Patent Publication Number: US-2012032976-A1

Title: Image display device, image display system, and image display method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2010-177166 filed in Japan on Aug. 6, 2010. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image display device that displays an arbitrary image on a display screen, and more particularly, to an image display device, an image display system, and an image display method in which an image drawn by a user on a display screen is displayed. 
     2. Description of the Related Art 
     Conventionally, at conferences or the like in companies, educational institutions, administrative agencies, and the like, an electronic blackboard is being used which displays a background image on a large-scaled display and allows a user to write a drawn image such as letters, numerical numbers, diagrams, or the like on the background image. 
     Currently, Smart Board available from Smart Technologies Co., Ltd., Star Board (a registered trademark) available from Hitachi Software Engineering Co., Ltd., Cyber Conference (a registered trademark) available from Pioneer Corporation, and the like are being provided as electronic blackboards. Japanese Patent Application Laid-open No. 2008-176802 discloses an electronic blackboard system including a coordinate input/detection device. These electronic blackboards have a function of displaying a background image functioning as a blackboard on a screen and displaying a drawn image drawn with a touch panel on the screen in a superimposed manner. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, there is provided an image display device that displays at lease one image frame, including: a receiving unit that receives the at least one image frame from an image frame provision device that provides the at least one image frame; a display unit that displays one of the at least one image frame that is received by the receiving unit; a touch panel that detects a contact position on the display unit, which configures configuration information of at least one drawn image to be superimposedly drawn on the at lease one image frame; a database in which the at least one image frame and the configuration information of the at least one drawn image to be displayed on the display unit are registered to be associated with each other; a judging unit that refers to the database to judge whether or not an image frame that is identical to one of the at least one image frame received from the image frame provision device is present in the database. The judging unit refers to the database to cause the display unit to display at least one of the at least one drawn image represented by a part of the configuration information associated with an image frame judged as being identical to a received one of the at least one image frame. 
     According to another aspect of the present invention, there is provided an image display system, including: an image display device that displays at least one image frame; and an image frame provision device that provides the image display device with the at least one image frame. The image display device includes a receiving unit that receives the at least one image frame from the image frame provision device, a display unit that displays one of the at least one image frame that is received by the receiving unit, a touch panel that detects a contact position on the display unit, which configures configuration information of at least one drawn image to be superimposedly drawn on the at lease one image frame, a database in which the at least one image frame and the configuration information of the at least one drawn image to be displayed on the display unit are registered to be associated with each other, and a judging unit that refers to the database to judge whether or not an image frame identical to one of the at least one image frame received from the image frame provision device is present in the database. The judging unit refers to the database to cause the display unit to display at least one of the at least one drawn image represented by a part of the configuration information associated with an image frame judged as being identical to a received one of the at least one image frame. 
     According to still another aspect of the present invention, there is provided an image display method executed by an image display device that displays at least one image frame, including: receiving, by the image display device, the at least one image frame from an image frame provision device that provides the at least one image frame; and judging, by the image display device, whether or not there is an image frame identical to one of the at least one image frame received from the image frame provision device by referring to a database in which the at least one image frame to be displayed on a display unit of the image display device and configuration information of at least one drawn image to be superimposedly drawn on the at least one image frame are registered to be associated with each other. The judging includes referring to the database to cause the display unit to display at least one of the at least one drawn image represented by a part of the configuration information associated with an image frame judged as being identical to a received one of the at least one image frame. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an image display system according to an embodiment; 
         FIG. 2  is a diagram illustrating a functional configuration of an image display device according to the embodiment; 
         FIG. 3  is a flowchart illustrating a process executed by an image display device according to the embodiment; 
         FIG. 4  is a diagram illustrating a use state of the image display device according to the embodiment; 
         FIG. 5  is a diagram illustrating a snapshot history information table used by the image display device according to the embodiment; 
         FIG. 6  is a diagram illustrating a functional configuration of an image display device according to another embodiment; 
         FIG. 7  is a flowchart illustrating a process executed by the image display device according to another embodiment; 
         FIG. 8  is a flowchart illustrating a process executed by the image display device according to another embodiment; and 
         FIG. 9  is a diagram illustrating an embodiment of an image frame displayed on an image display device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. 
       FIG. 1  illustrates an image display system  100  according to an embodiment. The image display system  100  includes an image display device  110  and a user personal computer (PC)  120 . 
     The image display device  110  is a device that displays an image frame that is a background image received from the user PC  120 , and displays a drawn image drawn by a user. The image display device  110  is configured to include a display unit  112 , operation buttons  114  and  116 , and an image frame input unit  118 . 
     The display unit  112  is a functional unit that displays the background image and the drawn image. The display unit  112  includes a touch panel installed on a screen thereof and can detect the user&#39;s contact position with a contact position sensor that configures the touch panel. 
     The operation buttons  114  and  116  are a functional unit used to instruct switching of an operation mode of the image display device  110 . Switching of the operation mode of the image display device  110  will be described later in detail. 
     The image frame input unit  118  is a functional unit that receives the image frame, which is a background image displayed on the display unit  112 , from the user PC  120 . In the present embodiment, a video graphic array (VGA) input connector configured by a VGA terminal may be employed as the image frame input unit  118 . In this case, a VGA signal is received from the user PC  120  through a cable  122  such as a VGA cable, and the VGA signal is supplied to a controller (not shown) included in the image display device  110 . In another embodiment, the image frame input unit  118  may receive image data from the user PC  120  through a radio communication that conforms to a radio communication protocol such as Bluetooth or WiFi. 
     The image display device  110  includes a Pentium (a registered trademark) processor or a compatible processor and executes a program of the present invention written in a program language such as assembler, C, C++, Java (a registered trademark), JavaScript (a registered trademark), Perl, Ruby, and Python under the management of an operating system (OS) such as Windows (a registered trademark) series, Unix (a registered trademark), Linux (a registered trademark), Itron, or μItron. The image display device  110  includes a random access memory (RAM) that provides an execution space used to execute the program of the present invention and a hard disk drive (HDD) device used to persistently retain a program, data, and the like. By executing the program of the present invention, each functional unit of the present embodiment is implemented in the image display device. 
     The program of the present invention may be distributed in the form stored in a device-readable recording medium such as a HDD, a compact disc-read only memory (CD-ROM), a magneto optical (MO), a flexible disk, an electrically erasable programmable read-only memory (EEPROM), and an erasable programmable read-only memory (EPROM). The program of the present invention may be configured in a form readably by foreign device and transmitted via a network. 
     The user PC  120  is an image frame provision device that provides the image display device  110  with the image frame which is the background image. In the embodiment illustrated in  FIG. 1 , a notebook computer is employed as the user PC  120 , but in another embodiment, an information processing device, which can supply image data, such as a desktop type PC, a personal digital assistant (PDA) may be used. The user PC  120  includes an interface, which serves to output the background image, and supplies a display image being displayed on a display screen thereof to the image display device  110 . In the present embodiment, the user PC  120  includes a VGA output connector as the interface and can supply the image display device  110  with the display image as a VGA signal. In another embodiment, the user PC  120  may transmit the display image through a radio communication that conforms to a variety of radio communication protocols. 
       FIG. 2  is a diagram illustrating a functional configuration of an image display device  200  according to the present embodiment. The functional configuration of the image display device  200  will be described below with reference to  FIG. 2 . 
     The image display device  200  is configured to include a controller  202 , a display unit  250 , and a touch panel  260 . The controller  202  is a functional unit that processes the background image received from a user PC  270  and the drawn image input through the touch panel  260 . The controller  202  is configured to include an image frame distribution unit  210 , a display output control unit  212 , and a display control unit  214 . 
     The image frame distribution unit  210  is a functional unit that acquires the image frame, which is to be displayed on the display unit  250 , received from the user PC  270  and distributes the image frame to the display output control unit  212  or an image frame acquisition unit  216 . In the present embodiment, the image frame distribution unit  210  distributes the image frame, which is configured by an analogue signal such as the VGA signal, according to an operation mode which can be designated by the operation button. Specifically, when an operation mode of the image display device  200  is an image frame display mode which is a first state, the image frame distribution unit  210  provides the display output control unit  212  with the image frame. Meanwhile, when a drawn image display mode which is a second state is designated, the image frame distribution unit  210  provides the image frame acquisition unit  216  with the image frame. 
     The display output control unit  212  is a function unit that selectively outputs an image to the display control unit  214  in response to a request by a mode switching control unit  238 . In the present embodiment, the display output control unit  212  may be configured with a two-input one-output toggle operation switch circuit. In the image frame display mode which is the first state, the display output control unit  212  provides the display control unit  214  with the image frame received from the image frame distribution unit  210 . When the drawn image display mode which is the second state is designated, the display output control unit  212  provides the display control unit  214  with an image received from a display image generation unit  236 . 
     The display control unit  214  is a functional unit that causes display of an image received from the display output control unit  212  on the display unit  250 . The display control unit  214  transmits an analogue signal configuring the image received from the display output control unit  212  to the display unit  250  to cause the display unit  250  to display the image. 
     Further, the controller  202  is configured to include the image frame acquisition unit  216 , a snapshot information storage unit  218 , and a dynamic display control unit  220 . 
     The image frame acquisition unit  216  is a functional unit that sequentially stores the image frames acquired from the image frame distribution unit  210  in the snapshot information storage unit  218  when the drawn image display mode, which is the second state, is designated. The image frame acquisition unit  216  performs analogue-to-digital (A/D) conversion to convert the image frame configured by the analogue signal such as the VGA signal into a digital image frame and then stores the digital image frame in the snapshot information storage unit  218 . Further, the image frame acquisition unit  216  acquires the image frame from the snapshot information storage unit  218  and provides the acquired image frame in response to a request by another functional unit. 
     The snapshot information storage unit  218  is a storing unit that stores a last image frame and information configuring last one of the drawn images (hereinafter, referred to as “drawn image configuration information”) that configure snapshot information of the image frame and the drawn image. In the snapshot information storage unit  218 , latest image frames, which are to be displayed on the display unit  250 , received from the user PC  270  are sequentially stored. Further, in the snapshot information storage unit  218 , stored is latest piece of the drawn image configuration information denoting latest drawn image which the user has drawn through the touch panel using a drawing processing unit which will be described later. 
     The dynamic display control unit  220  is a functional unit that associates the image frame, which is being displayed on the display unit  250 , with the drawn image, which have been drawn by the user on the display unit  250  using the touch panel  260 , when the drawing image display mode which is the second state is designated. The dynamic display control unit  220  is configured to include an image frame management unit  222 , an image frame buffer memory  224 , an image frame judgment unit  226 , a snapshot history information processing unit  228 , and a snapshot history information database  230 . 
     The image frame management unit  222  is a functional unit that acquires the image frame received from the user PC  270 ; and provides the acquired image frame in response to a request by another functional unit included in the dynamic display control unit  220  or stores the acquired image frame in the image frame buffer memory  224 . The image frame management unit  222  periodically issues a load request to the image frame acquisition unit  216  to acquire the latest image frame stored in the snapshot information storage unit  218 ; and temporarily stores the acquired latest image frame in a storage device (not shown) such as a RAM or a register. The image frame management unit  222  generates image frame identification information which is information for uniquely identifying the image frame acquired from the snapshot information storage unit  218 . In the present embodiment, the image frame is acquired at intervals of 1/30 seconds, but in another embodiment, the image frame may be acquired at arbitrary intervals. 
     The image frame buffer memory  224  is a storing unit that stores therein the image frame which the image frame management unit  222  has acquired from the snapshot information storage unit  218 . The image frames periodically acquired from the snapshot information storage unit  218  are sequentially stored in the image frame buffer memory  224 . 
     The image frame judgment unit  226  is a functional unit that judges the identity between two consecutive image frames acquired from the image frame acquisition unit  216  are identical to each other. That is, the image frame judgment unit  226  judges whether or not a new the image frame has been received from the user PC  270 . 
     In the present embodiment, the image frame judgment unit  226  sets one or more specific area included in each of the image frames as a reference area and compares images in the reference areas each included in one of the two image frames. When the images in the reference areas are identical to each other as a result of comparison, it is judged that the two image frames are identical to each other, whereas when the images in the reference areas are different from each other, it is judged that the image frames are different from each other. In the case of comparing the images in the reference areas, the image frame judgment unit  226  may judge whether or not the images in the reference areas are identical to each other by calculating the value of the sum of pixel values in each of the reference areas of the two image frames and compares the values of the sums of the pixel values in the reference areas corresponding to each other. An integrated image in the reference area may be used for calculating the value of the sum of the pixel values of the reference area. 
     In the present embodiment, the identity between the image frames may be judged using the following Equation (1). Here, Rk represents a sum value of pixel values in a reference area k of a first image frame, and Sk represents a sum value of pixel values in a reference area k in a second image frame. N represents the number of reference areas. 
     
       
         
           
             
               
                 
                   
                     
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                       N 
                     
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                           S 
                           k 
                         
                       
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     When the condition of Equation (1) is satisfied, it can be judged that the first image frame is identical to the second image frame. 
     In another embodiment, the image frame judgment unit  226  may judge whether or not the image frames are identical to each other by calculating sum values of all pixel values of two image frames. 
     When it is judged that the two consecutive image frames are different from each other as a result of the abovementioned judgment process, the image frame judgment unit  226  notifies the snapshot history information processing unit  228  of the fact that a new image frame has been received from the user PC  270 . 
     The snapshot history information processing unit  228  is a functional unit that registers the image frame received from the user PC  270  in a snapshot history information table in the snapshot history information database  230  so that the image frame is associated with the drawn image. When the image frame judgment unit  226  judges that the new image frame has been received from the user PC  270 , the snapshot history information processing unit  228  acquires the latest drawn image configuration information from the snapshot information storage unit  218  and stores the drawn image configuration information in the snapshot history information table so that the drawn image configuration information is associated with the image frame acquired by the image frame management unit  222 . 
     The snapshot history information database  230  is a storing unit that stores the snapshot history information table in which the snapshot history information which is history information of snapshot information is registered. The snapshot history information database  230  updates the data contents of the snapshot history information table in response to a request by the snapshot history information processing unit  228 . The snapshot history information table will be described later with reference to  FIG. 5  together with information to register. 
     Further, the controller  202  includes a touch panel control unit  232 , the drawing processing unit  234 , the display image generation unit  236 , and the mode switching control unit  238 . 
     The touch panel control unit  232  is a functional unit that controls the touch panel  260  and acquires the drawn image configuration information. When the drawn image display mode which is the second state is designated, the touch panel control unit  232  drives the touch panel  260 , acquires contact position coordinates, and provides the drawing processing unit  234  with the acquired contact position coordinates. 
     In the present embodiment, a device driver which can control the touch panel  260  may be employed as the touch panel control unit  232 . The touch panel control unit  232  can display, on the display unit  250 , a user interface through which drawn image attribute information such as the thickness or the color of line segments configuring the drawn image can be designated. Further, the touch panel control unit  232  can provide the drawing processing unit  234  with the drawn image attribute information as the drawn image configuration information. 
     The drawing processing unit  234  is a functional unit that stores the drawn image configuration information in the snapshot information storage unit  218  when the drawn image display mode which is the second state is designated. When the drawn image configuration information is received from the touch panel control unit  232 , the drawing processing unit  234  sequentially stores the drawn image configuration information in the snapshot information storage unit  218 . As a result, the latest drawn image configuration information is stored in the snapshot information storage unit  218 . 
     The display image generation unit  236  is a functional unit that generates a display image which is an image to be displayed on the display unit  250  when the drawn image display mode which is the second state is designated. The display image generation unit  236  periodically acquires the latest image frame and the latest drawn image configuration information from the snapshot information storage unit  218 ; generates the drawn image based on the drawn image configuration information; synthesizes the drawn image with the image frame; converts the synthesized image into an analog signal; and generates the display image. 
     In further detail, the display image generation unit  236  may generate the drawn image on the latest image frame by changing a pixel value at a coordinate position denoted by each contact position coordinates included in the drawn image configuration information to a pixel value according to the color denoted by the drawn image attribute information. Further, when the thickness of a line segment is designated, the drawn image may be generated by changing pixel values in a predetermined range centering on the coordinate position denoted by the contact position coordinates to a pixel value according to the color denoted by the drawn image attribute information. The predetermined range may be set in advance as a function of the thickness of a line segment. The display image generation unit  236  provides the display output control unit  212  with the display image generated in this way, so the display image is displayed on the display unit  250 . 
     In the present embodiment, the display image generation unit  236  preferably generates the display image and displays the display image on the display unit  250  at an interval equal to or shorter than an interval at which the image frame management unit  222  acquires the image frame. 
     The mode switching control unit  238  is a functional unit that performs control of switching between the operation modes of the image display device  200 . The mode switching control unit  238  switches the operation mode according to a type of an operation button pressed by the user or in response to a command from the image frame judgment unit  226 . In the present embodiment, when an operation button “A” is pressed down, the mode switching control unit  238  switches the operation mode of the image display device  200  to the image frame display mode which is the first state. Further, when an operation button “B” is pressed down, the mode switching control unit  238  switches the operation mode of the image display device  200  to the drawn image display mode which is the second state. 
     Specifically, the mode switching control unit  238  transmits to the display output control unit  212  a command to switch an input terminal receiving an image to be displayed so as to cause the display output control unit  212  to switch the input terminal. Further, the mode switching control unit  238  transmits a command to enable or disable an operation of the drawing processing unit  234  to the drawing processing unit  234  so as to enable or disable the operation of the drawing processing unit  234 . 
       FIG. 3  is a flowchart illustrating a process executed by an image display device according to the present embodiment when the drawn image display mode which is the second state is designated. Referring to  FIG. 3 , in Step S 300 , the process starts; and in Step S 301 , the image frame management unit  222  judges whether or not the image frame has been received from the user PC  270 . In the present embodiment, the presence and absence of reception of the image frame may be judged such that when the image frame acquisition unit  216  receives the image frame, the image frame acquisition unit  216  changes a value of a dedicated register representing the presence and absence of acquisition of the image frame to a value representing the fact that the image frame has been received, and the image frame acquisition unit  216  refers to the value of the dedicated register. Further, when the image frame has not been received during a predetermined time period, the image frame acquisition unit  216  changes the value of the dedicated register to a value representing the fact that the image frame has not been received. 
     When it is judged that the image frame has not been received (No in Step S 301 ), the process of Step S 301  is repeated. However, the process branches to Step S 302  when it is judged that the image frame has been received (Yes in Step S 301 ). In Step S 302 , the snapshot history information processing unit  228  initializes the snapshot history information table in the snapshot history information database  230  and an interruption flag representing the presence and absence of the image frame. In the present embodiment, the interruption flag is initialized such that it is set to a value representing that the image frame is present. In the present embodiment, “true” is used as an initial value. 
     When the process of Step S 302  is executed, a process of Step S 303  and a process of Step S 317  are subsequently executed. 
     In Step S 317 , the image frame management unit  222  judges whether or not another image frame has been received from the user PC  270 . When it is judged that another image frame has been received (Yes in Step S 317 ), the process of Step S 317  is repeated. However, the process branches to Step S 318  when it is judged that another image frame has not been received (No in Step S 317 ). In Step S 318 , the interruption flag is set to a value representing that an image frame has not been received. Then, in Step S 319 , the process is finished. In the present embodiment, “False” is used as the value representing that an image frame has not been received. 
     In Step S 303 , the image frame management unit  222  initializes the image frame buffer memory  224  and an image fame storage variable to be assigned the image frame identification information of the image frame stored in the image frame buffer memory  224 . In Step S 304 , the image frame management unit  222  judges whether or not the interruption flag has the initial value “true,” that is, whether or not the image frame has been received. When it is judged that the interruption flag does not have the initial value “true” (No in Step S 304 ), the process branches to Step S 308 . In Step S 308 , the snapshot history information processing unit  228  stores the contents of the snapshot history information table stored in the snapshot history information database  230  in a non-volatile storage device, for example, as a log or a journal. Then, in Step S 319 , the process is finished. 
     Meanwhile, when it is judged that the interruption flag has the initial value “true” (Yes in Step S 304 ), the process branches to Step S 305 . In Step S 305 , the image frame management unit  222  judges whether or not a predetermined time has elapsed. When it is judged that the predetermined time has not elapsed (No in Step S 305 ), the process of Step S 305  is repeated. However, the process branches to Step S 306  when it is judged that the predetermined time has elapsed (Yes in Step S 305 ). 
     In the present embodiment, time may be measured using a hardware timer or a software timer. For example, when it is judged in Step S 304  that the interruption flag has the initial value “true”, a timer starts to measure time; and, in Step S 305 , it may be judged whether or not a predetermined time (for example, 1/30 seconds) has elapsed by acquiring a measured time from the timer. 
     In Step S 306 , the image frame management unit  222  acquires the latest image frame from the snapshot information storage unit  218  through the image frame acquisition unit  216  and assigns the image frame identification information of the latest image frame to the image frame storage variable. In Step S 307 , the image frame judgment unit  226  judges whether or not a lastly stored image frame (hereinafter, referred to as “immediately previous image frame) among image frames already stored in the image frame buffer memory  224  is identical to an image frame denoted by the image frame identification information assigned to the image frame storage variable in Step S 306 . When it is judged that the image frames are identical to each other in the judgment in Step  307  (Yes in Step S 307 ), the process branches to Step S 304 . However, when it is judged that the image frames are different from each other (No in Step S 307 ), the process branches to Step S 309 . 
     In Step S 309 , the image frame judgment unit  226  adds the image frame identification information designated by the image frame storage variable and the image frame denoted by the image frame identification information to the image frame buffer memory  224  through the image frame management unit  222 . In Step S 310 , the snapshot history information processing unit  228  refers to the snapshot history information table in the snapshot history information database  230  to judge whether or not the image frame is present which is identical to the latest image frame denoted by the image frame identification information assigned to the image frame storage variable. 
     In the present embodiment, the snapshot history information processing unit  228  may employ a method of judging the identity between the image frames as that executed by the image frame judgment unit  226 . The snapshot history information processing unit  228  sequentially acquires the image frame denoted by the image frame identification information registered in the snapshot history information table from the image frame buffer memory  224  and compares the acquired image frame with the latest image frame using the method. Further, in another embodiment, the snapshot history information processing unit  228  may cause the image frame judgment unit  226  to judge the identity between the image frames. In this case, the image frame judgment unit  226  sequentially acquires the image frame denoted by the image frame identification information registered in the snapshot history information table from the image frame buffer memory  224  and compares the acquired image frame with the latest image frame. 
     Meanwhile, when it is judged that the image frame identical to the image frame designated by the image frame storage variable is not present (No in Step S 310 ), the process branches to Step S 311 . In Step S 311 , the snapshot history information processing unit  228  judges whether or not the drawn image configuration information is stored in the snapshot information storage unit  218 . When it is judged that the drawn image configuration information is not stored (No in Step S 311 ), the process branches to Step S 314 . However, when it is judged that the drawn image configuration information is stored (Yes in Step S 311 ), the process branches to Step S 312 . 
     In Step S 312 , the snapshot history information processing unit  228  acquires the latest drawn image configuration information from the snapshot information storage unit  218  and registers the acquired latest drawn image configuration information in the snapshot history information table so that the acquired latest drawn image configuration information is associated with the image frame identification information designated by the image frame storage variable. In Step S 313 , the snapshot history information processing unit  228  deletes the drawn image configuration information stored in the snapshot information storage unit  218 . 
     In Step S 314 , the snapshot history information processing unit  228  transmits to the mode switching control unit  238  a command to switch the operation mode of the image display device to the first state, so that the mode switching control unit  238  switches the operation mode. When the process of Step S 314  is finished, the process of Step S 304  is executed. 
     Meanwhile, when it is judged that the image frame identical to the image frame designated by the image frame storage variable is present (Yes in Step S 310 ), the process branches to Step S 315 . In Step S 315 , the snapshot history information processing unit  228  acquires the drawn image configuration information associated with the image frame judged as being identical in Step S 310  by referring to the snapshot history information table, and stores the acquired drawn image configuration information in the snapshot information storage unit  218 . Thus, when the background image previously displayed on the image display device is displayed again, the drawn image associated with the background image can be displayed again together with the background image. 
     In Step S 316 , the snapshot history information processing unit  228  transmits to the mode switching control unit  238  a command to switch the operation mode of the image display device to the second state, so that the mode switching control unit  238  switches the operation mode. When the process of Step S 316  is finished, the process of Step S 304  is executed. 
       FIG. 4  is a diagram illustrating a use state of an image display device according to the present embodiment. On a display unit of an image display device  400 , an image frame  410  which is identical to an image frame  422  displayed on a PC  420  and a drawn image  412  drawn by a user are displayed in a superimposed manner. In the present embodiment, contact position coordinates related to the drawn image  412  are given using a coordinate value for which a top left point of the display unit of the image display device  400  is used as an original point. 
       FIG. 5  illustrates a snapshot history information table  500  used by the image display device according to the present embodiment. In the snapshot history information table  500 , image frame identification information  510  and drawn image configuration information  520  are registered to be associated with each other. 
     The image frame identification information  510  is information for uniquely identifying the image frame received by the image display device. In an embodiment illustrated in  FIG. 5 , a combination of a digit string and a character string representing a storage type of an image frame is used as the image frame identification information. However, in another embodiment, a different digit string, a different character string, and/or a combination thereof may be used. 
     The drawn image configuration information  520  is information that configures the drawn image which the user has drawn using the touch panel. The drawn image configuration information  520  includes information representing the position at which the user touches the touch panel. The drawn image configuration information  520  can be represented by a coordinate value on the display unit of the image display device. In the embodiment illustrated in  FIG. 5 , only the coordinate value denoting a position at which the drawn image should be displayed is illustrated as the drawn image configuration information. However, in another embodiment, in addition to the coordinate value, attribute information of the drawn image such as the display color or the thickness of a line segment of the drawn image may be registered such that the attribute information is associated with the image frame identification information. 
       FIG. 6  is a diagram illustrating a functional configuration of an image display device  600  according to another embodiment. The function configuration of the image display device  600  will be described below with reference to  FIG. 6 . A redundant description regarding a functional unit having the same function as the functional unit of the image display device  200  illustrated in  FIG. 2  will not be repeated. 
     In the embodiment illustrated in  FIG. 6 , a dynamic display control unit  620  is configured to include an image frame management unit  622 , an image frame buffer memory  624 , an image frame judgment unit  626 , a snapshot history information processing unit  628 , a snapshot history information database  630 , and a scroll amount calculation unit  640 . 
     The scroll amount calculation unit  640  is a functional unit that calculates a scroll amount which is a movement amount when the image frame received from a user PC  670  has moved by scrolling. The scroll amount calculation unit  640  calculates an amount of scrolling in a horizontal direction or a vertical direction performed between two consecutive image frames acquired by the image frame management unit  622 . Specifically, the scroll amount calculation unit  640  calculates the scroll amount as follows: 
     (1) values of pixels each configuring one of two consecutive image frames are compared in units of one pixel or in units of a group of pixels, and a difference therebetween is calculated; 
     (2) an area obtained by excluding an area corresponding to a pixel with no difference between the image frames from each image frame is specified as a scroll area of each image frame; 
     (3) on the scroll area of each image frame, a reference area used to calculate the scroll amount is specified; and 
     (4) the identity between the reference areas of the image frames is judged by comparing the reference areas of the image frames, and a movement amount in a horizontal direction and/or a vertical direction between the reference areas judged as identical to each other is calculated as the scroll amount of the image frame. 
     In the process (3), when the scroll amount of the image frame in the vertical direction is calculated, the reference area is specified such that the scroll area is divided into a plurality of sub scroll areas in the vertical direction, and a sub scroll area other than a sub scroll area whose entire area is blank among the sub scroll areas is set as the reference area. Meanwhile, when the scroll amount of the image frame in the horizontal direction is calculated, the reference area is specified such that the scroll area is divided into a plurality of sub scroll areas in the horizontal direction, and a sub scroll area other than the sub scroll area whose entire area is blank among the sub scroll areas is set as the reference area. 
     In the process (4), whether or not one reference area of the latest image frame is identical to any one of reference areas of the immediately previous image frame is judged using a pattern recognition method such as template matching. When the reference areas identical to each other are present, the scroll amount is calculated by comparing coordinate values of the reference positions of the reference areas (for example, coordinate values of top left points of the reference areas). Further, it is judged whether or not there is any other reference area of the latest image frame which is identical to any one of reference areas of the immediately previous image frame. When there is any other identical reference area, the scroll amount can be calculated using coordinate value of the reference position of the reference area. 
     In the present embodiment, an average value of the movement amounts of the reference areas judged as being identical may be calculated as the scroll amount of the image frame using the following Equation (2). Here, Rxk represents an x coordinate value of a top left point of a reference area k of a first image frame, and Ryk represents a y coordinate value of the top left point of the reference area k of the first image frame. Sxk represents an x coordinate value of a top left point of a reference area k of a second image frame; and Syk represents a y coordinate value of the top left point of the reference area k of the second image frame. Further, N represents the number of reference areas which are judged as being identical between the first image frame and the second image frame. 
     
       
         
           
             
               
                 
                   
                     
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     In the present embodiment, in Equation (2), the latest image frame may be set as the first image frame, and the immediately previous image frame may be set as the second image frame. In another embodiment, the immediately previous image frame may be set as the first image frame, and the latest image frame may be set as the second image frame. 
     When calculation of the scroll amount is completed, the scroll amount calculation unit  640  notifies the snapshot history information processing unit  628  of the calculated scroll amount. 
     The snapshot history information processing unit  628  changes the drawn image configuration information using the scroll amount calculated by the scroll amount calculation unit  640  and stores the changed drawn image configuration information in a snapshot information storage unit  618  or deletes the drawn image configuration information stored in the snapshot information storage unit  618 . The snapshot history information processing unit  628  registers the drawn image configuration information changed according to the scroll amount and the image frame stored in the snapshot information storage unit  618  so as to be associated with each other in the snapshot history information table in the snapshot history information processing unit  628 . 
       FIGS. 7 and 8  are flowcharts illustrating a process executed by the image display device illustrated in  FIG. 6 . The process executed by the image display device will be described below with reference to  FIGS. 7 and 8 . Step S 700  to Step S 711  of the process illustrated in  FIG. 7  are the same as Step S 300  to Step S 308  and Step S 317  to Step S 319  of the process illustrated in  FIG. 3 , and thus a redundant description will not be repeated. 
     The process of  FIG. 8  corresponds to the process of Step S 309  to Step S 316  illustrated in  FIG. 3  and is executed when the image frame judgment unit  626  judges that two consecutive image frames are different from each other. In Step S 801 , the snapshot history information processing unit  628  refers to the snapshot history information table of the snapshot history information database  630  to judge whether or not there is an image frame identical to a latest image frame denoted by image frame identification information assigned to a first image frame storage variable in Step S 706 . 
     When it is judged that there is the image frame identical to the image frame designated by the first image frame storage variable (Yes in Step S 801 ), the process branches to Step S 817 . In Step S 817 , the snapshot history information processing unit  628  refers to the snapshot history information table to acquire the drawn image configuration information associated with the image frame identification information of the image frame judged as being identical to the image frame designated by the first image frame storage variable, and stores the drawn image configuration information in the snapshot information storage unit  618 . Thus, when the background image previously displayed on the image display device is displayed again, the drawn image associated with the background image can be displayed again along with the background image. In Step S 818 , the scroll amount calculation unit  640  causes a mode switching control unit  638  to switch the operation mode of the image display device  600  to the second state. 
     Meanwhile, when it is judged that there is no image frame that is identical to the image frame designated by the first image frame storage variable (No in Step S 801 ), the process branches to Step S 802 . In Step S 802 , the snapshot history information processing unit  628  assigns image frame identification information denoting an image frame lastly stored in the image frame buffer memory  624  to a second image frame storage variable. In Step S 803 , the snapshot history information processing unit  628  additionally puts the image frame identification information designated by the first image frame storage variable and the image frame denoted by the image frame identification information into the image frame buffer memory  624  through the image frame management unit  622 . 
     In Step S 804 , the scroll amount calculation unit  640  calculates the scroll amount (Δx, Δy) of the latest image frame using the image frame designated by the first image frame storage variable and the image fame designated by the second image frame storage variable and notifies the snapshot history information processing unit  628  of the calculated scroll amount (Δx, Δy). In Step S 805 , the snapshot history information processing unit  628  acquires the latest drawn image configuration information retained in the snapshot information storage unit  618 . In Step S 806 , the coordinate value in the drawn image configuration information is increased or decreased by the scroll amount (Δx, Δy). In Step S 807 , the snapshot history information processing unit  628  judges whether or not all positions denoted by coordinate values in the drawn image configuration information changed by increasing or decreasing by the scroll amount are inside a display screen area of the image display device  600 . 
     When it is judged that all positions denoted by coordinate values in the changed drawn image configuration information are inside the display screen area (Yes in Step S 807 ), the process branches to Step S 808 . In Step S 808 , the snapshot history information processing unit  628  registers the changed drawn image configuration information in the snapshot history information table so that the changed drawn image configuration information is associated with the image frame identification information of the image frame designated by the first image frame storage variable. In Step S 809 , the snapshot history information processing unit  628  stores the changed drawn image configuration information in the snapshot information storage unit  618 . Thus, the drawn image can be displayed in conjunction with the background image moved by scrolling. 
     In Step S 810 , the snapshot history information processing unit  628  causes the mode switching control unit  638  to switch the operation mode of the image display device  600  to the second state. Thereafter, Step S 704  of  FIG. 7  is executed. 
     Meanwhile, when it is judged that at least part of the positions denoted by the coordinate values in the changed drawn image configuration information is not inside the display screen area (No in Step S 807 ), the process branches to Step S 811 . In Step S 811 , the snapshot history information processing unit  628  judges whether all of the positions denoted by the coordinate values in the changed drawn image configuration information are outside the display screen area. In the present embodiment, when any one of the x coordinate and the y coordinate of one coordinate point configuring the drawn image is outside the display screen area, the one coordinate point is judged as being present outside the display screen area. 
     When it is judged that not all of the positions denoted by the coordinate values of the changed drawn image configuration information are outside the display screen area, that is, when it is judged that at least part of the positions denoted by the coordinate values is inside the display screen area (No in Step S 811 ), the process branches to Step S 812 . In Step S 812 , the snapshot history information processing unit  628  extracts the drawn image configuration information on a drawn image displayable inside the display area of a display unit  650  from the changed drawn image configuration information, and registers the extracted drawn image configuration information and the image frame identification information of the image frame designated by the first image frame storage variable to be associated with each other in the snapshot history information table. In Step S 813 , the snapshot history information processing unit  628  stores the drawn image configuration information, that is on the drawn image displayable inside the display area of the display unit  650 , extracted in Step S 812  in the snapshot information storage unit  618 . Thus, when the drawn image is displayed in conjunction with the background image moved by scrolling, it is possible to appropriately display the drawn image displayable inside the display screen area of the image display device. 
     In Step S 814 , the snapshot history information processing unit  628  causes the mode switching control unit  638  to switch the operation mode of the image display device  600  to the second state. Thereafter, Step S 704  in  FIG. 7  is executed. 
     Meanwhile, when it is judged that all of the positions denoted by the coordinate values in the changed drawn image configuration information are outside the display screen area (Yes in Step S 811 ), the process branches to Step S 815 . In Step S 815 , the snapshot history information processing unit  628  deletes the drawn image configuration information stored in the snapshot information storage unit  618 . In Step S 816 , the snapshot history information processing unit  628  causes the mode switching control unit  638  to switch the operation mode of the image display device  600  to the first state. Thereafter, Step S 704  in  FIG. 7  is executed. 
       FIG. 9  is a diagram illustrating an embodiment of an image displayed on an image display device. An image  910  represents an image frame in which a drawn image is not written. The image  910  is the same as an image frame displayed on the user PC that provides the image frame. An image  920  represents an image in which a drawn image  922  is additionally written on the image  910  on the image display device. An image  930  represents an image in which the image  920  on the display device moves upward as the image frame on the user PC is upward scrolled. In the present embodiment, as illustrated in  FIG. 9 , the drawn image  922  can be moved in conjunction with a change in the image frame. 
     According to the present invention, it is possible to provide an image display device, an image display system, an image display method, a program, and a recording medium in which a drawn image drawn on a background image displayed in the past can be displayed again, and a drawn image can be displayed in conjunction with movement of a background image. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.