Patent Publication Number: US-2009225153-A1

Title: Apparatus For Synthesizing Information, Apparatus For Controlling Information, And Computer Program Product

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for synthesizing information received, an apparatus for controlling information received, and a computer program product. 
     BACKGROUND ART 
     A multipoint teleconference system has been used to connect a plurality of teleconference terminals to one another via a network and exchange image data and sound data among the conference terminals. 
     Such a multipoint teleconference system often includes a media synthesizer. Resources of the media synthesizer used in the multipoint teleconference system are limited. An example of the media synthesizer is an image synthesizer that synthesizes the image data received from the conference terminal. 
     The image synthesizer receives the image data from the conference terminals, decompresses the received image data, synthesizes the decompressed data, compresses the synthesized data, and sends the compressed data. The image synthesizer uses software or hardware to perform the processes. However, the number of processable images is limited, and the number of the conference terminals that can be connected to the image synthesizer at the same time is also limited. 
     As described above, the image synthesizer can be connected to only limited number of conference terminals. However, there is a need to connect more conference terminals to a single image synthesizer. 
     JP-A H8-88842 (KOKAI) (hereinafter, “a first document”) discloses a technology that can realize a large conference by cascadingly connecting the image synthesizers (referred to as MCU&#39;s in the first document) to one another. 
     “Operating Instructions for SONY&#39;s Video Communication System PCS-111P, Chap. 8. “Multipoint Videoconference”; Sony Corporation; 2003” (hereinafter, “a second document”) describes a videoconference system that enables a multipoint videoconference at as many as ten locations by connecting a plurality of the image synthesizers to each other, each of which is connected to the limited number of the conference terminals. 
     According to the technologies described in the first and second documents, mapping between the media synthesizer and the conference terminals and mapping of the cascade connection between the media synthesizers is fixed in the multipoint teleconference system. In other words, a user needs to manually assign the conference terminals to be connected to each media synthesizer and the cascadingly connect between the media synthesizers. To realize the multipoint teleconference by taking advantage of the resources of the media synthesizer, a complicated setting work is required. 
     DISCLOSURE OF INVENTION 
     According to one aspect of the present invention, a communication controller that controls communications of devices via a multiplex communication network, the devices including at least two information synthesizers bilaterally connected with an exclusive line and at least two terminal devices that send information to be synthesized by the information synthesizers, the communication controller includes: a receiving unit that receives first ID and second ID from the information synthesizers, the first ID being ID of an information synthesizer bilaterally connected to one of the information synthesizers and second ID being ID of the one of the information synthesizers; a storage unit that stores the first ID received and the second ID received associated with each other; and an assigning unit that assigns a one of the terminal devices to the first information synthesizer and the other of the terminal devices to the second information synthesizer, when both of the first ID and the second ID received from the first information synthesizer match both of the first ID and the second ID received from the second information synthesizer. 
     According to another aspect of the present invention, an information synthesizer includes: a first communicating unit that receives ID of other information synthesizer, sends ID of the information synthesizer to the other information synthesizer, and receives first information to be synthesized via an exclusive line that bilaterally connects the information synthesizer to the other information synthesizer; a second communicating unit that sends the ID of the other information synthesizer to a communication controller along with the ID of the information synthesizer, receives the ID of a first terminal device from the communication controller, and receives second information to be synthesized from the first terminal device via a multiplex communication network for multiplex communication with the first terminal device, the communication controller, and the other information synthesizer; and a synthesizing unit that synthesizes the first information and the second information to generate synthesized information. 
     According to still another aspect of the present invention, a computer program product having a computer readable medium including programmed instructions for controlling communications of devices via a multiplex communication network, the devices including at least two information synthesizers bilaterally connected with an exclusive line and at least two terminal devices that send information to be synthesized by the information synthesizers, wherein the instructions, when executed by a computer, cause the computer to perform: receiving first ID and second ID from the information synthesizers, the first ID being ID of an information synthesizer bilaterally connected to one of the information synthesizers and second ID being ID of the one of the information synthesizers; storing the first ID received and the second ID received associated with each other; and assigning one of the terminal devices to the first information synthesizer and the other of the terminal devices to the second information synthesizer, when both of the first ID and the second ID received from the first information synthesizer match both of the first ID and the second ID received from the second information synthesizer. 
     According to still another aspect of the present invention, a computer program product having a computer readable medium including programmed instructions for synthesizing information to be synthesized and generating synthesized information, wherein the instructions, when executed by a computer, cause the computer to perform: receiving ID of an information synthesizer via an exclusive line; sending ID of the computer to the information synthesizer via the exclusive line; sending the ID of the information synthesizer to a communication controller via a multiplex communication network along with the ID of the computer; receiving ID of a terminal device from the communication controller via the multiplex communication network; receiving first information from the information synthesizer via the exclusive line; receiving second information to be synthesized from the terminal device via the multiplex communication network; sending the second information to the information synthesizer via the exclusive line; and synthesizing the first information and the second information to generate synthesized information. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of a multipoint teleconference system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of an image synthesizer shown in  FIG. 1 ; 
         FIG. 3  is a block diagram of a conference controller shown in  FIG. 1 ; 
         FIG. 4  is an example of a management table in a resource storage unit shown in  FIG. 3 ; 
         FIG. 5  is a flowchart of a registration process performed by the image synthesizer; 
         FIG. 6  is a sequence diagram of message transmission in the registration process; 
         FIG. 7  is a flowchart of a process of registering information to the resource storage unit; 
         FIG. 8  is an example of the management table after a registering unit shown in  FIG. 3  registers an entry of a first image synthesizer; 
         FIG. 9  is an example of the management table after the registering unit registers an entry of a second image synthesizer; 
         FIG. 10  is a flowchart of a process of the conference controller starting and terminating a conference; 
         FIG. 11  is a sequence diagram of the message transmission when the conference starts; 
         FIG. 12  is an example of the management table before the conference starts; 
         FIG. 13  is a flowchart of the image synthesizer synthesizing image data when it is clustered with another image synthesizer; 
         FIGS. 14A to 14D  are examples of a screen layout displayed on a conference terminal during the conference; 
         FIG. 15  is an example of a data configuration of layout information in a controlling unit of the image synthesizer; 
         FIG. 16  is an example of the data configuration of the layout information when the image synthesizer is clustered; 
         FIG. 17  is a sequence diagram of the message transmission when the conference is terminated; 
         FIG. 18  is a block diagram of hardware in the image synthesizer; and 
         FIG. 19  is a block diagram of hardware in the conference controller. 
     
    
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of the present invention are explained below in detail referring to the accompanying drawings. The explanation is given assuming that an information synthesizer according to the present embodiment is applied to an image synthesizer and an information controller according to the present embodiment is applied to a conference controller. The present invention is not limited to the embodiments explained below. The information synthesizer and the information controller can be applied to other devices. 
     As shown in  FIG. 1 , a multipoint teleconference system according to an embodiment of the present invention includes conference terminals  120   a  to  120   d , image synthesizers  110   a ,  110   b , and  110   c , a conference controller  100 , and a network  150  that connects them to one another. The network is a multiplex communication network. 
     The conference terminals  120   a  to  120   d  are used to participate in a teleconference. Each of the conference terminals  120   a  to  120   d  sends image data of a participant to an image synthesizer, receives image data of other participants from the image synthesizer, and displays the image data. 
     The image synthesizers  110   a ,  110   b , and  110   c  receive the image data from the conference terminals  120   a  to  120   d , synthesize image data of each conference terminal from the received image data, and send the synthesized data to the conference terminals  120   a  to  120   d.    
     The conference controller  100  controls connections between the conference terminals  120   a  to  120   d  and the image synthesizers  110   a ,  110   b , and  110   c  when the conference terminals  120   a  to  120   d  hold the conference. 
     Although the four conference terminals  120   a  to  120   d  are connected to the network in  FIG. 1 , the number of the conference terminals used in the teleconference is not limited to four. 
     The image synthesizers  110   a  and  110   b  are connected one-to-one via an exclusive line  151  so that they can communicate with each other via the exclusive line  151  for a so-called bilateral connection (hereinafter, “clustering”). The image synthesizer  110   c  is included in the multipoint teleconference system as an independent device that is not clustered with other image synthesizers. 
     To connect the conference controller  100 , the image synthesizers  110   a ,  110   b , and  110   c , and the conference terminals  120   a  to  120   d , a best-effort packet-switching network such as the Internet is used in the embodiment. The network that connects them is not limited to the packet-switching network, and any type of network can be used regardless of a wireless line or a wired line. 
     As shown in  FIG. 2 , the image synthesizer  110   a  includes a packet communicating unit  201 , a controlling unit  202 , an image-data communicating unit  203 , an image synthesizing unit  204 , image decompressing units  205   a  to  205   e , and image compressing units  206   a  to  206   e . The image synthesizer  110   b  includes the same units, and the explanation thereof is omitted. 
     The image synthesizer  110   a  basically follows an instruction from the conference controller  100 . It receives image data from conference terminals that are used in a teleconference specified by the conference controller  100 , synthesizes the received image data, and sends the synthesized data to other conference terminals. 
     The image decompressing units  205   a  to  205   e  perform decompression, and the image compressing units  206   a  to  206   e  perform compression. By using them, the image synthesizer  110   a  is connected to five conference terminals and performs the decompression and the compression of the image data with respect to each conference terminal. The number of the image decompressing units and the image compressing units depends on the number of the maximum conference terminals required by the image synthesizer  110   a , and it is not limited to five. 
     The packet communicating unit  201  includes a sending unit  221  and a receiving unit  222 . It sends and receives various types of data, session controlling information, or other protocol information with the conference terminals  120   a  to  120   d  via the network  150 . The types of the data will be explained with the sending unit  221  and the receiving unit  222 . 
     The packet communicating unit  201  uses communication protocols such as a session initiation protocol (SIP) (RFC3261: a signaling protocol to control initiation and termination of a session) standardized by Internet Engineering Task Force (IETF), a real-time transport protocol/RTP control protocol (RTP/RTCP) (RFC3550: a protocol to send and receive real-time data such as image and sound data or to monitor and control the real-time data), a transmission control protocol/user datagram protocol (TCP/UDP) that is a lower protocol to transfer protocol information, an internet protocol (IP), and Ethernet (registered trademark). 
     The protocols used by the packet communicating unit  201  comply with specifications of a conference system that uses image data, and other protocols can be used. 
     The sending unit  221  sends registration information for registering the image synthesizer  110   a  to the conference controller  100 , control data for connecting and disconnecting conference terminals, and image data synthesized from the image data sent from other conference terminals. 
     The registration information includes an identifier of the image synthesizer  110   a  (for example, a media access control (MAC) address) and an IP address. If the image synthesizer  110   a  is clustered with another image synthesizer, the registration information includes an identifier of the other image synthesizer. The registration process will be explained later. The registration information can include the maximum number of the resources. The maximum number of the resources is the number of the conference terminals that the image synthesizer  110   a  can operate, and it is five in the case of the image synthesizer  110   a.    
     The receiving unit  222  receives a request for starting the conference, the control data for connecting and disconnecting conference terminals from the conference controller  100 , and image data sent from the conference terminals  120   a  to  120   d.    
     The image decompressing units  205   a  to  205   e  decompress the image data received by the receiving unit  222 . A single image decompressing unit decompresses image data sent from a corresponding conference terminal. If the received image data is coded differently (for example, decompression coding method, resolution, frame rate, bit rate) with respect to each conference terminal, the image decompressing units  205   a  to  205   e  decompress each image data in an appropriate method. 
     The controlling unit  202  includes a detecting unit  211 , and controls the entire image synthesizer  110   a . The controlling unit  202  instructs the image-data communicating unit  203  to perform clustering and analyzes the protocol of the packet received by the packet communicating unit  201 . Moreover, the controlling unit  202  generates the registration information for registering the image synthesizer  110   a  to the conference controller  100 , and outputs it to the packet communicating unit  201 . The registration information is then sent by the sending unit  221 . 
     The controlling unit  202  issues an instruction of session control such as the start and the termination of the conference with other conference terminals based on the information from the conference controller  100 . 
     The controlling unit  202  controls to output the image data that the packet communication unit  201  received from other conference terminals to each image decompressing unit. The controlling unit  202  packetizes the compressed synthesized image data based on the protocol appropriate to the conference terminal to which it is sent, and outputs the packet to the packet communication unit  201 . The controlling unit  202  controls an image-synthesis arrangement pattern in the image synthesizing unit  204 . The controlling unit  202  instructs the image compressing units  206   a  to  206   e  and the image decompressing units  205   a  to  205   e  to set or change various parameters, methods, and the like. 
     The detecting unit  211  controls the image-data communicating unit  203  to detect an image synthesizer clustered via the exclusive line  151 . 
     The image-data communicating unit  203  exchanges the image data with the image synthesizer  110   b  clustered via an exclusive signal line other than the network  150 . The image-data communicating unit  203  sends the image data decompressed by the image decompressing units  205   a  to  205   e , and receives the image data decompressed by the image synthesizer  110   b.    
     The image-data communicating unit  203  detects the clustered image synthesizer and exchanges identifiers with the clustered image synthesizer  110   b  via the exclusive signal line. The identifier is a piece of unique information that identifies the image synthesizer, and a MAC address is used herein as the identifier. 
     A signal line for one-to-one connection is used as a clustering interface that clusters the image synthesizers, which is connected with a cable and connectors when the image synthesizers are installed. According to the embodiment, the image synthesizers are connected one-to-one. Therefore, for example, by the image-data communicating unit  203  monitoring voltage of the signal line for detection of the clustering, it is detected that the image synthesizers are clustered. Moreover, because the image synthesizers are connected one-to-one with the exclusive line  151 , it is guaranteed that the image-data communicating unit  203  exchanges information with an appropriate device without authorization or coding/decoding. 
     Apart from the embodiment, a wireless interface such as a wireless local area network (wireless LAN) or Bluetooth (registered trademark) can be used as the clustering interface. To cluster the image synthesizers using the wireless interface, a certain device is required to make sure that the connection is made with the desired image synthesizer or that appropriate information is exchanged with the appropriate image synthesizer. 
     Returning to the explanation of the embodiment, in the multipoint teleconference system, a plurality of image synthesizers clustered with one another can share the image data sent from a plurality of conference terminals. This allows more conference terminals to have a conference than the conference terminals that the image synthesizer  110   a  can handle alone. 
     The image synthesizing unit  204  synthesizes image data in different screen layouts for each conference terminal using the decompressed image data based on the instruction from the controlling unit  202 . The screen layout for each conference terminal will be explained later. 
     The image synthesizing unit  204  uses the image data received from the image-data communicating unit  203  as well as the image data decompressed by the image decompressing units  205   a  to  205   e . The image data from the image-data communicating unit  203  is decompressed by the image decompressing units of the other image synthesizer  110   b . In other words, the image synthesizer  110   a  synthesizes the image data of conference terminals that is not connected via the network  150  to it. 
     For example, it is assumed that the image synthesizer  110   a  is connected to the conference terminals  120   a  and  120   b , and the clustered image synthesizer  110   b  is connected to the conference terminals  120   c  and  120   d . The image synthesizer  110   a  receives the image data only from the conference terminals  120   a  and  120   b . However, the image synthesizer  110   a  receives the decompressed image data of the conference terminals  120   c  and  120   d  from the image synthesizer  110   b  via the exclusive signal line. In this manner, the image synthesizer  110   a  synthesizes all the image data from the conference terminals  120   a  to  120   d.    
     Because the image synthesizer  110   b  receives the decompressed image data of the conference terminals  120   a  and  120   b  from the image synthesizer  110   a  via the exclusive signal line, the image synthesizer  110   b  also synthesizes all the image data from the conference terminals  120   a  to  120   d . In this manner, even if the conference terminals  120   a  to  120   d  are connected to different image synthesizers, a synthesized image of the conference terminals  120   a  to  120   d  participating in a single conference is generated. 
     The image compressing units  206   a  to  206   e  compress the image data synthesized by the image synthesizing unit  204  with respect to each conference terminal connected to the image synthesizer  110   a . The image compressing units  206   a  to  206   e  compress the synthesized image data in a method appropriate to each conference terminal. 
     In other words, a single image compressing unit is dedicated to compress image data to be sent to a corresponding conference terminal. In this manner, the synthesized image data to be displayed in different layouts can be compressed in different compression coding methods or compression parameters with respect to each conference terminal. 
     For example, when the conference terminals  120   a  to  120   d  participate in a conference, the conference terminal  120   a  displays image data synthesized from the image data sent from the conference terminals  120   b ,  120   c , and  120   d , and the conference terminal  120   b  displays image data synthesized from the image data sent from the conference terminals  120   a ,  120   c , and  120   d.    
     Because each of the image compressing units  206   a  to  206   e  compresses image data of a single conference terminal, the compression coding method (for example, MPEG(Moving Picture Experts Group)-2, MPEG-4, H.263, H.264, and the like), the resolution, the frame rate, the bit rate, and the like can be set with respect to each conference terminal. 
     The image synthesizer  110   a  thereby generates and outputs image data in different layouts or image data compressed in different methods. 
     As shown in  FIG. 3 , the conference controller  100  includes a packet communicating unit  301 , a controlling unit  302 , a resource storage unit  303 , a layout managing unit  304 , and a clock  305 . The conference controller  100  controls the image synthesizer registered to the resource storage unit  303 , the connection between each conference terminal and the image synthesizer, the start of the conference, and the termination of the conference. 
     The conference controller  100  can control the screen layout of the synthesized image data. Each unit of the conference controller  100  can be realized by software. 
     The clock  305  measures the current time. The measured current time is used to control the time to start the conference and the like. 
     The resource storage unit  303  stores therein information of the image synthesizer that can perform the image synthesizing process. As shown in  FIG. 4 , a management table stored in the resource storage unit  303  associates an entry number, an identifier of the image synthesizer, an IP address of the image synthesizer, a maximum number of the resources, the number of the resources in use, a clustering flag, and an identifier of the clustered image synthesizer with one another. Each record is an entry indicative of registration of the image synthesizer. 
     The identifier of the image synthesizer uniquely identifies each image synthesizer. The identifier of the image synthesizer can be any kind of information that is guaranteed to be different from those of other image synthesizers. The MAC address assigned to the network interface of the image synthesizer is herein used as the identifier. 
     The IP address of the image synthesizer is used by each image synthesizer to communicate based on the IP. It can be used as self-ID (IDentification information). 
     The maximum number of the resources is the maximum number of the conference terminals that can be connected to the image synthesizer at the same time, which is five in the case of the image synthesizer  110   a  shown in  FIG. 2 . The maximum number of the resources is determined based on the hardware configuration of the image synthesizer and the like. 
     The number of the resources in use is the number of the resources consumed by the conference terminals currently connected to the image synthesizer. In general, one resource is consumed per conference terminal. However, more than one resource can be consumed if a single conference terminal participates in a plurality of conferences at a time. A difference between the maximum number of the resources and the number of the resources in use is an allowable number of the resources that can be used. 
     By storing the allowable number of the resources, upon receiving a request to start a conference from any one of the conference terminals  120   a  to  120   d , the conference controller  100  specifies the image synthesizer that holds the conference. Moreover, the multipoint teleconference system according to the embodiment can hold a plurality of conferences within the allowable number of the resources. 
     The clustering flag indicates whether the image synthesizer is clustered with another image synthesizer. 
     The identifier of the clustered image synthesizer is the identifier of another image synthesizer clustered therewith, if the image synthesizer is clustered with the other image synthesizer. It uniquely identifies the clustered image synthesizer. The identifier is defined as the same information as the identifier of the image synthesizer described above. The MAC address assigned to the network interface of the clustered image synthesizer is herein used as the identifier of the clustered image synthesizer. 
     Returning to the explanation of  FIG. 3 , the packet communicating unit  301  includes a sending unit  321  and a receiving unit  322 . The packet communicating unit  301  sends and receives various data, session control information for controlling the start and the termination of the conference between the image synthesizers  110   a ,  110   b , and  110   c  and the conference terminals  120   a  to  120   d , screen layout information for synthesizing the images, and other protocol information. The various data will be explained with the sending unit  321  and the receiving unit  322 . 
     Like the image synthesizer  110   a , the packet communicating unit  301  can use various communication protocols such as the SIP, the RTP/RTCP, the TCP/UDP, the IP, and the Ethernet (registered trademark). 
     The protocols used by the packet communicating unit  301  according to the embodiment comply with specifications of a conference system that uses image data, and other protocols can be used. 
     The receiving unit  322  receives the registration information from the image synthesizers  110   a ,  110   b , and  110   c , and receives a request for starting or terminating a conference from the conference terminals  120   a  to  120   d.    
     The sending unit  321  sends information of whether the conference can start to the conference terminals  120   a  to  120   d  and sends information to identify a conference terminal to be connected thereto to the image synthesizer that synthesizes the image data. The processes performed by the receiving unit  322  and the sending unit  321  will be explained later in detail. 
     The controlling unit  302  includes a registering unit  311 , a device extracting unit  312 , a determining unit  313 , and an assigning unit  314 , and controls the entire conference controller  100 . 
     The registering unit  311  registers the registration information received by the receiving unit  322  to the management table in the resource storage unit  303 . In other words, the registering unit  311  registers the identifier of the image synthesizer, the IP address of the image synthesizer, the maximum number of the image synthesizers, the number of resources in use, the clustering flag, and the identifier of the clustered image synthesizer associated with one another to the management table. The maximum number of the resources can be stored in the conference controller  100  in advance, or it can be sent from the image synthesizer. The registering unit  311  sets zero to the number of the resources in use as an initial value. The clustering flag will be described later. 
     If the received registration information includes the identifier of the clustered image synthesizer, the determining unit  313  determines whether the clustered image synthesizer is already stored in the resource storage unit  303 . 
     The determining unit  313  determines whether a combination of the identifier of the image synthesizer and the identifier of the clustered image synthesizer in the received registration information matches the same combination in any record stored in the resource storage unit  303 . 
     More specifically, the determining unit  313  determines whether the identifier of the clustered image synthesizer is included in the field of the identifier of the image synthesizer in the management table. If it is included, the determining unit  313  determines whether the identifier of the image synthesizer in the received registration information is included in the field of the identifier of the clustered image synthesizer in the record that includes the identifier. If it is included, the determining unit  313  determines that the clustered image synthesizer is already registered. 
     If the received registration information does not include any identifier of the clustered image synthesizer, or if the determining unit  313  determines that the clustered image synthesizer is not registered, the registering unit  311  sets “no” to the clustering flag as the initial value. 
     If the determining unit  313  determines that the clustered image synthesizer is already registered to the management table, the registering unit  311  sets “yes” to the clustering flag. In this case, the registering unit  311  updates the clustering flag in the record of the clustered image synthesizer to “yes”. By processing in this manner, the clustering flag indicates “yes” only when the image synthesizers recognize the clustering with each other. By checking the clustering flag, each of the image synthesizers identifies the image synthesizer recognized to be clustered with each other. The clustering flag does not indicate “yes” by only one of the image synthesizers erroneously detecting the clustered image synthesizer. As a result, accuracy of extracting the image synthesizers clustered with each other is improved by checking the clustering flag. 
     If the entry of the image synthesizer in the received registration information is already registered to the management table, the registering unit  311  updates the management table using the received registration information. Therefore, the latest information is recorded in the management table. 
     Using a receipt of the request for starting the conference from any one of the conference terminals  120   a  to  120   d  as a trigger, the device extracting unit  312  extracts the identifier, the IP address, and the like of the image synthesizer used to start the conference from the management table. 
     When the device extracting unit  312  extracts the image synthesizer to start the conference, the controlling unit  302  performs control to start the conference session between the conference terminals and the image synthesizer. 
     As the trigger to start the conference, other processes can be used instead of the receipt of the request for starting the conference from the conference terminals  120   a  to  120   d . For example, information reserved through a website can be used as the trigger to start the conference. 
     The assigning unit  314  determines the image synthesizer extracted by the device extracting unit  312  for assigning to synthesize images from conference terminals  120   a  to  120   d.    
     The layout managing unit  304  manages the screen layout information to determine the layout of the image data for each of the conference terminals in the conference. The layout managing unit  304  outputs the screen layout information to the sending unit  321 , and the sending unit  321  sends it to the image synthesizer. Therefore, the image synthesizer is controlled by the layout managing unit  304  to use the screen layout appropriate to each conference terminal to synthesize the image data. 
     In addition to direct control of the screen layout, the layout managing unit  304  can perform control based on a request from each conference terminal in the conference or an instruction from a conference terminal that leads the conference. The layout managing unit  304  can change the layout according to a voice detected from the conference terminal, i.e., a detected speaker. The layout managing unit  304  can control the screen layout based on an instruction from each conference terminal, if the image synthesizers  110   a ,  110   b , and  110   c  send different synthesized image data with respect to each conference terminal. 
       FIG. 5  is a flowchart of a registration process to the conference controller  100  performed by the image synthesizer  110   a . Although the explanation is given based on the image synthesizer  110   a , the same process is performed by other image synthesizers, and the explanation thereof is omitted. 
     Only by the registration from the image synthesizers  110   a ,  110   b , and  110   c , the conference controller  100  recognizes them. The recognition enables the conference controller  100  to connect the recognized image synthesizers  110   a ,  110   b , and  110   c  to the conference terminals  120   a  to  120   d  and start the conference. 
     The controlling unit  202  of the image synthesizer  110   a  performs initial setting at the time of startup (step S 401 ). The initial setting includes various network setup operations including setup of the IP address of the image synthesizer  110   a  and various hardware setup operations for the image synthesizer  110   a . To set the network for the image synthesizer  110   a  including the IP address and a default gateway, information acquired from a dynamic host configuration protocol (DHCP) server can be set, or the setup can be made based on information manually set by an administrator in advance. The DHCP server can send information of the conference controller  100  to be connected including the IP address and the port number to the image synthesizer  110   a  in addition to the network information. By using the DHCP to connect the network in the multipoint teleconference system in this manner, the image synthesizer connected to the system is easily operated with reduced load on the administrator. 
     The detecting unit  211  detects a clustered image synthesizer from the image-data communicating unit  203  (step S 402 ). The detection of the clustering is realized as a function of the image-data communicating unit  203 . For example, there is a method in which, in the exclusive line  151  used for the clustering, an electric signal is low when the device is not connected, and the electric signal is pulled up high when the device is clustered and the exclusive line  151  is physically conducted. However, any other method can be used. 
     When the detecting unit  211  detects the clustered image synthesizer (YES at step S 402 ), the image-data communicating unit  203  sends an identifier of the image synthesizer  110   a  to the clustered image synthesizer (step S 403 ). 
     The image-data communicating unit  203  receives an identifier of the clustered image synthesizer from the clustered image synthesizer (step S 404 ). 
     The sending unit  221  sends the registration information to the conference controller  100  (step S 405 ). The registration information includes the identifier of the image synthesizer  110   a , the IP address of the image synthesizer  110   a , and the maximum number of the resources if no clustered image synthesizer is detected. If a clustered image synthesizer is detected, the registration information includes the identifier of the image synthesizer  110   a , the IP address of the image synthesizer  110   a , the maximum number of the resources, and the identifier of the clustered image synthesizer. 
     The conference controller  100  thereby registers the image synthesizer  110   a  as an entry. The message transmission at the time of the registration will be explained later in detail. 
     The receiving unit  222  determines whether a message is received from the conference controller  100  as a request for starting the conference (step S 406 ). If it is not received (NO at step S 406 ), no particular process is performed. 
     If the receiving unit  222  receives the message from the conference controller  100  (YES at step S 406 ), a process for the conference is performed (step S 407 ). The process performed for the conference based on the message of starting the conference will be explained later in detail. 
     The detecting unit  211  detects any change in configuration of the clustered image synthesizer (step S 408 ). If there is no change (NO at step S 408 ), the detecting unit  211  determines whether the request for starting the conference is received. 
     If the detecting unit  211  detects any change in the configuration of the clustered image synthesizer (YES at step S 408 ), it determines again whether there is any clustering (step S 402 ). 
     In the multipoint teleconference system according to the embodiment, the image synthesizers  110   a ,  110   b , and  110   c  are automatically registered to the conference controller  100  in the process described above. Presence or absence of the clustering is also sent at the time of the registration. As a result, the conference controller  100  automatically recognizes the state of the image synthesizers  110   a ,  110   b , and  110   c  including the presence of the clustering. The multipoint teleconference system reduces the workload of the administrator manually registering each of the image synthesizers. 
     If the clustering is changed according to the processing procedure, the clustering is immediately detected and the registration information indicative of the change of the clustering is sent. The registering unit  311  of the conference controller  100  updates the management table in the resource storage unit  303  with the received registration information. Therefore, the conference controller  100  always recognizes the latest state of the image synthesizer. 
     Message transmission by the conference controller  100  to register the image synthesizers  110   a  and  110   b  is explained below referring to  FIG. 6 . 
     In the explanation of the embodiment, the SIP is used for controlling various sessions including the registration, the start of the conference, and the termination of the conference performed between the image synthesizers  110   a ,  110   b , and  110   c  and the conference controller  100 . The SIP is used to control a call in an IP network, and it is standardized by the Internet Engineering Task Force (IETF). The embodiment is not limited to use the SIP for controlling the sessions, and any other controlling method can be used. 
     The image synthesizers  110   a  and  110   b  detect the clustering with each other at the time of the startup based on the procedure shown in  FIG. 5 , and exchange the identifiers (steps S 501  and S 502 ). 
     The image synthesizer  110   a  sends a “REGISTER” message to the conference controller  100  (step S 503 ). 
     The conference controller  100  detects that the image synthesizer  110   a  requests registration by receiving the “REGISTER” message from the image synthesizer  110   a . The conference controller  100  sends a “200OK” message indicative of success to the image synthesizer  110   a  (step S 504 ). 
     By receiving the “200OK” message, the image synthesizer  110   a  recognizes that the request was accepted, and sends a “SUBSCRIBE” message to the conference controller  100  (step S 505 ). 
     The “SUBSCRIBE” message is a request for notification of an event. It includes the registration information such as the identifier of the image synthesizer  110   a  (00:00:aa:00:00:01), the IP address of the image synthesizer  110   a  (192.168.10.10), the maximum number of the resources (5), and the identifier of the clustered image synthesizer  110   b  (00:00:aa:00:00:02). 
     Upon receiving the “SUBSCRIBE” message, the conference controller  100  sends the “200OK” message to the image synthesizer  110   a  (step S 506 ). 
     The conference controller  100  registers the received registration information to the management table in the resource storage unit  303  (step S 507 ). The information registered to the management table and the process of registering it will be described later. 
     The image synthesizer  110   b  sends the “REGISTER” message to the conference controller  100  (step S 508 ). 
     When the conference controller  100  receives the “REGISTER” message from the image synthesizer  110   b , the conference controller  100  recognizes that the image synthesizer  110   b  requests the registration. The conference controller  100  sends the “200OK” message to the image synthesizer  110   b  (step S 509 ). 
     Upon receiving the “200OK” message, the image synthesizer  110   b  sends the “SUBSCRIBE” message to the conference controller  100  (step S 510 ). The “SUBSCRIBE” message sent from the image synthesizer  110   b  includes the registration information such as the identifier of the image synthesizer  110   b  (00:00:aa:00:00:02), the IP address of the image synthesizer  110   b  (192.168.10.11), the maximum number of the resources (5), and the identifier of the clustered image synthesizer  110   a  (00:00:aa:00:00:01). 
     Upon receiving the “SUBSCRIBE” message, the conference controller  100  sends the “200OK” message to the image synthesizer  110   b  (step S 511 ). 
     The conference controller  100  registers the received registration information to the management table (step S 512 ). Because the clustered image synthesizer  110   a  is already registered in this case, the management table is updated with “yes” set to the clustering flag and the like. The registration process will be explained later in detail. 
     By performing the process described above, the conference controller  100  registers the information of the image synthesizers  110   a  and  110   b  to the resource storage unit  303  along with the information that they are clustered. 
     In the sequence diagram shown in  FIG. 6 , the steps S 503  to S 506  and the steps S 508  to S 511  are performed in order with respect to each device. However, the process is indicated in a simple manner for convenience of explanation, and it can be performed in different orders. For example, the process by the image synthesizer  110   a  and the conference controller  100  and the process by the image synthesizer  110   b  and the conference controller  100  can be independently performed in parallel with each other. 
     A flowchart of a process of registering information received from the conference controller  100  to the resource storage unit  303  is explained based on  FIG. 7 . 
     The receiving unit  322  in the conference controller  100  determines whether the registration information is received from any one of the image synthesizers (step S 701 ). If the receiving unit  322  does not receive any registration information (NO at step S 701 ), the receiving unit  322  repeats the step S 701  until it receives the registration information. The receiving process corresponds to the receipt of the “SUBSCRIBE” message shown in  FIG. 6 . The conference controller  100  sends the “200OK” message (not shown). 
     If the receiving unit  322  receives the registration information (YES at step S 701 ), the determining unit  313  determines whether the registration information includes the identifier of the clustered image synthesizer (step S 702 ). 
     If the registration information includes the identifier of the clustered image synthesizer (YES at step S 702 ), the determining unit  313  determines whether the clustered image synthesizer is registered to the management table as an entry (step S 703 ). The determining unit  313  determines that the clustered image synthesizer is registered as the entry when it detects a record that includes the identifier of the clustered image synthesizer in the corresponding field of the management table. 
     For example, if the clustered image synthesizer is already registered as the entry at the step S 703 , the determining unit  313  determines whether the identifier of the clustered image synthesizer in the entry matches the identifier of the image synthesizer of which the registration information is sent. Only if they match, they both recognize the clustering with each other, and the process proceeds to a step S 704 . 
     If the registration information does not include the identifier of the clustered image synthesizer (NO at step S 702 ) or if the entry of the clustered image synthesizer is not registered (NO at step S 703 ), the registering unit  311  registers the identifier of the image synthesizer, the IP address of the image synthesizer, the maximum number of the resources, the number of the resources in use, the clustering flag “no”, and the identifier of the clustered image synthesizer in the registration information associated with one another (step S 706 ). The identifier of the image synthesizer, the IP address of the image synthesizer, the maximum number of the resources, and the number of the resources in use are same as those used at the step S 704 . The identifier of the clustered image synthesizer is always registered if it is included in the registration information. 
     In the example shown in  FIG. 4 , the conference controller  100  cannot detect a record that includes the same identifier of the clustered image synthesizer  110   b  (00:00:aa:00:00:02) in the registration information from the image synthesizer  110   a  in the field of the identifier of the image synthesizer at the step S 703 . This means that an entry indicative of the clustered image synthesizer  110   b  is not registered. In such a case, the clustering flag is set to “no”. 
     As shown in  FIG. 8 , in the management table after the entry of the image synthesizer  110   a  is registered, the clustering flag of the image synthesizer  110   a  is “no” because the entry of the image synthesizer  110   b  is not registered. 
     Returning to the explanation of  FIG. 7 , if the entry of the clustered image synthesizer is registered (YES at step S 703 ), the registering unit  311  registers the identifier of the image synthesizer, the IP address of the image synthesizer, the maximum number of the resources, the number of the resources in use, the clustering flag “yes”, and the identifier of the clustered image synthesizer in the registration information associated with one another (step S 704 ). The identifier of the image synthesizer, the IP address of the image synthesizer, the identifier of the clustered image synthesizer, and the maximum number of the resources are included in the registration information. The number of the resources in use is set to zero as an initial value. 
     The registering unit  311  updates the clustering flag of the entry of the clustered image synthesizer to “yes” in the management table (step S 705 ). 
     In the example shown in  FIG. 8 , the conference controller  100  confirms at the step S 703  that the identifier of the clustered image synthesizer  110   a  (00:00:aa:00:00:01) in the registration information of the image synthesizer  110   b  is registered to the management table as an entry (see an entry number  2  in the management table shown in  FIG. 8 ). In such a case, the clustering flag of the image synthesizer  110   b  is set to “yes”. In this case, the clustering flag of the image synthesizer  110   a  is also updated to “yes”. 
     As shown in  FIG. 9 , in the management table after the entry of the image synthesizer  110   b  is registered, the clustering flag of the image synthesizer  110   b  is “yes” (see the entry number  3 ) because the entry of the image synthesizer  110   a  is already registered. The clustering flag of the image synthesizer  110   a  is also updated to “yes” (see the entry number  2 ). 
     A flowchart of a process of the conference controller  100  starting and terminating a conference is explained using  FIG. 10 . 
     The receiving unit  322  in the conference controller  100  determines whether the request for starting a conference is received from a conference terminal (step S 1001 ). The request for starting the conference herein includes the number of the resources required to start the conference. 
     If the request is received (YES at step S 1001 ), the device extracting unit  312  extracts an image synthesizer that can spare the required number of the resources in the request from the management table (step S 1002 ). 
     If the device extracting unit  312  cannot extract the image synthesizer that can spare the required number of the resources (NO at step S 1002 ), the device extracting unit  312  extracts a plurality of image synthesizers clustered with each other that can spare the required number of the resources from the management table (step S 1003 ). In other words, though the conference cannot be held with a single image synthesizer, the device extracting unit  312  determines whether the conference can be held by combining a plurality of the image synthesizers. For example, if the total number of the resources of available image synthesizers clustered with each other is larger than the number of the resources in the request for starting the conference, the image synthesizers are extracted. The image synthesizers share the processes to start the conference. 
     If the device extracting unit  312  extracts one or more image synthesizers that can spare the number of the resources in the request (YES at step S 1002  or S 1003 ), the sending unit  321  sends a message that the conference can start to the conference terminal that requested to start the conference (step S 1004 ). The conference controller  100  then generates a session to start the conference (step S 1005 ). 
     If the device extracting unit  312  cannot extract a plurality of the image synthesizers (NO at step S 1003 ), the sending unit  321  sends a message that the conference cannot be held to the conference terminal (step S 1006 ). 
     If the received information is not a request for starting a conference (NO at step S 1001 ), the receiving unit  222  determines whether the received information is a request for terminating the conference (step S 1007 ). 
     If the received information is the request for terminating the conference (YES at step S 1007 ), the conference controller  100  performs control to terminate the current conference (step S 1008 ). The controlling unit  302  performs a process to reduce the number of the resources consumed by the conference in the entry of the image synthesizer used to hold the conference in the management table (step S 1009 ). At this point, the conference resource is released. 
     The message transmission by the conference controller  100  when the image synthesizers  110   a  and  110   b  synthesize image data in response to the request for starting a conference from the conference terminals  120   a  to  120   d  is explained below referring to  FIGS. 11 and 12 . 
     As shown in  FIG. 12 , according to the management table before the process shown in  FIG. 11  is performed, all the resources of the image synthesizer  110   c  with the entry number  1  are consumed. In the image synthesizer  110   a  with the entry number  2 , the maximum number of the resources is five, the number of the resources in use is three, and therefore the extra number of the resources is two. Similarly, in the image synthesizer  110   b  with the entry number  3 , the maximum number of the resources is five, the number of the resources in use is three, and therefore the extra number of the resources is two. The image synthesizers  110   a  and  110   b  are clustered with each other. 
     In other words, none of the image synthesizers can hold a conference among more than two conference terminals alone; however, a conference can be held among up to four conference terminals by the image synthesizers  110   a  and  110   b  sharing the synthesizing process. In this case, if the conference controller  100  receives the request for starting the conference from more than four conference terminals, it returns a message that the conference cannot be held because sufficient resources cannot be acquired even by using the clustering. 
     Although the SIP is used as a transmission control protocol in the sequence diagram shown in  FIG. 11 , the transmission control protocol is not limited to the SIP. 
     The conference terminal  120   a  sends a request for starting a conference with the conference terminals  120   b ,  120   c , and  120   d  to the conference controller  100  (step S 1101 ). The conference terminal  120   a  sends a “NOTIFY” message indicative of transmission of notification of the requested event as the request for starting the conference. The request can be sent from another conference terminal instead of the conference terminal  120   a.    
     The conference can be started using another incident as a trigger instead of the request sent from the conference terminal to the conference controller  100 . For example, in the case of a scheduled conference, the conference controller  100  can perform control to start the conference when the current time reaches the starting time of the scheduled conference stored in the conference controller  100 . Such a configuration naturally requires the clock  305  in the conference controller  100  to measure the current time. Because the conference is scheduled in advance, the conference is held with reduced load of a user and the resources to hold the conference can be saved in advance. 
     The conference controller  100  sends the “200OK” message to the conference terminal  120   a  for confirmation (step S 1102 ). 
     The conference controller  100  extracts an image synthesizer with available resources (step S 1103 ). For example, with the conference resources shown in  FIG. 12 , the device extracting unit  312  recognizes that a new conference can start by combining the remaining resources of the image synthesizers  110   a  and  110   b  that are clustered with each other. If the device extracting unit  312  recognizes that a new conference can start, the controlling unit  302  increases the consumed number of the resources in the corresponding record in the management table. The assigning unit  314  assigns the image synthesizer  110   a  for the conference terminals  120   a  and  120   b , and it assigns the image synthesizer  110   b  for the conference terminals  120   c  and  120   d.    
     To notify that the conference can be held, the sending unit  321  sends a “NOTIFY” message that indicates the conference can start to the conference terminal  120   a  (step S 1104 ). If there are not sufficient resources to start the conference, the sending unit  321  sends a “NOTIFY” message that indicates the conference cannot start. 
     Upon receiving the “NOTIFY” message, the conference terminal  120   a  sends the “200OK” message to the conference controller  100  for confirmation (step S 1105 ). 
     The conference controller  100  sends a “REFER(INVITE)” message to the image synthesizer  110   a  to establish a session between the image synthesizer  110   a  and the conference terminal  120   a  (step S 1106 ). The “REFER” message is referred to as a third-party call control, which is used by the conference controller  100  as the third party to control the establishment and the termination of the session between the image synthesizer  110   a  and the conference terminal  120   a.    
     Upon receiving the “REFER(INVITE)” message, the sending unit  221  of the image synthesizer  110   a  sends a “202ACCEPTED” message to the conference controller  100  (step S 1107 ). 
     The controlling unit  202  sets screen layout information for synthesizing the image in the “REFER(INVITE)” message to the image synthesizing unit  204  (step S 1108 ). 
     Because the received “REFER(INVITE)” message includes an instruction to the image synthesizer  110   a  to be connected to the conference terminal  120   a , the sending unit  221  sends an “INVITE” message to the conference terminal  120   a  to establish a session between the image synthesizer  110   a  and the conference terminal  120   a  (step S 1109 ). 
     Upon receiving the “INVITE” message, the conference terminal  120   a  sends the “200OK” message to the image synthesizer  110   a  (step S 1110 ). The conference terminal  120   a  starts to exchange image data with the image synthesizer  110   a  based on the RTP (step S 1111 ). The “INVITE” message can include various parameters of the image data such as the compression coding method, the resolution, the frame rate, and the bit rate. 
     Similarly, based on the third-party call control from the conference controller  100 , the session is established and the image is exchanged between the image synthesizer  110   a  and the conference terminal  120   b  (steps S 1112  to S 1117 ). 
     The image synthesizer  110   a  thereby acquires the image data from the conference terminals  120   a  and  120   b . After the image synthesizer  110   a  decompresses the received image data, the image-data communicating unit  203  of the image synthesizer  110   a  starts to send the decompressed image data to the image synthesizer  110   b  (step S 1118 ). However, not all of the decompressed image data need to be sent to the image synthesizer  110   b . For example, the image-data communicating unit  203  can send only image data requested by the image synthesizer  110   b.    
     The conference controller  100  sends a “REFER(INVITE)” message to establish a session between the image synthesizer  110   b  and the conference terminal  120   c  (step S 1119 ). 
     Upon receiving the “REFER(INVITE)” message, the sending unit  221  of the image synthesizer  110   b  sends the “202ACCEPTED” message to the conference controller  100  (step S 1120 ). 
     The controlling unit  202  of the image synthesizer  110   b  sets screen layout information in the “REFER(INVITE)” message for synthesizing the image to the image synthesizing unit  204  (step S 1121 ). 
     Because the received “REFER(INVITE)” message includes an instruction to be connected to the conference terminal  120   c , the sending unit  221  of the image synthesizer  110   b  sends the “INVITE” message to the conference terminal  120   c  to establish a session with the conference terminal  120   c  (step S 1122 ). 
     Upon receiving the “INVITE” message, the conference terminal  120   c  sends the “200OK” message to the image synthesizer  110   b  (step S 1123 ). The conference terminal  120   c  starts to exchange image data with the image synthesizer  110   b  based on the RTP (step S 1125 ). 
     Similarly, based on the third-party call control from the conference controller  100 , the session is established and the image is exchanged between the image synthesizer  110   b  and the conference terminal  120   d  (steps S 1126  to S 1130 ). 
     The image synthesizer  110   b  thereby acquires the image data from the conference terminals  120   c  and  120   d . After the image synthesizer  110   b  decompresses the received image data, the image-data communicating unit  203  of the image synthesizer  110   b  starts to send the decompressed image data to the image synthesizer  110   a  (step S 1131 ). However, not all of the decompressed image data need to be sent to the image synthesizer  110   a . For example, the image-data communicating unit  203  can send only image data requested by the image synthesizer  110   a.    
     According to the procedure described above, the conference can be held among the conference terminals  120   a  to  120   d . The conference terminals  120   a  and  120   b  are connected to the image synthesizer  110   a , and the conference terminals  120   c  and  120   d  are connected to the image synthesizer  110   b . In other words, the image synthesizers  110   a  and  110   b  receive difference image data via the network  150 . However, the image synthesizers  110   a  and  110   b  exchange decompressed image data via the exclusive line  151 . Therefore, the image data synthesized by the image synthesizing unit  204  in each image synthesizer to be sent to other image synthesizers includes all of the images of the conference terminals  120   a  to  120   d.    
     In the sequence diagram shown in  FIG. 11 , the steps S 1106  to S 1111 , the steps S 1112  to S 1117 , the steps S 1119  to S 1124 , and the steps S 1125  to S 1130  are performed in order with respect to each device. However, the process is indicated in a simple manner for convenience of explanation, and it can be performed in different orders. More specifically, the process performed by the image synthesizer  110   a , the conference controller  100 , and the corresponding conference terminals and the process performed by the image synthesizer  110   b , the conference controller  100 , and the corresponding conference terminals are independently performed in parallel with each other. 
       FIG. 13  is a flowchart of a process of synthesizing the image data from the conference terminals when the image synthesizer  110   a  is clustered with the image synthesizer  110   b  to share the decompression and the compression of the image data. The image synthesizers  110   b  and  110   c  can perform the same process when they are clustered with each other, and the explanation thereof is omitted. 
     The controlling unit  202  of the image synthesizer  110   a  determines whether the conference is terminated (step S 1301 ). The conference is terminated, for example, when the receiving unit  222  receives a request for terminating the conference. 
     If the conference is not terminated (NO at step S 1301 ), the controlling unit  202  determines whether the received information is image data from the connected conference terminals (step S 1302 ). 
     If the received information is the image data (YES at step S 1302 ), one of the image decompressing units  205   a  to  205   e  in charge of the image data decompresses the received image data (step S 1303 ). 
     If the clustered image synthesizer  110   b  operates the same conference, the image-data communicating unit  203  sends the decompressed image data (step S 1304 ). In this case, the image-data communicating unit  203  receives the decompressed image data (step S 1305 ). 
     The image synthesizing unit  204  of the image synthesizer  110   a  synthesizes the decompressed image data to generate a single image data with respect to each conference terminal (step S 1306 ). The image layout will be described later. 
     Each of the image compressing units  206   a  to  206   e  of the image synthesizer  110   a  compresses the synthesized image data (step S 1307 ). 
     The sending unit  221  sends the compressed image data with respect to each conference terminal (step S 1308 ). 
     The controlling unit  202  determines whether the receiving unit  222  has received the screen layout for synthesizing the images (step S 1309 ). If the screen layout is received (YES at step S 1309 ), the controlling unit  202  sets the layout information to the image synthesizing unit  204  (step S 1310 ). 
     The screen layout shown in  FIG. 14A  is realized by the controlling unit  202  setting as described above. The screen layout is not limited to the example shown in  FIG. 14A . For example, when a speaker is detected, an image of the speaker can be enlarged. 
     As shown in  FIG. 14B , the image data of the conference terminal  120   d  used by a participant D, who is the speaker, is enlarged and images of participants A to C can be shrunk to be displayed at the bottom. 
     In an example shown in  FIG. 14C , the images of the participants A to C are shrunk more than those in the example shown in  FIG. 14B . 
     In an example shown in  FIG. 14D , the images of the participants A to C are shrunk and displayed at the corners. As shown in  FIGS. 14A to 14D , the screen layout can change as needed. The screen layout is not limited to those described above, and any other layout can be used. 
     To set a layout, layout information is used. The layout information is sent from the conference controller  100  and stored in the controlling unit  202 . As shown in  FIG. 15 , an image number, a use status, a reduction ratio, and an arrangement position are associated with one another in the layout information with respect to each conference terminal. The information includes five images because the image synthesizer can connect five conference terminals. 
     In  FIG. 15 , screen size of the conference terminal is assumed 720×480 dots. Images  1  to  4  to be input to the image synthesizer hold a single conference, and therefore, the use status of the image data  1  to  4  indicates “yes”. The layout information is based on the quartered layout as shown in  FIG. 14A . The arrangement position in the layout information includes layer information as well as an X coordinate and a Y coordinate as an origin. The layer information indicates priority to be displayed. It indicates which image data has a higher priority to be displayed when the images are superposed. 
     According to the embodiment, because the image data is synthesized with respect to each conference terminal, self-image data can be excluded. For example, when there are the participants A to D, the image data to be sent to the participant A can be synthesized to display only the participants B to D. 
     Moreover, according to the embodiment, because the image data is synthesized based on the layout information for each conference terminal regardless of the image synthesizer, the image data is provided in any layout as the user likes. 
     A data configuration of the layout information shown in  FIG. 16  when the image synthesizer is clustered with another includes the same fields as the layout information shown in  FIG. 15 ; however, the number of the images is different. In other words, the image data can be synthesized using five image data from the image synthesizer and five image data from the clustered image synthesizer. The layout information is based on the quartered layout using the image data  1  and  2  from the image synthesizer and image data  1 ′ and  2 ′ from the clustered image synthesizer. 
     By setting the layout information in this manner, the image data can be synthesized in a layout proper to each conference terminal. 
     Returning to the explanation of  FIG. 13 , if the conference terminates (YES at step S 1301 ), the controlling unit  202  of the image synthesizer  110   a  performs control to stop the conference (step S 1311 ). 
     By performing the process as described above, the image synthesizers  110   a ,  110   b , and  110   c  synthesize image data received from the connected conference terminals based on the layout information, and send the image data in the layout proper to each conference terminal. When the image synthesizer does not share the decompression and the compression of the image data with another image synthesizer, the image-data communicating unit  203  does not send or receive the image data at the steps S 1304  and S 1305 ; however, other processes are performed as described above. 
     Message transmission performed when the multipoint teleconference system according to the embodiment terminates a conference is explained referring to  FIG. 17 . 
     The explanation is given assuming that the conference terminal  120   a  sends a request for terminating the conference among the conference terminals  120   a  to  120   d  to the conference controller  100 ; however, the message transmission is not limited to such a case. For example, another conference terminal can send the request instead of the conference terminal  120   a . Otherwise, the conference can be terminated using another incident as a trigger instead of one of the conference terminals  120   a  to  120   d  sending the request to the conference controller  100 . For example, the conference controller  100  can perform control to terminate the conference when the current time reaches the termination time of the scheduled conference stored in the conference controller  100  or the like. 
     Although the SIP is used as the transmission control protocol in the sequence diagram shown in  FIG. 17 , it is not limited to use the SIP. 
     During the conference, the conference terminals  120   a  and  120   b  exchange image data with the connected image synthesizer  110   a  (steps S 1401 , S 1402 ). The conference terminals  120   c  and  120   d  exchange image data with the connected image synthesizer  110   b  (steps S 1403 , S 1404 ). Decompressed image data is also exchanged between the image synthesizers  110   a  and  110   b  that are clustered with each other (step S 1405 ). 
     The conference terminal  120   a  sends a “NOTIFY” message indicative of a request for terminating the conference among the conference terminals  120   a  to  120   d  to the conference controller  100  (step S 1406 ). 
     The sending unit  321  in the conference controller  100  sends the “200OK” message to the conference terminal  120   a  for confirmation (step S 1407 ). 
     The sending unit  321  sends a “REFER(BYE)” message to terminate the session between the image synthesizer  110   a  and the conference terminal  120   a  (step S 1408 ). 
     Upon receiving the “REFER(BYE)” message, the controlling unit  202  of the image synthesizer  110   a  sends the “202ACCEPTED” message to the conference controller  100  (step S 1409 ). 
     Because the “REFER(BYE)” message instructs termination of the session between the image synthesizer  110   a  and the conference terminal  120   a , the image synthesizer  110   a  sends a “BYE” message to the conference terminal  120   a  to terminate the session with the conference terminal  120   a  (step S 1410 ). 
     Upon receiving the “BYE” message, the conference terminal  120   a  sends the “200OK” message to the image synthesizer  110   a  (step S 1411 ). 
     The conference terminal  120   a  thereby stops the exchange of the image data with the image synthesizer  110   a  based on the RTP (step S 1412 ). 
     By performing a process similar to the steps S 1408  to S 1412 , the conference controller  100  performs the third-party call control to terminate a session between the image synthesizer  110   a  and the conference terminal  120   b  and stop the transmission of the image data (steps S 1413  to S 1416 ). 
     The sending unit  321  sends a “REFER(BYE)” message to terminate a session between the image synthesizer  110   b  and the conference terminal  120   c  (step S 1418 ). 
     Upon receiving the “REFER(BYE)” message, the controlling unit  202  of the image synthesizer  110   b  sends the “202ACCEPTED” message to the conference controller  100  (step S 1419 ). 
     Because the image synthesizer  110   b  is instructed by the received “REFER(BYE)” message to terminate the session with the conference terminal  120   c , it sends the “BYE” message to the conference terminal  120   c  to terminate the session (step S 1420 ). 
     Upon receiving the “BYE” message, the conference terminal  120   c  sends the “200OK” message to the image synthesizer  110   b  (step S 1421 ). 
     The conference terminal  120   c  thereby stops the exchange of the image data with the image synthesizer  110   b  based on the RTP (step S 1422 ). 
     By performing a process similar to the steps S 1418  to S 1422 , the conference controller  100  performs the third-party call control to terminate a session between the image synthesizer  110   b  and the conference terminal  120   d  and stop the transmission of the image data (steps S 1423  to S 1427 ). 
     The conference controller  100  releases the conference resources that were used for the conference (step S 1428 ). For example, the controlling unit  302  reduces the numbers of the resources of the image synthesizers  110   a  and  110   b  in use by two, respectively. 
     With the procedure described above, the conference among the conference terminals  120   a  to  120   d  is terminated. In the sequence diagram shown in  FIG. 17 , the termination process is performed in order with respect to each device at the steps S 1408  to S 1412 , the steps S 1413  to S 1417 , the steps S 1418  to S 1422 , and the steps S 1423  to S 1427 . However, the process is indicated in the order for convenience of explanation, and it can be performed in different orders. For example, the process between the image synthesizer  110   a  and the conference controller  100  and the process between the image synthesizer  110   b  and the conference controller  100  are independently performed in parallel with each other. 
     The image synthesizers  110   a ,  110   b , and  110   c  decompress, synthesize, and compress the image data received from a plurality of conference terminals. However, considering an amount of the image data to be processed and the number of the steps performed in the process, it is difficult to perform the process by software using a single central processing unit (CPU). The functions of the image synthesizers  110   a ,  110   b , and  110   c  according to the embodiment can be realized using hardware. 
     As shown in  FIG. 18 , the image synthesizer  110   a  includes a CPU  1801 , a random access memory (RAM)  1802 , a network interface (network I/F)  1803 , image compressing/decompressing circuits  1804  to  1808 , an image synthesizing circuit  1809 , a clustering interface (clustering I/F)  1810 , and a bus  1811  that connects the units. 
     The image decompressing units  205   a  to  205   e  and the image compressing units  206   a  to  206   e  are realized by the image compressing/decompressing circuits  1804  to  1808 . Image-compressing circuits and image-decompressing circuits can be realized by separate circuits. The image synthesizing unit  204  is realized by the image synthesizing circuit  1809 . 
     The packet communicating unit  201  controls transmission of information via the network I/F  1803  (for example, a network interface based on a 100 Mbps Ethernet (registered trademark)). The image-data communicating unit  203  controls transmission of image data and the like via the clustering I/F  1810 . 
     The controlling unit  202  is realized by software using the CPU  1801  and the RAM  1802  that is used by the CPU  1801 . 
     The bus  1811  (for example, a PCI (Peripheral Component Interconnect)) connects the CPU  1801 , the image compressing/decompressing circuits  1804  to  1808 , the image synthesizing circuit  1809 , and the network I/F  1803 . Although the units are connected with a single bus in  FIG. 18 , they can be connected with a plurality of buses at different bit rates via a bus bridge. 
     The image synthesizing circuit  1809  is connected to the image compressing/decompressing circuits  1804  to  1808  using a bus to exchange uncompressed image data (for example, ITU-R BT.656 bus). 
     Each of the internal bus interfaces is specified by a design guide. 
     The clustering I/F  1810  is an exclusive signal line to transmit a necessary amount of the compressed image data as required for the synthesizing process in real time. According to the embodiment, the clustering I/F  1810  is an about 10 Gbps signal line based on a low voltage differential signaling (LVDS). The amount of the uncompressed image data for the conference terminals can be thereby transmitted between the clustered image synthesizers after the image data is decoded with a standard definition quality at about 250 Mbps. However, the signal line is not limited to the LVDS. 
     Although the image data is transmitted using the clustering I/F  1810  according to the embodiment, the data to be transmitted can include other data than the image data. As described above, the clustering I/F  1810  reliably communicates at a higher speed than the network I/F  1803 . For example, the clustering I/F  1810  reliably transmits uncompressed image data that the network I/F  1803  cannot handle due to lack of the bit rate. 
     Otherwise, the entire image synthesizer can be realized by a computer program. 
     As shown in  FIG. 19 , the conference controller  100  includes a CPU  1901 , a RAM  1903 , a network I/F  1904 , a hard disk drive (HDD)  1906 , and a bus  1905 , as the hardware. The CPU  1901  controls each unit of the conference controller  100  according to a program in the HDD  1906 . The RAM  1903  stores therein various data required to control the conference controller  100 . The network I/F  1904  is connected to the network  150  for communication. The HDD  1906  stores therein a conference controlling program and various types of information. The bus  1905  connects the units in the conference controller  100 . The conference controller  100  can be realized by a common computer configured as described above. 
     The conference controlling program executed by the conference controller  100  according to the embodiment is recorded in a computer-readable recording medium such as an HDD, a compact disk read only memory (CD-ROM), a flexible disk (FD), a compact disk readable (CD-R), and a digital versatile disk (DVD) in an installable or executable format. 
     In this case, when the conference controller  100  reads the conference controlling program from the recording medium and executes it, the conference controlling program is loaded into a main storage unit and the controlling unit  302  is generated in the main storage unit. The resource storage unit  303  and the layout managing unit  304  are stored in the main storage unit at the time of executing the conference controlling program. 
     The conference controlling program can be stored in a computer connected to a network such as the Internet to be provided by downloading it via the network. The conference controlling program can be provided or distributed via the network. 
     The conference controlling program according to the embodiment can be recorded in a ROM and the like to be provided. 
     The conference controlling program includes modules of the units described above. When a CPU reads the conference controlling program from the recording medium and executes it, the units are loaded into the main storage unit and generated in it. 
     The image synthesizers are not limited to be clustered one-to-one, and they can be clustered one-to-n. 
     The embodiment was explained assuming that the multipoint teleconference system uses image data. Communication via the exclusive line for the clustering is even more effective when the transmitted data is large as the image data. However, the multipoint teleconference system according to the embodiment is not limited to transmission of the image data, and it can be configured to transmit various media data such as sound data and application data including a text chat application and a spreadsheet. 
     According to the embodiment, the conference controller  100  stores the registration information sent from an image synthesizer in the resource storage unit  303 . The conference controller  100  assigns an image synthesizer appropriate to the conference based on the registration information when it receives a request for starting a conference from a conference terminal. If it is registered that image synthesizers are clustered, the conference controller  100  automatically controls the clustered image synthesizers to hold the conference. In this manner, the administrator does not need to manually perform setup, which reduces the workload of the setup to realize the multipoint conference. 
     Moreover, in the multipoint teleconference system according to the embodiment, without the administrator&#39;s operation, the conference controller  100  registers the image synthesizers and controls to hold a multipoint teleconference taking advantage of resources of the image synthesizers. 
     The present invention is not limited to the embodiment described above, and it can be carried out in various modifications as described below. 
     The image synthesizers  110   a ,  110   b , and  110   c  according to the embodiment include the same number of the image compressing units  206   a  to  206   e  as the maximum number of the resources; however, the number of the image compressing units included in the image synthesizer is not limited. 
     According to a first modification, the image synthesizer includes only one image compressing unit. A single image synthesizer operates a single conference. If the image synthesizer is clustered with another, a plurality of the clustered image synthesizers can operate a single conference. The image synthesizer generates synthesized image data with a screen arrangement, a compression coding method, a resolution, a frame rate, and a bit rate that are common to all the conference terminals connected to the image synthesizer, and sends the synthesized image data to all of the conference terminals connected thereto. 
     The image synthesizer can be configured to use a scalable coding as a compressing coding method. With the scalable coding, even if the image synthesizer includes only one image compressing unit, the image synthesizer can extract a plurality of image data with different resolutions or bit rates from the compressed image data, whereby sending image data processed as appropriate to each conference terminal. 
     In the embodiment described above, if the image synthesizers  110   a  and  110   b  are clustered with each other, by receiving the registration information from the image synthesizer  110   a , the entry of the image synthesizer  110   a  is registered to the resource storage unit  303  in the conference controller  100 . At this time, the fact that the image synthesizers are clustered is not yet registered. Upon receiving the registration information from the image synthesizer  110   b , the registering unit  311  registers the fact of the clustering to the entries of the image synthesizers  110   a  and  110   b  in the resource storage unit  303 . However, the embodiment is not limited to the registration method as described above. 
     For example, if the image synthesizers  110   a  and  110   b  are clustered with each other, the conference controller  100  can be configured to register the fact of the clustering to the resource storage unit  303  at the time of receiving the identifiers of the clustered image synthesizers from one of the image synthesizers  110   a  and  110   b.    
     The present invention is not limited to the embodiment, and it can be realized with the units modified within the scope of the invention. 
     For example, according to the embodiment, the conference controller  100  and the image synthesizers  110   a ,  110   b , and  110   c  are independent devices. However, the conference controller  100  can include the function of the image synthesizers. If the multipoint teleconference system is realized by a plurality of the conference controllers that include the image synthesizers, any one of the conference controllers intensively performs the registration process for other image synthesizers. 
     A plurality of the units disclosed in the embodiment and the modification can be combined as needed. For example, some of the units can be removed. Moreover, the units separately disclosed in the embodiment and the modification can be combined as needed. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.