Patent Publication Number: US-2020296145-A1

Title: Communication terminal, communication system, and display control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-046139, filed on Mar. 13, 2019, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a communication terminal, a communication system, and a display control method. 
     Description of the Related Art 
     Communication systems are now in widespread use, which allows communication terminals at remote locations to communicate through a communication network such as the Internet. Examples of the communication systems include a conference system that performs a video conference. In a general conference system, a communication terminal used by one of parties involved in the videoconference converts an image or sound of a subject in a conference room into digital data and transmits the digital data to a communication terminal used by the other party. The communication terminal used by the other party displays an image on a display and outputs a sound from a speaker based on the received digital data. This enables to carry out a conference between remote locations, in a state close to an actual conference. Further, one communication terminal transmits image data indicating documents or the like being held or displayed on the one communication terminal to the other communication terminal through a communication network, to cause the image data to be shared between the communication terminals. 
     Furthermore, in recent years, communication terminals such as electronic whiteboards are widely used in companies, educational institutions, or government institutions. The electronic whiteboards display an image on a display and allows users to draw stroke images such as text, numbers, figures, or the like on the image. The communication terminal electronically converts content drawn by a user&#39;s operation of touching and moving an electronic pen or the user&#39;s hand on a surface of a display of the communication terminal, to generate stroke data such as coordinates data. The stroke image is rendered based on the stroke data. When a stroke image is drawn on one communication terminal, stroke data for reproducing the stroke image is transmitted to the other communication terminal through a communication network. The other communication terminal displays the same stroke image based on the received stroke data. 
     The techniques as describe heretofore enable one communication terminal to transmit stroke data for reproducing a stroke image as well as image data of documents such as documents (e.g., slides for presentation) that are stored in the communication terminal or that are currently being displayed at the communication terminal through a communication network. The other communication terminal on the receiving side displays a document image of the documents or a screen background as well as the stroke image. Still further, a technique is known that enables a communication terminal of a user who joins a remote conference part way through to receive stroke data already generated in the remote conference. 
     SUMMARY 
     According to one or more embodiments, a communication terminal is communicable with another communication terminal. The communication terminal includes circuitry configured to: start a download of a plurality of stroke data items corresponding to a plurality of stroke images drawn at the another communication terminal from a communication management system that stores data to be shared between the communication terminal and the another communication terminal; receive capture image data obtained by capturing a display screen displayed at the another communication terminal by the another communication terminal, from an image storage device that stores the capture image data; when the download of the plurality of stroke data items is not completed, control a display of the communication terminal to display a capture image corresponding to the received capture image data; and when the download of the plurality of stroke data items is completed, control the display to switch an image to be displayed on the display from the capture image currently being displayed to the plurality of stroke images corresponding to the downloaded plurality of stroke data items. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram illustrating a communication route, according to an embodiment of the present disclosure; 
         FIG. 2  is an illustration for explaining how an electronic whiteboard is used, according to an embodiment of the present disclosure; 
         FIG. 3  is a block diagram illustrating an example of a hardware configuration of the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 4  is a block diagram illustrating an example of a hardware configuration of a communication management system, a relay device, and an image storage device, according to an embodiment of the present disclosure; 
         FIG. 5  is a schematic diagram illustrating an example of a configuration of a communication system, according to an embodiment of the present disclosure; 
         FIG. 6A  and  FIG. 6B  are block diagrams illustrating an example of a functional configuration of the communication system, according to an embodiment of the present disclosure; 
         FIG. 7  is a conceptual diagram illustrating an authentication management table, according to an embodiment of the present disclosure; 
         FIG. 8  is a conceptual diagram illustrating a terminal management table, according to an embodiment of the present disclosure; 
         FIG. 9  is a conceptual diagram illustrating a destination list management table, according to an embodiment of the present disclosure; 
         FIG. 10  is a conceptual diagram illustrating a session management table, according to an embodiment of the present disclosure; 
         FIG. 11  is a conceptual diagram illustrating a relay device management table, according to an embodiment of the present disclosure; 
         FIG. 12  is a conceptual diagram illustrating a stroke data management table, according to an embodiment of the present disclosure; 
         FIG. 13  is a conceptual diagram illustrating a storage location information management table, according to an embodiment of the present disclosure; 
         FIG. 14  is a sequence diagram illustrating operation for preparing to start remote communication between electronic whiteboards, according to an embodiment of the present disclosure; 
         FIG. 15  is a diagram illustrating an example of destination list displayed on the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 16  is a sequence diagram illustrating operation of starting remote communication, according to an embodiment of the present disclosure; 
         FIG. 17  is a sequence diagram illustrating operation for sharing document image data and stroke data, according to an embodiment of the present disclosure; 
         FIG. 18  is a sequence diagram illustrating operation for sharing document image data and stroke data, according to an embodiment of the present disclosure; 
         FIG. 19A  is a diagram illustrating an example of a screen on the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 19B  is a diagram illustrating an example of another screen on the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 19C  is a diagram illustrating an example of another screen on the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 19D  is a diagram illustrating an example of another screen on the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 20  is a sequence diagram illustrating operation of sharing document image data and stroke data with a communication terminal that joins part way through an on-going remote conference, according to an embodiment of the present disclosure; 
         FIG. 21  is a sequence diagram illustrating operation of sharing document image data and stroke data with a communication terminal that joins part way through an on-going remote conference, according to an embodiment of the present disclosure; 
         FIG. 22  is a sequence diagram illustrating operation of sharing document image data and stroke data with a communication terminal that joins part way through an on-going remote conference, according to an embodiment of the present disclosure; 
         FIG. 23  is a diagram illustrating an example of a screen in which a capture image is displayed on the electronic whiteboard, according to an embodiment of the present disclosure; 
         FIG. 24  is a diagram illustrating an example of a screen in which a document image and a stroke image are displayed on the electronic whiteboard, according to an embodiment of the present disclosure, and 
         FIG. 25  is a diagram illustrating an example how plural images are superimposed in a display image displayed on the electronic whiteboard, according to an embodiment of the present disclosure. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Hereinafter, one or more embodiments of the present disclosure is described with reference to the drawings. 
     Overview of Communication System: 
     Communication Route: 
     A communication system for conducting a videoconference between a plurality of electronic whiteboards including an electronic whiteboard  1   a  and an electronic whiteboard  1   b  while drawing images on the electronic whiteboards is described with reference to  FIG. 1 .  FIG. 1  is a schematic diagram illustrating a communication route according to the present embodiment. The “videoconference” is sometimes called a “teleconference”. In this disclosure, a videoconference is described as an example, and other examples may be a meeting or a simple conversation. 
     The communication system includes a plurality of electronic whiteboard including the electronic whiteboards  1   a  and the electronic whiteboard  1   b , a relay device  3 , a communication management system  5 , and an image storage device  7 . The electronic whiteboard  1   a  and the electronic whiteboard  1   b  perform mutual communication of image data and audio data for calls and content data such as image data and stroke data for sharing. The stroke data is data necessary for reproducing a stroke image. The stroke data includes coordinate data, line width data, line color data, vector data, and the like. The electronic whiteboards  1   a  and the electronic whiteboard  1   b  exchange image data and audio data for a call with each other to reproduce an image and sound of a site where the communication counterpart resides, thereby making a remote video call. In this disclosure, an image of a site where each of the plurality of electronic whiteboards such as the electronic whiteboard la and the electronic whiteboard  1   b  reside may be referred to as a “site image”, hereinafter. Further, data for such site image may be referred to as “site image data”. 
     The electronic whiteboard  1   a  and the electronic whiteboard  1   b  exchange image data of a document image to be shared, thereby allowing participants using the communication system to share the same document image. The document image is an image displayed on the display of the electronic whiteboard  1 . For example, the document image is an image of a documents for the conference such as presentation slides, and a background image displayed on the display. Further, the electronic whiteboard  1   a  and the electronic whiteboard  1   b  exchange stroke data of a stroke image, thereby allowing participants using the communication system to share the same stroke image. The stroke image is a line or the like drawn by a user with a handwritten stroke with an electronic pen or the like. The stroke image is displayed by stroke data indicating points that indicate coordinates on the display. 
     Although in the example of  FIG. 1 , the communication system includes the two electronic whiteboards, i.e., the electronic whiteboard  1   a  and the electronic whiteboard  1   b , in another example, the communication system includes three or more electronic whiteboards. The electronic whiteboard  1   a  and the electronic whiteboard  1   b  are collectively referred to as an “electronic whiteboard  1 ” or “electronic whiteboards  1 ” hereinafter, to simplify the description, unless they need to be distinguished from one to another. 
       FIG. 1  illustrates an electronic whiteboard equipped with a videoconferencing function as an example of the electronic whiteboard  1   a  and the electronic whiteboard  1   b . A site image according to site image data can be either a moving image or a still image. 
     In this disclosure, an electronic whiteboard that sends a request for starting a videoconference is referred to as a “source terminal”, and an electronic whiteboard as a request destination to which the request is to be transmitted is referred to as a “destination terminal”. In  FIG. 1 , the electronic whiteboard  1   a  is the source terminal and the electronic whiteboard  1   b  is the destination terminal. When the start of the videoconference is requested from the electronic whiteboard  1   b , the electronic whiteboard  1   b  is the source terminal and the electronic whiteboard  1   a  is the destination terminal. Note that the electronic whiteboard  1   a  and the electronic whiteboard  1   b  may be used not only for communication between different offices or for communication between different rooms in the same office, but also for communication within the same room or for outdoor-indoor communication or outdoor-outdoor communication. 
     The relay device  3 , which is implemented by one or more computers, performs a process of relaying content data for a call between the electronic whiteboard  1   a  and the electronic whiteboard  1   b.    
     The communication management system  5  is implemented by one or more computers. The communication management system  5  centrally controls login authentication from the electronic whiteboard  1   a  and the electronic whiteboard  1   b , the communication status of each of the electronic whiteboard  1   a  and the electronic whiteboard  1   b , a destination list, the communication status of the relay device  3 , and the like. Further, the communication management system  5  relays stroke data to be shared between the electronic whiteboard  1   a  and the electronic whiteboard  1   b.    
     The image storage device  7 , which is implemented by one or more computers, stores image data of a document image to be shared, the image data being uploaded from the electronic whiteboard  1   a , and being downloaded to the electronic whiteboard  1   b , and vice versa. In other words, the image storage device  7  stores the image data uploaded from the electronic whiteboard  1   b , which is to be downloaded to the electronic whiteboard  1   a.    
     In one example, each of the relay device  3 , the communication management system  5 , and the image storage device  7  is configured as a single computer. In another example, each of the relay device  3 , the communication management system  5 , and the image storage device  7  is configured as a plurality of computers to which one or more units (functions, means, or storages) are arbitrarily allocated. In other words, each of the relay device  3 , the communication management system  5 , and the image storage device  7  can be implemented by a plurality of servers that operate in cooperation with one another. In this example, the communication management system  5  and the image storage device  7  is configured as a server system  6  that controls data to be shared the electronic whiteboard  1   a  and the electronic whiteboard  1   b . In the server system  6 , the communication management system  5  and the image storage device  7  may be configured as a single computer. 
     In the communication system of  FIG. 1 , a management information session sei for exchanging various types of management information is established between the electronic whiteboard  1   a  and the electronic whiteboard  1   b  through the communication management system  5 . In addition, four sessions are established between the electronic whiteboard  1   a  and the electronic whiteboard  1   b  to exchange four types of data including site image data of high resolution, site image data of medium resolution, site image data of low resolution, and audio data, through the relay device  3 . In  FIG. 1 , these four sessions are collectively referred to as an image and audio data session sed. Note that the video and audio data session sed does not necessarily have to be four sessions and may have a smaller or larger number of sessions than the four sessions. In addition, a communication session may be established directly between the source terminal and the destination terminal without using the relay device  3 . In still another example, the communication management system  5  can have functions of the relay device  3 . In this example, the image and audio data session sed is established between the electronic whiteboard  1   a  and the electronic whiteboard  1   b  through the communication management system  5 . 
     Further, in the communication system of  FIG. 1 , stroke data is exchanged between the electronic whiteboard  1   a  and the electronic whiteboard  1   b  using the management information session sei. 
     The description is now given of the resolution of an image of the site image data handled in the present embodiment. For example, the site image data of low resolution consists of 160 pixels in the horizontal direction and 120 pixels in the vertical direction. Such site image data of the low resolution is a base image. The site image data of medium resolution consists of, for example, 320 pixels in the horizontal direction and 240 pixels in the vertical direction. The site image data of high resolution consists of, for example, 640 pixels in the horizontal direction and 480 pixels in the vertical direction. In the case of a narrow band path, low-quality image data that only includes the site image data of low resolution as a base image is relayed. In the case of a relatively wide band path, intermediate-quality image data including the site image data of low resolution as a base image and the site image data of medium resolution is relayed. In the case of a very wide band path, high-quality image data including the site image data of low resolution as a base image, the site image data of medium resolution, and the site image data of high resolution is relayed. Since audio data is relatively small in data size compared with the site image data, the audio data is relayed even in the case of a narrow band path. 
       FIG. 2  is an illustration for explaining how an electronic whiteboard is used. As illustrated in  FIG. 2 , the electronic whiteboard  1  includes a plurality of legs  151  each having a plurality of casters on the lower side, a support  152  provided on the upper side of the legs  151 , and a main body  153  provided on top of the support  152 , and a display  180  provided on the front surface of the main body  153 . The main body  153  includes a central processing unit (CPU)  101  and the like which is described below. The user can input (draw) a stroke image such as characters on the display  180  using an electronic pen  190 . 
     Hardware Configuration: 
     A description is given hereinafter of a hardware configuration according to the present embodiment. 
     Hardware Configuration of Electronic Whiteboard: 
       FIG. 3  is a block diagram illustrating an example of a hardware configuration of the electronic whiteboard  1 , according to the embodiment. As illustrated in  FIG. 3 , the electronic whiteboard  1  includes a CPU  101 , a read only memory (ROM)  102 , a random access memory (RAM)  103 , a solid state drive (SSD)  104 , a network interface (I/F)  105 , and an external device connection I/F  106 . 
     The CPU  101  controls entire operation of the electronic whiteboard  1 . The ROM  102  stores a control program for controlling the CPU  101  such as an initial program loader (IPL) to boot the CPU  101 . The RAM  103  is used as a work area for the CPU  101 . The SSD  104  stores various data such as a control program for the electronic whiteboard  1 . The network I/F  105  controls communication with extraneous sources through a communication network  100 . The external device connection I/F  106  is an interface for connecting the electronic whiteboard to various extraneous sources. Examples of the extraneous sources include a universal serial bus (USB) memory  130  and external devices (a microphone  140 , a speaker  150 , and a camera  160 ). 
     The electronic whiteboard  1  further includes a capture device  111 , a graphics processing unit (GPU)  112 , a display controller  113 , a contact sensor  114 , a sensor controller  115 , an electronic pen controller  116 , a short-range communication circuit  119 , an antenna  119   a  of the short-range communication circuit  119 , a power switch  122 , and selection switches  123 . 
     The capture device  111  displays image data (image information) as a still image or a moving image on a display of a computer (PC)  170 , which is external to the electronic whiteboard  1 . The GPU  112  is a semiconductor chip dedicated to graphics processing. The display controller  113  controls display of screens to output an image output from the GPU  112  to the display  180  or the like. The contact sensor  114  detects a touch onto the display  180  with the electronic pen  190  or a user&#39;s hand H. The sensor controller  115  controls operation of the contact sensor  114 . The contact sensor  114  senses a touch input to a specific coordinate on the display  180  using the infrared blocking system. More specifically, the display  180  is provided with two light receiving elements disposed on both upper side ends of the display  180 , and a reflector frame surrounding the sides of the display  180 . The light receiving elements emit a plurality of infrared rays in parallel to a surface of the display  180 . The light receiving elements receive lights passing in the direction that is the same as an optical path of the emitted infrared rays, which are reflected by the reflector frame. The contact sensor  114  outputs an identifier (ID) of the infrared ray that is blocked by an object (such as the user&#39;s hand) after being emitted from the light receiving elements, to the sensor controller  115 . Based on the ID of the infrared ray, the sensor controller  115  detects a specific coordinate that is touched by the object. The electronic pen controller  116  communicates with the electronic pen  190  to detect a touch by the tip or bottom of the electronic pen  190  to the display  180 . The short-range communication circuit  119  is a communication circuit that communicates in compliance with the near field communication (NFC), the Bluetooth (Registered Trademark), and the like. The power switch  122  turns on or off the power of the electronic whiteboard  1 . The selection switches  123  are a group of switches for adjusting brightness, hue, etc., of display on the display  180 , for example. 
     The electronic whiteboard  1  further includes a bus line  110 . The bus line  110  is, for example, an address bus or a data bus, which electrically connects the elements such as the CPU  101  illustrated in  FIG. 3 . 
     The contact sensor  114  is not limited to the infrared blocking system type, and may be a different type of detector, such as a capacitance touch panel that identifies the contact position by detecting a change in capacitance, a resistance film touch panel that identifies the contact position by detecting a change in voltage of two opposed resistance films, or an electromagnetic induction touch panel that identifies the contact position by detecting electromagnetic induction caused by contact of an object to a display. In addition to or in alternative to detecting a touch by the tip or bottom of the electronic pen  190 , the electronic pen controller  116  may also detect a touch by another part of the electronic pen  190 , such as a part held by a hand of the user. 
     Hardware Configuration of Communication Management System, Relay Device, and Image Storage Device: 
       FIG. 4  is a block diagram illustrating an example of a hardware configuration of a communication management system  5 , according to the embodiment. The communication management system  5  is implemented by, for example, one or more computers. The computer includes a CPU  501 , a ROM  502 , a RAM  503 , a hard disk (HD)  504 , a hard disk drive (HDD) controller  505 , a display  506 , an external device connection I/F  508 , a network I/F  509 , a keyboard  511 , a pointing device  512 , a digital versatile disc-rewritable (DVD-RW) drive  514 , a medium I/F  516 , and a bus line  510 . 
     The CPU  501  controls entire operation of the communication management system  5 . The ROM  502  stores a control program such as an IPL to boot the CPU  501 . The RAM  503  is used as a work area for the CPU  501 . The HD  504  stores various data such as a communication control program. The HDD controller  505  controls reading or writing of various data to or from the HD  504  under control of the CPU  501 . The display  506  displays various information such as a cursor, menu, window, characters, or image. The external device connection I/F  508  is an interface that connects the computer as the communication management system  5  to various extraneous sources. The network I/F  509  is an interface for performing data communication using the communication network  100  such as the Internet. The keyboard  511  is one example of an input device (input means) provided with a plurality of keys for enabling a user to input characters, numerals, or various instructions. The pointing device  512  is an example of an input device (input means) that allows a user to select or execute a specific instruction, select a target for processing, or move a cursor being displayed. The DVD-RW drive  514  controls reading or writing (storing) of data to the DVD-RW. Alternatively, the DVD-RW drive  514  may control the reading or writing (storing) of data to a disc such as a Blu-ray (registered trademark) Disc Rewritable (BD-RE) instead of the DVD-RW drive. The medium I/F  516  controls reading or writing (storing) of data with respect to a storage medium  515  such as a flash memory. 
     The bus line  510  is an address bus, a data bus or the like, which electrically connects the elements illustrated in  FIG. 4  such as the CPU  501 . 
     The hardware configurations of the relay device  3  and the image storage device  7  illustrated in  FIG. 1  are the same or substantially the same as the hardware configuration of the communication management system  5 , and thus redundant description thereof is omitted below. The relay device  3 , however, stores a relay control program in the HD  504  in alternative to the communication control program. In addition, the image storage device  7  stores an image storage control program in the HD  504  in alternative to the communication control program. 
     Overview of Configuration of Communication System: 
     A description is now given of an overall configuration of the communication system according to embodiments of the present disclosure, with reference to  FIG. 5 .  FIG. 5  is a schematic diagram illustrating an overview of the communication system according to embodiments of the present disclosure. 
     In  FIG. 5 , the electronic whiteboard  1   a  is provided at a site A, the electronic whiteboard  1   b  is provided at a site B, and an electronic whiteboard  1   c  is provided at a site C. For example, the site A is a Tokyo office in Japan, the site B is a Beijing office in China, and the site C is a Washington D.C. office in the United States. User A 1  uses the electronic whiteboard  1   a  at the site A, Users B 1  and B 2  use the electronic whiteboard  1   b  at the site B, and User C 1  uses the electronic whiteboard  1   c  at the site C. 
     The electronic whiteboard  1   a , the electronic whiteboard  1   b , the electronic whiteboard  1   c , the relay device  3 , the communication management system  5 , and the image storage device  7  mutually communicate data through the communication network  100  such as the Internet. The communication network  100  may include a wireless network, in addition to a wired network. 
     The electronic whiteboard  1   a , the electronic whiteboard  1   b , and the electronic whiteboard  1   c  illustrated in  FIG. 5  are electronic whiteboards configured to perform video communication. 
     Functional Configuration of Communication System: 
     A description is now given of a functional configuration of the communication system according to embodiments of the present disclosure, with reference to  FIG. 6A  to  FIG. 13 .  FIG. 6A  and  FIG. 6B  are block diagrams illustrating an example of a functional configuration of the communication system according to embodiments of the present disclosure. 
     Functional Configuration of Electronic Whiteboard  1   a:    
     As illustrated in  FIG. 6A , the electronic whiteboard  1   a  includes a data exchange unit  11   a , an acceptance unit  12   a , an image/audio processing unit  13   a , a display control unit  14   a , a determination unit  15   a , a communication capacity measuring unit  16   a , an image processing unit  17   a , a short-range communication unit  18   a , and a storing/reading processing unit  19   a . These units are functions implemented by or caused to function by operating any of the hardware elements illustrated in  FIG. 3  in cooperation with the instructions of the CPU  101  according to the control program expanded from the SSD  104  to the RAM  103 . The electronic whiteboard  1   a  further includes a storage unit  1000   a , which is implemented by the RAM  103  illustrated in  FIG. 3  and the SSD  104  illustrated in  FIG. 3 . 
     Functional Units of Electronic Whiteboard  1   a:    
     A description is now given of the functional units of the electronic whiteboard  1   a  according to embodiments. The data exchange unit  11   a  exchanges various data (or information) with other terminals, apparatuses, systems, etc. through the communication network  100 . Further, the data exchange unit  11   a  also serves as a start unit and executes a process for starting communication with another communication terminal such as the electronic whiteboard  1   b . The acceptance unit  12   a  accepts various inputs from the user using the electronic pen  190  or the like. 
     The image/audio processing unit  13   a  performs main processing of the videoconferencing function. For example, the image/audio processing unit  13   a  performs digital processing such as encoding of site image data and audio data in accordance with the output signal of the microphone  140  and the output signal of the camera  160 . Further, the image/audio processing unit  13   a  generates an image signal and an audio signal based on the site image data and the audio data received at the data exchange unit  11   a . The image/audio processing unit  13   a  performs processing for combining site image data having different resolutions. 
     The display control unit  14   a  performs control for outputting an image signal or the like to the display  180 . The determination unit  15   a  makes various determinations. 
     The communication capacity measuring unit  16   a  measures a communication bandwidth of the management information session sei established between the electronic whiteboard  1   a  and the communication management system  5 . The communication capacity measuring unit  16   a  measures the communication bandwidth of a communication path that has received a band information request in response to the band information request received by the data exchange unit  11   a.    
     The image processing unit  17   a  performs main processing of the electronic whiteboard function. For example, the image processing unit  17   a  generates a stroke image and stroke data based on a stroke of the electronic pen  190  or the like accepted by the acceptance unit  12   a  or generates a stroke image based on stroke data received by the data exchange unit  11   a . The image processing unit  17   a  generates an image signal based on image data of a document image received by the data exchange unit  11   a.    
     The short-range communication unit  18   a  acquires and provides data through a short-range wireless communication network with each terminal having the short-range communication circuit. 
     The storing/reading processing unit  19   a  stores various data in the storage unit  1000 a or a storage medium  1010   a  such as the USB memory  130  and reads various types of data stored in the storage unit  1000   a  or the storage medium  1010   a.    
     Further, every time site image data and audio data are received in performing communication with other terminal, the storing/reading processing unit  19   a  overwrites the site image data and audio data stored in the storage unit  1000   a . The display  180  displays an image based on site image data before being overwritten, and the speaker  150  outputs sound based on audio data before being overwritten. 
     Further, stroke data corresponding to a stroke image drawn on the display  180  and document image data corresponding to a document image displayed on the display  180  are stored in the storage unit  1000   a.    
     Functional Configuration of Electronic Whiteboard  1   b:    
     As illustrated in  FIG. 6A , the electronic whiteboard  1   a  includes a data exchange unit  11   b , an acceptance unit  12   b , an image/audio processing unit  13   b , a display control unit  14   b , a determination unit  15   b , a communication capacity measuring unit  16   b , an image processing unit  17   b , a short-range communication unit  18   b , and a storing/reading processing unit  19   b . These units are functions implemented by or caused to function by operating any of the hardware elements illustrated in  FIG. 3  in cooperation with the instructions of the CPU  101  according to the control program expanded from the SSD  104  to the RAM  103 . The electronic whiteboard  1   b  further includes a storage unit  1000   b , which is implemented by the RAM  103  illustrated in  FIG. 3  and the SSD  104  illustrated in  FIG. 3 . The data exchange unit  11   b , the acceptance unit  12   b , the image/audio processing unit  13   b , the display control unit  14   b , the determination unit  15   b , the communication capacity measuring unit  16   b , the image processing unit  17   b , the short-range communication unit  18   b , the storing/reading processing unit  19   b , and the storage unit  1000   b  respectively have the same or substantially the same functions as the data exchange unit  11   a , the acceptance unit  12   a , the image/audio processing unit  13   a , the display control unit  14   a , the determination unit  15   a , the communication capacity measuring unit  16   a , the image processing unit  17   a , the short-range communication unit  18   a , the storing/reading processing unit  19   a , and the storage unit  1000   a  of the electronic whiteboard  1   a , and therefore the redundant descriptions thereof are omitted below. 
     Functional Configuration of Electronic Whiteboard  1   c:    
     As illustrated in  FIG. 6B , the electronic whiteboard  1   c  includes a data exchange unit  11   c , an acceptance unit  12   c , an image/audio processing unit  13   c , a display control unit  14   c , a determination unit  15   c , a communication capacity measuring unit  16   c , an image processing unit  17   c , a short-range communication unit  18   c , and a storing/reading processing unit  19   c . These units are functions implemented by or caused to function by operating any of the hardware elements illustrated in  FIG. 3  in cooperation with the instructions of the CPU  101  according to the control program expanded from the SSD  104  to the RAM  103 . For example, the CPU  101  of the electronic whiteboard  1   c  executes a program according to the present disclosure to implement a display control method according to the present disclosure. The electronic whiteboard  1   c  further includes a storage unit  1000   c , which is implemented by the RAM  103  illustrated in  FIG. 3  and the SSD  104  illustrated in  FIG. 3 . The data exchange unit  11   c , the acceptance unit  12   c , the image/audio processing unit  13   c , the display control unit  14   c , the determination unit  15   c , the communication capacity measuring unit  16   c , the image processing unit  17   c , the short-range communication unit  18   c , the storing/reading processing unit  19   c , and the storage unit  1000   c  respectively have the same or substantially the same functions as the data exchange unit  11   a , the acceptance unit  12   a , the image/audio processing unit  13   a , the display control unit  14   a , the determination unit  15   a , the communication capacity measuring unit  16   a , the image processing unit  17   a , the short-range communication unit  18   a , the storing/reading processing unit  19   a , and the storage unit  1000   a  of the electronic whiteboard  1   a , and therefore the redundant descriptions thereof are omitted below. 
     Functional Configuration of Relay Device: 
     As illustrated in  FIG. 6B , the relay device  3  includes a data exchange unit  31 , which also functions as a transfer unit, a determination unit  35 , and a storing/reading processing unit  39 . These units are functions implemented by or caused to function by operating any of the hardware elements illustrated in  FIG. 4  in cooperation with the instructions of the CPU  501  according to the relay control program expanded from the HD  504  to the RAM  503 . Further, the relay device  3  includes a storage unit  3000  implemented by the RAM  503  and the HD  504  illustrated in  FIG. 4 . 
     Functional Units of Relay Device: 
     A description is now given of the functional units of the relay device  3  in detail. The data exchange unit  31  of the relay device  3  illustrated in  FIG. 6B  exchanges various data (or information) with other terminals, devices, or systems through the communication network  100 . The data exchange unit  31  also functions as a transfer unit, and transfers site image data and audio data transmitted from one terminal to another terminal. The determination unit  35  performs various determinations such as determination of whether data transmission is delayed. 
     The storing/reading processing unit  39  stores various types of data in the storage unit  3000  or reads various types of data stored in the storage unit  3000 . 
     Functional Configuration of Communication Management System: 
     As illustrated in  FIG. 6A , the communication management system  5  includes a data exchange unit  51 , an authentication unit  52 , a generation unit  53 , a selector  54 , a determination unit  55 , and a storing/reading processing unit  59 . These units are functions implemented by or caused to function by operating any of the hardware elements illustrated in  FIG. 4  in cooperation with the instructions of the CPU  501  according to the communication control program expanded from the HD  504  to the RAM  503 . Further, the communication management system  5  includes a storage unit  5000 , which is implemented by HD  504  illustrated in  FIG. 4 . 
     Authentication Management Table: 
       FIG. 7  is an illustration of an example data structure of an authentication management table. The storage unit  5000  stores an authentication management database (DB)  5001 , which is implemented by the authentication management table as illustrated in  FIG. 7 . The authentication management table stores, for each one of the electronic whiteboards  1  managed by the communication management system  5 , a terminal ID of the electronic whiteboard and a password in association with each other. For example, the authentication management table illustrated in  FIG. 7  indicates that the terminal ID of the electronic whiteboard  1   a  (communication terminal) is “01aa” and the password of the electronic whiteboard  1   a  is “aaaa”. The password is an example of authentication information. The authentication information also includes an access token. 
     Terminal Management Table: 
       FIG. 8  is an illustration of an example data structure of a terminal management table. The storage unit  5000  stores a terminal management DB  5002 , which is implemented by the terminal management table as illustrated in  FIG. 8 . The terminal management table stores, for each one of the terminal IDs identifying the electronic whiteboards  1  (communication terminals), a terminal name to be used when each electronic whiteboard  1  is a destination terminal, an operating status of each electronic whiteboard  1 , reception date and time when login request described below is received by the communication management system  5 , and the internet protocol (IP) address of each electronic whiteboard  1  (communication terminal) in association with each other. For example, the terminal management table illustrated in  FIG. 8  indicates that the electronic whiteboard  1   a  whose terminal ID is “01aa” has the terminal name “JAPAN TOKYO OFFICE TERMINAL AA” and the operating status of the electronic whiteboard  1   a  is “Online (Ready)”. Further, the terminal management table indicates that the date and time when the login request is received by the communication management system  5  is “13:40 on Feb. 10, 2019” and the IP address of the electronic whiteboard  1   a  is “1.2.1.3”. Note that the terminal ID, the terminal name, and the IP address of the communication terminal are stored when each electronic whiteboard  1  is pre-registered to receive service provided by the communication management system  5 . 
     Destination List Management Table: 
       FIG. 9  is an illustration of an example data structure of a destination list management table. The storage unit  5000  stores a destination list management DB  5003 , which is implemented by the destination list management table as illustrated in  FIG. 9 . The destination list management table stores the terminal ID of the source terminal (electronic whiteboard  1 ) that sends a request for a start of communication in association with the terminal IDs of all destination terminals (electronic whiteboards  1 ) registered as destination terminal candidates for the source terminal. For example, the destination list management table illustrated in  FIG. 9  indicates that the destination terminal candidates to which the source terminal (electronic whiteboard  1   a ) whose terminal ID is “01aa” can send the request for the start of communication are the electronic whiteboard  1   b  whose terminal ID is “01ba” and other communication terminal such as the electronic whiteboard whose terminal IDs are “01ab”, “01bb”, etc. The destination terminal candidates are updated by addition or deletion in response to an addition or deletion request transmitted from any source terminal to the communication management system  5 . 
     The destination list is an example of destination information. In another example, the destination information can be managed in various ways other than the list format as illustrated in  FIG. 9 , as long as information on destination such as a terminal ID can be obtained. 
     Session Management Table: 
       FIG. 10  is an illustration of an example data structure of a session management table. The storage unit  5000  stores a session management DB  5004 , which is implemented by the session management table as illustrated in  FIG. 10 . The session management table stores, for each communication session ID for identifying a session to perform mutual communication between each electronic whiteboard  1  (communication terminal) and the relay device  3 , a relay device ID of the relay device  3  to be used, a source terminal ID of the electronic whiteboard  1  (source terminal), a destination terminal ID of electronic whiteboard  1  (destination terminal), a delay time (ms) in receiving site image data at the destination terminal, and reception date and time when delay information indicating this delay time is transmitted from the destination terminal and received by the communication management system  5 , in association. For example, the session management table illustrated in  FIG. 10  indicates that, in a communication session executed using the session ID “se01”, the relay device whose relay device ID is “111a” relays site image data and audio data between the electronic whiteboard whose terminal ID is “01aa” and the electronic whiteboard whose terminal ID “01db”. Further, for example, this session management table indicates that at the electronic whiteboard (destination terminal), the delay time of the site image data at “13:41 on Feb. 10, 2019” is 200 ms. 
     Relay Device Management Table: 
       FIG. 11  is an illustration of an example data structure of a relay device management table. The storage unit  5000  stores a relay device management DB  5005 , which is implemented by the relay device management table as illustrated in  FIG. 11 . The relay device management table stores, for each one of the relay devices  3 , the relay device ID, the operation status of the relay device  3 , the date and time when the communication management system  5  received status information indicating the operation status, an IP address of the relay device  3 , and maximum data transfer rate (Mbps) of the relay device  3 , in association. For example, the relay device management table illustrated in  FIG. 11  indicates that the relay device  3  whose relay device ID is “111a” has the operation status “Online”, the date and time when the communication management system  5  received the operation status is “13:30 on Feb. 10, 2019”, the IP address of the relay device  3  is “1.2.1.2”, and the maximum data transfer rate of the relay device  3  is 100 Mbps. 
     Stroke Data Management Table: 
       FIG. 12  is an illustration of an example data structure of a stroke data management table. The storage unit  5000  stores a stroke data management DB  5006 , which is implemented by the stroke data management table as illustrated in  FIG. 12 . The stroke data management table stores, for each session ID identifying a session to perform mutual communication between each electronic whiteboard  1  (communication terminal) and the relay device  3 , the session ID and stroke data in association with each other. The stroke data includes information of a stroke ID, a sequence number, a start time, an end time, a color of a drawn stroke, a width indicating a line width of the drawn stroke, vertices of the drawn stroke, and an image ID. The stroke ID identifies stroke data generated by a stroke drawing event. The sequence number indicates the order in which the stroke drawing events occur. The start time indicates the time when drawing of the stroke starts. The end time indicates the time when drawing of the stroke ends. The image ID identifies a document image, which is a background image on which the stroke is drawn. 
     In the embodiments, the “stroke drawing” is an input process of drawing data by a user. For example, the stroke drawing is an event detected from when a user presses the electronic pen  190  against the display  180  and moves the electronic pen  190  with the electronic pen in contact with the surface of the display  180  until when the user releases the electronic pen  190  from the display  180 . The color of the stroke is represented in a data format of RGBA (Red Green Blue Alpha), and each element is represented by a numerical value of 0 to 255. Further, the width of the line of the drawn stroke is represented by the number of pixels. The vertices of the drawn stroke are represented by X-Y coordinates. A Bezier curve obtained from the vertices forms a line segment indicating the stroke. Using this stroke data management table, the communication management system  5  manages stroke data generated during a videoconference or the like conducted through a particular communication session. 
     Storage Location Information Management Table: 
       FIG. 13  is an illustration of an example data structure of a storage location information management table. The storage unit  5000  stores a storage location information management DB  5007 , which is implemented by the storage location information management table as illustrated in  FIG. 13 . The storage location information management table stores, for each communication session ID identifying a session to perform mutual communication between each electronic whiteboard  1  and the relay device  3 , an image ID identifying image data, a uniform resource locator (URL) indicating a storage location where the image data is stored, and information on data type of the image data to be stored in the URL in association. 
     The URL is an example of storage location information. In another example, the storage location information includes a uniform resource identifier (URI). Examples of the data type include a document image and a capture image. The document image is an image displayed on the display  180  of the electronic whiteboard  1 , and also is a background image on which a stroke image is to be superimposed. The capture image is an image obtained by capturing a display screen displayed on the display  180  of the electronic whiteboard  1 . Using the storage location information management table, the communication management system  5  manages image data such as data of a document image or the like generated curing the videoconference or the like conducted through a particular communication session. 
     Functional Units of Communication Management System: 
     Referring again to  FIG. 6A , the data exchange unit  51  of the communication management system  5  transmits and receives various data (or information) to and from other terminals, devices, or systems through the communication network  100 . For example, the data exchange unit  51  transmits a capture image data generation request indicating a request for generation of capture image data to the electronic whiteboard  1  that is performing a remote conference. 
     The authentication unit  52  searches the authentication management DB  5001  stored in the storage unit  5000  using the terminal ID and password included in a login request received through the data exchange unit  51  as search keys, to perform authentication by determining whether the same pair of a terminal ID and a password are stored in the authentication management DB  5001 . The generation unit  53  generates a session ID for identifying a communication session based on a communication start request from the electronic whiteboard  1  (described below as step S 32 ). The selector  54  performs a process of selecting one relay device  3  from the plurality of relay devices  3 . The determination unit  55  performs various determinations. The storing/reading processing unit  59  stores various types of data in the storage unit  5000  or reads various types of data stored in the storage unit  5000 . 
     Functional Configuration of Image Storage Device: 
     As illustrated in  FIG. 6B , the image storage device  7  includes a data exchange unit  71  and a storing/reading processing unit  79 . These units are functions implemented by or caused to function by operating any of the hardware elements illustrated in  FIG. 4  in cooperation with the instructions of the CPU  501  according to the image storage control program expanded from the HD  504  to the RAM  503 . Further, the image storage device  7  includes a storage unit  7000 , which is implemented by the RAM  503  and the HD  504  illustrated in  FIG. 4 . The storage unit  7000  stores document image data or capture image data at a storage location indicated by the URL generated by the communication management system  5 . 
     Functional Units of Image Storage Device: 
     A description is now given of the functional units of the image storage device  7  in detail. The data exchange unit  71  of the image storage device  7  illustrated in  FIG. 6B  transmits and receives various data (or information) to and from other terminals, devices, or systems through the communication network  100 . The storing/reading processing unit  79  stores various types of data in the storage unit  7000  or reads various types of data stored in the storage unit  7000 . 
     Operation: 
     A description is now given of operations performed by the communication system according to embodiments of the present disclosure, with reference to  FIG. 14  to  FIG. 25 . 
     Operation of Preparing for Remote Communication: 
     First, a description is given of operation of preparing for remote communication, performed by the electronic whiteboard  1   a  as a login request sender terminal, with reference to  FIG. 14  and  FIG. 15 .  FIG. 14  is a sequence diagram illustrating operation for preparing to start remote communication between the electronic whiteboards.  FIG. 15  is a diagram illustrating an example of a destination list displayed on the electronic whiteboard  1   a . The electronic whiteboard  1   b  performs login operation in the same or substantially the same manner as the electronic whiteboard  1   a , and thus the redundant description thereof is omitted below. 
     In step S 11 , when the power switch  122  of the electronic whiteboard  1   a  is turned on, the acceptance unit  12   a  accepts a request to turn on the power of the electronic whiteboard  1   a.    
     In step S 12 , the data exchange unit  11   a  transmits login request information indicating a request for login authentication to the communication management system  5  through the communication network  100 . This login request information includes the terminal ID and password of the electronic whiteboard  1   a.    
     In step S 13 , the authentication unit  52  of the communication management system  5  searches the authentication management table ( FIG. 7 ) using the terminal ID and the password included in the login request information received through the data exchange unit  51  as search keys, to authenticate the communication terminal (electronic whiteboard  1 ) by determining whether the same terminal ID and the same password are stored in the authentication management table. In the embodiment, the description is continued assuming that the same terminal ID and the same password are stored in the authentication management table. 
     When the same terminal ID and the same password are stored in the authentication management table, the authentication unit  52  determines that the login request is received from the communication terminal (electronic whiteboard) that has a usage authorization. Accordingly, in step S 14 , the storing/reading processing unit  59  of the communication management system  5  updates the operation status associated with the terminal ID received in step S 12  in the terminal management table ( FIG. 8 ) “Online (Ready)”. Further, the storing/reading processing unit  59  stores the reception date and time when the login request information is received in step S 12  in the reception date and time field in the terminal management table. As a result, in the terminal management table, the terminal ID “01aa”, the operation status “Online (Ready)”, the reception date and time “2019.2.10.13:40”, and the IP address “1.2.1.3” are stored in association with each other. In another example, the IP address of the communication terminal (electronic whiteboard) may not be registered is transmitted from the electronic whiteboard  1   a  in step S 12 , instead of being stored in advance. 
     In step S 15 , the storing/reading processing unit  59  adds a new record including the terminal ID of the electronic whiteboard  1   a  received in step S 12  in the session management table as illustrated in  FIG. 10 . In step S 16 , the data exchange unit  51  of the communication management system  5  transmits the authentication result obtained in step S 13  to the electronic whiteboard  1   a  that has sent the login request through the communication network  100 . 
     In step S 17 , when the data exchange unit  11   a  of the login request sender terminal (electronic whiteboard  1   a ) receives the authentication result indicating that the communication terminal (electronic whiteboard  1   a ) has a usage authorization, the data exchange unit  11   a  transmits destination list request information indicating a request for a destination list to the communication management system  5  through the communication network  100 . Accordingly, the data exchange unit  51  of the communication management system  5  receives the destination list request information. 
     In step S 18 , the storing/reading processing unit  59  searches the destination list management table ( FIG. 9 ) using the terminal ID “01aa” of the login request sender terminal (electronic whiteboard  1   a ) as a search key, to read the terminal IDs of the destination terminal candidates that can communicate with the login request sender terminal (electronic whiteboard  1   a ). Further, the storing/reading processing unit  59  reads the destination names associated with the terminal IDs from the terminal management table ( FIG. 8 ). As a result, the terminal ID and terminal name of each of the destination terminal candidates corresponding to the terminal ID “01aa” of the login request sender terminal (electronic whiteboard  1   a ) are extracted. 
     In step S 19 , the data exchange unit  51  of the communication management system  5  reads destination list frame data and icon data indicating the operation status from the storage unit  5000  through the storing/reading processing unit  59 . In step S 20 , the data exchange unit  51  of the communication management system  5  transmits destination list information including the destination list frame, the icon, the terminal IDs, and the terminal names that are read by the storing/reading processing unit  59  to the login request sender terminal (electronic whiteboard  1   a ). Accordingly, the data exchange unit  11   a  of the login request sender terminal (electronic whiteboard  1   a ) receives the destination list information. In step S 21 , the storing/reading processing unit  19   a  stores the destination list information in the storage unit  1000   a.    
     As described above, in the present embodiment, the communication management system  5  centrally manages the destination list information of all terminals, instead of that each terminal manages the destination list information. For example, when a new electronic whiteboard  1  is added to the communication system, when a terminal already included in the communication system is replaced with a new model, or when appearance of the destination list frame is to be changed, since the communication management system  5  according to the present embodiment centrally manages all destination candidates, time and efforts for change the destination list information on each terminal are saved. 
     In step S 22 , the storing/reading processing unit  59  of the communication management system  5  searches the terminal management table ( FIG. 8 ) using the read-out terminal IDs of the destination terminal candidates as search keys, to read the operation statuses associated with the terminal IDs, thereby acquiring the operation status of each of the electronic whiteboards  1  as the destination terminal candidates. 
     In step S 23 , the data exchange unit  51  transmits terminal status information including the terminal IDs that are used as the search keys in step S 22  and the operation statuses of the corresponding destination terminals through the communication network  100  to the login request sender terminal (electronic whiteboard  1   a ). 
     In step S 24 , the storing/reading processing unit  19   a  of the login request sender terminal (electronic whiteboard  1   a ) successively stores the operation status information of the terminals received from the communication management system  5  in the storage unit  1000   a . As a result, the login request sender terminal (electronic whiteboard  1   a ) acquires the operation status of the destination terminal candidate (electronic whiteboard  1   b ) that can communicate with the login request sender terminal (electronic whiteboard  1   a ). 
     In step S 25 , the display control unit  14   a  of the login request sender terminal (electronic whiteboard  1   a ) generates a destination list reflecting the operation statuses of the destination terminal candidates based on the destination list information and the operation status information stored in the storage unit  1000   a . Further, the display control unit  14   a  controls the display  180  of the electronic whiteboard  1   a  to a destination list screen  1100  as illustrated in  FIG. 15 . On the destination list screen  1100 , the icon indicating the operation status, the terminal ID, and the terminal name are displayed for each destination terminal candidate. In the example of  FIG. 15 , the icons representing the operation statuses of the communication terminals displayed from the top of the screen indicate the operation status “Offline”, “Online (Ready)”, and “Online (Ready)”. 
     In step S 26 , the storing/reading processing unit  59  of the communication management system  5  searches the destination list management table ( FIG. 9 ) based on the terminal ID “01aa” of the login request sender terminal (electronic whiteboard  1   a ), to extract the terminal IDs of other communication terminals that have registered the terminal ID “01aa” of the login request sender terminal (electronic whiteboard  1   a ) as the destination terminal candidate. In the destination list management table illustrated in  FIG. 9 , the terminal IDs of other terminals to be read in step S 26  are “01ab”, “01ba”, and “01da”. 
     In step S 27 , the storing/reading processing unit  59  of the communication management system  5  searches the terminal management table ( FIG. 8 ) based on the terminal ID “01aa” of the login request sender terminal (electronic whiteboard  1   a ), to acquire the operation status of the login request terminal (electronic whiteboard  1   a ). 
     In step S 28 , the data exchange unit  51  transmits the terminal status information including the terminal ID “01aa” of the login request sender terminal (electronic whiteboard  1   a ) and the terminal status information including the operation status “Online” acquired in step S 27 , to the communication terminal(s) whose operation status is “Online” in the terminal management table ( FIG. 8 ) among the communication terminals identified by the terminal IDs extracted in step S 26 . When the data exchange unit  51  transmits the terminal status information to the electronic whiteboard  1   b , the IP address of the electronic whiteboard stored in the terminal management table ( FIG. 8 ) is referred to based on each terminal ID. Thus, the terminal ID “01aa” and the operation status “Online” of the login request sender terminal (electronic whiteboard  1   a ) are transmitted to each of the other destination terminals that can communicate with the login request sender terminal (electronic whiteboard  1   a ) as a destination terminal candidate. Accordingly, in step S 29 , the operation status of the other destination terminal candidate is displayed on the destination terminal candidate (such as the electronic whiteboard  1   b ). 
     Operation of Starting Remote Communication: 
     A description is now given of operation in which the electronic whiteboard  1   a  starts remote communication with the electronic whiteboard  1   b , with reference to  FIG. 16 .  FIG. 16  is a sequence diagram illustrating operation of starting remote communication. 
     In step S 31 , when the user A 1  of a request source terminal (electronic whiteboard  1   a ) selects the electronic whiteboard  1   b  by selecting a destination terminal candidate (terminal ID “01ba”) illustrated in  FIG. 15 , the acceptance unit  12   a  illustrated in  FIG. 6A  accepts a request to start communication with the selected destination terminal (electronic whiteboard  1   b ). In step  32 , the data exchange unit  11   a  of the request source terminal (electronic whiteboard  1   a ) transmits start request information indicating a request for starting communication to the communication management system  5 . The start request information includes the terminal ID “01aa” of the request source terminal (electronic whiteboard  1   a ) and the terminal ID “01ba” of the destination terminal (electronic whiteboard  1   b ). Accordingly, the data exchange unit  51  of the communication management system  5  receives the start request information and the IP address of the request source terminal (electronic whiteboard  1   a ) from which the start request information is transmitted. 
     In step S 33 , the storing/reading processing unit  59  changes the operation status in each of records including the terminal ID “01aa” and the terminal ID “01ba” to “busy” in the terminal management table ( FIG. 8 ) based on the terminal ID “01aa” of the request source terminal (electronic whiteboard  1   a ) and the terminal ID “01ba” of the destination terminal (electronic whiteboard  1   b ) included in the start request information. In this state, the request source terminal (electronic whiteboard  1   a ) and the destination terminal (electronic whiteboard  1   b ) have not started a videoconference but are in a call status. In this case, when a third electronic whiteboard (e.g., electronic whiteboard  1   c ) tries to make a call with the request source terminal (electronic whiteboard  1   a ) or the destination terminal (electronic whiteboard  1   b ), a notification sound or display indicating a busy status is output. 
     Next, a description is given of operation of executing a session for selecting the relay device  3  to be used. In step S 34 , the generation unit  53  of the communication management system  5  generates session ID used for execution of the session for selecting the relay device  3 . In the embodiment, a description is given of an example case in which the session ID “se01” is generated. 
     In step S 35 , the storing/reading processing unit  59  stores the session ID “se01” generated in step S 34 , the terminal ID “01aa” of the request source terminal (electronic whiteboard  1   a ), and the terminal ID “01ba” of the destination terminal (electronic whiteboard  1   b ) in association with each other in the session management table ( FIG. 10 ). 
     In step S 36 , the selector  54  of the communication management system  5  illustrated in  FIG. 6A  selects the relay device  3  for relaying a call between the two sites where the request source terminal (electronic whiteboard  1   a ) and the destination terminal (electronic whiteboard lb) are located, based on the relay device management table ( FIG. 11 ) and the terminal management table illustrated ( FIG. 8 ). Specifically, the selector  54  selects, among the relay device IDs corresponding to the relay devices whose operation statuses are “ONLINE” in the relay device management table ( FIG. 11 ), the relay device ID corresponding to the relay device  3  whose IP address is close to the IP address of the request source terminal (electronic whiteboard  1   a ) in the terminal management table ( FIG. 8 ). In the embodiment, the following description is given of an example case where the relay device  3  identified by the relay device ID “111a” is selected. 
     In step S 37 - 1 , when the relay device selection process in step S 36  is completed, the data exchange unit  51  of the communication management system  5  transmits relay device selection information to the request source terminal (electronic whiteboard  1   a ). The relay device selection information includes the IP address of the relay device  3  selected in step S 36  and the session ID “se01” generated in step S 34 . Thereby, the request source terminal (electronic whiteboard  1   a ) acquires the IP address of the communication management system  5 , which is the transmission source of the relay device selection information. 
     In step S 37 - 2 , the data exchange unit  51  of the communication management system  5  transmits the relay device selection information to the destination terminal (electronic whiteboard  1   b ). The relay device selection information includes the IP address of the relay device  3  selected in step S 36 , the terminal ID “01aa” of the request source terminal (electronic whiteboard  1   a ), and the session ID “se01” generated in step S 34 . Thereby, the destination terminal (electronic whiteboard  1   b ) acquires the IP address of the communication management system  5 , which is the transmission source of the relay device selection information, in execution of a session with the session ID “se01”. 
     In step S 38 - 1 , in response to the process of S 37 - 1 , the data exchange unit  11   a  of the request source terminal (electronic whiteboard  1   a ) transmits reception completion information indicating that the relay device selection information is received in step S 37 - 1  to the communication management system  5 . The reception completion information includes the session ID exchanged in the process of step S 37 - 1 . Thereby, the communication management system  5  acknowledges that the transmission of the relay device selection information, executed with the specific session ID “se01” has been completed. 
     In step  38 - 2 , in response to the process of step S 37 - 2 , the destination terminal (electronic whiteboard  1   b ) also transmits the reception completion information indicating that the relay device selection information has been received through the process of step S 37 - 2  to the communication management system  5 . The communication management system  5  acknowledges that the transmission of the relay device selection information, executed with the specific session ID “se01”, has been completed. 
     Thus, the electronic whiteboard  1   a  and the electronic whiteboard  1   b  perform a videoconference by exchanging site image data and audio data through the relay device  3  selected in step S 36 . 
     Operation of Communicating Document Image and Stroke Image: 
     A description is now given of operation of communicating a document image displayed on the electronic whiteboard  1  and a stroke image input to the electronic whiteboard  1 , with reference to  FIG. 17  to  FIG. 19D .  FIG. 17  and  FIG. 18  are sequence diagrams illustrating operation for sharing document image data and stroke data. In the present embodiment, the electronic whiteboard  1   a  and the electronic whiteboard  1   b  share data, such as the document image data and the stroke data. Here, a case where the document image and the stroke image displayed on the electronic whiteboard  1   a  are also displayed on the electronic whiteboard  1   b  is described. FIG. 19 A is a screen example of the electronic whiteboard  1   a .  FIG. 19B  is a screen example of the electronic whiteboard  1   b .  FIG. 19C  is another screen example of the electronic whiteboard  1   a .  FIG. 19D  is another screen example of the electronic whiteboard  1   b.    
     On the electronic whiteboard  1   a  at the site A, the screen illustrated in  FIG. 19A  is displayed on the display  180  by the display control unit  14   a . A document image d 1 , a site image v 2  of the site B, and a Share button b 1  to be pressed when sharing the document image d 1  are displayed on the electronic whiteboard  1   a  at the site A. In step S 51 , the acceptance unit  12   a  accepts an instruction to share data, such as the document image, in response to a pressing of the Share button b 1  by the user A 1  of the electronic whiteboard  1   a . In step S 52 , the data exchange unit  11   a  transmits request information indicating a request for a URL indicating the storage location of document image data, which is image data of the document image, to the communication management system  5 . Thereby, the data exchange unit  51  of the communication management system  5  receives the request information. The URL is an example of storage location information. In another example, the storage location information includes a URI. 
     In step S 53 , the generation unit  53  of the communication management system  5  generates a URL indicating the storage location of the document image data. In step S 54 , the storing/reading processing unit  59  stores the URL generated by the generation unit  53  in the storage location information management DB  5007  ( FIG. 13 ). Specifically, the storing/reading processing unit  59  stores, in the storage location information management table, the URL generated by the generation unit  53  in association with an image ID (e.g., “m001”), which is newly assigned, and a data type “document image”. In step S 55 , the data exchange unit  51  transmits the URL indicating the storage location of the document image data generated by the generation unit  53  to the electronic whiteboard  1   a . Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the URL indicating the storage location of the document image data. 
     In step S 56 , the data exchange unit  11   a  of the electronic whiteboard  1   a  transmits an upload start notice of the document image data to the communication management system  5 . Thereby, the data exchange unit  51  of the communication management system  5  receives the upload start notice. 
     In step S 57 , the data exchange unit  51  transfers the upload start notice to the electronic whiteboard  1   b  participating in the remote conference. Thereby, the data exchange unit  11   b  of the electronic whiteboard  1   b  receives the upload start notice. In step S 58 , the display control unit  14   b  of the electronic whiteboard  1   b  controls the display  180  of the electronic whiteboard  1  to display a screen as illustrated in  FIG. 19B . Specifically, in response to receiving the upload start notice in step S 57 , the display control unit  14   b  of the electronic whiteboard  1   b  displays an hourglass icon c 2  that visually indicates the progress in downloading the document image data, in addition to a site image v 1  of the site A and a Share button b 2 , which are displayed in advance. 
     In this example, elapsed time, which reflects the progress, can be indicated by a still-image hourglass or an animated hourglass with the change in amount of sand in the hourglass. The hourglass icon c 2  is an example of progress information indicating that downloading of the document image data is taking place. Other examples of progress information may be any icon other than hourglass icon, for example, a clock icon, character (and/or symbol) instead of icon, or combination of icon and character (and/or symbol). The progress information may be output as a notification by sound. In this case, instead of the display control unit  14   a , the image/audio processing unit  13   a  controls the speaker  150  to output the sound. 
     The electronic whiteboard  1   a  may perform the process of step S 52  after the process of step S 56 . The communication management system  5  may transmit the URL indicating the storage location of the document image data in step S 55  after receiving the upload start notice of the document image data in step S 56 . 
     In step S 59 , the data exchange unit  11   a  of the electronic whiteboard  1   a  transmits an upload request of the document image data to the communication management system  5 . This upload request includes the document image data and the URL indicating the storage location of the document image data received in step S 55 . Thereby, the data exchange unit  51  of the communication management system  5  receives the upload request from the electronic whiteboard  1   a . In step S 60 , the data exchange unit  51  of the communication management system  5  uploads the document image data to a storage location of the image storage device  7  indicated by the URL received in step S 59 . Thereby, the data exchange unit  71  of the image storage device  7  receives the document image data. In step S 61 , the storing/reading processing unit  79  of the image storage device  7  stores the document image data in the storage location indicated by the URL in the storage unit  7000 . In another example, the electronic whiteboard  1   a  may upload the document image data directly to the storage location indicated by the URL at the image storage device  7  not through the communication management system  5 . 
     In step S 62 , at the site A, the user A 1  moves the electronic pen  190  or the hand H in contact with the display  180  of the electronic whiteboard  1   a , and the acceptance unit  12   a  accepts an input of a stroke movement (trajectory). In step S 63 , the image processing unit  17   a  creates stroke data (for example, coordinate data (x, y)) for displaying a stroke image on the two-dimensional display  180  based on the stroke. The display control unit  14   a  controls the display  180  of the electronic whiteboard  1   a  to display the stroke image based on the stroke data. Thereby, a stroke image stA is displayed as illustrated in  FIG. 19C . 
     In step S 64 , the data exchange unit  11   a  transmits the stroke data generated in step S 63  for reproducing the stroke image to the communication management system  5 . Thereby, the data exchange unit  51  of the communication management system  5  receives the stroke data of the stroke image. In step S 65 , the storing/reading processing unit  59  of the communication management system  5  stores the stroke data received in step S 64  in the stroke data management DB  5006  ( FIG. 12 ). Specifically, as illustrated in  FIG. 12 , the storing/reading processing unit  59  stores, in the stroke data management table, items of the stroke data received by the data exchange unit  51  in the order of occurrence (sequence order) in association with the session ID of the communication session with which the remote conference is conducted. 
     In step S 66 , the data exchange unit  51  transfers the stroke data to the electronic whiteboard  1   b , which is the counterpart terminal. Thereby, the data exchange unit  11   b  of the electronic whiteboard  1   b  receives the stroke data. In step S 67 , the image processing unit  17   b  of the electronic whiteboard  1   b  generates a stroke image based on the stroke data, and the display control unit  14   b  controls the display  180  of the electronic whiteboard  1   b  to display the stroke image. Thereby, the same stroke image stA as that of displayed on the electronic whiteboard  1   a  as the counterpart terminal is displayed, as illustrated in  FIG. 19D . 
     Now referring to  FIG. 18 , in step S 71 , the data exchange unit  71  of the image storage device  7  transmits an upload completion notice indicating that the upload of the document image data is completed to the communication management system  5 . Thereby, the data exchange unit  51  of the communication management system  5  receives the upload completion notice. In step S 72 , the data exchange unit  51  of the communication management system  5  transfers the upload completion notice indicating that the document image data is completed to the electronic whiteboard  1   a . Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the upload completion notice. In another example, the image storage device  7  may transmit the upload completion notice directly to the electronic whiteboard la not through the communication management system  5 . In this case, in response to receiving the upload completion notice from the image storage device  7 , the electronic whiteboard la transmits, to the communication management system  5 , the upload completion notice including the URL indicating the storage location of the document image data received in step S 55 . 
     In step S 73 , the data exchange unit  51  of the communication management system  5  transmits the upload completion notice including the URL indicating the storage location of the document image data to the electronic whiteboard  1   b  as the counter terminal. Thereby, the data exchange unit  11   b  of the electronic whiteboard  1   b  receives the upload completion notice. 
     In step S 74 , the data exchange unit  11   b  of the electronic whiteboard  1   b  transmits download request information indicating a request for downloading the document image data by accessing the URL indicating a storage location at the image storage device  7  received in step S 73 . Thereby, the data exchange unit  71  of the image storage device  7  receives the download request information. 
     In step S 75 , the storing/reading processing unit  79  of the image storage device  7  reads the document image data, which is a request target, from the storage unit  7000  based on the URL. In step S 76 , the data exchange unit  71  transmits the requested document image data to the electronic whiteboard  1   b  as the request source terminal. Thereby, the data exchange unit  11   b  of the electronic whiteboard  1   b  downloads (receives) the document image data. In step S 77 , the display control unit  14   b  of the electronic whiteboard  1   b  displays the same document image as the document image d 1  illustrated in  FIG. 19C  on the screen illustrated in  FIG. 19D  and stops displaying the hourglass icon c 2  displayed until then. 
     Operation of Communicating Document Image and Stroke Image to Halfway Participating Terminal: 
     A description is now given of an example in which another electronic whiteboard  1  provided at another site joins part way through a remote conference (videoconference) that is already started among the plurality of electronic whiteboards  1  provided at plural sites, with reference to  FIG. 20  to  FIG. 25 .  FIG. 20  to  FIG. 22  are sequence diagrams illustrating operation of sharing document image data and stroke data with a communication terminal that joins part way through an on-going remote conference or the like.  FIG. 23  is a diagram illustrating an example of a screen in which a capture image is displayed on the electronic whiteboard  1   c .  FIG. 24  is a diagram illustrating an example of a screen in which a document image and a stroke image are displayed on the electronic whiteboard  1   c .  FIG. 25  is a diagram illustrating an example how plural images are superimposed in a display image displayed on the electronic whiteboard  1 . In the following, an example is described in which the electronic whiteboard  1   c  joins part way through a remote conference (videoconference) that is already conducted between the electronic whiteboard  1   a  and the electronic whiteboard  1   b . Any electronic whiteboard  1  that joins part way through an already-started remote conference (videoconference) may be referred to as a “halfway participating terminal” hereinafter. In the following example, the electronic whiteboard  1   c  is the halfway participating terminal. The timing when the halfway participating terminal joins the remote conference can be any time after the remote conference begins. 
     In step S 101 , the data exchange unit  11   c  of the electronic whiteboard  1   c , which is to join part way through an already-started remote conference (videoconference) between the electronic whiteboard  1   a  and the electronic whiteboard  1   b  transmits a conference participation request to the communication management system  5 . The following description is given on the assumption that at the electronic whiteboard  1   c , a communication session whose session ID “se01” is established by the operation described above with reference to  FIG. 14  and  FIG. 16 . 
     In step S 102 , the data exchange unit  51  of the communication management system  5  transmits a conference participation notice indicating that the electronic whiteboard  1   c  has participated in the remote conference, to the electronic whiteboard  1   a  that is conducting the remote conference using the communication session whose session ID “se01”. Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the conference participation notice from the communication management system  5 . In step S 103 , the data exchange unit  51  of transmits the conference participation notice also to the electronic whiteboard  1   b . Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   b  receives the conference participation notice from the communication management system  5 . Thus, the user A 1  of the electronic whiteboard  1   a  and the users B 1  and B 2  of the electronic whiteboard  1   b  recognizes that the electronic whiteboard  1   c  has participated in the remote conference. 
     In S 104 , the storing/reading processing unit  59  of the communication management system  5  searches the storage location information management table ( FIG. 13 ) using the session ID as a search key, to read a URL indicating a storage location where document image data generated during the remote conference is stored. In this example, the storing/reading processing unit  59  reads out the URLs corresponding to the image IDs “m001”, “m002”, and “m004” identifying the document images associated with the session ID “se01”. 
     In step S 105 , the data exchange unit  51  of the communication management system  5  transmits the URLs indicating the storage location of the document image data read in step S 104  to the halfway participating terminal (electronic whiteboard  1   c ). Thereby, the data exchange unit  11   c  of the electronic whiteboard  1   a  receives the URLs indicating the storage locations of the document image data. 
     In step S 106 , the data exchange unit  11   c  of the halfway participating terminal (electronic whiteboard  1   c ) transmits download request information indicating a request for downloading document image data by accessing the URLs indicating storage locations at the image storage device  7  received in step S 105 . Thereby, the data exchange unit  71  of the image storage device  7  receives the download request information. In step S 107 , the storing/reading processing unit  79  of the image storage device  7  reads the requested document image data from the storage unit  7000  based on the URLs received by the data exchange unit  71 . In step S 108 , the data exchange unit  71  starts transmitting the requested document image data to the electronic whiteboard  1   c , which is a request source terminal. Thereby, the data exchange unit  11   c  of the electronic whiteboard  1   c  starts downloading (receiving) the document image data. Further, in step S 109 , the data exchange unit  11   c  of the halfway participating terminal (electronic whiteboard  1   c ) transmits request information indicating a request for downloading stroke data to the communication management system  5 . Thereby, the data exchange unit  51  of the communication management system  5  receives the request information. In step S 110 , the storing/reading processing unit  59  of the communication management system  5  searches the stroke data management table ( FIG. 12 ) using the session ID as a search key, to read the stroke data generated during the remote conference. In this example, the storing/reading processing unit  59  reads the stroke data associated with the session ID “se01”. In step S 111 , the data exchange unit  51  starts transmitting the stroke data read in step S 110  to the electronic whiteboard  1   c . Thereby, the data exchange unit  11   c  of the electronic whiteboard  1   c  starts downloading (receiving) the stroke data. 
     Now referring to  FIG. 21 , in step  131 , the data exchange unit  51  of the communication management system  5  transmits a bandwidth information request indicating a request for bandwidth information to the electronic whiteboard  1   a . Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the bandwidth information request. In step S 132 , the communication capacity measuring unit  16   a  of the electronic whiteboard  1   a  measures the communication bandwidth of the communication path through which the bandwidth information request is received. In step S 133 , the data exchange unit  11   a  transmits bandwidth information indicating information on the communication bandwidth measured by the communication capacity measuring unit  16   a  to the communication management system  5 . In this example, the data exchange unit  11   a  transmits the bandwidth information indicating 2000 kbps, for example, as the communication bandwidth measured by the communication capacity measuring unit  16   a . Thereby, the data exchange unit  51  of the communication management system  5  receives the bandwidth information. 
     Also, in step  134 , the data exchange unit  51  of the communication management system  5  transmits a bandwidth information request indicating a request for bandwidth information to the electronic whiteboard  1   b . Thereby, the data exchange unit  11   b  of the electronic whiteboard  1   b  receives the bandwidth information request. In step S 135 , the communication capacity measuring unit  16   b  of the electronic whiteboard  1   b  measures the communication bandwidth of the communication path through which the bandwidth information request is received. In step S 136 , the data exchange unit  11   b  transmits bandwidth information indicating information on the communication bandwidth measured by the communication capacity measuring unit  16   b  to the communication management system  5 . In this example, the data exchange unit  11   b  transmits the bandwidth information indicating 512 kbps, for example, as the communication bandwidth measured by the communication capacity measuring unit  16   b . Thereby, the data exchange unit  51  of the communication management system  5  receives the bandwidth information. 
     In step S 137 , the determination unit  55  of the communication management system  5  performs a bandwidth determination process using the bandwidth information received in steps S 133  and S 136 . Specifically, the determination unit  55  identifies a communication terminal having the largest communication bandwidth based on the information on the communication bandwidths included in the received bandwidth information. In this example, since the communication bandwidth of the electronic whiteboard  1   a  is 2000 kbps and the communication bandwidth of the electronic whiteboard  1   b  is 512 kbps, the determination unit  55  identifies the electronic whiteboard  1   a  as the communication terminal having the largest communication bandwidth. 
     In step S 138 , the data exchange unit  51  of the communication management system  5  transmits a capture image data generation request indicating a request for generating capture image data, to the electronic whiteboard  1   a , which is identified by the determination unit  55  as the communication terminal having the largest communication bandwidth. Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the capture image data generation request. In step S 139 , the image processing unit  17   a  of the electronic whiteboard  1   a  performs capturing of a screenshot displayed on the display  180  to generate capture image data. In the embodiment, the term “screenshot” refers to a display screen (still image or moving image) displayed on the display  180 . The “capturing” refers to a process of acquiring the display screen displayed on the display  180  as image data. The capturing may be alternatively referred to as snapshot or image recognition. In step S 140 , the data exchange unit  11   a  transmits, to the communication management system  5 , request information indicating a request for a URL indicating a storage location where the capture image data generated by the image processing unit  17   a  is stored. Thereby, the data exchange unit  51  of the communication management system  5  receives the request information. 
     In step S 141 , the generation unit  53  of the communication management system  5  generates a URL indicating a storage location in which the capture image data is to be stored. In step S 142 , the storing/reading processing unit  59  stores the URL generated by the generation unit  53  in the storage location information management DB  5007  ( FIG. 13 ). In this example, the storing/reading processing unit  59  stores, in the storage location information management table, the URL generated by the generation unit  53  in association with an image ID (e.g., “m003”), which is newly assigned, and a data type “capture image”, as illustrated in  FIG. 13 . In step S 143 , the data exchange unit  51  transmits the URL indicating the storage location in which the capture image data is to be stored generated by the generation unit  53  to the electronic whiteboard  1   a . Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the URL indicating the storage location at which the capture image data is to be stored. 
     In step S 144 , the data exchange unit  11   a  transmits an upload request indicating a request for uploading the capture image data to the communication management system  5 . This upload request includes the capture image data and the URL indicating the storage location for the capture image data received in step S 143 . Thereby, the data exchange unit  51  of the communication management system  5  receives the upload request from the electronic whiteboard la. In step S 145 , the data exchange unit  51  of the communication management system  5  uploads the capture image data to the storage location of the image storage device  7  indicated by the URL included in the upload request received in step S 144 . Thereby, the data exchange unit  71  of the image storage device  7  receives the capture image data. In step S 146 , the storing/reading processing unit  79  of the image storage device  7  stores the capture image data in the storage location indicated by the URL in the storage unit  7000 . In another example, the electronic whiteboard  1   a  may upload the capture image data directly to the storage location indicated by the URL at the image storage device  7  not through the communication management system  5 . 
     In step S 147 , the data exchange unit  71  transmits an upload completion notice indicating that the upload of the capture image data has been completed to the communication management system  5 . Thereby, the data exchange unit  51  of the communication management system  5  receives the upload completion notice. In step S 148 , the data exchange unit  51  of the communication management system  5  transfers the upload completion notice indicating that the upload of the capture image data has been completed to the electronic whiteboard  1   a . Thereby, the data exchange unit  11   a  of the electronic whiteboard  1   a  receives the upload completion notice. In another example, the image storage device  7  may transmit the upload completion notice directly to the electronic whiteboard  1   a  not through the communication management system  5 . In this case, in response to receiving the upload completion notice from the image storage device  7 , the electronic whiteboard  1   a  transmits, to the communication management system  5 , the upload completion notice including the URL indicating the storage location of the capture image data received in step S 143 . 
     Note that the processes of steps S 131  to S 137  illustrated in  FIG. 21  may be omitted. For example, in a case where a time required for the processes of steps S 131  to S 137  illustrated in  FIG. 21  exceeds a time required for the processes of steps S 144  and S 145  illustrated in  FIG. 21 , there is small effect of reducing the time required for uploading the capture image data. In this case, the communication management system  5  may transmit the capture image data generation request to any one of the electronic whiteboards  1  that are conducting the remote conference in step S 138  of  FIG. 21 . 
     Now referring to  FIG. 22 , in step S 149 , the data exchange unit  51  of the communication management system  5  transfers the upload completion notice including the URL to the halfway participating terminal (electronic whiteboard  1   c ). Thereby, the data exchange unit  11   c  of the electronic whiteboard  1   c  receives the upload completion notice. 
     In step S 150 , the data exchange unit  11   c  of the electronic whiteboard  1   c  transmits download request information indicating a request for downloading the capture image data by accessing the URL received in step S 149  indicating the storage location at the image storage device  7 . Thereby, the data exchange unit  71  of the image storage device  7  receives the download request information. 
     In step S 151 , the storing/reading processing unit  79  of the image storage device  7  reads the capture image data, which is a request target, from the storage unit  7000  based on the URL. In step S 152 , the data exchange unit  71  transmits the requested capture image data to the electronic whiteboard  1   c  as the request source terminal. Thereby, the data exchange unit  11   c  of the electronic whiteboard  1   c  downloads (receives) the capture image data. In step S 153 , the display control unit  14   c  of the electronic whiteboard  1   c  controls the display  180  to display a capture image h 1  as illustrated in  FIG. 23 . The display image illustrated in  FIG. 23  includes the capture image h 1 , which is generated by the electronic whiteboard  1   a , and a notification image ml indicating that download of document image data and stroke data is in progress. 
     Thus, the user C 1  of the electronic whiteboard  1   c  recognizes that download of the document image data and the stroke data is in progress as well as grasps the outline of the remote conference so far from the capture image h 1 . The notification image ml illustrated in  FIG. 23  is just example, and any image, text, icon, or a combination thereof can be used provided that it can notify the user C 1  that data is being downloaded. For example, the notification image m 1  can be an indicator indicating the progress of the download, a numerical value of the percentage (%) of the downloaded data, or an hourglass icon. The notification image m 1  is an example of a predetermined message indicating that the download of the stroke data is in progress. 
     Note that the order of the processing from step S 104  to S 110  and the processing from steps S 131  to S 153  may be reversed or may be executed in parallel. 
     Further, the electronic whiteboard  1   c  downloads document image data and stroke data, while displaying the display image illustrated in  FIG. 23  on the display  180 . Since the communication management system  5  stores a plurality pieces of stroke data generated during the remote conference, and the data size of the plurality of pieces of stroke data to be downloaded is large. It takes more time to download stroke data than it does to download capture image data as described above and document image data from the image storage device  7 . For this reason, the electronic whiteboard  1   c  displays the display image as illustrated in  FIG. 23  on the display  180  until the download of all the stroke data is completed. 
     When download of the document image data is completed, the data exchange unit  11   c  of the electronic whiteboard  1   c  receives, from the image storage device  7 , a document image data transmission completion notice indicating that download of the document image data is completed. Further, when download of the plurality pieces of stroke data is completed, the data exchange unit  11   c  of the electronic whiteboard  1   c  receives a stroke data transmission completion notice indicating that download of the plurality of pieces of stroke data is completed. For example, the document image data transmission completion notice is transmitted in the same packet as the document image data. Alternatively, the document image data transmission completion notice may be transmitted in the different packet from a packet in which the document image data is transmitted. Similarly, for example, the stroke data transmission completion notice is transmitted in the same packet as the plurality of pieces of stroke data. Alternatively, the stroke data transmission completion notice may be transmitted in the different packet from a packet in which the plurality of pieces of stroke data is transmitted. 
     In step S 154 , in response to receiving the document image data transmission completion notice and the stroke data transmission completion notice by the data exchange unit  11   c , the display control unit  14   c  of the electronic whiteboard  1   c  controls the display  180  to display an image obtained by combining the document image d 2  and the stroke images (st 1  to st 5 ) as illustrated in  FIG. 24 , replaced with the capture image h 1  as illustrated in  FIG. 23 . The display image illustrated in  FIG. 24  includes the document image d 2  based on the document image data transmitted from the image storage device  7  and the stroke images st 1  to st 5  based on the stroke data transmitted from the communication management system  5 . 
     The display image illustrated in  FIG. 24  is configured by a layer for a stroke image and a layer for a document image, as illustrated in  FIG. 25 . The display image illustrated in  FIG. 24  is obtained by superimposing the stroke image as a first layer (closest layer viewed from a user) on the document image as a second layer (second closest layer viewed from the user). The stroke image is, for example, an image including drawing data received by the acceptance unit  12 . The document image is an image including image data displayed on the display  180 , and is an image arranged at the rearmost plane of the superimposed images. The electronic whiteboard  1   c  controls the display  180  to display the display image (shared image) including the document image d 2  and the stroke images st 1  to st 5  as illustrated in  FIG. 24 , and thereby making it possible to perform stroke drawing to be shared with other electronic whiteboards  1  at different sites. 
     As described heretofore, the halfway participating terminal (electronic whiteboard  1   c ) downloads document image data corresponding to a document image displayed on the electronic whiteboard  1   a  from the image storage device  7 , and downloads stroke data corresponding to stroke images drawn on the electronic whiteboard  1   a  and the electronic whiteboard  1   b  from the communication management system  5 . Further, the halfway participating terminal (electronic whiteboard  1   c ) downloads capture image data obtained by capturing a display screen displayed on the electronic whiteboard  1   a  or the electronic whiteboard  1   b  from the image storage device  7 . While download of the document image data and the stroke data is in progress, the halfway participating terminal (electronic whiteboard  1   c ) controls the display  180  to display the capture image h 1  corresponding to the capture image data. When the download of the document image data and the stroke data is completed, the electronic whiteboard  1   c  replaces the capture image h 1  displayed on the display  180  with the stroke images st 1  to st 5 ) and the document image d 2 . 
     In order to cause the halfway participating terminal (electronic whiteboard  1   c ) to display a shared image shared in a remote conference, all the records of document image data and stroke data generated in the remote conference have to be transmitted to the electronic whiteboard  1   c . For example, since the halfway participating terminal has to download all the records of stroke data to reproduce the state of the shared image, it takes a very long time for the halfway participating terminal to draw stroke images corresponding all pieces of stroke data. To address this issue, in the present embodiment, when the electronic whiteboard  1   c  joins part way through a remote conference, the electronic whiteboard  1   c  controls the display  180  to display the capture image h 1 , which is obtained by capturing the display screen displayed on the electronic whiteboard  1   a  (or the electronic whiteboard  1   b ), until download of stroke data is completed. Thus, the communication system according to the present embodiment can reduce the time to display an image representing the current state of the remote conference (a display screen displayed on other communication terminals) on the halfway participating terminal. This enables the user of the halfway participating terminal join the conference smoothly, thereby reducing the risk of inconsistency in communication. 
     Further, based on the bandwidth information indicating the communication bandwidths of the electronic whiteboard  1   a  and the electronic whiteboard  1   b  that are conducting the remote conference, the communication management system  5  requests the electronic whiteboard  1  (e.g., the electronic whiteboard  1   a ), which has a larger communication bandwidth, to generate capture image data obtained by capturing a display screen displayed on the electronic whiteboard  1   a . In response to receiving the request from the communication management system  5 , the electronic whiteboard  1   a  uploads the obtained capture image data to the image storage device  7 . The halfway participating terminal downloads the uploaded capture image data from the image storage device  7 . This reduces the time required for uploading the capture image data to the image storage device  7 , thereby further reducing the time to display the capture image h 1  on the halfway participating terminal. 
     Furthermore, the description given with reference to  FIG. 20  to  FIG. 25  is of the operation performed when document image data and stroke data generated during a remote conference conducted between the electronic whiteboard  1   a  and the electronic whiteboard lb are shared to the halfway participating terminal (electronic whiteboard  1   c ). However, the operation illustrated in  FIG. 20  to  FIG. 25  is not limited to the operation for the communication terminal that is to join part way through the remote conference. For example, the operation illustrated in  FIG. 20  to  FIG. 25  can be performed to share document image data and stroke data displayed on the display  180  of the starting terminal (electronic whiteboard  1   a ) before the start of a remote conference with the destination terminal (electronic whiteboard  1   b  or electronic whiteboard  1   c ) at the start of the remote conference. In this case, at the time of establishing a communication session between the starting terminal and the destination terminal, the starting terminal uploads the document image data and the stroke data to the server system  6 , and the destination terminal downloads the document image data and the stroke data uploaded to the server system  6 . 
     As described heretofore, the electronic whiteboard  1   c  (an example of a communication terminal) according to an embodiment of the present disclosure is a communication terminal that shares data with another communication terminals (e.g., the electronic whiteboard  1   a  or the electronic whiteboard  1   b ). The electronic whiteboard  1   c  starts downloading a plurality of stroke data items corresponding to a plurality of stroke images drawn on the other communication terminal from the communication management system  5  that stores data to be shared between with communication terminal and the other communication terminal. The electronic whiteboard  1   c  also receives capture image data obtained by capturing a display screen displayed on the other communication terminal by the other communication terminal, from the image storage device  7  that stores the capture image data. When download of the plurality of stroke data items is not completed, the electronic whiteboard  1   c  controls the display  180  (an example of a display unit) to display the capture image h 1  corresponding to the received capture image data. When the download of the plurality of stroke data items is completed, the electronic whiteboard  1   c  controls the display  180  to display the plurality of stroke images (st 1  to st 5 ) corresponding to the received plurality of stroke data items, replaced with the capture image h 1 . Thus, the electronic whiteboard  1   c  displays the display screen (capture image h 1 ) displayed on the other communication terminal until the download of the stroke data is completed. This enables the user of the electronic whiteboard  1   c  join the remote conference smoothly, thereby reducing the risk of inconsistency in communication. 
     Further, the electronic whiteboard  1   c  (an example of a communication terminal) according to an embodiment of the present disclosure receives, from the communication management system  5 , a URL (an example of storage location information) indicating a storage location where document image data corresponding to a document image displayed on another communication terminal (e.g., the electronic whiteboard  1   a  or the electronic whiteboard  1   b ), and transmits a download request for the document image data to the storage location indicated by the received URL at the image storage device  7 . The electronic whiteboard  1   c  starts downloading the document image data that is transmitted from the image storage device  7  in response to the download request. When the download of the plurality of stroke data items is completed and the download of the document image data is completed, the electronic whiteboard  1   c  controls the display  180  to display the plurality of stroke images (st 1  to st 5 ) superimposed on the document image data d 2 , in place of the capture image h 1 . Thus, the electronic whiteboard  1   c  displays the display screen (capture image h 1 ) displayed on the other communication terminal until the download of the document image data and the stroke data is completed, and when the download is completed, displays on the electronic whiteboard  1   c  a shared image on which a stroke to be shared with the other communication terminal can be drawn. Accordingly, the communication terminal displays various types of images based on various types of data shared between a plurality of communication terminals in a suitable manner, thereby implementing smooth communication. 
     Furthermore, the electronic whiteboard  1   c  (an example of a communication terminal) according to an embodiment of the present disclosure displays, from the start to the completion of download of the plurality of stroke data items, a notification image m 1  (an example of a predetermined message) indicating that the download of a plurality of stroke data items is in progress. Thus, the user C 1  of the electronic whiteboard  1   c  recognizes that download of the stroke data is in progress as well as grasps the current status of the remote conference from the capture image h 1 . 
     A communication system according to an embodiment of the present disclosure includes an electronic whiteboard  1   c  (an example of a communication terminal), another communication terminal (e.g., the electronic whiteboard  1   a  or the electronic whiteboard  1   b ), and the communication management system  5 . The communication management system  5  transmits a capture image data generation request to the other communication terminal in response to a request from the electronic whiteboard  1   c . The other communication terminal transmits the capture image data generated in response to the generation request transmitted from the communication management system  5  to the image storage device  7 . Thus, when the electronic whiteboard  1   c  joins part way through a remote conference, the communication system according to an embodiment of the present disclosure allows the other communication terminal to upload the capture image data generated by the other communication terminal to the image storage device  7 . Thereby, the communication system allows the electronic whiteboard  1   c  to acquire an image (a display screen displayed on the other communication terminal) indicating the current state of the remote conference and to display the acquired image on the electronic whiteboard  1   c.    
     Furthermore, the communication system according to an embodiment of the present disclosure includes a plurality of other communication terminals (e.g., the electronic whiteboard  1   a  and the electronic whiteboard  1   b ). Each of the plurality of other communication terminals transmits bandwidth information indicating a communication bandwidth used for sharing data to the communication management system  5 . The communication management system  5  transmits a generation request requesting generation of capture image data to a particular communication terminal (e.g., the electronic whiteboard  1   a ) of the plurality of other communication terminals, the particular communication terminal having the largest communication bandwidth indicated by the bandwidth information transmitted from the plurality of other communication terminals. The particular communication terminal transmits the capture image data generated in response to the generation request transmitted from the communication management system  5  to the image storage device  7 . Accordingly, the communication system according to an embodiment of the present disclosure reduces the time required for uploading the capture image data to the image storage device  7 , thereby further reducing the time to display the capture image h 1  on the electronic whiteboard  1   c.    
     Variations: 
     In the above-described embodiment, an office device that is an electronic whiteboard has been described as an example of a communication terminal, however, the communication terminal in the present disclosure is not limited to the office devices. Other examples of the communication terminal include a PC, a smartphone, a smartwatch, a car navigation system, a telepresence robot. Further, the communication terminal includes a medical device. In a case where the communication terminal is a medical device, the document image is a patient image. 
     When a number of stroke images are being drawn at a counterpart communication terminal, it takes time for a communication terminal to download all the stroke data generated during the remote conference. During such download time, the communication terminal cannot display a shared image that is to be shared with the counterpart communication terminal. Therefore, the user of the communication terminal cannot check the shared image to be shared with the counterpart communication terminal until all the stroke data has been downloaded. This may degrade smoothness of communication between the user of the communication terminal and the user of the counterpart communication terminal. 
     According to one or more embodiments, a communication terminal displays various types of images based on various types of data shared between a plurality of communication terminals in a suitable manner. This contributes to implementation of smooth communication. 
     Further, in the above embodiment, a case where the videoconference is performed by the communication system has been described. However, the present disclosure is not limited to this case and may be used for meetings, general conversations between family members, friends, remote diagnosis, or presentation of information in one direction. 
     The functions of each embodiment can be implemented by a computer executable program described in a legacy programming language such as assembler, C, C++, C#, Java (registered trademark) or an object-oriented programming language, etc. Programs for executing the functions of each embodiment can be distributed through telecommunication lines. 
     The programs for executing the functions of the embodiments may be stored and distributed on equipment readable recording media such as a ROM, an electrically erasable programmable read-only memory (EEPROM), an erasable programmable read-only memory (EPROM), a flash memory, a flexible disc, a compact disc-read only memory (CD-ROM), a compact disc-rewritable (CD-RW), a digital versatile disc-read only memory (DVD-ROM), a digital versatile disc-random access memory (DVD-RAM), a digital versatile disc-rewritable (DVD-RW), a Blu-ray disc (registered trademark), a Secure Digital (SD) card, a magneto-optical (MO) disc, and the like. 
     In addition, some or all of the functions of the embodiments may be mounted on a programmable device (PD) such as a field programmable gate array (FPGA) or implemented as an application specific integrated circuit (ASIC), and distributed by the recording medium as a circuit configuration data (bit stream data) downloaded to the PD in order to implement the functions of the embodiments on the PD, or as data described by Hardware Description Language (HDL), Very High Speed Integrated Circuits Hardware Description Language (VHDL), Verilog-HDL, etc. for generating circuit configuration data. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), digital signal processor (DSP), field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions.