Patent Publication Number: US-2023162437-A1

Title: Image processing device, calibration board, and method for generating 3d model data

Description:
TECHNICAL FIELD 
     The present technique relates to an image processing device, a calibration board, and a method for generating 3D model data and particularly relates to an image processing device, a calibration board, and a method for generating 3D model data, by which synchronization among devices can be easily achieved. 
     BACKGROUND ART 
     A technique is available to provide a free viewpoint image by generating a 3D model of a subject from moving images captured from multiple viewpoints and generating a virtual viewpoint image of the 3D model according to any viewing position. This technique is also called volumetric capture. 
     A plurality of imaging devices for capturing moving images for generating a 3D model are disposed at different locations to capture a subject from different directions (viewpoints), and the positional relationship among the imaging devices is calculated. The calculation of the positional relationship among the imaging devices requires the use of moving images synchronized among the imaging devices. 
     Various techniques have been proposed for the calibration of imaging devices (for example, see PTL 1 and PTL 2). 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] 
         JP 2018-111166A 
         [PTL 2] 
         JP 2007-129709A 
       
    
     SUMMARY 
     Technical Problem 
     However, a method for synchronization among a plurality of imaging devices is not disclosed. 
     The present technique has been made in view of such a situation and is configured to easily achieve synchronization among devices. 
     Solution to Problem 
     An image processing device according to a first aspect of the present technique includes: an image synchronization unit that performs time synchronization on a plurality of images of a board on a basis of lighting conditions of a plurality of light-emitting parts included in the plurality of images captured by a plurality of imaging devices, the board including the plurality of light-emitting parts and a predetermined image pattern; and a calibration unit that calculates camera parameters of the plurality of imaging devices by using the plurality of images having been subjected to the time synchronization. 
     In the first aspect of the present technique, time synchronization is performed on a plurality of images of a board on a basis of lighting conditions of a plurality of light-emitting parts included in the plurality of images captured by a plurality of imaging devices, the board including the plurality of light-emitting parts and a predetermined image pattern, and the camera parameters of the plurality of imaging devices are calculated by using the plurality of images having been subjected to the time synchronization. 
     A calibration board according to a second aspect of the present technique includes a plurality of light-emitting parts that change lighting conditions at each lapse of a unit time, and a predetermined image pattern, wherein the plurality of light-emitting parts are caused to illuminate to perform time synchronization on a plurality of images captured by a plurality of imaging devices. 
     Provided in the second aspect of the present technique are a plurality of light-emitting parts that change lighting conditions at each lapse of a unit time and a predetermined image pattern. The plurality of light-emitting parts are caused to illuminate to perform time synchronization on a plurality of images captured by a plurality of imaging devices. 
     A method for generating 3D model data according to a third aspect of the present technique, the method including: performing time synchronization on a plurality of images of a board on a basis of lighting conditions of a plurality of light-emitting parts included in the plurality of images captured by a plurality of imaging devices, the board including the plurality of light-emitting parts and a predetermined image pattern; calculating camera parameters of the plurality of imaging devices by using the plurality of images having been subjected to the time synchronization; generating a 3D model of a predetermined subject from a plurality of subject images of the predetermined subject, the subject images being captured by the plurality of imaging device by using the calculated camera parameters; and generating a virtual viewpoint image by viewing the generated 3D model of the predetermined subject from a predetermined viewpoint. 
     In the third aspect of the present technique, time synchronization is performed on a plurality of images of a board on a basis of lighting conditions of a plurality of light-emitting parts included in the plurality of images captured by a plurality of imaging devices, the board including the plurality of light-emitting parts and a predetermined image pattern; the camera parameters of the plurality of imaging devices are calculated by using the plurality of images having been subjected to the time synchronization; a 3D model of a predetermined subject is generated from a plurality of subject images of the predetermined subject, the subject images being captured by the plurality of imaging device by using the calculated camera parameters; and a virtual viewpoint image is generated by viewing the generated 3D model of the predetermined subject from a predetermined viewpoint. 
     The image processing device according to a first aspect of the present technique can be realized by causing a computer to execute a program. The program to be executed by the computer can be provided by transmission through a transmission medium or recording on a recording medium. 
     The image processing device may be a standalone device or an internal block constituting one device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an explanatory drawing of the generation of a 3D model of a subject and the display of a free viewpoint image. 
         FIG.  2    is a block diagram illustrating a configuration example of an image processing system to which the present technique is applied. 
         FIG.  3    is an explanatory drawing of the synchronization of moving images. 
         FIG.  4    is a diagram illustrating an example of a calibration board. 
         FIG.  5    is a diagram illustrating a lighting example of the light-emitting parts of the calibration board. 
         FIG.  6    is an explanatory drawing illustrating a method of using the time display part of the calibration board. 
         FIG.  7    is an explanatory drawing illustrating a method of using the position display part of the calibration board. 
         FIG.  8    is an explanatory drawing illustrating a method of using the position display part of the calibration board. 
         FIG.  9    is an explanatory drawing illustrating the calibration of cameras with the calibration board. 
         FIG.  10    is an explanatory drawing illustrating the calibration of cameras with the calibration board. 
         FIG.  11    is a block diagram illustrating a configuration example of the calibration board. 
         FIG.  12    is a block diagram illustrating a configuration example of an image processing device. 
         FIG.  13    is a flowchart for explaining the position pattern assignment of the calibration board. 
         FIG.  14    is a flowchart for explaining the time information lighting of the calibration board. 
         FIG.  15    is a flowchart for explaining the position information lighting of the calibration board. 
         FIG.  16    is a flowchart for explaining the image extraction of the image processing device. 
         FIG.  17    is a flowchart for explaining the calibration of the image processing device. 
         FIG.  18    is a diagram illustrating a modification example of the calibration board. 
         FIG.  19    is a block diagram illustrating a configuration example of the image processing device for generating a 3D model. 
         FIG.  20    is a flowchart for explaining 3D model generation. 
         FIG.  21    is a block diagram illustrating a configuration example of an embodiment of a computer to which the present technique is applied. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A mode for embodying the present disclosure (hereinafter referred to as an embodiment) will be described below with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration will be denoted by the same reference numerals, and thus repeated descriptions thereof will be omitted. The description will be given in the following order. 
     1. Outline of volumetric capture
 
2. Configuration example of image processing system
 
3. Calibration using calibration board
 
4. Block diagram
 
5. Position pattern assignment
 
6. Time information lighting
 
7. Position information lighting
 
8. Image extraction
 
     9. Calibration 
     10. Modification example of calibration board
 
11. Configuration example of 3D model generation
 
12. Flowchart of 3D model generation
 
13. Computer configuration example
 
     &lt;1. Outline of Volumetric Capture&gt; 
     An image processing system according to the present disclosure relates to volumetric capture for providing an image with a free viewpoint (free viewpoint image) by generating a 3D model of a subject from images captured from multiple viewpoints and generating a virtual viewpoint image of the 3D model according to any viewing position. 
     First, referring to  FIG.  1   , the generation of a 3D model of a subject and the display of a free viewpoint image using the 3D model will be briefly described below. 
     For example, a plurality of captured images can be obtained by imaging a predetermined imaging space, in which subjects such as a person are disposed, from the outside by using a plurality of imaging devices. The captured image includes, for example, a moving image. In the example of  FIG.  1   , three imaging devices CAM 1  to CAM 3  are disposed around a subject Ob 1 . The number of imaging devices CAM is not limited to three, and any number of imaging devices CAM may be provided. The number of imaging devices CAM during imaging is equivalent to the known number of viewpoints when a free viewpoint image is generated, and thus the larger the number of imaging devices CAM, the higher the accuracy of presentation of the free viewpoint image. The subject Ob 1  in  FIG.  1    is assumed to be a person making a predetermined motion. 
     A 3D object MO 1 , which is a 3D model of the subject Ob 1  to be displayed in the imaging space, is generated by using the captured images obtained from the plurality of imaging devices CAM in different directions (3D modeling). The 3D object MO 1  is generated by using, for example, a scheme such as Visual Hull in which a three-dimensional shape of a subject is cut out using images captured in different directions. 
     From one or more 3D objects in the imaging space, data on the one or more 3D objects (hereinafter also referred to as 3D model data) is transmitted to a device on the reproduction side and is reproduced therein. In other words, by rendering the acquired 3D object based on the data on the 3D object in the device on the reproduction side, a two-dimensional image of the 3D object is displayed on the viewing device of a viewer.  FIG.  1    illustrates an example in which the viewing device is a display D 1  or a head-mounted display (HMD) D 2 . 
     On the reproduction side, only a 3D object to be viewed can be requested from one or more 3D objects in the imaging space and can be displayed on the viewing device. For example, on the reproduction side, a virtual camera is assumed to have an imaging range equivalent to the viewing range of a viewer, and only a 3D object to be captured by the virtual camera is requested from multiple 3D objects in the imaging space and is displayed on the viewing device. The viewpoint (virtual viewpoint) of the virtual camera may be set at any position such that a viewer can view a subject from any viewpoint in the real world. A background image representing a predetermined space can be optionally combined with the 3D object. 
     &lt;2. Configuration Example of Image Processing System&gt; 
       FIG.  2    illustrates a configuration example of the image processing system to which the present technique is applied, the image processing system being configured to capture moving images for generating a 3D model. 
     An image processing system  1  of  FIG.  2    includes an image processing device  11 , N (N&gt;1) cameras  12 - 1  to  12 -N, and a display device  13 . 
     The N cameras  12 - 1  to  12 -N are imaging devices that capture subject images. As described with reference to  FIG.  1   , the cameras  12 - 1  to  12 -N are disposed at different locations around a subject. Hereinafter, the N cameras  12 - 1  to  12 -N will be simply referred to as cameras  12  unless otherwise specified. 
     The image processing device  11  includes, for example, a personal computer and a server. The image processing device  11  controls the timing of imaging by the cameras  12 - 1  to  12 -N, acquires moving images captured by the cameras  12 , and performs predetermined image processing such as the generation of a 3D model based on the acquired moving images. 
     In order to allow the image processing device  11  to generate a 3D model of a subject by using moving images captured by the cameras  12 , the positional relationship among the cameras  12  needs to be a known relationship. Moreover, in order to calculate the positional relationship among the cameras  12 , the moving images of the cameras  12  need to be synchronized with one another. Thus, before imaging for generating a 3D model, the image processing device  11  performs calibration for calculating the positional relationship among the cameras  12 , specifically, the positions and orientations of the cameras  12  on world coordinates by using the synchronized moving images of the cameras  12 . The positions and orientations of the cameras  12  are the external parameters of the cameras  12 . The internal parameters of the cameras  12  are assumed to be known parameters. 
     When the image processing device  11  causes the cameras  12  to capture images, the image processing device  11  generates a control signal for providing an instruction to start or terminate the capturing of images and a synchronizing signal, and supplies the signals to each of the cameras  12 - 1  to  12 -N. 
     Referring to  FIG.  3   , the synchronization of moving images at the time of imaging by the plurality of cameras  12  will be described below. 
       FIG.  3    illustrates four moving images  14 - 1  to  14 - 4 . The four moving images  14 - 1  to  14 - 4  are divided at regular intervals in the time direction. One section of the moving image  14  corresponds to a period from the start to the end of a single exposure and represents (a frame image of) one frame. 
     The timings to start capturing the moving image  14 - 1  and the moving image  14 - 2  are not synchronized with each other, and the phases of exposure timing (the timings to start and terminate exposure) are not synchronized with each other. 
     The timings to start capturing the moving image  14 - 2  and the moving image  14 - 3  are not synchronized with each other, but the phases of exposure timing are synchronized with each other. 
     The timings to start capturing the moving image  14 - 2  and the moving image  14 - 4  are synchronized with each other, and the phases of exposure timing are also synchronized with each other. 
     Moving images generated by the cameras  12 - 1  to  12 -N on the basis of the control signal for starting and terminating the capturing of images and the synchronizing signal have the synchronization relationship like the moving image  14 - 2  and the moving image  14 - 3 , the control signal and the synchronizing signal being supplied from the image processing device  11 . In other words, the moving images are generated by the cameras  12 - 1  to  12 -N such that the phases of exposure timing are synchronized with each other, but the timings to start capturing the moving images are not synchronized with each other. 
     Returning to  FIG.  2   , the image processing device  11  causes the cameras  12  to capture images of a predetermined subject in order to calculate the positional relationship among the cameras  12 . The subject is, for example, a calibration board (e.g., a calibration board  21  in  FIG.  4   ) having a predetermined image pattern. The image processing device  11  acquires the moving images of the calibration board from the cameras  12 , synchronizes the timings to start capturing the moving images, and performs calibration for calculating the external parameters of the cameras  12 . 
     In a state where the positional relationship among the cameras  12  is known due to the calibration, the image processing device  11  causes the cameras  12  to capture images of a predetermined subject serving as a target of 3D model generation. For example, the cameras  12  capture images of a person making a predetermined motion, as a predetermined subject serving as a target of 3D model generation. The image processing device  11  generates a 3D model of an object, which is a person imaged as a subject, from multiple moving images supplied from the cameras  12 - 1  to  12 -N. 
     Furthermore, the image processing device  11  can generate a virtual viewpoint image by viewing the generated 3D model of the object from any virtual viewpoint and display the image on the display device  13 . The display device  13  includes, for example, the display D 1  or the head-mounted display (HMD) D 2  that are illustrated in  FIG.  1   . 
     Communications between the image processing device  11  and the cameras  12 - 1  to  12 -N and communications between the image processing device  11  and the display device  13  may be direct communications via a cable or the like or communications via a predetermined network, e.g., a LAN (Local Area Network) or the Internet. The communications may be wire communications or radio communications. The image processing device  11  and the display device  13  may be integrated into a single unit. 
     In the image processing system  1  configured thus, calibration for calculating the positional relationship among the cameras  12 , that is, the externa parameters of the cameras  12  is first performed by using moving images of a predetermined calibration board imaged by the cameras  12 . 
     When the positional relationship among the cameras  12  is known, a predetermined subject as a target of 3D model generation is imaged by the cameras  12 , and a 3D model of an object, which is the predetermined subject, is generated on the basis of multiple moving images captured by the cameras  12 . 
     &lt;3. Calibration Using Calibration Board&gt; 
     First, calibration using the calibration board will be described in detail. 
       FIG.  4    illustrates an example of the calibration board used for calibration. 
     A calibration board  21  in  FIG.  4    has an image pattern  22  in a so-called checkered pattern (chess pattern), in which square black patterns and white patterns are alternately disposed in the vertical direction and the horizontal direction, on a predetermined plane serving as the front side of a thin-plate shape. A light-emitting part  23  is disposed in each of the black patterns of the image pattern  22  in the checkered pattern. The image pattern  22  in the checkered pattern in  FIG.  4    has 44 black patterns, so that the number of light-emitting parts  23  is 44. 
     Moreover, at least one operation button  24  is disposed at a predetermined point of the calibration board  21 . The operation button  24  is operated by a user when an operation is performed to start or terminate a light-emitting operation in the 44 light-emitting parts  23 . 
     The light-emitting part  23  disposed in each of the black patterns of the image pattern  22  is composed of an LED (Light Emitting Diode) or the like. The light-emitting part  23  can provide two lighting conditions, for example, an illuminated state and an unilluminated state of white light. Alternatively, the light-emitting parts  23  may illuminate in multiple colors, for example, red and green. 
     The 44 light-emitting parts  23  are grouped into a time display part  31  that illuminates according to a time, and a position display part  32  that illuminates according to a position. In the example of  FIG.  4   , from among the 44 light-emitting parts  23 , the 39 light-emitting parts  23  in the upper part are allocated to the time display part  31 , whereas the other five light-emitting parts  23  are allocated to the position display part  32 . 
     The time display part  31  illuminates to perform time synchronization on moving images captured by the plurality of cameras  12 . The time display part  31  associates “illuminated” or “unilluminated” of the light-emitting part  23  with a bit of “0” or “1.” The 39 light-emitting parts  23  display 39-bit time information. 
     The position display part  32  associates “illuminated” or “unilluminated” of the light-emitting part  23  with a bit of “0” or “1.” The five light-emitting parts  23  display 5-bit position information. 
     For example, as illustrated in  FIG.  5   , the time display part  31  constitutes 39-bit bit strings in which the light-emitting part  23  on the upper left end serves as a least significant bit (LSB) and the light-emitting part  23  on the lower right end serves as a most significant bit (MSB) in a raster scan sequence. When the light-emitting parts  23  of the time display part  31  are illuminated or unilluminated as illustrated in the example of  FIG.  5   , the time display part  31  displays “000000000000000000000000000000011010101.” 
     The position display part  32  constitutes 5-bit bit strings in which the light-emitting part  23  on the left end serves as a least significant bit (LSB) and the light-emitting part  23  on the right end serves as a most significant bit (MSB). When the light-emitting parts  23  of the position display part  32  are illuminated or unilluminated as illustrated in the example of  FIG.  5   , the position display part  32  displays “00011.” 
     If the light-emitting parts  23  can provide illumination in multiple colors, “0” or “1” may be represented by a color difference instead of illuminated/unilluminated. For example, “red illumination” may be represented as “1” and “green illumination” may be represented as “0.” 
     Referring to  FIG.  6   , a method of using the time display part  31  of the calibration board  21  in the calibration will be described below. 
     The time display part  31  increments (updates) a 39-bit bit value on the basis of the internal timer of the calibration board  21  at each lapse of a predetermined unit time. In the calibration, the cameras  12  capture the moving images of the calibration board  21 , and the lighting pattern of the time display part  31  of the calibration board  21  is identified in the moving images, thereby determining an imaging time. 
       FIG.  6    illustrates an example of the moving images of the calibration board  21  when the moving images are captured by the cameras  12 - 1  and  12 - 2 . 
     The lighting pattern of the time display part  31  of the calibration board  21  in the moving images of  FIG.  6    is originally represented by 39-bit bit values. However, the higher-order 31 bits of the lighting pattern of the time display part  31  in the moving images of  FIG.  6    are all “0” and thus only the lower-order 8 bits are described. 
     The p-th frame (p is a natural number) of the moving image captured by the camera  12 - 1  in the calibration includes the calibration board  21  in which the time display part  31  has a lighting pattern “11010011.” The (p+1)-th frame includes the calibration board  21  in which the time display part  31  has a lighting pattern “11010100.” The (p+2)-th frame includes the calibration board  21  in which the time display part  31  has a lighting pattern “11010101.” 
     The p-th frame of the moving image captured by the camera  12 - 2  in the calibration includes the calibration board  21  in which the time display part  31  has a lighting pattern “11010101.” The (p+1)-th frame includes the calibration board  21  in which the time display part  31  has a lighting pattern “11010110.” The (p+2)-th frame includes the calibration board  21  in which the time display part  31  has a lighting pattern “11010111.” 
     Thus, in the (p+2)-th frame of the camera  12 - 1  and the p-th frame of the camera  12 - 2 , the frames being surrounded by frames in  FIG.  6   , the time display part  31  has the common lighting pattern “11010111,” proving that the frames have been captured at the same time. 
     As described with reference to  FIG.  3   , the phases of the exposure timing of the moving images captured by the plurality of cameras  12  are synchronized with each other, but the timings to start capturing the moving images are not synchronized with each other. Thus, synchronization is necessary between the timings to start capturing the moving images. 
     As illustrated in  FIG.  6   , the times when the moving images are captured are detected on the basis of the lighting patterns of the time display part  31  in the frame images of the captured moving images, thereby detecting the frame images captured at the same time. In other words, synchronization can be achieved between the timings to start capturing the moving images. 
     Referring to  FIGS.  7  and  8   , a method of using the position display part  32  of the calibration board  21  in the calibration will be described below. 
       FIG.  7    is a plan view (top view) illustrating, when N cameras in the image processing system  1  are eight cameras (N=8), the layout of eight cameras  12 - 1  to  12 - 8  and an example of an imaging space. 
     An imaging space  41  is determined on the basis of the imaging range of the eight cameras  12 - 1  to  12 - 8 . In the example of  FIG.  7   , the imaging space  41  is set as a cubic (square) region in a region inside the eight cameras  12 - 1  to  12 - 8 . 
     In the present embodiment, on the assumption that a user or a self-propelled robot moves with the calibration board  21  on the floor of the imaging space  41  when an image is captured, only a two-dimensional region corresponding to the floor of the imaging space  41  is examined. The imaging space  41  is also referred to as an imaging region  41 . 
     The N cameras  12 - 1  to  12 -N are annularly disposed at predetermined intervals (for example, regular intervals) outside the imaging region  41  so as to face the center of the imaging region  41 . 
     The square imaging region  41  is divided into a plurality of sections  42 . A predetermined bit value that can be represented by the position display part  32  is allocated to each of the sections  42 . 
     For example, as illustrated in  FIG.  7   , the square imaging region  41  is equally divided into four sections  42 A to  42 D. As illustrated in  FIG.  8   , a bit value of “00000” is allocated to the section  42 A, a bit value of “00001” is allocated to the section  42 B, a bit value of “00010” is allocated to the section  42 C, and a bit value of “00011” is allocated to the section  42 D. 
     The calibration board  21  includes a position information detection unit, for example, a GPS module capable of acquiring position information. The 5-bit position information of the position display part  32  is controlled according to the position of the calibration board  21  in one of the four sections  42 A to  42 D of the imaging region  41 . In the calibration, the cameras  12  capture the moving images of the calibration board  21 , and the lighting pattern of the position display part  32  of the calibration board  21  is identified in the moving images, thereby determining the position of the calibration board  21 , specifically, which one of the sections  42 A to  42 D has the calibration board  21  when an image is captured. 
     It is known that the accuracy of calibration for calculating the positional relationship among the cameras  12  is increased by detecting the feature points of the image pattern  22  of the calibration board  21  at various positions evenly in the imaging region (imaging space)  41 . 
     The section of the frame image captured as the calibration board  21  can be determined by identifying the lighting pattern of the position display part  32  of the calibration board  21  included in the frame images of in the moving images captured by the cameras  12 , so that the frame image used for calibration can be evenly selected from the four sections  42 A to  42 D in the imaging region  41 . 
     In the example of  FIGS.  7  and  8   , the imaging region  41  is divided into four sections  42 . The number of divisions in the imaging region  41  is not limited to four and thus may be two, three, or five or more. In the foregoing example, only a two-dimensional region corresponding to the floor of the imaging space  41  is examined. The cubic imaging space  41  may be divided into a plurality of sections in a three-dimensional space. For example, even at the same plane position in the imaging space  41 , different bit values may be allocated to a height H 1  close to the floor and a height H 2  remote from the floor. 
     Referring to  FIGS.  9  and  10   , the calibration of the cameras  12  with the calibration board  21  will be described below. 
     As illustrated in  FIG.  9   , if the calibration board  21  is disposed in an imaging range  46   (1,2)  shared by an imaging range  45 - 1  of the camera  12 - 1  and an imaging range  45 - 2  of the camera  12 - 2 , the image processing device  11  detects the feature points of the image pattern  22  of the calibration board  21  in frame images captured by the cameras  12 - 1  and  12 - 2  and performs matching, thereby calculating the positional relationship between the cameras  12 - 1  and  12 - 2 . 
     In this way, if the two cameras  12  have the common imaging range  46 , the positional relationship between the two cameras  12  can be directly calculated on the basis of the feature points of the image pattern  22  of the calibration board  21  in frame images captured in synchronization with each other by the two cameras  12 . 
     For example, even if the two cameras  12 - 1  and  12 -N do not have the common imaging range  46  as illustrated in  FIG.  10   , imaging ranges  45  are indirectly coupled between the camera  12 - 1  and the camera  12 -N via the imaging ranges  45  of one or more other cameras  12  (cameras  12 - 2  to  12 -(N−1)), for example, the cameras  12 - 1  and  12 - 2  have a common imaging range  46   (1,2) , the cameras  12 - 2  and  12 - 3  have a common imaging range  46   (2,3) , and the cameras  12 -(N−1) and  12 -N have a common imaging range  46   (N-1,N) , so that the positional relationship between the cameras  12 - 1  and  12 -N can be indirectly calculated by sequentially calculating the positional relationships among the cameras  12 - 1  to  12 -N having the common imaging ranges  46 . 
     &lt;4. Block Diagram&gt; 
       FIG.  11    is a block diagram illustrating a configuration example of the calibration board  21 . 
     The calibration board  21  includes a position information detection unit  51 , an operation unit  52 , a control unit  53 , and an information display unit  54 . 
     The position information detection unit  51  includes, for example, a GPS (Global Positioning System) module. The position information detection unit  51  detects current position information on the calibration board  21  and supplies the information to the control unit  53 . 
     The operation unit  52  corresponds to the operation button  24  of  FIG.  4   . The operation unit  52  receives a user operation and supplies, to the control unit  53 , an operation signal corresponding to the received user operation. 
     The control unit  53  controls the display of the information display unit  54 , specifically, the lighting of the 44 light-emitting parts  23  on the basis of the position information supplied from the position information detection unit  51  and the operation signal supplied from the operation unit  52 . 
     The information display unit  54  corresponds to the 44 light-emitting parts  23  of  FIG.  4    and includes the time display part  31  and the position display part  32 . The information display unit  54  illuminates or does not illuminate each of the 44 light-emitting parts  23  under the control of the control unit  53 . The time display part  31  illuminates according to a time. The position display part  32  illuminates according to the position of the calibration board  21 , specifically, the four sections  42 A to  42 D of the imaging region  41 . If the light-emitting parts  23  can illuminate in multiple colors, the light-emitting parts  23  illuminates in a predetermined color under the control of the control unit  53 . 
       FIG.  12    is a block diagram illustrating a configuration example of the image processing device  11 . 
     The image processing device  11  includes a moving image acquisition unit  71 , an image extraction unit  72 , an extracted image storage unit  73 , an image synchronization unit  74 , a calibration unit  75 , and a camera parameter storage unit  76 . 
     The moving image acquisition unit  71  acquires the moving images of the calibration board  21  from the plurality of cameras  12  and supplies the moving images to the image extraction unit  72 . 
     The image extraction unit  72  performs image extraction for extracting a time lighting-pattern changed frame image from the moving image supplied from the plurality of cameras  12 . More specifically, the image extraction unit  72  extracts, as a time lighting-pattern changed frame image, a frame image after the lighting pattern of the time display part  31  of the calibration board  21  in the moving image changes from that of the preceding frame image, and the image extraction unit  72  supplies the extracted time lighting-pattern changed frame image to the extracted image storage unit  73 . 
     The extracted image storage unit  73  stores a plurality of time lighting-pattern changed frame images that are extracted from the moving images of the cameras  12  in the image extraction unit  72 . 
     The image synchronization unit  74  selects the time lighting-pattern changed frame images such that the four sections  42 A to  42 D of the imaging region  41  are allocated in a predetermined ratio on the basis of the lighting conditions of the position display part  32  of the calibration board  21  in the time lighting-pattern changed frame images stored in the extracted image storage unit  73 . For example, the image synchronization unit  74  selects the time lighting-pattern changed frame image such that the four sections  42 A to  42 D are equally allocated. 
     Furthermore, the image synchronization unit  74  performs time synchronization on the plurality of time lighting-pattern changed frame images, which are selected such that the sections  42  are allocated in a predetermined ratio, on the basis of the lighting conditions of the time display part  31  in the frame images. In other words, the image synchronization unit  74  collects frame images in which the lighting conditions of the time display part  31  indicate the same time. The plurality of time lighting-pattern changed frame images captured at the same time are supplied to the calibration unit  75 . 
     The calibration unit  75  performs calibration for calculating the external parameters of the N cameras  12  by using the plurality of lighting-pattern changed frame images that are time-synchronized images. More specifically, by using a plurality of time lighting-pattern changed frame images captured by two cameras  12 -A and  12 -B (A and B are natural numbers of 1 to N and are different from each other) at the same time, the calibration unit  75  sequentially performs, on the N cameras  12 - 1  to  12 -N, processing for calculating the positional relationship between the cameras  12 -A and the camera  12 -B. The external parameters of the N cameras  12  are stored in the camera parameter storage unit  76 , the external parameters being obtained by the calibration. 
     The camera parameter storage unit  76  accommodates the external parameters of the N cameras  12 , the external parameters being supplied from the calibration unit  75 . 
     The image processing device  11  is configured as above: 
     &lt;5. Position Pattern Assignment&gt; 
     Referring to the flowchart of  FIG.  13   , the position pattern assignment of the calibration board  21  will be described below. The position pattern assignment is performed as preparation for capturing images of the calibration board  21  by the cameras  12 . This processing is performed by the calibration board  21  when an operation to start the position pattern assignment is performed in, for example, the operation unit  52 . 
     First, in step S 1 , the control unit  53  of the calibration board  21  acquires the position information of the imaging region  41 . For example, when a user carrying the calibration board  21  moves in the outer edge of the imaging region  41 , position information corresponding to the outer edge of the imaging region  41  is supplied from the position information detection unit  51  to the control unit  53  and is stored in internal memory, so that the position information of the imaging region  41  is acquired. The method of acquiring the position information of the imaging region  41  is not particularly limited. For example, position information on the four corners of a rectangle corresponding to the imaging region  41  may be inputted. 
     In step S 2 , the control unit  53  divides the imaging region  41  into the plurality of sections  42  after the position information is acquired. For example, as illustrated in  FIG.  7   , it is determined in advance that the rectangular imaging region  41  is to be equally divided into the four sections  42 A to  42 D. The control unit  53  divides the imaging region  41  into the four sections  42  after the position information is acquired. The method of dividing the imaging region  41  and the number of divisions are optionally determined and are not particularly limited. For example, a user carrying the calibration board  21  may enter the number of divisions of the sections  42  via the operation unit  52 , and then the imaging region  41  may be divided according to the inputted number of divisions. 
     In step S 3 , the control unit  53  sets a correlation between the plurality of sections  42  split in the imaging region  41  and the lighting pattern of the position display part  32  and stores the correlation. Specifically, as illustrated in  FIG.  8   , the control unit  53  correlates a predetermined 5-bit bit value with each other the sections  42 A to  42 D split in the imaging region  41 , and stores the correlation result in the internal memory. Any method may be used for correlating the sections  42  with 5-bit bit values. For example, a user may sequentially specify the four sections  42 A to  42 D split in step S 2 , and the control unit  53  may assign “00000,” “00001,” “00010,” and “00011” to the sections in the order in which the sections are specified. 
     In step S 3 , when the correlation between the plurality of sections  42  split in the imaging region  41  and the lighting pattern of the position display part  32  is stored in the control unit  53 , the position pattern assignment is completed. 
     At the completion of the position pattern assignment of  FIG.  13   , the preparation for capturing the images of the calibration board  21  with the plurality of cameras  12  is completed. Thus, processing is performed to capture the images of the calibration board  21  in the imaging region  41  with the cameras  12 . 
     In the processing for capturing the images of the calibration board  21 , the control signal for providing an instruction to start capturing images and the synchronizing signal are supplied from the image processing device  11  to the cameras  12 . The cameras  12  start capturing images in response to the control signal for providing an instruction to start capturing images, and capture moving images (capture a moving image for each frame) in response to the synchronizing signal. 
     While the cameras  12  capture moving images, for example, the user carrying the calibration board  21  moves in the imaging region  41 . The cameras  12  capture at least the moving images of the calibration board  21  in the imaging region  41 . 
     &lt;6. Time Information Lighting&gt; 
       FIG.  14    is a flowchart of time information lighting performed on the calibration board  21  while the cameras  12  capture images. This processing is started, for example, when the user carrying the calibration board  21  operates the operation unit  52  to start illuminating the information display unit  54 . 
     First, in step S 21 , the control unit  53  sets “0” for a variable tb corresponding to the time information of the time display part  31 . The variable tb corresponds to a value obtained by expressing, as a decimal number, a 39-bit bit value in binary form. 
     In step S 22 , the control unit  53  illuminates the time display part  31  (39 light-emitting parts  23 ) in a lighting pattern corresponding to a time tb. The lighting pattern corresponding to the time tb is a pattern in which the variable tb in decimal form is represented as a 39-bit bit string (binary), “0” represents an unilluminated state, and “1” represents an illuminated state. 
     In step S 23 , the control unit  53  determines whether a predetermined unit time has elapsed. The processing of step S 23  is repeated until it is determined that the predetermined unit time has elapses. The predetermined unit time corresponds to the time of one bit of the time display part  31 . 
     If it is determined that the predetermined unit time has elapsed in step S 23 , the processing advances to step S 24 , and the control unit  53  increments the variable tb, which corresponds to time information, by “1.” 
     In step S 25 , it is determined whether an operation to terminate the lighting of the information display unit  54  has been performed. 
     If it is determined in step S 25  that the operation to terminate the lighting of the information display unit  54  has not been performed, the processing returns to step S 22 , and the processing of steps S 22  to S 25  is performed again. In other words, the time display part  31  (39 light-emitting parts  23 ) is illuminated for the predetermined unit time in the lighting pattern corresponding to the variable tb incremented by “1.” 
     In step S 25 , if it is determined that the operation to terminate the lighting of the information display unit  54  has been performed, the time information lighting is terminated. 
     As described above, the information display unit  54  of the calibration board  21  changes the lighting condition at each lapse of the unit time. 
     &lt;7. Position Information Lighting&gt; 
       FIG.  15    is a flowchart of position information lighting performed on the calibration board  21  concurrently with the time information lighting in  FIG.  14    while the cameras  12  capture images. This processing is started, for example, when the user carrying the calibration board  21  operates the operation unit  52  to start illuminating the information display unit  54 . 
     First, in step S 41 , the control unit  53  acquires current position information from the position information detection unit  51  and illuminates the position display part  32  (5 light-emitting parts  23 ) in a lighting pattern corresponding to the current position. The lighting pattern corresponding to the current position is a pattern in which “0” represents an unilluminated state, and “1” represents an illuminated state in a 5-bit bit string (binary) assigned to the section  42  including the current position. 
     In step S 42 , the control unit  53  determines whether position information supplied from the position information detection unit  51  has changed. The processing of step S 42  is repeated until it is determined that the position information has changed. 
     In step S 42 , if it is determined that the position information has changed, the processing advances to step S 43 . The control unit  53  determines whether a movement across the section  42  of the imaging region  41  has been made before and after a change of the position information. 
     In step S 43 , if it is determined that a movement across the section  42  has been made before and after a change of the position information, the processing advances to step S 44 , and the control unit  53  illuminates the position display part  32  (five light-emitting parts  23 ) in the lighting pattern corresponding to the current position. 
     If it is determined in step S 43  that a movement across the section  42  has not been made, step S 44  is skipped. 
     In step S 45 , the control unit  53  determines whether an operation to terminate the lighting of the information display unit  54  has been performed. 
     If it is determined in step S 45  that the operation to terminate the lighting of the information display unit  54  has not been performed, the processing returns to step S 42 , and the processing of steps S 42  to S 45  is performed again. In other words, the processing for illuminating the position display part  32  (five light-emitting parts  23 ) is continued in the lighting pattern corresponding to the current position. 
     In step S 45 , if it is determined that the operation to terminate the lighting of the information display unit  54  has been performed, the position information lighting is terminated. 
     As described above, the position display part  32  of the calibration board  21  changes the lighting condition according to the sections  42 . 
     The time information lighting in  FIG.  14    and the position information lighting in  FIG.  15    are simultaneously started in response to an operation to start the lighting of the information display unit  54  and are simultaneously terminated in response to an operation to terminate the lighting of the information display unit  54 . 
     &lt;8. Image Extraction&gt; 
     Referring to the flowchart of  FIG.  16   , image extraction performed by the moving image acquisition unit  71 , the image extraction unit  72 , and the extracted image storage unit  73  of the image processing device  11  will be described below. 
     The moving images of the calibration board  21  are inputted from the plurality of cameras  12  to the image processing device  11 . The image extraction of  FIG.  16    is performed on the inputted moving images of the cameras  12 . Specifically, for example, if the moving images of the calibration board  21  are captured by the eight cameras  12 - 1  to  12 - 8 , the image extraction of  FIG.  16    is performed on each of the eight moving images. 
     In the present embodiment, it is assumed that the unit time of one frame based on the frame rate of a moving image is shorter than a unit time during which the lighting pattern of the time display part  31  of the calibration board  21  changes and timings to start capturing images are synchronized with each other in a frame immediately after the lighting pattern of the time display part  31  changes. 
     First, in step S 61 , the moving image acquisition unit  71  acquires (a frame image of) one frame of a moving image inputted from the camera  12  and supplies the frame to the image extraction unit  72 . 
     In step S 62 , the image extraction unit  72  determines whether the frame image supplied from the moving image acquisition unit  71  includes the calibration board 
     If it is determined in step S 62  that the frame image supplied from the moving image acquisition unit  71  does not include the calibration board  21 , the processing returns to step S 61  and the processing of steps S 61  and S 62  is performed again. Thus, the frame images of moving images are searched until it is determined that the frame image includes the calibration board  21 . 
     In step S 62 , if it is determined that the frame image includes the calibration board  21 , the processing advances to step S 63 . The image extraction unit  72  identifies the lighting pattern of the time display part  31  of the calibration board  21  in the frame image and stores the lighting pattern therein. 
     Subsequently, in step S 64 , the moving image acquisition unit  71  determines whether (a frame image of) the subsequent frame of a moving image is present, in other words, whether a subsequent frame has been inputted from the camera  12 . 
     In step S 64 , if it is determined that the subsequent frame of a moving image is absent, the image extraction is terminated. 
     If it is determined in step S 64  that the subsequent frame of a moving image is present, the processing advances to step S 65 . The moving image acquisition unit  71  acquires (a frame image of) the subsequent frame inputted from the camera  12  and supplies the frame to the image extraction unit  72 . The frame acquired in step S 61  will be referred to as the preceding frame, and the frame acquired in step S 65  will be referred to as the current frame. 
     In step S 66 , the image extraction unit  72  determines whether the frame image of the current frame supplied from the moving image acquisition unit  71  includes the calibration board  21 . 
     If it is determined in step S 66  that the frame image of the current frame does not include the calibration board  21 , the processing returns to step S 61  and the processing of step S 61  and later is performed again. Specifically, if the calibration board  21  is not included in any one of the frame images of the preceding frame and the current frame, the image processing device  11  acquires a preceding frame again. 
     If it is determined in step S 66  that the frame image of the current frame includes the calibration board  21 , the processing advances to step S 67 . The image extraction unit  72  determines whether the lighting pattern of the time display part  31  in the frame image of the current frame has changed from the frame image of the preceding frame. 
     If it is determined in step S 67  that the lighting pattern of the time display part  31  of the current frame has not changed from the frame image of the preceding frame, the processing returns to step S 64  and the foregoing steps S 64  to S 67  are repeated. In steps S 64  to S 67 , the subsequent frame of the moving image is acquired as the current frame, and it is determined whether the lighting pattern of the time display part  31  has changed. 
     If it is determined in step S 67  that the lighting pattern of the time display part  31  of the current frame has changed from the frame image of the preceding frame, the processing advances to step S 68 . The image extraction unit  72  associates time information identified from the lighting pattern of the time display part  31  and position information (section  42 ) identified from the lighting pattern of the position display part  32  with the frame image of the current frame, and stores the frame image in the extracted image storage unit  73 . The stored frame image of the current frame in the extracted image storage unit  73  is the time lighting-pattern changed frame image. 
     After step S 68 , the processing returns to step S 63 , and the foregoing processing is repeated. Specifically, the lighting pattern of the time display part  31  of the calibration board  21  in the frame image of the current frame is internally stored as the information of the preceding frame, the subsequent frame is acquired as the current frame, it is determined whether the lighting pattern of the time display part  31  has changed in the current frame from the preceding frame, and if the lighting pattern has been changed, time information and position information are stored while being associated with the frame image of the current frame. If it is determined that the subsequent frame of the moving image is absent, the image extraction is terminated. 
     Through the image extraction, at least one time lighting-pattern changed frame image is extracted from a moving image and is stored in the extracted image storage unit  73  along with the time information and position information of the calibration board  21  in the frame image. 
     The image extraction of  FIG.  16    is performed on a moving image inputted from each of the cameras  12 . Thus, a time lighting-pattern changed frame image is collected for a moving image captured by each of the cameras  12  and is stored in the extracted image storage unit  73 . 
     The image processing device  11  may temporarily store moving images, which are outputted from the cameras  12 , in the device and then perform the image extraction of  FIG.  16    for each of the cameras  12 . Alternatively, the image processing device  11  may simultaneously perform the image extraction of  FIG.  16    on two or more moving images. 
     In the image extraction, as described above, a frame image is extracted when the lighting pattern of the time display part  31  changes. Thus, the unit time of one frame of a moving image is preferably shorter than the unit time during which the lighting pattern of the time display part  31  of the calibration board  21  changes. The unit time of one frame of a moving image may be set longer than or as long as the unit time during which the lighting pattern of the time display part  31  changes. 
     &lt;9. Calibration&gt; 
     Referring to the flowchart of  FIG.  17   , calibration using the time lighting-pattern changed frame image, which is time-synchronized, will be described below. The calibration is performed by the image synchronization unit  74 , the calibration unit  75 , and the camera parameter storage unit  76  of the image processing device  11 . The processing is performed after the completion of the image extraction of  FIG.  16   . 
     First, in step S 81 , the image synchronization unit  74  selects the time lighting-pattern changed frame images such that the four sections  42 A to  42 D of the imaging region  41  are allocated in a predetermined ratio on the basis of position information associated with the time lighting-pattern changed frame images of the extracted image storage unit  73 . For example, the image synchronization unit  74  selects the time lighting-pattern changed frame image such that the four sections  42 A to  42 D are equally allocated. 
     In step S 82 , the image synchronization unit  74  performs time synchronizations on the time lighting-pattern changed frame images on the basis of time information associated with the time lighting-pattern changed frame images of the extracted image storage unit  73 . In other words, the image synchronization unit  74  selects (collects) the time lighting-pattern changed frame images captured at the same time, on the basis of the time information associated with the time lighting-pattern changed frame images. The selected time lighting-pattern changed frame images are supplied to the calibration unit  75 . 
     In step S 83 , the calibration unit  75  performs calibration for calculating the external parameters of the N cameras  12  by using the time lighting-pattern changed frame images that are time-synchronized and supplied from the image synchronization unit  74 . More specifically, by using time lighting-pattern changed frame images captured by the two cameras  12 -A and  12 -B at the same time, the calibration unit  75  sequentially performs, on the N cameras  12 - 1  to  12 -N, processing for calculating the positional relationship between the cameras  12 -A and the camera  12 -B. The external parameters of the N cameras  12  are supplied to the camera parameter storage unit  76  and are stored therein, the external parameters being obtained by the calibration. 
     Hence, the calibration using the time lighting-pattern changed frame images, which are time-synchronized, is completed. 
     By the calibration of  FIG.  17   , the time lighting-pattern changed frame images can be selected such that the four sections  42 A to  42 D of the imaging region  41  are allocated in a predetermined ratio on the basis of position information associated with the time lighting-pattern changed frame images. For example, if the user carrying the calibration board  21  moves in the imaging region  41  and the cameras  12  capture images of the calibration board  21 , the frame images of the sections  42  in the imaging region  41  may be unevenly distributed. Also in this case, the time lighting-pattern changed frame images of the sections  42  can be uniformly selected. 
     Moreover, on the basis of time information associated with the time lighting-pattern changed frame images, the plurality of time lighting-pattern changed frame images captured at the same time can be easily selected. In other words, synchronization can be easily achieved among the devices. Hence, calibration for calculating the external parameters of the cameras  12  can be performed by using the synchronized time lighting-pattern changed frame images. 
     The foregoing example described equal allocation in which the four sections  42 A to  42 D are allocated in a predetermined ratio. The sections  42 A to  42 D do not always need to be equally allocated. For example, if the locations of subjects serving as targets of 3D model generation are biased in the imaging region  41 , the sections may be distributed according to the ratio of the locations. 
     &lt;10. Modification Example of Calibration Board&gt; 
     In the example of  FIG.  4   , the 44 light-emitting parts  23  corresponding to the information display unit  54  of the calibration board  21  are disposed in the pattern of the image pattern  22 . This configuration is advantageous in that the light-emitting part  23  can be always detected by detecting the image pattern  22 . In other words, in a moving image of the calibration board  21 , the light-emitting parts  23  are always captured with the image pattern  22 . Since the light-emitting parts  23  are located near the feature points of the image pattern  22 , the light-emitting parts  23  are easily detected. The image pattern  22  may have any shape, for example, a pattern with circles in addition to a chess pattern. 
     The image pattern  22  is formed over a wide range in the calibration board  21 , so that the multiple light-emitting parts  23  ( 44  in the example of  FIG.  4   ) can be disposed in the pattern. 
     Since the multiple light-emitting parts  23  are provided, a sufficient amount of information can be obtained even if the information display unit  54  displays two kinds of information in the time display part  31  that illuminates according to a time and the position display part  32  that illuminates according to a position. Specifically, a large number of light-emitting parts  23  is allocated to the time display part  31  and thus the same lighting pattern does not periodically appear from the start to the end of image capturing, so that an elapsed time can be uniquely indicated. This can easily achieve synchronization among the plurality of cameras  12  that start capturing images at different times or start capturing the calibration board  21  at different times. 
     &lt;Another Layout Example of Light-Emitting Parts  23 &gt; 
     However, the light-emitting parts  23  of the calibration board  21  do not always need to be disposed in the pattern of the image pattern  22  and may be disposed in a region other than the region of the image pattern  22 . 
     For example, as illustrated in  FIG.  18   , the plurality of light-emitting parts  23  constituting the time display part  31  and the plurality of light-emitting parts  23  constituting the position display part  32  may be disposed outside the region of the image pattern  22 . In the example of  FIG.  18   , the time display part  31  and the position display part  32  are each composed of the eight light-emitting parts  23  and display position information and time information in units of 8 bits. 
     In addition to the time display part  31  and the position display part  32 , the information display unit  54  may further include a board display part that illuminates to identify the calibration board  21  when the plurality of calibration boards  21  are used at the same time. In this case, images can be captured by using the plurality of calibration boards  21 , and frame images are selected with the board display part in the same lighting condition, so that frame images including the same calibration board  21  can be selected. 
     &lt;Manual Display Example of Position Display Part  32 &gt; 
     In the foregoing example, the calibration board  21  is provided with the position information detection unit  51  including a GPS module, and the lighting condition of the position display part  32  changes according to the detection result of the position information detection unit  51 . 
     However, the lighting condition of the position display part  32  may be changed by a user operation on the operation button  24  serving as the operation unit  52 . For example, the floor is marked with the plurality of sections  42  determined by dividing the imaging region  41 , and the operation button  24  is operated according to the section  42  where a user carrying the calibration board  21  is located, so that the lighting condition can be changed according to the section  42 . In this case, the position information detection unit  51  can be omitted. Moreover, the lighting condition of the position display part  32  may include an unavailable state indicating unavailability to calibration. The user operates the operation button  24  to set the lighting condition of the position display part  32  to an unavailable state during a movement between the sections  42  or at a location unavailable to calibration, so that the frame image can be excluded in the image extraction of the image extraction unit  72 . 
     &lt;Addition of Communication Function&gt; 
     The calibration board  21  can be configured with the communication function of radio communications of Wi-Fi (registered trademark) and bluetooth (registered trademark) or cable communications. This achieves a configuration that transmits, for example, the detection result of the position information detection unit  51  to a self-propelled robot via radio communications and causes the self-propelled robot to move in the imaging region  41  according to received position information. Alternatively, the calibration board  21  transmits the detection result of the position information detection unit  51  to the smartphone (portable terminal) of the user carrying the calibration board  21 , allowing the user to move in the imaging region  41  while confirming position information displayed on a map application of the smartphone. Alternatively, the detection result of the position information detection unit  51  may be transmitted to the image processing device  11  and the position of the calibration board  21  may be displayed on the display device  13 , allowing the user carrying the calibration board  21  to move while confirming position information displayed on the display device  13 . 
     &lt;11. Configuration Example of 3D Model Generation&gt; 
     When calibration for calculating the positional relationship among the cameras  12  is performed by using the moving images of the calibration board  21  imaged by the cameras  12  and the external parameters of the cameras  12  are stored in the camera parameter storage unit  76 , a predetermined subject as a target of 3D model generation is prepared for imaging with the cameras  12 . 
     3D model generation will be described below. The 3D model generation includes processing for capturing the moving images of the predetermined subject as a target of 3D model generation with the cameras  12  in the image processing system  1  and generating a 3D model of an object, which is the predetermined subject, on the basis of the moving images captured by the cameras  12 , and rendering for displaying a two-dimensional image of a 3D object on the viewing device of a viewer on the basis of the generated 3D model. 
       FIG.  19    is a block diagram illustrating a configuration example of 3D model generation by the image processing device  11 . 
     The image processing device  11  includes the camera parameter storage unit  76  and a 3D-model operation unit  81 . The 3D-model operation unit  81  includes a moving image acquisition unit  91 , a 3D-model generation unit  92 , a 3D model DB  93 , and a rendering unit  94 . 
     The moving image acquisition unit  91  acquires the images (moving images) of a subject from the N cameras  12 - 1  to  12 -N and supplies the images to the 3D-model generation unit  92 . 
     The 3D-model generation unit  92  acquires the camera parameters of the N cameras  12 - 1  to  12 -N from the camera parameter storage unit  76 . The camera parameters include at least an external parameter and an internal parameter. 
     The 3D-model generation unit  92  generates a 3D model of the subject on the basis of the images captured by the N cameras  12 - 1  to  12 -N and the camera parameters, and stores moving image data on the generated 3D model (3D-model data) in the 3D model DB  93 . 
     The 3D model DB  93  stores the 3D-model data generated in the 3D-model generation unit  92  and supplies the data to the rendering unit  94  in response to a request from the rendering unit  94 . The 3D model DB  93  and the camera parameter storage unit  76  may be the same storage medium or different storage media. 
     The rendering unit  94  acquires, from the 3D model DB  93 , moving image data (3D model data) on a 3D model specified by a viewer of the reproduced image of the 3D model. The rendering unit  94  then generates (reproduces) a two-dimensional image by viewing the 3D model from the viewing position of the viewer and supplies the two-dimensional image to the display device  13 , the viewing position being supplied from the operation unit, which is not illustrated. On the assumption that a virtual camera is used with an imaging range equivalent to the viewing range of the viewer, the rendering unit  94  generates the two-dimensional image of the 3D object captured by the virtual camera and displays the two-dimensional image on the display device  13 . The display device  13  includes the display D 1  or the head-mounted display (HMD) D 2  as illustrated in  FIG.  1   . 
     &lt;12. Flowchart of 3D Model Generation&gt; 
     Referring to the flowchart of  FIG.  20   , 3D model generation by the image processing device  11  in  FIG.  19    will be described below. This processing is started in response to an instruction to start processing for capturing, with the cameras  12 , images of a predetermined subject serving as a target of 3D model generation in, for example, the image processing device  11 . 
     First, in step S 81 , the moving image acquisition unit  91  acquires the images (moving images) of a subject from the N cameras  12 - 1  to  12 -N and supplies the images to the 3D-model generation unit  92 . 
     In step S 82 , the 3D-model generation unit  92  acquires the camera parameters of the N cameras  12 - 1  to  12 -N from the camera parameter storage unit  76 . 
     In step S 83 , the 3D-model generation unit  92  generates a 3D model of the subject on the basis of the images captured by the N cameras  12 - 1  to  12 -N and the camera parameters, and stores moving image data on the generated 3D model (3D-model data) in the 3D model DB  93 . 
     In step S 84 , the rendering unit  94  acquires, from the 3D model DB  93 , moving image data (3D model data) on a 3D model specified by a viewer. The rendering unit  94  then generates (reproduces) a two-dimensional image by viewing the 3D model from the viewing position of the viewer and causes the display device  13  to display the two-dimensional image, the viewing position being supplied from the operation part, which is not illustrated. 
     The process of step S 84  is continuously performed until the end of viewing of the reproduced image of the 3D model. When an exit operation is detected, the 3D model generation is terminated. 
     The 3D model generation in steps S 81  to S 83  and rendering for displaying a two-dimensional image of a 3D object on the viewing device of a viewer in step S 84  do not have to be consecutively performed and may be performed at different timings. 
     As described above, the image processing device  11  can perform processing for calculating the camera parameters of the cameras  12  on the basis of the moving images of the calibration board  21 , processing for generating a 3D model of an object, which is the predetermined subject, on the basis of the moving images of the predetermined subject imaged by the cameras  12  by using the calculated camera parameters, and processing for generating a two-dimensional image as a virtual viewpoint image obtained by viewing the generated 3D model of the object from a predetermined viewpoint. 
     &lt;13. Computer Configuration Example&gt; 
     The series of processing can be performed by hardware or software. When the series of processing is performed by software, a program constituting the software is installed in a computer. In this configuration, the computer includes a microcomputer embedded in dedicated hardware or includes, for example, a general-purpose personal computer in which various functions can be performed by installing various programs. 
       FIG.  21    is a block diagram illustrating a hardware configuration example of a computer in which the series of processing is performed by the programs. 
     In the computer, a CPU (Central Processing Unit)  101 , a ROM (Read Only Memory)  102 , and a RAM (Random Access Memory)  103  are connected to one another via a bus  104 . 
     An input/output interface  105  is further connected to the bus  104 . An input unit  106 , an output unit  107 , a storage unit  108 , a communication unit  109 , and a drive  110  are connected to the input/output interface  105 . 
     The input unit  106  includes a keyboard, a mouse, a microphone, a touch panel, and an input terminal. The output unit  107  includes a display, a speaker, and an output terminal. The storage unit  108  includes a hard disk, a RAM disc, and a nonvolatile memory. The communication unit  109  includes a network interface. The drive  110  drives a removable recording medium  111 , e.g., a magnetic disk, an optical disc, a magneto-optical disc, or a semiconductor memory. 
     In the computer configured thus, for example, the CPU  101  performs the series of processing by loading a program, which is stored in the storage unit  108 , into the RAM  103  via the input/output interface  105  and the bus  104  and executing the program. In the RAM  103 , data necessary for performing a variety of processing by the CPU  101  is also optionally stored. 
     The program to be executed by the computer (the CPU  101 ) can be recorded on, for example, the removable recording medium  111  serving as a package medium and provided in this form. The program can also be provided via wire or wireless transmission media such as a local area network, the Internet, or digital satellite broadcasting. 
     In the present description, the steps having been described in the flowcharts may be carried out in parallel or with necessary timing, for example, when evoked, even if the steps are not executed in time series along the order having been described therein, as well as when the steps are executed in time series. 
     In the present specification, a system means a collection of a plurality of constituent elements (devices, modules (components), or the like) regardless of the presence or absence of all the constituent elements in the same casing. Accordingly, a plurality of devices stored in separate casings and connected via a network and a single device including a plurality of modules stored in a casing are all systems. 
     Note that embodiments of the present disclosure are not limited to the foregoing embodiment and can be modified in various manners without departing from the gist of the present disclosure. 
     Furthermore, for example, a plurality of techniques relating to the present technique can be independently implemented as separate techniques if no contradiction arises. Of course, any present techniques may be implemented in combination. For example, at least a part of the present technique described in any one of the embodiments can be implemented in combination with at least a part of the present technique described in other embodiments. Alternatively, at least a part of the present technique can be implemented in combination with other techniques that are not described above. 
     For example, the present technique may be configured for cloud computing in which one function is shared and cooperatively processed by a plurality of devices via a network. 
     In addition, the steps described in the flowchart can be executed by a single device or can be shared among a plurality of devices. 
     Furthermore, if one step includes a plurality of processes, the plurality of processes included in the step can be executed by a single device or can be shared among a plurality of devices. 
     The advantageous effects described in the present specification are merely exemplary and are not limited, and other advantageous effects may be achieved in addition to the advantageous effects described in the present specification. 
     The present technique can be configured as follows: 
     (1) 
     An image processing device including: an image synchronization unit that performs time synchronization on a plurality of images of a board on a basis of lighting conditions of a plurality of light-emitting parts included in the plurality of images captured by a plurality of imaging devices, the board including the plurality of light-emitting parts and a predetermined image pattern; and a calibration unit that calculates camera parameters of the plurality of imaging devices by using the plurality of images having been subjected to the time synchronization. 
     (2) 
     The image processing device according to (1), wherein the plurality of light-emitting parts have a time display part that illuminates according to an imaging time, and the image synchronization unit performs the time synchronization on the plurality of images by selecting the images in which the time display part has the same lighting condition. 
     (3) 
     The image processing device according to (2), wherein the plurality of light-emitting parts further have a position display part that illuminates according to a position of the board, and 
     the image synchronization unit performs the time synchronization on the plurality of images by selecting the images in which the time display part has the same lighting condition from the images selected such that the different lighting conditions of the position display part are provided in a predetermined ratio. 
     (4) 
     The image processing device according to (3), wherein an imaging range of the plurality of imaging devices is divided into a plurality of sections, and the position display part of the plurality of light-emitting parts illuminates according to the sections. 
     (5) 
     The image processing device according to (3) or (4), wherein the image synchronization unit selects the images such that the different lighting conditions of the position display part are equally allocated. 
     (6) 
     The image processing device according to any one of (1) to (5), wherein the plurality of light-emitting parts have a board display part that illuminates to identify the board, and 
     the image synchronization unit performs the time synchronization on the plurality of images by selecting the images in which the board display part has the same lighting condition. 
     (7) 
     The image processing device according to any one of (1) to (6), wherein the board is configured with the light-emitting parts disposed in a pattern of the predetermined image pattern. 
     (8) 
     The image processing device according to any one of (1) to (6), wherein the board is configured with the light-emitting parts disposed in a region different from a region of the predetermined image pattern. 
     (9) 
     The image processing device according to any one of (2) to (8), wherein the time display part of the board changes the lighting condition at each lapse of a unit time. 
     (10) 
     The image processing device according to any one of (3) to (9), wherein the board further includes a position information detection unit that detects position information, and 
     the position display part changes the lighting condition according to a detection result of the position information detection unit. 
     (11) 
     The image processing device according to any one of (3) to (10), wherein the board further includes an operation unit that receives a user operation, and the position display part changes the lighting condition in response to an operation on the operation unit. 
     (12) 
     The imaging device according to any one of (1) to (11), wherein the light-emitting parts of the board illuminate in different colors according to 0 or 1. 
     (13) 
     The imaging processing device according to any one of (1) to (11), wherein the light-emitting parts of the board are illuminated or unilluminated according to 0 or 1. 
     (14) 
     The imaging processing device according to any one of (1) to (13), further including an extraction unit that determines whether the lighting conditions of the plurality of light-emitting parts included in the images have changed, and extracts the changed images, 
     wherein the image synchronization unit performs time synchronization on the plurality of images on the basis of the lighting conditions of the plurality of light-emitting parts included in the plurality of extracted images. 
     (15) 
     A calibration board including a plurality of light-emitting parts that change lighting conditions at each lapse of a unit time, and a predetermined image pattern, wherein the plurality of light-emitting parts are caused to illuminate to perform time synchronization on a plurality of images captured by a plurality of imaging devices. 
     (16) 
     A method for generating 3D model data, the method comprising: performing time synchronization on a plurality of images of a board on a basis of lighting conditions of a plurality of light-emitting parts included in the plurality of images captured by a plurality of imaging devices, the board including the plurality of light-emitting parts and a predetermined image pattern; calculating camera parameters of the plurality of imaging devices by using the plurality of images having been subjected to the time synchronization; 
     generating a 3D model of a predetermined subject from a plurality of subject images of the predetermined subject, the subject images being captured by the plurality of imaging device by using the calculated camera parameters; and generating a virtual viewpoint image by viewing the generated 3D model of the predetermined subject from a predetermined viewpoint. 
     REFERENCE SIGNS LIST 
     
         
           1  Image processing system 
           11  Image processing device 
           12 - 1  to  12 -N Camera (imaging device) 
           13  Display device 
           21  Calibration board 
           22  Image pattern 
           23  Light-emitting part 
           24  Operation button 
           31  Time display part 
           32  Position display part 
           41  Imaging region (imaging space) 
           42  ( 42 A to  42 D) Section 
           46  Common imaging range 
           51  Position information detection unit 
           52  Operation unit 
           53  Control unit 
           54  Information display unit 
           71  Moving image acquisition unit 
           72  Image extraction unit 
           73  Extracted image storage unit 
           74  Image synchronization unit 
           75  Calibration unit 
           76  Camera parameter storage unit 
           81  3D-model operation unit 
           91  Moving image acquisition unit 
           92  3D-model generation unit 
           93  3D model DB 
           94  Rendering unit 
           101  CPU 
           102  ROM 
           103  RAM 
           106  Input unit 
           107  Output unit 
           108  Storage unit 
           109  Communication unit 
           110  Drive