 802.16 (WiMAX) standards or other wireless data standard. Although  FIG. 1  shows only a single user for the sake of clarity, practical networks typically communicate with a large number of users and terminals. Although the description that follows refers to a single network, system  20  may operate with any desired number of communication networks, which may conform to different standards or protocols. 
     System  20  comprises a location tracking subsystem  44 , which measures the geographical locations of wireless communication terminals  28  in area  22 . The example of  FIG. 1  refers to a single location tracking subsystem. 
     Alternatively, the system may comprise two or more location tracking subsystems, which may be of different types. Location tracking subsystem  44  may apply any suitable location tracking technique available in the network, or a combination of such techniques, in order to measure terminal locations. 
     Some location tracking techniques, referred to as network-based techniques, are carried out by base stations and other network-side components of the network, without necessarily using special hardware or software in terminals  28 . Other location tracking techniques are terminal-based, i.e., use special hardware or software in wireless terminals  28 . Terminal-based techniques may comprise, for example, techniques that use Global Navigation Satellite Systems (GNSS) such as GPS or GALILEO. The location tracking techniques may be passive or active. Passive techniques perform unobtrusive probing of the signaling information transmitted in the network, and extract location information from the monitored signaling. Active techniques, on the other hand, proactively request the network or the terminal to provide location information. 
     Some examples of location tracking techniques that can be used for this purpose are described in U.S. patent application Ser. No. 12/497,799, filed Jul. 6, 2009, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. Location tracking subsystem  44  thus measures the geographical locations of at least some of terminals  28 , and produces location indications that indicate the measured terminal locations. 
     System  20  comprises a correlation system  48 , which interacts with location tracking subsystem  44  and video subsystem  36 . For a given individual  24 , correlation system  48  selects the appropriate video camera for tracking this individual based on the location indications of terminal  28  operated by this individual, as provided by subsystem  44 . In particular, the correlation system uses the location indications to decide when to perform camera hand-off (i.e., switch to a different camera), and to which camera. In a typical implementation, correlation system  48  notifies video subsystem  36  of the estimated geographical location of the individual, and the video subsystem selects the camera that is best suited for viewing this location. Alternatively, the camera selection may be carried out by the correlation system itself. 
     The video from the selected video camera is provided from video subsystem  36  via correlation system  48  to a monitoring center  52 . The video is typically displayed to an operator  56  using an output device such as a display  60 . In the example of  FIG. 1 , operator  56  is presented with real-time video images showing user  24  and his or her vicinity. Operator  56  may control the display or provide other input using an input device  64 , such as a keyboard or mouse. Additionally or alternatively to presenting the video to operator  56 , correlation system  48  or monitoring center  52  may store the video for later retrieval and analysis. 
       FIG. 2  is a block diagram that schematically illustrates components of system  20 , in accordance with an embodiment that is described herein. Video subsystem  36  comprises a networked video server  70 , which manages the operation of cameras  32 , receives the video images captured by the cameras and sends the video to correlation system  48 . Video server  70  stores captured video in a video records database  74  for off-line viewing and analysis. Subsystem  36  also comprises an image-to-location mapping database  78 . Database  78  stores a predefined mapping of image coordinates to geographical coordinates for each camera  36 . By querying this database with a certain geographical location, server  70  can determine which of cameras  32  has a field-of-view that covers this geographical location. 
     Correlation system  48  comprises interfaces  82  and  86  for communicating with location tracking subsystem  44  and video subsystem  36 , respectively. System  48  further comprises a correlation processor  90 , which carries out the correlation functions described herein. 
     Monitoring center  52  comprises a Location-Based Monitoring (LBM) server  94 , which accepts the video from the selected camera (or cameras) from correlation system and presents it to operator  56  using an operator terminal  98 . In the example of  FIG. 2 , LBM server  94  and correlation system  48  interact directly. In alternative embodiments, however, system  48  and server  94  may interact via a database that stores the selected video (e.g., database  74  or a separate database). In some embodiments, server  94  interacts with a Geographic Information System (GIS)  102 , which provides map information and other geographic data for presentation purposes. The GIS may hold any suitable kind of geographic information, such as Points of Interest (POIs), clutter data and blueprints of area  22 . The geographic information is stored in a map database  106 . 
     The configurations of  FIGS. 1 and 2  are example configurations, which were selected purely for the sake of conceptual clarity. In alternative embodiments, any other suitable system configuration can also be used. For example, correlation system  48  and monitoring center  56  may be collocated, and the functions of processor  90  can be integrated into server  94 . In some embodiments, some functions of correlation system  52  may be implemented as part of a switch, such as a Mobile Switching Center (MSC), of communication network  40 . 
     The different databases in system  20  (e.g., databases  74 ,  78  and  106 ) may be implemented using any suitable data structures and storage devices. Typically, processor  90  and server  94  comprise general-purpose computers, which are programmed in software to carry out the functions described herein. The software may be downloaded to the computers in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on tangible media, such as magnetic, optical, or electronic memory. 
     Automatic Camera Hand-Off Process 
       FIG. 1  above shows an example scenario, in which user  24  moves through area  22  between three locations denoted X 0 , X 1  and X 2 . When the user is at location X 0  he is best viewed by camera  32 A, when the user is at location X 1  he is best viewed by camera  32 D, and when the user is at location X 2  he is best viewed by camera  32 E. In this example, video surveillance of this user should begin with camera  32 A, then hand-off to camera  32 D, and finally hand-off to camera  32 E. Note that some locations along the user&#39;s path are obstructed from view by buildings and cannot be viewed by any of the cameras. 
     As user  24  moves through area  22 , location tracking subsystem  44  measures the geographical location of terminal  28  and sends location indications to correlation system  48 . Note that the user need not necessarily operate the terminal, e.g., conduct calls. It is usually sufficient that the terminal is switched on. Correlation system  48  sends the estimated geographical location of the user to video subsystem  36 . Based on the user&#39;s estimated location, and on the known fields-of-view of the different cameras, server  70  decides which camera is best suited for viewing the user. 
     Typically, server  70  queries database  78  with the user&#39;s estimated geographical location, so as to determine one or more cameras whose field-of-view currently covers the user. If the user appears in the fields-of-view of multiple cameras, server  70  may apply any suitable criteria or logic to select one of the cameras. For example, server  70  may select the camera in which the user appears closest to the center of the field-of-view, or the camera that is geographically closest to the user. The video from the selected camera is sent to monitoring center  52  and presented to operator  56 . 
     The above camera selection process is typically carried out continually, in real-time. In other words, as user  24  moves through area  22 , location subsystem  44  sends up-to-date location indications to correlation system  48 , and the correlation system sends up-to-date location estimates to video subsystem  36 . The video subsystem updates the camera selection (i.e., performs camera hand-off) whenever needed to match the changing location estimates, and sends the video captured by the currently-selected camera. 
       FIG. 3  is a diagram that schematically illustrates the video displayed to operator  56  in the above example scenario, in accordance with an embodiment that is described herein. The left hand side of the figure shows the cameras selected by server  70  as user  24  moves from location X 0  to location X 1  and then to location X 2 . When the user is in the vicinity of X 0  around a time denoted to, server  70  selects camera  32 A (presented as “CAMERA A” to the operator). When the user moves away from X 0  and approaches X 1  around a time denoted t 1 , server  70  hands-off to camera  32 D (“CAMERA B”). When the user approaches X 2  around a time denoted t 2 , server  70  hands-off to camera  32 E (“CAMERA C”). 
     The video displayed to operator  56  in this scenario is shown on the right hand side of  FIG. 3 . Displays  110 A . . .  110 C show the video that is presented to the operator around times t 0  . . . t 2 , respectively. Around time t 0 , user  24  is located in the vicinity of location X 0 , and therefore system  20  displays the video captured by camera A ( 32 A) at the center of the screen. This view is shown in display  110 A. In the present example, the system also displays a smaller window, showing the video of camera B ( 32 D) to which hand-off is anticipated. 
     Between t 0  and t 1 , the user moves away from X 0  and approaches X 1 . At some point in time between t 0  and t 1 , server  70  decides to hand-off from camera A ( 32 A) to camera B ( 32 D). As a result, around time t 1  the system displays the video of camera B ( 32 D) at the center of the screen, as shown in display  110 B. In this example, the display also shows smaller windows with the video of camera A ( 32 A) from which the previous hand-off was performed, and of camera C ( 32 E) to which the next hand-off is anticipated. 
     Between t 1  and t 2 , the user moves away from X 1  toward X 2 . At a certain time between t 1  and t 2 , server  70  decides to hand-off from camera B ( 32 D) to camera C ( 32 E). Therefore, around time t 2  system  20  displays the video of camera C ( 32 E) at the center of the screen, as shown in display  110 C. Display  110 C also shows a smaller window with the video of camera B ( 32 D) from which the previous hand-off was performed. 
     As can be appreciated, the above camera hand-off process is performed by system  20  automatically without operator intervention. At any point in time, system  20  selects the video camera that best covers the current geographical location of the user, based on the location measurements performed on the user&#39;s communication terminal. As a result, the operator is automatically presented with continuous video footage of the user&#39;s location, even though the user moves in and out of the fields-of-view of different video cameras. 
     The user interface shown in  FIG. 3  is an example user interface, which is shown purely for the sake of conceptual clarity. In alternative embodiments, any other suitable user interface can also be used to present the video from the selected camera or cameras to the operator. 
     Surveillance Method Description 
       FIG. 4  is a flow chart that schematically illustrates a surveillance method, in accordance with an embodiment that is described herein. The method describes the process of tracking a given user of interest, referred to as a target user, as he or she moves through area  22 . The method begins with LBM server  94  displaying to operator  56  video from a certain video camera, at a video output step  120 . The camera has been selected by server  70  based on location indications of the target user&#39;s communication terminal (e.g., cellular phone) provided by location tracking subsystem  44 . 
     Correlation system  48  receives up-to-date location indications from subsystem  44 , at a location updating step  124 . In response to the location indications, system sends up-to-date estimates of the target user&#39;s geographical location to video server  70 . 
     Based on the estimated user location, server  70  evaluates whether it is necessary to perform camera hand-off (i.e., switch to displaying the video of a different camera), at a switching evaluation step  128 . Server  70  may apply various conditions or criteria for deciding whether to perform camera hand-off, and to which camera. In some embodiments, the criterion is defined with respect to the user&#39;s image location within the field-of-view of the currently-selected camera, and/or the user&#39;s image location within the fields-of-view of neighboring cameras. Note, however, that server  70  determines these image locations using the location indications and using database  78 , i.e., irrespective of the video images themselves. Server  70  typically does not rely on image processing to determine the user&#39;s image position in the cameras&#39; fields-of-view. 
     For example, server  70  may check whether the user&#39;s estimated location corresponds to the edge of the currently-selected camera&#39;s field-of-view. If the user&#39;s location is in the middle of the current camera&#39;s field-of-view, hand-off may not be necessary. Otherwise, server  70  may attempt to find a camera that better covers the user&#39;s location. In some embodiments, the decision to switch to a different camera is based only on the estimated geographical location of the user, regardless of the user&#39;s position at the camera&#39;s field-of-view. Additionally or alternatively, server  70  may apply any other suitable criterion to decide whether, and to which camera, to perform hand-off. 
     Server  70  checks whether hand-off is needed, at a checking step  132 . If no hand-off is needed, the method loops back to step  120  above, and system  20  continues to display video from the currently-selected camera. If hand-off is needed, server  70  performs camera hand-off, at a hand-off step  136 . Server  70  selects a camera whose field-of-view better covers the current user location, as explained above. The method loops back to step  120  above, and system  20  begins to display video from the newly-selected camera. 
     The embodiments described herein mainly refer to video surveillance of individuals based on location measurements performed on the mobile terminals they carry. Alternatively, however, the methods and systems described herein can also be used to perform video surveillance (and in particular camera hand-off) on various other types of objects. Tracking such objects may be performed using various other kinds of location indications. For example, the methods and systems described herein can be used with object fitted with Radio Frequency Identification (RFID) tags, or Automatic Vehicle Location (AVL) or Automatic Person Location (APL) transponders. In these embodiments, location measurements of an RFID tag or transponder can be used as location indications of the object. 
     The embodiments described herein refer mainly to automatic selection of camera, i.e., camera hand-off. Additionally, when a given camera has an adjustable field-of-view (e.g., a PTZ camera), server  70  may also adjust the selected camera&#39;s field-of-view based on the location indications, in order to best view the user. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present disclosure is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present disclosure includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.