Abstract:
A position detector displays a target on a given plane and adds a standard on the given plane in the vicinity of the target with the location of the standard known. An image of the given plane is formed on an image plane of an image sensor with an image of the standard included, a point in the image of the given plane which is formed at a predetermined position of the image plane corresponding to the point to be detected. An image processor identifies the image of the standard on the image plane to calculate the position of the point to be detected. The standard includes asymmetric pattern. The standard includes a first standard and a second standard sequentially added on the given plane, the difference being calculated accompanied with the plus sign or the minus sign. The image on the given plane is formed by means of a scanning, the image sensor reads out the sensed image upon the termination of at least one period of the scanning. The second standard is added upon the initiation of the scanning after the completion of reading out of the image of the first standard.

Description:
BACKGROUND OF THE INVENTION 
   This application is based upon and claims priority of Japanese Patent Applications No. 2001-081908 filed on Mar. 22, 2001 and No. 2001-102934 filed on Apr. 2, 2001, the contents being incorporated herein by reference. 
   1. Field of the Invention 
   The present invention relates to a position detector and an attitude detector. 
   2. Description of Related Art 
   In this field of the art, especially in a robot vision, game machine and pointing device, various methods of detecting a position on a screen have been proposed. The typical one of the methods detects the desired position on the basis of the image of a standard or marks on the screen taken by a camera. 
   Examples of the above position detector are disclosed in Japanese Patent Publication Nos. Hei 6-35607, Hei 7-121293 and Hei 11-319316. 
   Also, a system for adjusting a video projector has been well known, in which a video camera captures a test pattern image displayed on a screen. However, if the video projector and the video camera have different vertical scanning frequencies from each other, a flickering pattern of bright and dark bands would be caused in the image taken by the video camera. In order to solve the problem, various proposals have been made, such as in Japanese Patent Publication Nos. Hei 5-30544, Hei 8-317432 and Hei 11-184445. 
   For example, Japanese Patent Publication No. Hei 11-184445 discloses an imaging system in which the timing of the start and the end of photographing in a video camera is controlled by generating a shutter control signal in accordance with the vertical synchronizing signal of a display apparatus. 
   However, there have been problems and disadvantages still left in the related arts, especially as to the convenience, accuracy or quickness of the detection. 
   SUMMARY OF THE INVENTION 
   In order to overcome the problems and disadvantages, the invention provides a position detector for detecting a position on a given plane. The position detector comprises a first controller for displaying a target point on the given plane and a second controller for displaying a known standard on the given plane in the vicinity of the target point with the location of the standard being known. The position detector further comprises an image sensor having an image plane on which an image that includes an image of the standard is formed, the image plane having a predetermined position. Also in the position detector according to the present invention, an image processor identifies the image of the standard on the image plane, and a processor calculates a position of a point on the given plane corresponding to the predetermined position on the image plane using parameters of an attitude of the image plane relative to the given plane based on the identified image of the standard. 
   Thus, the known standard can always be sensed on the image plane of the image sensor as long as the target point is aimed at even if the field angle of the image sensor is not so wide. 
   The above advantage is typical in accordance with a detailed feature of the present invention. In the detailed feature, the first controller displays the target point at different positions on the given plane, and the second controller displays the known standard at different positions on the given plane in correspondence to the different positions of the target point. Alternatively, the first controller displays one of different target points on the given plane, and the second controller displays the known standard in the vicinity of the one of the different target points on the given plane. Thus, the known standard always keeps up with the target no matter where the aimed target point is located or moved on the given plane. 
   According to another feature of the present invention, the known standard includes an asymmetric pattern. For example, the asymmetric pattern includes four marks forming a rectangle, one of the four marks being distinguishable from the others. This makes it possible to determine the rotary attitude of the image plane of the image sensor relative to the given plane. 
   According to still another feature of the present invention, the known standard includes a first standard and a second standard sequentially displayed on the given plane, wherein the image sensor senses a first image that includes an image of the first standard and a second image that includes an image of the second standard, and wherein the processor includes a calculator that calculates the difference between the first image and the second image to identify the image of the standard. In more detail, the processor determines whether the difference is positive or negative at the identified standard. 
   According to a further feature of the present invention the first standard and the second standard include a plurality of marks, respectively, the marks of the second standard being located at the same positions as the marks of the first standard with the pattern formed by the marks in the second standard being a reversal of that in the first standard. 
   The above features give the standard a high advantage in the detection thereof as well as the realization of its asymmetry. 
   According to another feature of the present invention, the first controller forms an image by scanning the given plane, the target point is displayed as a part of the image formed by the scanning, and the second controller displays the known standard as a part of the image formed by the scanning. 
   In more detail, the image sensor reads out the sensed image upon the termination of at least one period of the scanning. 
   According to another detailed feature the known standard includes the first standard and the second standard sequentially displayed on the given plane, the second controller starts displaying the second standard upon the initiation of the scanning after the image sensor completes the reading out of the sensed image that includes the first standard. 
   The above features are advantageous for the image sensor to sense the image on the given plane in synchronism with the scanning of the given plane by the first controller. 
   The above features and advantages according to the present invention are not only applicable to the position detector, but also to an attitude detector in its essence. Further, the above features and advantages relating to synchronization of the function of the image sensor with the scanning of the given plane is not only applicable to the position detector or the attitude detector, but also to a detector in general for detecting a standard on a given plane in its essence. 
   Other features and advantages according to the invention will be readily understood from the detailed description of the preferred embodiment in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  represents a perspective view of the first embodiment of a shooting game machine. 
       FIG. 2  represents a block diagram of the embodiment according to the present invention. 
       FIG. 3  represents a detailed block diagram of image processor  50 . 
       FIG. 4  represents a perspective view of controller  100 . 
       FIG. 5  represents a cross sectional view of the optical system in the controller  100 . 
       FIG. 6  represents a flowchart of the basic operation of the shooting game according to the present invention. 
       FIG. 7  shows the manner of calculating the coordinate of the target point. 
       FIG. 8  represents the image q taken by the controller  100 . 
       FIG. 9  is to explain the coordinate conversion. 
       FIG. 10  is an explanation of the spatial relationship between X-Y-Z coordinate and X*-Y* coordinate. 
       FIG. 11  represents the pair of standard images Kt 1  and Kt 2  both with four marks. 
       FIG. 12  represents a flowchart of the functions of sensing image 
       FIG. 13  represents sensed image q taken by CCD  101  of controller  100 . 
       FIG. 14  represents timing charts of the function of controller  100  in sensing images. 
       FIG. 15  represents a flowchart of the function of controller  100 . 
       FIG. 16  represents the image signals for the four marks. 
       FIG. 17  represents an illustration of images for explaining the identification of the mark position. 
       FIG. 18  represents a flowchart for identifying the mark positions. 
       FIG. 19  represents the projected image on the wide screen 
       FIG. 20  represents a timing chart of the second embodiment. 
       FIG. 21  represents a flowchart of the function of controller  100  according to the second embodiment. 
       FIG. 22  represents a timing chart of the function of controller  100  according to the third embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   [First Embodiment] 
     FIG. 1  represents a perspective view of the first embodiment of a shooting game machine on the basis of the position and attitude detecting system according to the present invention. Projector  130  projects on wide screen  110  a scene according to the shooting game story. 
   Projected scene  111  includes target object A, which is a flying object, as well as a standard image including four detection marks mQ 1 , mQ 2 , mQ 3 , mQ 4  surrounding target object A, the positions of the detection marks relative to target object A being predetermined in the projected scene  111 . The player at point PS in front of wide screen  110  is to shoot the target object A at a predetermined point Ps with controller  100  formed as a gun, controller  100  serving as a sensor of the position and attitude detecting system. 
   In  FIG. 1 , the respective centers of gravity of the four marks are defined as characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4 , which in combination form a rectangular. The position of image on the screen is identified with X*-Y* coordinate named “screen coordinate” with its origin at predetermined point Ps. 
   Though four detection marks are adopted as the standard image in the above embodiment, any alternative may be adopted as the standard as long as it can define a rectangle. 
     FIG. 2  represents a block diagram of the embodiment according to the present invention, in which the manner of sensing image is explained. 
   Controller  100  includes objective lens  102 , image sensor  101  such as CCD (hereinafter referred to as CCD  101 ), trigger switch  103  for the player to take the picture on CCD  101  upon shooting the target, A/D converter  104  for converting the output of CCD  101  into digital image data, timing generator  105  for generating various clock signals necessary for CCD  101  to sense the image, synchronization signal detector  106  for picking up only the vertical synchronization signals among image signals transmitted to the image projector, and interface  107  for communicating with main body  120  of the game machine (or the personal computer). Though controller  100  also includes other conventional elements, such as power source, they are omitted from  FIG. 2  for simplification. 
   The image signal taken by controller  100  is output from interface  107  for transmission to interface  121  of main body  120 . As interface  107  and  121 , various wired or wireless means may be adopted, such as USB, IEEE1294, IrDA or Bluetooth or the like. If main body  120  is provided with the conventional video board within the housing, the analog signal generated by CCD  101  may be directly input into main body  120  since such a video board normally includes an A/D converter. 
   Main body  120  includes timing generator  123  for synchronization with image signal, display controller  124  for controlling the video signal on display, and image processor  50 , which processes image according to a predetermined program. Image processor according to the embodiment carries out the extraction of the four marks and necessary calculations thereon for detecting the position of target object. 
   Display controller  124  outputs video signal to projector  130  at the same cycle as the vertical synchronization signals generated by timing generator  123 . 
   Synchronizing signal detector  106  located within controller  100  in the embodiment may be modified to locate within main body  120 . 
   The display according to the embodiment, in which projector  130  projects image on wide screen  110 , may modified to be replaced by a cathode ray tube (CRT) display, a liquid crystal device (LCD) display, or the like. 
     FIG. 3  represents a detailed block diagram of image processor  50 . 
   In  FIG. 3 , image processor  50  includes characteristic point detector  51 , position calculator  6  and image generator  9 . 
   Characteristic point detector  51  includes difference calculator  511  for extracting marks characterizing a rectangle on the basis of a difference between a pair of standard images of different illumination. Characteristic point detector  51  also includes a binary processor  512 . 
   Mark identifier  513  is for calculating the coordinate of the center of gravity of each mark and distinguishes a mark from the others. 
   Position calculator  52  includes attitude calculator  521  for calculating the attitude of the wide screen relative to controller  100  and target point calculator  522  for calculating the coordinate of the target object. 
   Hit comparator  8  judges whether or not one of the objects is shot in one of its portions by means of comparing the position of each portion in each object with the position calculated by coordinate calculator  522 . Hit comparator  8  is informed of positions of all portions in all objects to identify the shot object with its specific portion. 
   Image generator  9  superimposes the relevant objects on the background virtual reality space for display on screen  110  by projector  130 . In more detail, image generator  9  includes movement memory  91  for storing a movement data predetermined for each portion of each object, the movement data being to realize a predetermined movement for any object if it is shot in any portion. Further included in image generator  9  is coordinate calculator  92  for converting a movement data selected from movement memory  91  into a screen coordinate through the perspective projection conversion viewed from an image view point, i.e. an imaginary camera view point, along the direction defined by angles α, γ and ψ. Image generator superimposes the calculated screen coordinate on the data of the background virtual reality space by means of picture former  93 , the superimposed data thus obtained being stored in frame memory  94 . 
   Picture former  93  controls the picture formation of the objects and the background virtual reality space in accordance with the advance of the game. For example, a new object will appear in the screen or an existing object will move within the screen in accordance with the advance of the game. 
   The superimposed data of objects and the background virtual reality space temporarily stored in frame memory  94  is combined with the scroll data to form a final frame image data to be projected on screen  110  by projector  130 . 
     FIG. 4  represents a perspective view of controller  100 . 
   In  FIG. 4 , the controller  100  has the shutter release button  103  of a camera  100  to be transmitted toward the target for visually pointing the target point on the screen plane. The sighting device  200  is the light beam emitter or the optical finder for the purpose of aiming the target point so that the target point is sensed at the predetermined point on the image sensing plane of CCD  101 . 
   Controller  100  further has control buttons  14 ,  15  to have an object character jump or go up and down, or backward and forward, which is necessary for advancing the game. Input/output interface  3  processes the image data by A/D converter, and transfers the result to image processor. 
     FIG. 5  represents a cross sectional view of the optical system in the controller  100  using the light beam emitter as the sighting device  200 . If a power switch is made on, the laser beam is emitted at light source point  200 A and collimated by collimator  200 B to advance on the optical axis of camera lens  102  toward rectangular plane  110  by way of mirror  200 C and semitransparent mirror  13 A. Camera  100  includes objective lens  102  and CCD  101  for sensing image through semitransparent mirror  13 A, the power switch of the laser being made off when the image is sensed by camera  100 . Therefore, mirror  13 A may alternatively be a full refractive mirror, which is retractable from the optical axis when the image is sensed by camera  100 . 
   The followings will give the explanation of the manner of detecting the position and attitude. 
   (a) Position Calculation 
   Position calculator calculates a coordinate of a target point Ps on a screen plane defined by characteristic points, the screen plane being located in a space. 
     FIG. 6  represents a flowchart of the basic operation of the shooting game according to the present invention. 
   In step S 100 , the main power of the controller is turned on. In step S 101 , the target point on a screen plane having the plurality of characteristic points is aimed so that the target point is sensed at the predetermined point on the image sensing plane of CCD  101 . According to the first embodiment, the predetermined point is specifically the center of image sensing plane of CCD  101  at which the optical axis of the objective lens  102  of camera intersects. 
   In step S 102 , the image is taken in response to shutter switch (trigger switch)  103  of the camera  100  with the image of the target point at the predetermined point on the image sensing plane of CCD  101 . 
   In step S 103 , the characteristic points defining the rectangular plane are identified each of the characteristic points being the center of gravity of each of predetermined marks, respectively. The characteristic points are represented by coordinate q 1 , q 2 , q 3  and q 4  on the basis of image sensing plane coordinate. 
   Step S 104  is for processing the rotational parameters for defining the attitude of the screen plane in a space relative to the image sensing plane, and step S 105  is calculating the coordinate of the target point on the screen plane, which will be explained later in detail. 
   In step S 106 , the coordinate of position of the target point is compared with the coordinate of position calculated in step S 105  to find whether the distance from the position calculated by the processor to the position of the target point is less than a limit. In other words it is judged in step S 106  whether or not one of the objects is shot in one of its portions. If no object is shot in any of its portions in step S 106 , the flow returns to step S 101  to wait for next trigger by the player since it is shown in step  106  that the player fails in shooting the object. 
   If it is judged in step  106  that one of the objects is shot in one of its portions, the flow advances to step S 107 , where a predetermined movement is selected in response to the identified shot portion. In more detail, in step S 107 , the movement data predetermined for the shot portion is retrieved from movement memory  91  to realize the movement for the shot portion. If such movement data includes a plurality of polygon data for a three-dimensional object, a movement with high reality of the object is realized by means of selecting the polygon data in accordance with the attitude calculated in step S 104 . 
   In step S 108  the data of movement of the target given through step S 109  is combined with the data of position and direction of the player given through step for forming a final image to be displayed on screen  110  by projector  130 . The data of position of the player will give a high reality of the change in the target and the background space on screen  110  in accordance with the movement of the player relative to screen  110 . 
     FIG. 7  shows the manner of calculating the coordinate of the target point and corresponds to the details of step  105  in  FIG. 6 . 
     FIG. 8  represents the image q taken by the controller  100 . In  FIG. 8  image of target point Ps is in coincidence with predetermined point Om, which is the origin of the image coordinate. Characteristic points q 1 , q 2 , q 3  and q 4  are the images on the image sensing plane of the original of characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4  on the rectangular plane represented by X*-Y* coordinate. 
   (a1) Attitude Calculation 
   Now, the attitude calculation, which is the first step of position calculation, is to be explained in conjugation with the flow chart in  FIG. 7 . 
   The parameters for defining the attitude of the given plane with respect to the image sensing plane are rotation angle γ around X-axis, rotation angle ψ around Y-axis, and rotation angle α or β around Z-axis. 
   Referring to  FIG. 7 , linear equations for lines q 1 q 2 , q 2 q 3 , q 3 q 4  and q 4 q 1  are calculated on the basis of coordinates for detected characteristic points q 1 , q 2 , q 3  and q 4  in step S 201 , lines q 1 q 2 , q 2 q 3 , q 3 q 4  and q 4 q 1  being defined between neighboring pairs among characteristic points q 1 , q 2 , q 3  and q 4 , respectively. In step S 202 , vanishing points T 0  and S 0  are calculated on the basis of the liner equations. 
   The vanishing points defined above exist in the image without fail if a rectangular plane is taken by a camera. The vanishing point is a converging point of lines. If lines q 1 q 2  and q 3 q 4  are completely parallel with each other, the vanishing point exists in infinity. 
   According to the first embodiment, the plane located in a space is a rectangular having two pairs of parallel lines, which cause two vanishing points on the image sensing plane, one vanishing point approximately on the direction along the X-axis, and the other along the Y-axis. 
   In  FIG. 8 , the vanishing point approximately on the direction along the X-axis is denoted with S 0 , and the other along the Y-axis with T 0 . Vanishing point T 0  is an intersection of lines q 1 q 2  and q 3 q 4 . 
   In step S 203 , linear vanishing lines OmS 0  and OmT 0 , which are defined between vanishing points and origin Om, are calculated. 
   Further in step S 203 , vanishing characteristic points qs 1 , qs 2 , qt 1  and qt 2 , which are intersections between vanishing lines OmS 0  and OmT 0  and lines q 3 q 4 , q 1 q 2 , q 4 q 1  and q 2 q 3 , respectively, are calculated. 
   The coordinates of the vanishing characteristic points are denoted with qs 1  (Xs 1 ,Ys 1 ), qs 2  (Xs 2 ,Ys 2 ), qt 1  (Xt 1 ,Yt 1 ) and qt 2  (Xt 2 ,Yt 2 ). Line qt 1 qt 2  and qs 1 qs 2  defined between the vanishing characteristic points, respectively, will be called vanishing lines as well as OmS 0  and OmT 0 . 
   Vanishing lines qt 1 qt 2  and qs 1 qs 2  are necessary to calculate target point Ps on the given rectangular plane. In other words, vanishing characteristic points qt 1 , qt 2 , qs 1  and qs 2  on the image coordinate (X-Y coordinate) correspond to points T 1 , T 2 , S 1  and S 2  on the plane coordinate (X*-Y* coordinate) in  FIG. 1 , respectively. 
   If the vanishing point is detected in infinity along X-axis of the image coordinate in step S 202 , the vanishing line is considered to be in parallel with X-axis. 
   Instep S 204 , image coordinate (X-Y coordinate) is converted into X′-Y′ coordinate by rotating the coordinate by angle β around origin Om so that X-axis coincides with vanishing line OmS 0 . Alternatively, image coordinate (X-Y coordinate) may be converted into X″-Y″ coordinate by rotating the coordinate by angle α around origin Om so that Y-axis coincides with vanishing line OmT 0 . Only one of the coordinate conversions is necessary according to the first embodiment. 
     FIG. 9  is to explain the coordinate conversion from X-Y coordinate to X′-Y′ coordinate by rotation by angle β around origin Om with the clockwise direction is positive.  FIG. 9  also explains the alternative case of coordinate conversion from X-Y coordinate to X″-Y″ coordinate by rotating the coordinate by angle α. 
   The coordinate conversion corresponds to a rotation around Z-axis of a space (X-Y-Z coordinate) to determine one of the parameters defining the attitude of the given rectangular plane in the space. 
   By means of the coincidence of vanishing line qs 1 qs 2  with X-axis, lines mQ 1 mQ 2  and mQ 3 mQ 4  are made in parallel with X-axis. 
   In step S 205 , characteristic points q 1 , q 2 , q 3  and q 4  and vanishing characteristic points qt 1 , qt 2 , qt 3  and qt 4  on the new image coordinate (X′-Y′ coordinate) are related to characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4  and points T 1 , T 2 , S 1  and S 2  on the plane coordinate (X*-Y* coordinate). This is performed by perspective projection conversion according to the geometry. By means of the perspective projection conversion, the attitude of the given rectangular plane in the space (X-Y-Z coordinate) on the basis of the image sensing plane is calculated. In other words, the pair of parameters, angle ψ around Y-axis and angle γ around X-axis for defining the attitude of the given rectangular plane are calculated. 
   In step S 206 , the coordinate of target point Ps on the plane coordinate (X*-Y* coordinate) is calculated on the basis of the parameters gotten in step S 205 . The details of the calculation to get the coordinate of target point Ps will be discussed later in section (a2). 
   Perspective projection conversion is for calculating the parameters (angles ψ and angle γ) for defining the attitude of the given rectangular plane relative to the image sensing plane on the basis of the four characteristic points identified on image coordinate (X-Y coordinate). 
     FIG. 10  is an explanation of the spatial relationship between X-Y-Z coordinate (hereinafter referred to as “image coordinate”) representing the equivalent image sensing plane in a space and X*-Y* coordinate (hereinafter referred to as “plane coordinate”) representing the given rectangular plane. Z-axis of image coordinate intersects the center of the equivalent image sensing plain perpendicularly thereto and coincides with the optical axis of the objective lens. View point O for the perspective projection conversion is on Z-axis apart from origin Om of the image coordinate by f. Rotation angle γ around X-axis, rotation angle ψ around Y-axis, and two rotation angles α and β both around Z-axis are defined with respect to the image coordinate, the clockwise direction being positive for all the rotation angles. With respect to view point O, Xe-Ye-Ze coordinate is set for perspective projection conversion, Ze-axis being coincident with Z-axis and Xe-axis and Ye-axis being in parallel with which will X-axis and Y-axis, respectively. 
   Equations (1) and (2) are conclusion of defining angle γ an ψ which are the other two of parameters for defining the attitude of the given rectangular plane relative to the image sensing plane. The value for tan γ given by equation (1) can be practically calculated by replacing tan ψ by the value calculated through equation (2). Thus,all of the three angles β, γ and ψ are obtainable. 
               tan   ⁢           ⁢   γ     =       -     1     tan   ⁢           ⁢   ϕ         ·       X   t1   ′       Y   t1   ′                 (   1   )                 tan   ⁢           ⁢   ϕ     =           Y   1   ′     -     Y   t1   ′             X   t1   ′     ⁢     Y   1   ′       -     X   1   ′     -     Y   t1   ′         ·   f             (   2   )             
 
   In the case of equations (1) and (2), at least one coordinate of characteristic point q 1  (X′ 1 , Y′ 1 ), at least one coordinate of a vanishing characteristic point qt 1  (X′t 1 , Y′t 1 ) and distance f are only necessary to get angles γ and ψ. 
   (a2) Coordinate Calculation 
   Now, the coordinate calculation for determining the coordinate of the target point on the given rectangular plane is to be explained. The position of target point Ps on given rectangular plane  110  with the plane coordinate (X*-Y* coordinate) in  FIG. 1  is calculated by coordinate calculator  522  in  FIG. 3  on the basis of the parameters for defining the attitude of the given rectangular plane obtained by attitude calculator  521 . 
   The coordinate of the target point Ps on the given rectangular plane can be expressed as in the following equation (3) using ratio m=OmS 1 /OmS 2  and ratio n=OmT 1 /OmT 2 . 
                 P   s     ⁡     (     u   ,   v     )       =     (         m     m   +   1       ·     U   max       ,       n     n   +   1       ·     V   max         )             (   3   )               m   =             O   m     ⁢     S   1       _           O   m     ⁢     S   2       _       =         |     X   s1   ′     |       |     X   s2   ′     |       ·       |           X   s2   ′     ·   tan     ⁢           ⁢   ϕ     +   f     |       |           X   s1   ′     ·   tan     ⁢           ⁢   ϕ     +   f     |                   (   4   )               n   =             O   m     ⁢     T   1       _           O   m     ⁢     T   2       _       =         |     X   t1   ′     |       |     X   t2   ′     |       ·         |   f     ⁣           ·   tan     ⁢           ⁢   ϕ     -     X   t2   ′       |           |   f     ⁣           ·   tan     ⁢           ⁢   ϕ     -     X   t1   ′       |                     (   5   )             
 
(b) Characteristic Point Detection
 
   The function of the characteristic point detector is as follows: 
   (b1) The Standard Image 
   According to the embodiment, the mark is extracted by means of the difference method, For the difference method, a pair of standard images of different illumination are prepared, the images being displayed according to the time sharing. In the embodiment, the pair of standard images consists of a first image and a second image both with four marks, the color of which differs between green in the first state and black in the second state. 
     FIG. 11  represents the pair of standard images Kt 1  and Kt 2  both with four marks. 
     FIG. 11A  represents first standard image Kt 1 , in which the upper-left mark is green in the first state and the others are black in the second state. On the other hand,  FIG. 11B  represents second standard image Kt 2 , in which the upper-left mark is black in the second state and the others are green in the first state. The relationship of the four marks is reversed between first standard image Kt 1  and second standard image Kt 2 . Further, one of the marks is distinguishable from the other three, which causes an asymmetry color arrangement of the four marks. 
   The one mark of the color different from those of the other three marks makes it possible to determine the rotary attitude of the image plane of CCD  101  relative to wide screen  110 . Further, only two colors, i.e., black and green, are used to represent all the marks in the pair of standard images, CCD  101  can easily extract the four marks without any difficulty of sensing a color difficult to detect, which removes conditions necessary for successful extraction of the marks. 
   (b2) Sensing of the Projected Standard Image 
     FIG. 12  represents a flowchart of the functions of sensing image to detecting the characteristic points. In the flowchart, steps S 301  and S 302  correspond to the sensing of the image projected on the wide screen. Steps S 303  to S 308  correspond to the difference calculation to the characteristic points detection, which will be referred to in subparagraph (b3). 
   The first standard image Kt 1  is projected on the wide screen and sensed by the image sensor in step S 301 , while the second standard image Kt 2  is projected on the wide screen and sensed by the image sensor in step S 302 . 
   As shown in  FIG. 1 , the target object is projected on the wide screen. If a player operates trigger switch  103  with controller  100  aimed at a specific portion of the target object, step S 301  and step S 302  are successively carried out to sense the four marks located at a predetermined position relative to the target object. 
     FIG. 13  represents sensed image q taken by CCD  101  of controller  100  located at point Ps in  FIG. 1 . Detected point Ps is set at the center of the sensed image, which is the origin of image coordinate X-Y and coincides with a specific point of the target object, e.g., a wing of the flying object, if the specific point is correctly aimed. 
   According to the embodiment, the marks are prepared and indicated at a predetermined position adjacent to the target object in conformity with the advance of game story. 
     FIG. 14  represents timing charts of the function of controller  100  in sensing images.  FIG. 14(   a ) represents the timing of START pulse for starting the image sensing, which is generated when trigger switch  103  of controller  100  is operated by a player. START pulse is input into timing generator  105  and also into main body  120  through interfaces  107  and  121 .  FIG. 14(   b ) represents the timing of PJ signal, which is a composite video signal formed by adding vertical synchronization signals to the video signal transmitted from main body  120  to projector  130 .  FIG. 14(   c ) represents the timing of VDp signal, which is the vertical synchronization signal component extracted from the composite video signal transmitted from main body  120 . Main body  120  transmits to projector  130  the video signal for projecting the first standard image during period Tv between t 1  and t 2  directly following the generation of START pulse. 
     FIG. 14(   d ) represents the timing of RST pulse, which is the reset pulse for CCD  101 . Timing generator  105  generates and transmits RST pulse to CCD  101  at time t 1  in synchronism with VDp signal. Following RST pulse, CCD  101  starts to sense the projected first standard image.  FIG. 14(   e ) represents the timing of RD start pulse to start reading out the accumulated charge on CCD. Timing generator  105  generates RD start pulse at time t 2  with period Tv passed after the transmission of RST pulse to CCD  101 . RD start pulse causes RD out signal in  FIG. 14(   f ) for reading out the accumulated charge on CCD  101 , RD out signal going on for time Tc. Then main body  120  repeats to generate PJ signals for a period three times as long as Tv, which continues the projection of the first standard image until time t 3 . The projection of the first standard image by projector  130  is substituted by that of the second standard image at time t 3 , which starts with the first period Tv until time t 4 . 
   As in  FIG. 14(   d ) on the other hand, timing generator  105  generates and transmits to CCD  101  RST pulse at time t 3  to remove unnecessary charges. Then the charge on CCD  101  is read out during time Tc in  FIG. 14(   f ) starting with time t 4  when RD start pulse in  FIG. 14(   e ) is generated in synchronism with VDp pulse in  FIG. 14(   c ). The first standard image is not necessarily continued to be projected for a period three times as long as Tv, but to be projected for the first period Tv in a modified embodiment. 
     FIG. 15  represents a flowchart of the function of controller  100 . The flow begins with the operation of trigger switch  103 , which causes START pulse as in  FIG. 14(   a ) to be transmitted to timing generator  105  and main body  120 . Step S 401  wait for a first VDp signal for the projection of the first standard image. When the first VDp signal comes at time t 1 , the flow advances to step S 402 , in which main body  120  transmits PJ signal to projector  130  for projecting the first standard image. In step S 403 , timing generator  105  generates and transmits RST pulse to CCD  101  for removing unnecessary charges. In step S 404 , CCD  101  is exposed to the first standard image. The exposure is continued until the generation of the second VDp signal is detected in step S 405 . In step S 406 , when the exposure time is over at time t 2 , timing generator  105  generates and transmits RD start pulse as in  FIG. 14(   e ) to CCD  101 , which causes the reading out of the charge on CCD  101  during period time Tc. 
   If it is detected that the fourth VDp signal comes at time t 3  in step S 407 , the flow advances to step S 408 , in which main body  120  switches the projection of the first standard image into the second standard image. In step S 409 , timing generator  105  generates and transmits RST pulse to CCD  101 . In step  410 , CCD  101  is exposed to the second standard image. The exposure is continued until the generation of the fifth VDp signal is detected in step S 411 . In step S 412 , when the second exposure time is over at time t 4 , timing generator  105  generates and transmits RD start pulse as in  FIG. 14(   e ) to CCD  101 , which causes the reading out of the second standard image form CCD  101  during time Tc. 
   (b3) Difference Method and Characteristic Point Detection 
   Referring back to  FIG. 12 , difference method is carried out in steps S 303  on the basis of difference between the first standard image gotten in step S 301  and the second standard image gotten in step S 302 . 
     FIG. 16  represents the image signals for the four marks, in which  FIG. 16A  represents the first standard image with marks mQ 1 , mQ 2 , mQ 3  and mQ 4 ,  FIG. 16B  the second standard image with the four marks, and  FIG. 16C  the difference between the first and second images. 
   In step  304  of the flowchart in  FIG.12 , the portions relating to the four marks are extracted from the difference in  FIG. 16C  by means of the binarization with respect to a predetermined threshold level. In step S 305 , the sign of the extracted portion of difference in  FIG. 16C  is determined for each mark between plus sign and minus sign, which is recorded for each mark. 
   In step S 306 , the position of center of gravity for each of the extracted marks is calculated. And, the individual positions of the four marks are identified in step S 307  on the basis of the position of center of gravity calculated in step S 306  and the sign recorded in step S 305 . In other words, mark mQ 4  can be distinguished from the other marks my means of the plus sign thereof different from the minus sign of the others. And, the other three marks can be identified in accordance with the predetermined arrangement as in  FIG. 11  if mark mQ 4  is once identified. If the individual positions of the marks are identified through step S 307 , the flow goes to step S 308  to close the function. 
     FIG. 17  represents an illustration of images for explaining the identification of the mark position, in which  FIG. 17A  represents the projected image on the wide screen. On the other hand,  FIG. 17B  represents the sensed image taken by CCD  101  of controller  100 , in which the origin of X-Y coordinate is the position to be calculated on the basis of the identified mark positions. 
     FIG. 18  represents a flowchart for identifying the mark positions. The flow starting with step S 500  makes the identification of mQ 4  in step S 501 . In step S 502 , three formulas are calculated to represent three straight lines h 1 , h 2  and h 3  defined between the position of mQ 4  and the other three mark positions, respectively. With respect to the other three mark positions in this stage, no one can tell which is which. 
   In step S 503 , three formulas are calculated to represent three straight lines g 1 , g 2  and g 3  defined between all possible pairs among the other three mark positions, respectively. 
   In step S 504 , all possible intersections between one group of straight lines h 1  to h 3  and the other groups of straight lines g 1  to g 3  are calculated. In step  505 , the positions of the calculated intersections are compared with the four mark positions to find out intersection mg located at a position other than the four mark positions. 
   And, a pair of straight lines causing intersection mg is found out in step S 506 . Thus, straight line h 2  and straight line g 3  are identified as the pair of straight lines causing intersection mg. Then mQ 2  can be identified on line h 2  on the other side of mg than mQ 4  in step S 507 . 
   With respect to mQ 1  and mQ 3  on straight line g 3 , discrimination is made in steps S 508  and S 509  to tell which is which. In step S 508 , one of the remaining mark positions is selected so that the coordinates of the selected mark position are substituted for x and y of the formula, y=a2x+b2 representing straight line h 2 . And, it is tested in step S 509  whether or not the following conditions are both fulfilled:
 
 y&gt;a 2 x+b 2 and  a 2&gt;0
 
   If the answer is affirmative, the flow goes to step S 510  for determining that the mark position selected in step S 508  is mQ 3 . On the other hand, the flow goes to step S 511  for determining that the mark position selected in step S 508  is mQ 1  if the answer is negative. 
   Thus, the last one mark position can be identified, and the flow is closed in step S 512 . 
   According to the present invention, the four marks necessary for calculating the aimed position, which is the origin of X-Y coordinate in the sensed image taken by CCD  101 , is located close to the target object in the projected image. And the positions of the four marks are shifted along with the movement of the target object over the wide screen. Accordingly, the four marks can always be sensed on the image plane of CCD  101  as long as the player aims the target object with controller  100  even if the field angle of objective lens  102  is not so wide. 
     FIG. 19  represents the projected image on the wide screen in various cases for explaining the above feature.  FIG. 19A  represents a case in which flying object A as the target object is located in the upper-right portion of the wide screen, characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4  being located close to flying object.  FIG. 19B  represents a case in which flying object A moves toward the lower-left direction, characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4  keeping up flying object A.  FIG. 19C  represents a case in which spire B of a steeple as another target object is located in the central portion of the wide screen, characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4  being located not close to flying object A, but to spire B. This means that the player does not aim at flying object A, but at spire in the case of  FIG. 19C . 
   For changing the target object to be aimed at, controller  100  includes a selector button for the player to designate one of the selectable target objects. Alternatively, an automatic designation of the target object is possible by means of automatically identifying a target object within the field angle of objective lens  102 . Such identification is possible by having each of the target objects flicker with a predetermined different frequency. Thus, the target object coming into the field angle of objective lens  102  is identified in dependence on its frequency of flicker to automatically change the designation of the target object with characteristic points mQ 1 , mQ 2 , mQ 3  and mQ 4  located close thereto. 
   In all the cases in  FIG. 19 , the positions of the characteristic points are known no matter where the characteristic points located. Thus, the point where the player aims with controller  100  can be calculated as long as controller  100  senses the characteristic points. 
   [Second Embodiment] 
   Ordinary game machine outputs video signal with display scan frequency of 50 to 80 Hz (i.e., display scan period Tv of 1/50sec. to 1/80sec.) On the other hand, it takes time Tc (e.g., 1/50sec for PAL or 1/60sec. for NTSC in the case of ordinary video signal) for CCD to output signal for one entire image. 
   If period Tv does not so differ from time Tc with the former being shorter than the latter, the period three times as long as Tv for projecting the first standard image is sufficient for CCD to output the sensed first standard image as in the first embodiment. 
   On the contrary, a period two times as long as Tv for projecting the first standard image may be sufficient for CCD to output the sensed first standard image if period Tv is relatively longer than time Tc. This may also be possible if time Tc is successfully shortened so as to be shorter than period Tc. Thus, the projection of the first standard image by projector  130  may be substituted by that of the second standard image after a lapse of the period two times as long as Tv. The second embodiment in  FIG. 20  is prepared to realize such a prompt substitution of the first standard image by the second standard image succeeding the termination of time Tc. 
     FIG. 20(   a ) to  FIG. 20(   f ) are similar to  FIG. 14(   a ) to  FIG. 14(   f ). In the second embodiment, however, RD end pulse as in  FIG. 20(   g ) to be generated from timing generator  105  at time t 5  upon the termination of reading out the sensed image from CCD is added. RD end pulse in  FIG. 20(   g ) has main body  120  substitute the projection of the first standard image by that of the second standard image at time t 3 , at which the fist VDp pulse in  FIG. 20(   c ) comes after the generation of RD end pulse at time t 5 . Further, RD end pulse in  FIG. 20(   g ) causes main body  120  to generate RST pulse in  FIG. 20(   d ). 
   In the case of  FIG. 20  itself, the substitution of the first standard image by the second standard image succeeds the period three times as long as Tv, which is similar to  FIG. 14 , because period Tv is shorter than time Tc. If period Tv is made relatively longer than time Tc, however, the substitution of the first standard image by the second standard image would promptly succeed the period two times as long as Tv according to the second embodiment. According to the second embodiment, an additional controlling cable connects between controller  110  and main body  120  to transmit RD end pulse. 
   The prompt substitution of the first standard image by the second standard image succeeding the termination of reading out the sensed first standard image from CCD is also possible by modifying the first embodiment, in which RD end pulse and the cable for transmitting it as in the second embodiment are not necessary. In such a modification, main body  120  in the first embodiment includes a switch for changing the repetition of generating PJ signals from the period three times as long as Tv to a period two times as long as Tv if period Tv is relatively longer than time Tc. In other words, step S 407  in  FIG. 15  is modified to detect whether the third VDp signal (instead of the fourth VDp signal) comes if period Tv is relatively longer than time Tc. 
     FIG. 21  represents a flowchart of the function of controller  100  according to the second embodiment. Steps S 601  to S 606  from the projection of the first standard image to the reading out of the charge on CCD  101  are similar to steps S 401  to S 406  in  FIG. 15 . 
   In step S 607   a , it is checked whether or not the reading out of the charge on CCD is over. If the reading out is over at time t 5 , the flow advances to step S 607   b , in which timing generator  105  generates RD end pulse as in  FIG. 20(   g ) for transmitting it through interfaces  107  and  121  to main body  120 . In response to RD end pulse, the flow waits for the next VDp signal in step S 608 . If it is detected that the next VDp signal comes at time t 3  in step S 608 , the flow advances to step S 609 , in which main body  120  switches the projection of the first standard image into the second standard image. 
   In step S 610 , timing generator  105  generates and transmits RST pulse to CCD  101 . In step  611 , CCD  101  is exposed to the second standard image. The exposure is continued until the generation of the next VDp signal is detected in step S 612 . In step S 613 , when the second exposure time is over at time t 4 , timing generator  105  generates and transmits RD start pulse as in  FIG. 20(   e ) to CCD  101 , which causes the reading out of the second standard image form CCD  101  during time Tc. 
   [Third Embodiment] 
   In the first and second embodiments, CCD  101  is exposed to the standard image for period Tv, which is one display scan period. However, in the case of CCD of lower sensitivity, the exposure for only one display scan period would be insufficient for getting the expected level of image signal. The third embodiment is designed with such a case taken into consideration. 
     FIG. 22  represents a flowchart of the function of controller  100  according to the third embodiment.  FIG. 22(   a ) to  FIG. 20(   g ) can be understood in the similar manner to that in  FIG. 20(   a ) to  FIG. 20(   g ) in the second embodiment. 
   In the third embodiment, however, timing generator  105  is modified to generate RD start pulse at time t 2  with period two times as long as Tv passed after the transmission of RST pulse to CCD  101  as in  FIG. 22(   e ). Thus, CCD  101  is exposed to the standard image with double amount of light, which increases the level of image signal with undesired influence of flicker modulated. According to the concept of the third embodiment, timing generator  105  may be further modified to generate RD start pulse with period three or more times as long as Tv if CCD requires more amount of light exposed. 
   According to the present invention, various types of further modification of the embodiment are possible. For example, the four detection marks forming a rectangular may be modified into other type of geometric pattern. Or, the first and second standard images of different illumination may be modified into a pair of standard images of different contrast. 
   As in  FIG. 15  or  FIG. 20  or  FIG. 22 , the embodiment according to the present invention designs CCD to be exposed for display scan period Tv or a period integer times as long as Tv. Thus, the detection marks or the like can be completely sensed by CCD without being chipped regardless of the location thereof in the wide screen. This makes it possible for the detection marks or the like to be located close to the target object in the projected image no matter where the target object is located in the wide screen.