Patent Publication Number: US-10313627-B2

Title: Apparatus for playing back recorded video images related to event, and method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of U.S. application Ser. No. 12/763,897, filed Apr. 20, 2010, which claims priority from Japanese Patent Application No. 2009-105560 filed Apr. 23, 2009, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an apparatus for playing back recorded video images related to an event and a method thereof. 
     Description of the Related Art 
     Installation of a digital monitoring system including a plurality of network cameras, a recording apparatus to record camera video images, and a viewer to play back live video images or recorded video images is proceeding at a rapid rate. Japanese Patent Application National Publication (Laid-Open) No. 2007-505523 discusses a method for enabling a corresponding event on a time line when a camera is specified to clarify the correspondence between a specific camera and an event. Also, a method for playing back recorded video images captured by the corresponding camera from the corresponding time when an event is specified is discussed. U.S. Pat. No. 6,144,375 discusses a method for configuring a three-dimensional model of scenes from video images taken from a plurality of cameras, establishing correspondences between events and video images, and displaying events for each object. 
     An event in which a person carrying an object appears during capturing of moving images and the person then goes away while leaving behind the object can be detected as a desertion detection event. Here, if a desertion detection event occurs, it is useful if the person who has brought in the deserted object can be checked. However, according to a conventional technique, in order to play back video images of an object related to an event, it is usual to sequentially specify related events after the relationship between the object and events is considered by the user. Also, according to a conventional technique, a window dedicated to the object is separately displayed to know correspondence relations between the object and events. Thus, it is difficult to intuitively understand the correspondence relations between the object and events. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an apparatus includes a recording unit configured to record a video image, an index generation unit configured to generate, when a first type of event in which a state of an object changes is detected from the video image, an index associating the first type and a second type of event related to the first type, and a playback unit configured to play back, when the first type in the video image is specified, a video image concerning the second type corresponding to the specified first type based on the index. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a diagram illustrating an example of a system configuration of a network camera system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a diagram illustrating an example of a hardware configuration of a playback apparatus. 
         FIG. 3  is a diagram illustrating an example of a display screen. 
         FIG. 4  is a diagram illustrating an example of event information. 
         FIG. 5  is a diagram illustrating a presence time of objects and an event when desertion detection occurs. 
         FIGS. 6A and 6B  are diagrams illustrating an example of generation of an index by extracting video images before and after events related to the desertion detection. 
         FIG. 7  is a diagram illustrating an example of index information. 
         FIG. 8  is a flow chart illustrating an example of a processing procedure for one process by the playback apparatus. 
         FIG. 9  is a diagram illustrating a presence time of objects and events when taking away detection occurs. 
         FIGS. 10A and 10B  are diagrams illustrating an example of generation of an index by extracting video images before and after events related to the taking away detection. 
         FIG. 11  is a flow chart illustrating an example of a processing procedure of one process by the playback apparatus. 
         FIG. 12  is a diagram illustrating an example of a correspondence relation between an object region display and an event display on a time line. 
         FIG. 13  is a diagram illustrating another example emphasizing events concerning the same object on the time line. 
         FIG. 14  is a diagram illustrating an example of a screen of a recording application when objects and events among a plurality of cameras are displayed by establishing correspondences therebetween. 
         FIG. 15  is a flow chart illustrating an example of a processing procedure of one process by the playback apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     In a first exemplary embodiment of the present invention, when an event concerning an object occurs, a playback apparatus for network cameras generates an index describing playback timing of events concerning an object related to the object. When, for example, a desertion detection event occurs, the playback apparatus generates an index describing the desertion detection event, an approximate time at which a person who has brought in a deserted object appears, and an approximate start time of desertion. Next, when a user specifies a symbol indicating a desertion detection event on a time line where symbols indicating events are displayed at positions of their occurrence times, the playback apparatus continuously plays back recorded video images based on an index including the desertion detection event. As a result, there is no need for the user of a network camera system (or a playback apparatus) in the present exemplary embodiment to search for related events and manually play back such related events, so that the user can immediately check important video images. 
     A system configuration of a typical network camera system in the present exemplary embodiment is illustrated in  FIG. 1 . In the network camera system, as illustrated in  FIG. 1 , a plurality of network cameras  101 ,  102 , and  103  and a playback apparatus  200  including a recording apparatus  2100  are connected via a network. The network camera  101  has a function to transmit video images via the network, a motion detection function of video images, and a detection function of an object region in a video image. 
     The playback apparatus  200  has a function to acquire a video image from a network camera, a function to save the acquired video image in the recording apparatus  2100 , a function to playback the received/saved video images, a motion detection function of video images, and a detection function of an object region in a video image. 
     These functions of the playback apparatus  200  are realized by programs stored in a read-only memory (ROM)  207  or a hard disk drive (HDD)  209  being executed by a central processing unit (CPU)  206 . 
       FIG. 2  is a diagram illustrating an example of the hardware configuration of the playback apparatus  200 . 
     A display apparatus  201  displays, for example, windows, icons, messages, menus and other user interface information. A video random access memory (VRAM)  202  stores video images to be displayed on the display apparatus  201 . The display apparatus  201  may be hardware constituting the playback apparatus  200  or an apparatus (hardware) that is connected to the playback apparatus  200  and different from the playback apparatus  200 . In the example in  FIG. 2 , the display apparatus  201  is different from hardware constituting the playback apparatus  200 . 
     Image data stored in the VRAM  202  is transferred to the display apparatus  201  according to predetermined rules and video images are thereby displayed on the display apparatus  201 . A compact disk drive (CDD)  203  is an apparatus that reads/writes various control programs and data from/to recording media such as CD-ROM and CD-R. Instead of the CDD, a digital versatile disc (DVD) drive may be used. 
     A keyboard  204  has various keys to input characters. A pointing device (PD)  205  is used to point to icons, menus, or other objects displayed, for example, on a display screen of the display apparatus  201 . The CPU  206  controls each device based on a control program stored in the ROM  207 , the hard disk drive (HDD)  209 , a flexible disk drive (FDD)  210 , or an external storage apparatus (CDD)  203 . As already described above, the functions of the playback apparatus  200  are realized by programs being executed by the CPU  206  of the playback apparatus  200 . 
     The read-only memory (ROM)  207  stores various control programs and data. A random access memory (RAM)  208  has a work area for the CPU  206 , a data save area during abnormality handling, a load area of control programs and the like. 
     The HDD  209  stores various control programs and various types of data. A network interface (Net-I/F)  211  performs communication with other information processing apparatuses, printers and the like via a network  213 . A CPU bus  212  includes an address bus, a data bus, and a control bus. A control program can be provided to the CPU  206  from the ROM  207 , the HDD  209 , the FDD  210 , or the CDD  203  or from other information processing apparatuses via the network  213 . 
     The function of the recording apparatus  2100  can be realized by external storage apparatuses such as a hard disk drive being controlled by the CPU  206  inside the playback apparatus  200 . The playback apparatus  200  performs communication with the network camera  101  via a network control unit to acquire video images, which are saved in the recording apparatus  2100 . At this point, communication between the network camera  101  and the playback apparatus  200  is performed based on, for example, HyperText Transfer Protocol (HTTP), but the present exemplary embodiment is not limited to a specific protocol. 
     Here, the motion detection function of video images of the playback apparatus  200  calculates a quantity of motion in video images based on, for example, a difference between adjacent frames and, if the quantity of motion in video images exceeds a certain reference quantity, causes a motion detection event. The detection function of an object region in a video image of the playback apparatus  200  detects a region of a certain preset size or more obtained by performing region connection processing on, for example, a background difference image as an object region. In this case, a technique of improving detection accuracy by using knowledge about objects obtained from object region detection results in the past is generally used. The motion detection function of video images and the detection function of an object region may be provided on the network camera  101  side or the playback apparatus  200  side. In the present exemplary embodiment, techniques of motion detection and object region detection are not to be limited. 
     Next, an example of the display screen of the playback apparatus  200  in the present exemplary embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a diagram illustrating an example of the display screen.  FIG. 3  illustrates a window in the screen and the window includes a window frame  300 , image display regions  310 ,  320 , and  330 , and a time line  340 . Frames  311  and  312  indicating an object region are displayed in the image display region  310 . In addition to the illustrated frame as a display example of the object region, representations of any shape such as a block and ellipse can be used. The time line  340  indicates recording states and occurrence times of events. More specifically, graphic symbols indicating events are arranged along the time axis in the time line  340 . 
     The right end of the time line  340  indicates the current time and the playback apparatus  200  displays display events so that, with the passage of time, display events shift from right to left. If the user specifies the current time or a past time, recorded video images from the currently enabled camera are played back from the specified time. Events include the activation/stop of a system, the start/stop of recording, changes of the input state of an external sensor, changes of the motion detection state, the appearance/exit of an object, desertion, and taking away. In  FIG. 3 , while an event  341  is represented as a rectangular form, other forms than the rectangular form can also be represented. 
     Next, an example of event information in the present exemplary embodiment will be described with reference to  FIG. 4 .  FIG. 4  is a diagram illustrating an example of event information. The playback apparatus  200  manages events by assigning IDs in order of occurrence time. Management content includes the occurrence date/time, type, priority, device ID, camera ID, object ID, and event details. The type is grouping and the details are a more concrete description of an event. For desertion detection, for example, the type is “abnormality detection” and the details are “standing-still detection”. The device ID indicates an ID of a sensor or a camera device that became an event source. The priority is a priority concerning an event notification and the display color and the alarm notification to the user are set separately for each level of priority. The object ID is a serial number assigned to each detected object individually. 
     Next, the method for generating an index in the present exemplary embodiment will be described. In the present exemplary embodiment, desertion detection is taken as an example.  FIG. 5  is a diagram illustrating a presence time of objects and an event when desertion detection occurs. An object B is a bag and an object A is a person who carries the bag. The object A appears at time  411 , leaves behind the object B at time  421 , and exits at time  431 . The object A appears at time  421  and is determined to be at rest for a predetermined time or longer by the playback apparatus  200  at time  441 . As a result, a desertion detection event occurs at time  441 . At this point, the object B is an object separated from the object A and thus, a relationship between the objects A and B is defined by the playback apparatus  200 . 
     Separation and integration of objects become possible by analyzing motions of the objects. If, for example, a new object arises from the same position as an existing object immediately after the size of the existing object increases rapidly, the playback apparatus  200  can determine that the existing object has separated. If, on the other hand, two existing objects or more are present at the same position and the number of apparent objects is one for a predetermined time or longer thereafter, the playback apparatus  200  can determine that the existing objects have integrated. 
     Here, the playback apparatus  200  can improve detection accuracy of separation/integration of objects by using a human detection technique. The human detection technique is a technique to determine a human region or a position by using body features or face features in a video image. When the human detection technique is applied to separation of objects, a rule is set down that while a person can separate into a person and an object other than the person, a person cannot separate into a plurality of persons. Conversely, when the human detection technique is applied to integration of objects, a rule is set down that while a person and an object can be integrated into a person, a plurality of persons cannot be integrated into a person. Accordingly, an erroneous detection concerning separation/integration by an overlapping operation of persons in the playback apparatus  200  can be reduced. 
     Next, the generation method of an index when a desertion detection occurs will be described with reference to  FIGS. 6A and 6B .  FIGS. 6A and 6B  are diagrams illustrating an example of generation of an index by extracting video images before and after events related to the desertion detection.  FIG. 6A  illustrates recorded video images.  FIG. 6B  is an example of played back video images based on the index. In  FIG. 6A , the respective events at the time  411  of the appearance of a person, the time  421  of the start of desertion, the time  431  of the exit of the person, and the time  441  of the detection of desertion described with reference to  FIG. 5  are illustrated in the recorded video images  500 . 
     When a desertion detection event  442  occurs, what the user wants to know is a person who has left behind the deserted object and the moment of the desertion. Thus, as illustrated in  FIG. 6B , the playback apparatus  200  plays back video images before and after the time  411  of the appearance of a person and the time  421  of the start of desertion continuously. If the playback time before or after an event is set as ΔT, video images  510  of the duration time 4ΔT will be played back by the playback apparatus  200 . Here, video images before and after the desertion detection event  442  are changeless video images and thus, the playback apparatus  200  omits the playback thereof. The playback time before or after an event is not limited to ΔT and may be set optionally for each event. 
     An example of information stored as an index is illustrated in  FIG. 7 .  FIG. 7  is a diagram illustrating an example of index information stored by the playback apparatus  200  as an index. As illustrated in  FIG. 7 , index information includes three types of information: attention event information, related event information, and operation information. The attention event information is an event that causes generation of an index and is, for example, the desertion detection event in  FIG. 5 . An event ID of the event is stored as the attention event information. The related event information is, for example, the time  421  of the appearance of an object corresponding to the attention event information or the time  411  of the appearance of a separation source object of the object. The event ID of the event is stored as the related event information. However, a comment illustrated in  FIG. 7  is not necessarily needed. The operation information is a list of the start time and stop time of the playback. More specifically, the operation information includes operation content such as the start and end, camera ID, time, and related event IDs. As already described above, the playback time and stop time are determined by the playback apparatus  200  depending on the occurrence times of related events. 
     Various techniques can be used for how to represent event information in  FIG. 4  or the index in  FIG. 7 . For example, the event information or the index can be represented in a list form of a structure in a program or saved in a file or transmitted in the XML form. 
     The user causes the playback apparatus  200  to play back video images by specifying a desertion detection event on the time line  340 . Methods for specifying an event include double-clicking and a method for using a context menu, but the present exemplary embodiment does not limit the method thereof. 
     Next, the processing procedure for one process by the playback apparatus  200  in the present exemplary embodiment will be described with reference to  FIG. 8 .  FIG. 8  is a flow chart illustrating an example of a processing procedure for one process by the playback apparatus  200 . The process is a process of the graphical user interface (GUI) and more specifically an event processing process. For simplification of description, only events directly related to the present exemplary embodiment are illustrated. As already described above, an event processing process is executed by a program being executed by the CPU  206 . 
     In step S 701 , the CPU  206  performs initialization. In step S 702 , the CPU  206  waits for an operational input from the user. When an operation event occurs (YES in step S 702 ), then in step S 703 , the CPU  206  determines whether the event is a desertion/taking away detection event. If the operation event is a desertion/taking away detection event (YES in step S 703 ), then in step S 704 , the CPU  206  generates an index according to the technique described above. 
     If the operation event is other than a desertion/taking away detection event in step S 703  (NO in step S 703 ), then in step S 710 , the CPU  206  determines whether the detection event is a playback instruction input. The playback instruction input is generated as a result of an event on the time line being designated. If the detection event is the playback instruction input (YES in step S 710 ), then in step S 711 , the CPU  206  plays back the index of recorded video images related to the designated event. If the detection event is not the playback instruction input (NO in step S 710 ), then in step S 720 , the CPU  206  performs other event processing before returning. 
     According to the processing described above, when an event concerning an object occurs, the playback apparatus  200  generates an index of video images of other objects related to the object and plays back video images according to the designation. More specifically, when a desertion detection event occurs, the playback apparatus  200  generates an index capturing moments of the appearance of a person who has brought in a deserted object and the desertion of the object and plays back related recorded video images based on the index according to the designation of an event on the time line. 
     Next, a second exemplary embodiment of the present invention will be described. There is a method for generating a video index when an event concerning an object occurs after waiting until all objects related to the object exit. This method is effective if the index should include, when, for example, a taking away detection event occurs, up to the exit of a person who has taken away an object from the screen. On the other hand, a delay may arise between the time when the taking away detection event occurs and the time when an index is generated and video images are played back. 
     The system configuration of the present exemplary embodiment is similar to that of the first exemplary embodiment and thus, a description thereof will not be repeated and the description below focuses on different portions from the first exemplary embodiment. The method for generating an index is illustrated in  FIGS. 9 and 10 . 
       FIG. 9  is a diagram illustrating the presence time of objects and events when taking away detection occurs. An object A is a person and an object B is a background region that appears after taking away occurs. The object B is the background region that newly appears after taking away occurs because when object region extraction using background differences is used, the playback apparatus  200  stores scenes before taking away occurs as a background and thus, a region such as a wall that appears after the taking away is first determined to be a new object region. Events concerning the objects A include an appearance of a person  811 , a start of taking away  821 , and an exit  831 . The object B appears after being separated from the object A at detection time of the start of taking away  821 , and a taking away detection event  841  occurs because the object B is in a resting state for a predetermined time or longer.  FIG. 9  is different from  FIG. 5  in that an event of the exit  831  of the taking-away person occurs after the taking away detection event  841  occurs. Here, the playback apparatus  200  can measure the fact that the object B has separated from the object A by using an identification unit or method like the one described in the first exemplary embodiment. 
       FIGS. 10A and 10B  are diagrams illustrating an example of the generation of the index by extracting video images before and after events related to the taking away detection.  FIG. 10A  illustrates recorded video images from the appearance of the object A to the exit of the object B and events.  FIG. 10B  illustrates played back video images. In  FIG. 10A , the recorded video images  910  include the appearance of a person  811 , the start of taking away  821 , confirmation of the taking away detection (the exit  831  of the taking-away person), and the event of exit of the person (the taking away detection event  841 ). When the taking away detection occurs, it is necessary for the user to verify a taking-away act. For this purpose, the appearance of a person who has taken away an object, the moment of taking away the object, and the exit while carrying the object are important moments. 
     Thus, as illustrate in  FIG. 10B , the playback apparatus  200  in the present exemplary embodiment generates an index storing times of ΔT before or after the appearance of a person  811 , the start of taking away  821 , and the exit of the person  842 . As a result, video images  920  of the duration time 6ΔT will be played back. Like the first exemplary embodiment, the playback time ΔT before or after an event may be set optionally for each event. 
     Next, the processing procedure for one process by the playback apparatus  200  in the present exemplary embodiment will be described with reference to  FIG. 11 .  FIG. 11  is a flow chart illustrating an example of the processing procedure of one process by the playback apparatus  200 . This process is a process concerning the GUI and more specifically, an event processing process. Like  FIG. 8 , for simplification of description, only events directly related to the present exemplary embodiment are illustrated. A different portion from  FIG. 8  will be described. 
     In step S 1003 , the CPU  206  determines whether the event is a desertion/taking away detection event. If the event is a desertion/taking away detection event (YES in step S 1003 ), then in step S 1004 , the CPU  206  determines whether any related object is present in video images. This is because of waiting until all related objects exit. If any related object is present in video images (YES in step S 1004 ), the CPU  206  continues processing. If there is no related object (NO in step S 1004 ), then in step S 1005 , the CPU  206  generates a video index. If the operation event is other than a desertion/taking away detection event (NO in step S 1003 ), then in step S 1010 , the CPU  206  determines whether the detection event is a playback instruction input. This occurs as a result of an event on the time line being designated. If the detection event is the playback instruction input (YES in step S 1010 ), then in step S 1011 , the CPU  206  plays back recorded video images according to the index. If the detection event is not the playback instruction input (NO in step S 1010 ), then in step S 1020 , the CPU  206  performs other event processing before returning. 
     According to the processing described above, when an event occurs, a video index in which only important scenes are edited is generated after waiting until related objects exit. More specifically, when a taking away detection event occurs, an index including the entry of a person who would take a taking-away act, the moment of taking away, and up to the exit of the person who has taken a taking-away act is generated to play back video images. 
     In a third exemplary embodiment of the present invention, processing to make a correspondence relation between the object region display and the event display on a time line intuitively understandable in a monitoring system having the time line will be described. 
     The method for indicating a correspondence relation between the object region display and the event display on the time line in the present exemplary embodiment will be described with reference to  FIG. 12 .  FIG. 12  is a diagram illustrating an example of a correspondence relation between the object region display and the event display on the time line. In  FIG. 12 , an image display region  310  and a frame  312  indicating an object region in a camera are in a selected state. This state occurs by the frame  312  indicating an object region being clicked using a mouse pointer by a user. At this point, events of the object related to the frame  312  are highlighted on the time line. Events of an object are, for example, the appearance and exit of the object. In the event information illustrated in  FIG. 4 , the playback apparatus  200  can identify correspondence relations based on the same object ID. At this point, the playback apparatus  200  can make correspondences between objects and events clear by highlighting, like the frame  312  indicating an object region, the frame  312  indicating an object region and events  342 ,  343 , and  344  related to the object. More specifically, the playback apparatus  200  uniforms the frame thickness, line type, color, and the like. Accordingly, the user can easily identify only events corresponding to a specified object from among many events on the time line. 
     The playback apparatus  200  can change the color of the frame  312  indicating an object region in a video image and the display of the event  342  on the time line according to changes in attributes of the object. Assume, for example, that the display of the event  342  is an appearance event of the object. Here, a desertion detection event is defined as an event and it is assumed that when the object is in a stopped state for a predetermined time or longer, the event is generated. In such a case, the playback apparatus  200  can indicate that an occurrence of the event is approaching by changing the color of the frame  312  like blue-green-yellow-red according to a stop time of the object. In this case, by also changing the color of the display of the event  342  on the time line  340  in conjunction with the frame  312  indicating an object region, the playback apparatus  200  can maintain the correspondence relation. The playback apparatus  200  indicates the correspondence relation between an object and the event display and can also indicate stop time information of the object only by the event display on the time line. 
     The playback apparatus  200  in the present exemplary embodiment highlights, when an event is specified, events concerning the same object and can also highlight corresponding video images and the object display. This means that if the user selects the display of, for example, the event  342 , the playback apparatus  200  highlights the display of the events  342 ,  343 , and  344  concerning the same object and also highlights the corresponding image display region  310  and the frame  312  indicating the object region. If the user specifies the display of the event  342  by double-clicking the mouse in this state, the playback apparatus  200  starts the playback of video images of the corresponding time. 
     Another example of emphasizing events concerning the same object on the time line is illustrated in  FIG. 13 .  FIG. 13  is a diagram illustrating another example emphasizing events concerning the same object on the time line. In  FIG. 13 , like the time line in  FIG. 12 , the playback apparatus  200  highlights events concerning the same object and further colors a background  350  between an appearance event of an object and an exit event separately from the normal background. The playback apparatus  200  may display, as illustrated in  FIG. 13 , events concerning other objects than the object selected by the user simultaneously or keep such events hidden behind. 
     Incidentally, the playback apparatus  200  can establish correspondence relations between the object display and the event display even if there is a plurality of cameras. In other words, the playback apparatus  200  displays the object so as to make clear all the correspondence relations between the object and the event among the plurality of cameras, if the same object is taken by the plurality of cameras disposed at different positions. 
       FIG. 14  is a diagram illustrating an example of the screen of a recording application when objects and events among a plurality of cameras are displayed by establishing correspondences therebetween. In  FIG. 14 , it is assumed that the frame  312  indicating an object region present in the image display region  310  and an object  321  present in the camera video image  320  are the same object. In  FIG. 14 , there is a plurality of methods for the playback apparatus  200  to determine whether objects in a plurality of camera video images are the same object. If there are events of the same type occurring at the same time in a plurality of video images, the events can be considered to concern the same object. This means that if desertion detection occurs at the same time in different video images, the playback apparatus  200  considers the objects to be the same. A method for verifying the identity of objects in scenes by the playback apparatus  200  based on the physical relationship of a plurality of cameras is known as another method. 
     If the user selects the frame  312  indicating an object region in the image display region  310  of the network camera  101 , the frame  312  is highlighted by the playback apparatus  200 . At the same time, event information is referenced by the playback apparatus  200  to highlight events  345 ,  347 , and  348  related to the object on the time line  340 . Next, the playback apparatus  200  checks whether there is any applicable event concerning the same object and, if there is any, highlights such an event. In  FIG. 14 , an event  346  is highlighted. It is assumed here that a rule that events of the same type occurring at the same time are considered to concern the same object is used. In the example in  FIG. 14 , desertion detection events occur at the same time in the image display region  310  of the network camera  101  and the camera video image  320  of the camera  102 . To show that desertion detection events has occurred at the same time, the playback apparatus  200  displays a plurality of events by shifting the position. Further, the playback apparatus  200  highlights the object  321  corresponding to the event  346  in the same manner. With the above processing, the playback apparatus  200  can display the same object  321  in video images different from events related to the frame  312  indicating an object region by establishing a correspondence therebetween. 
     At this point, the playback apparatus  200  also adds correspondences between objects to the event management in  FIG. 4 . For example, the playback apparatus  200  considers a standing-still detection ON event (for example, the device ID: Cam02, object ID: 003) that has occurred in a different camera at the same time as a standing-still detection ON event (event ID: 00128, device ID: Cam01, object ID: 002) in  FIG. 4  as events concerning the same object. Then, the playback apparatus  200  stores the object ID: 002 and the object ID: 003 as the same object. 
     Next, the processing procedure for one process by the playback apparatus  200  in the present exemplary embodiment will be described with reference to  FIG. 15 .  FIG. 15  is a flow chart illustrating an example of the processing procedure of one process by the playback apparatus  200 . This process is a process concerning the GUI and more specifically, an event processing process. Like  FIGS. 8 and 11 , for simplification of description, only events directly related to the present exemplary embodiment are illustrated. 
     In step S 1400 , the CPU  206  initializes the display. In step S 1401 , the CPU  206  waits for an event. Next, in step S 1402 , the CPU  206  determines whether any video frame is specified. This determines whether a specific cameral video frame is specified by the user using a mouse pointer. If a video frame is specified (YES in step S 1402 ), the processing proceeds to step S 1404 . In step S 1404 , the CPU  206  highlights the specified frame and, then in step S 1405 , highlights events related to the specified camera. 
     If the event is not an event to specify a video frame (NO in step S 1402 ), then in step S 1410 , the CPU  206  determines whether the event is an event to specify an object. If the event is an event to specify an object (YES in step S 14102 ), then in step S 1411 , the CPU  206  detects the specified range. This is because a plurality of objects may be specified. Next, in step S 1412 , the CPU  206  highlights events concerning the same object. In step S 1413 , the CPU  206  highlights objects related to the specified event. If the event is not an event to specify an object (NO in step S 1410 ), then in step S 1420 , the CPU  206  determines whether the event is an event to specify an event. If the event is an event to specify an event (YES in step S 1420 ), then in step S 1421 , the CPU  206  detects the specified range. This is because a plurality of events may be specified. In step S 1422 , the CPU  206  highlights events concerning the same object. In step S 1423 , the CPU  206  highlights objects related to the specified event. If the event is not an event to specify an event (NO in step S 1420 ), then in step S 1430 , the CPU  206  performs other event processing. 
     According to the present exemplary embodiment, as described above, correspondence relations between objects and events can be made clear by uniforming a highlighting method of the object display in video images and the event display on the time line. Also according to the present exemplary embodiment, a display method that makes correspondences between objects and events in a plurality of camera video images easily understandable can be realized for an event display form in which all events occurring in different camera video images are displayed on one time line. 
     According to each of the above exemplary embodiments, when an event occurs, the playback apparatus  200  plays back short video images including before and after the appearance time of a parent object of an object related to the event and the time when the object separates from the parent object. Accordingly, a user can check video images most desired to know concerning the event. Moreover, with only events corresponding to the object specified by the user being highlighted by the playback apparatus  200 , correspondence relations between the object display in a plurality of video images and the event display on the time line becomes clear. Further, changes in object attributes can be known on the time line by both the object display and the event display being changed by the playback apparatus  200  according to changes in object attributes. 
     Therefore, when an event concerning an object occurs, suitable video images concerning objects related to the object can swiftly be played back. 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment (s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions.