Patent Publication Number: US-10761600-B2

Title: Simulation control device and information storage medium

Description:
Japanese Patent Application No. 2016-064752, filed on Mar. 28, 2016, is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a simulation control device that utilizes a wearable image display device (e.g., head-mounted display), and an information storage medium. 
     A head-mounted display (hereinafter referred to as “HMD”) is a head-mounted image display device that displays an image in front of the user. For example, JP-A-2015-231443 discloses a game system that links an image to the motion of the head of the player using a motion sensor (acceleration sensor and gyro sensor) provided to the HMD, and a camera that externally captures the HMD, and allows the player to experience virtual reality as if he/she were situated in the game space. The game system disclosed in JP-A-2015-231443 changes the favorable impression parameter of the game character with respect to the player corresponding to the line of sight of the player detected from the motion of the head of the player so that the player can experience a situation in which the player holds eye contact with the game character. 
     In recent years, a game that utilizes the motion of a head in addition to an operation performed on a controller provided in the real space has been increasingly developed (e.g., the system disclosed in JP-A-2015-231443). When the game is designed so that the player moves in the real space, it is desirable that the player can keep a hands-free state as much as possible from the viewpoint of safety when the player falls. 
     SUMMARY 
     Several aspects of the invention may provide a simulation control device that can implement a user interface with high operability by utilizing a wearable image display device, and a computer-readable non-transitory information storage medium that stores a program that causes a computer to implement a simulation. 
     According to a first aspect of the invention, there is provided a simulation control device including: 
     a display processing section that displays a virtual space on a wearable image display device; 
     a measurement section that measures a gaze time of a user with respect to an object that is placed in the virtual space, and decreases a measured value obtained by the measurement corresponding to a time in which the user does not gaze at the object when the user has removed his/her gaze from the object; and 
     a reception section that determines that the object has been selected when the measured value has reached a first threshold value, determines that the selection of the object has been confirmed when the measured value has reached a second threshold value that is larger than the first threshold value, and determines that the selection of the object has been canceled when the measured value has decreased to a third threshold value before the selection of the object is confirmed, 
     wherein the measurement section sets a determination standard for determining that the user is gazing at the object that has been selected, to be less severe than a determination standard for determining that the user is gazing at the object that is not selected. 
     According to a second aspect of the invention, there is provided a computer-readable non-transitory information storage medium storing a program that causes a computer to implement a simulation, the program being a simulation control program that causes the computer to function as: 
     a display processing section that displays a virtual space on a wearable image display device; 
     a measurement section that measures a gaze time of a user with respect to an object that is placed in the virtual space, and decreases a measured value obtained by the measurement corresponding to a time in which the user does not gaze at the object when the user has removed his/her gaze from the object; and 
     a reception section that determines that the object has been selected when the measured value has reached a first threshold value, determines that the selection of the object has been confirmed when the measured value has reached a second threshold value that is larger than the first threshold value, and determines that the selection of the object has been canceled when the measured value has decreased to a third threshold value before the selection of the object is confirmed, 
     wherein the measurement section sets a determination standard for determining that the user is gazing at the object that has been selected, to be less severe than a determination standard for determining that the user is gazing at the object that is not selected. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a configuration diagram illustrating a schematic configuration of a game system according to one embodiment of the invention. 
         FIG. 2  illustrates a virtual three-dimensional space (simulation space) that can be experienced by means of a game system according to one embodiment of the invention. 
         FIG. 3  is a plan view illustrating the configuration of a structure according to one embodiment of the invention. 
         FIG. 4  is a cross-sectional view illustrating the configuration of a structure according to one embodiment of the invention. 
         FIGS. 5A and 5B  are respectively a perspective view and a side view illustrating the configuration of an HMD that is used to implement a game system according to one embodiment of the invention. 
         FIG. 6  is a configuration diagram illustrating the block configuration of a simulation control device according to one embodiment of the invention. 
         FIGS. 7A and 7B  illustrate an example of an effect object and a moving path member (effect device) according to one embodiment of the invention. 
         FIG. 8  is a flowchart illustrating the operation of a game system according to one embodiment of the invention. 
         FIG. 9  is a flowchart illustrating the operation of a game system according to one embodiment of the invention. 
         FIG. 10  illustrates the relationship with respect to a virtual three-dimensional space, a line-of-sight area with respect to a player P, and a space that is viewed from the player P (an example in which the line-of-sight area is situated at the center of the virtual three-dimensional space). 
         FIG. 11  illustrates the relationship with respect to a virtual three-dimensional space, a line-of-sight area with respect to a player P, and a space that is viewed from the player P (an example in which the line-of-sight area is situated in the upper left area of the virtual three-dimensional space). 
         FIGS. 12A and 12B  illustrates the relationship between a hit area and a line-of-sight area. 
         FIGS. 13A to 13D  illustrates the relationship between an elapsed time from the start of gaze, and a charge amount with respect to a gaze time. 
         FIG. 14  illustrates an example of a visual effect with respect to an object. 
         FIG. 15  illustrates a change in visual effect with respect to implementation of a combo process. 
         FIG. 16  illustrates integration of charge amounts. 
         FIG. 17  illustrates a partial cancelation process. 
         FIG. 18  illustrates separation of a charge amount. 
         FIG. 19  illustrates another example of a visual effect with respect to an object. 
         FIGS. 20A to 20C  illustrate a degree-of-lock control process. 
         FIG. 21  is a flowchart illustrating an eye input reception process. 
         FIG. 22  is a flowchart illustrating a process that is performed in connection with a combo process. 
         FIG. 23  is a flowchart illustrating a degree-of-lock control process. 
         FIG. 24  illustrates an application example of an eye input. 
         FIG. 25  illustrates an application example of an eye input. 
         FIG. 26  illustrates a further example of a visual effect with respect to an object. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     (1) According to one embodiment of the invention, there is provided a simulation control device including: 
     a display processing section that displays a virtual space on a wearable image display device; 
     a measurement section that measures a gaze time of a user with respect to an object that is placed in the virtual space, and decreases a measured value obtained by the measurement corresponding to a time in which the user does not gaze at the object when the user has removed his/her gaze from the object; and 
     a reception section that determines that the object has been selected when the measured value has reached a first threshold value, determines that the selection of the object has been confirmed when the measured value has reached a second threshold value that is larger than the first threshold value, and determines that the selection of the object has been canceled when the measured value has decreased to a third threshold value before the selection of the object is confirmed, 
     wherein the measurement section sets a determination standard for determining that the user is gazing at the object that has been selected, to be less severe than a determination standard for determining that the user is gazing at the object that is not selected. 
     According to this configuration, the user can select the object by gazing at the object until the measured value exceeds the first threshold value. On the other hand, the user can maintain the object to be in an unselected state by gazing at (glancing) the object so that the measured value does not exceed the first threshold value. After the object has been selected, the user can input an instruction that confirms the selection of the object to the simulation control device by merely gazing at the object until the measured value reaches the second threshold value. The user can input an instruction that cancels the selection of the object to the simulation control device by merely removing his/her gaze from the object until the measured value decreases to the third threshold value. Specifically, the user can change the state of the object between an unselected state, a selected state, and a confirmed state, by gazing at the object. 
     The measurement section sets the determination standard for determining that the user is gazing at the object that has been selected, to be less severe than the determination standard for determining that the user is gazing at the object that is not selected. Therefore, the user cannot select the object that is not selected without intentionally gazing at the object, but can easily maintain the selection of the object (that has been selected) without intentionally gazing at the object. 
     (2) In the simulation control device, the measurement section may set the determination standard to be less severe as the measured value increases. 
     In this case, the degree of selection (i.e., the degree by which selection is canceled) of the object increases as the gaze time of the user with respect to the object increases. 
     (3) In the simulation control device, the measurement section may increase a size of an area within the virtual space that is considered to intersect a line of sight of the user in order to set the determination standard to be less severe. 
     In this case, the user cannot select the object without gazing at the object at a position near the center of the object (e.g., when the user hesitates to select the object). On the other hand, the selection of the object is maintained unless the user moves his/her gaze to a position away from the center of the object after the object has been selected. 
     (4) In the simulation control device, the measurement section may increase a size of an area within the virtual space that is considered to include the object in order to set the determination standard to be less severe. 
     In this case, the user cannot select the object without gazing at the object at a position near the center of the object (e.g., when the user hesitates to select the object). On the other hand, the selection of the object is maintained unless the user moves his/her gaze to a position away from the center of the object after the object has been selected. 
     (5) In the simulation control device, the measurement section may set an enlargement ratio of the size in a rightward-leftward direction with respect to the user to be larger than an enlargement ratio of the size in an upward-downward direction with respect to the user. 
     According to this configuration, the degree of selection of the object in the rightward-leftward direction is relatively higher than the degree of selection of the object in the upward-downward direction. Since the line-of-sight direction of the user (that is determined by the attitude of the head, or the direction of the eyeball) is normally unstable in the rightward-leftward direction as compared with the upward-downward direction, it is considered that an erroneous input (e.g., a situation in which the user unintentionally cancels the selection of the object) can be reduced without impairing operability when the degree of selection in the rightward-leftward direction is set to be relatively higher than the degree of selection in the upward-downward direction. 
     (6) In the simulation control device, the measurement section may decrease a speed at which the measured value approaches the third threshold value as the measured value increases. 
     In this case, the user cannot select the object without gazing at the object for a time equal to or longer than a specific time (e.g., when the user hesitates to lock the object). On the other hand, the selection of the object is maintained unless the user removes his/her gaze from the selected object for a long time. 
     (7) The simulation control device may further include a notification section that notifies the user of a magnitude of the measured value. 
     When the measured value is visualized in this manner, the user can determine the object at which the user gazes, and determine whether or not the user is gazing at the desired object. The user can also visually determine the remaining time until the object at which the user gazes is selected, the gaze time required for the selection to be confirmed, and the like. 
     (8) In the simulation control device, the notification section may provide a visual effect that corresponds to the measured value to the object that has been selected. 
     When the measured value is visualized in this manner, the user can determine the object at which the user gazes, and determine whether or not the user is gazing at the desired object. The user can also visually determine the remaining time until the object at which the user gazes is selected, the gaze time required for the selection to be confirmed, and the like. 
     (9) In the simulation control device, the notification section may distinguish the object that has been selected from the object that is not selected. 
     According to this configuration, the user can determine whether or not each object (one object, or a plurality of objects) has been selected. 
     (10) In the simulation control device, the visual effect may be an animation. 
     It is possible to notify the user of a temporal change in the measured value in real time (or successively) by utilizing an animation. 
     (11) In the simulation control device, the animation may place a ring-shaped or subring-shaped gauge around an outer edge of the object, and change a length of the gauge corresponding to the measured value. 
     When the measured value is thus reflected in the length of the gauge, the user can intuitively determine the magnitude of the measured value, whether the measured value has increased or decreased, the speed at which the measured value increases, the speed at which the measured value decreases, and the like. 
     (12) In the simulation control device, the animation may place a line of marks around an outer edge of the object, and change a number of marks that form the line of marks corresponding to the measured value. 
     When the measured value is thus reflected in the number of marks that form the line of marks, the user can intuitively determine the magnitude of the measured value, whether the measured value has increased or decreased, the speed at which the measured value increases, the speed at which the measured value decreases, and the like. 
     (13) In the simulation control device, the animation may change an enhancement level with respect to the object corresponding to the measured value. 
     When the user is notified of the measured value using a change in the enhancement level with respect to the object, it is possible to provide a sufficient space around the object, and improve the degree of freedom with respect to the layout of the object. This configuration is effective when a number of objects are closely placed, for example. Note that the enhancement level with respect to the object may be adjusted using at least one of the following parameters, for example.
         Density of object   Brightness of object   Color of object   Opacity of object   Saturation of object   Shape of object   Change pattern of at least one of density, brightness, color, opacity, saturation, and shape of object       

     (14) The simulation control device may further include an execution section that performs a predetermined process that is linked to the object when the selection of the object has been confirmed. 
     According to this configuration, the predetermined process is automatically performed when the selection of the object has been confirmed. Note that the predetermined process may be performed at a timing immediately after the selection of the object has been confirmed, or may be performed when a predetermined time has elapsed after the selection of the object has been confirmed. The predetermined process is a process that sets a simulation parameter, a process that attacks the object, a process that moves the object, a process that is performed on another object, or a process that executes a predetermined program, for example. 
     (15) According to another embodiment of the invention, there is provided a computer-readable non-transitory information storage medium storing a program that causes a computer to implement a simulation, the program being a simulation control program that causes the computer to function as: 
     a display processing section that displays a virtual space on a wearable image display device; 
     a measurement section that measures a gaze time of a user with respect to an object that is placed in the virtual space, and decreases a measured value obtained by the measurement corresponding to a time in which the user does not gaze at the object when the user has removed his/her gaze from the object; and 
     a reception section that determines that the object has been selected when the measured value has reached a first threshold value, determines that the selection of the object has been confirmed when the measured value has reached a second threshold value that is larger than the first threshold value, and determines that the selection of the object has been canceled when the measured value has decreased to a third threshold value before the selection of the object is confirmed, 
     wherein the measurement section sets a determination standard for determining that the user is gazing at the object that has been selected, to be less severe than a determination standard for determining that the user is gazing at the object that is not selected. 
     Note that some or all of the functions that are implemented by the simulation control device may be implemented by a server device or a terminal device. Part or the entirety of the program may be recorded (stored) in an information storage medium. 
     The exemplary embodiments of the invention are described below. Note that the following exemplary embodiments do not in any way limit the scope of the invention defined by the claims laid out herein. All of the elements described below in connection with the exemplary embodiments should not necessarily be taken as essential elements of the invention. The exemplary embodiments are described below taking an example in which a head-mounted display (HMD) is used as the wearable image display device, and the simulation control device is applied to a game system that provides a game while generating a virtual three-dimensional space in connection with (in synchronization with, or so as to be linked to) the movement of the user in a space (i.e., real space) defined by a structure. 
     1. Outline of Game System 
     An outline of a game system  1  according to one embodiment of the invention is described below with reference to  FIGS. 1 and 2 .  FIG. 1  is a configuration diagram illustrating a schematic configuration of the game system  1 , and  FIG. 2  illustrates a virtual three-dimensional space (hereinafter may be referred to as “simulation space”) that can be experienced by means of the game system  1 . 
     The game system  1  mainly includes a structure  10  that defines a real space in which a player P (i.e., user) can move (hereinafter simply referred to as “real space”), and an HMD  20  that is worn by the player P, and displays a simulation image of the virtual three-dimensional space (i.e., simulation space) that is linked to the real space. 
     The game system  1  is a simulator that generates a simulation image that is viewed from the player P and represents the simulation space that corresponds to the real space, and allows the player P to experience various environments and situations within a pseudo-space. 
     The game system  1  is configured to (1) detect a player&#39;s state (i.e., the position and the attitude of the player in the real space) that represents the state of the player P in the real space, (2) perform an image generation process that generates a simulation image corresponding to the detected player&#39;s state, the simulation image being viewed from the player P and representing the simulation space that corresponds to the real space, the simulation image including a virtual moving path that is linked to a moving path R (see  FIG. 2 , for example), (3) display the generated simulation image on the HMD  20 , (4) determine that the player P is in a specific state in the simulation space when the player&#39;s state has satisfied a given condition within the moving path R, and (5) generate a simulation image that produces an effect based on the specific state when it has been determined that the player P is in the specific state. 
     The game system  1  includes a hanging unit  30  that hangs the HMD  20  from the structure  10 . The hanging unit  30  hangs the HMD  20  independently of the player P so that the HMD  20  follows the motion of the player P when the player P moves in the real space, or a predetermined part (e.g., head) of the player P makes a motion, and the HMD  20  is continuously worn by the player P (or does not fall) even when the player P has fallen down. 
     As illustrated in  FIG. 1 , the game system  1  further includes (in addition to the structure  10 , the HMD  20 , and the hanging unit  30 ) (1) a fall prevention unit  40  that prevents a situation in which the player P who moves in the real space falls down; (2) a hanging control unit  50  that changes the hanging position of the HMD  20  with respect to the hanging unit  30  corresponding to the movement of the player P in the real space, and changes the hanging position of the player P corresponding to the movement of the player P in the real space; (3) a marker unit  60  that is attached to a predetermined part (e.g., head, both hands, and both feet) of the player P, and an imaging camera  70  that detects the direction and the position of the each part by detecting the marker unit  60 , and detects the state (player&#39;s state) of the player P in the real space; (4) effect devices  90  to  93  and an effect object  80  that are disposed in the real space, and allow the player P to experience a given effect in synchronization with the simulation image; and (5) a simulation control device  100  that generates a simulation image that is viewed from the player P and represents the simulation space (virtual three-dimensional space) that is linked to the real space, and controls the effect devices  90  to  93  in synchronization with the simulation image corresponding to the detected player&#39;s state. 
     The game system  1  that is configured as described above can allow the player P to experience the specific state (simulation) when the player P is in the specific state in synchronization with the state of the player P, and can accurately reproduce the environment or the situation to be experienced by the player P (particularly an environment or a situation that is difficult to actually experience (e.g., a moving environment or situation at a height (including a situation in which the player P falls when the player P has run off the moving path R))). 
     For example, the game system  1  can reproduce an environment or a situation in a dangerous place (e.g., height (high place), closed place, special space, hot place, or cold place) or a space that is difficult to experience, corresponding to the state of the player P as a specific state or an environment that produces the specific state. 
     Specifically, the game system  1  can reproduce an arbitrary place or space (e.g., a dangerous place and a space that is difficult to experience), and allows the player P to have a more realistic experience even in a pseudo-space. 
     Since the game system  1  is configured so that the HMD  20  can be hung independently of the player P, it is possible to ensure that the HMD  20  is continuously worn by the player P, and does not fall even when the HMD  20  has been removed from the player P, even when the HMD  20  has moved in the forward-backward direction, the rightward-leftward direction, or the upward-downward direction, due to the movement of the player P in the real space, or the motion of a predetermined part (e.g., head) of the player P, or the player P has lost his/her balance, and fallen down. 
     Therefore, the game system  1  can prevent an unexpected situation in which the player P is injured when the player P has fallen down in a state in which the player P wears the HMD  20 , or the HMD  20  collides with a floor or a wall surface due to unintentional removal, and breaks or malfunctions, for example. 
     Since the game system  1  is configured so that the HMD  20  can be hung at a suitable position, and can move or make a motion in an arbitrary direction as long as the hanging position of the HMD  20  can be changed using a rail, an arm, or the like so as to follow the movement of the player P, for example, it is possible to ensure that the player P who wears the HMD  20  can safely move in the real space, prevent a situation in which the player P is injured, and prevent an unexpected situation in which the HMD  20  breaks or malfunctions, for example. 
     For example, when an image is supplied to the HMD  20  through a cable, or the HMD  20  is controlled through a cable (e.g., when a high-resolution image is used), it is possible to provide a line that connects the HMD  20  and a control device that controls the HMD  20  through the hanging unit  30 , and it is possible to prevent a situation in which the movement of the player P is limited, or the player P feels uncomfortable due to the presence of the line that is situated on the side of the player P, or around the feet of the player P. 
     Specifically, the above configuration makes it possible for the game system  1  to ensure smooth movement and safety with respect to the player P who wears the HMD  20  in the real space, and prevent an unexpected situation in which the player P is injured, for example. 
     Note that the embodiments of the invention are described below taking an example in which the game system  1  implements a game that allows the player to experience a fear of heights (hereinafter referred to as “fear of heights experience game”). 
     2. Configuration of Game System 
     2-1. Structure 
     The structure  10  included in the game system  1  is described below with reference to  FIGS. 1, 3, and 4 .  FIG. 3  is a plan view illustrating the configuration of the structure  10 , and  FIG. 4  is a cross-sectional view illustrating the configuration of the structure  10 . 
     The structure  10  is a housing that defines the real space in which the player P can move and the game is implemented. As illustrated in  FIGS. 1, 3, and 4 , the structure  10  has a box-like structure that is in the shape of a rectangular parallelepiped, and has a ceiling  15 , a floor  16 , and a wall  17  that defines (covers) each side of the real space, for example. 
     The structure  10  includes a standby area  11  in which the player P stands by before playing the experience-type game, and a play area  12  in which the player P plays the experience-type game. The play area  12  includes a start zone  13  in which the player P stands when starting the game, and a movement experience zone  14  in which the player P moves to experience a predetermined environment and situation, and the moving path R in which each player P can move during the game is formed. 
     A plurality of hanging control units  50  are provided to the ceiling  15  from the standby area  11  to the play area  12  so as to move along the moving path R in the movement experience zone  14 , the plurality of hanging control units  50  being slidably provided with the hanging unit  30  that hangs the HMD  20 , and the fall prevention unit  40  that prevents a situation in which the player P falls down. 
     Each hanging control unit  50  is provided with the simulation control device  100  that corresponds to each moving path. 
     The ceiling  15  is provided with a plurality of imaging cameras  70  that are used to detect the player&#39;s state with respect to the player P and the state of the effect object  80 , the plurality of imaging cameras  70  being provided at predetermined positions. The player&#39;s state with respect to the player P includes the position and the direction (attitude) of the head of the player P. 
     The floor  16  differs in configuration corresponding to the standby area  11 , the play area  12 , the start zone  13 , and the movement experience zone  14 . 
     More specifically, the floor  16  is formed by a panel (spring floor panel)  92  that is provided with a spring that produces an elevator environment (i.e., effect means) in the start zone  13  included in the play area  12 . 
     The movement experience zone  14  included in the play area  12  includes a moving path R in which the player P walks, and which is formed by a predetermined member (moving path member  93  described later) (e.g., metal), a non-moving path NR in which the player P cannot move, and which is formed by a mat or the like that protects the player P when the player P has fallen down. 
     The start zone  13  has a structure that provides a virtual three-dimensional space formed by the inner space of an elevator. An automatic door  91  (effect device) that functions as the door of an elevator and is opened and closed under control of the simulation control device  100 , is provided at the boundary between the start zone  13  and the movement experience zone  14 . 
     The effect object  80  is placed on the moving path R (i.e., at the end point of the moving path member  93 ). An effect device (e.g., blower  90 ) is optionally provided on the non-moving path NR, and a sensor unit (e.g., contact sensor) may also optionally be provided on the non-moving path NR. 
     The wall  17  is formed by a predetermined wall panel, or a mat that protects the player P from being injured due to a collision, for example. 
     2-2. HMD and Hanging Unit 
     The HMD  20  that is used to implement the game system  1 , and the hanging unit  30  that hangs the HMD  20 , are described below with reference to  FIGS. 4, 5A, and 5B . 
       FIGS. 5A and 5B  are respectively a perspective view and a side view illustrating the configuration of the HMD  20  that is used to implement the game system  1 . The HMD  20  implements the wearable image display device according to the embodiments of the invention, for example. 
     The HMD  20  is a non-see-through wearable display device that is worn on the head of the player P, and displays an image of the virtual three-dimensional space under control of the simulation control device  100 . The HMD  20  allows the player P to view only the displayed image (i.e., does not allow the player P to view the state of the external world), and allows the player P to visually experience augmented reality. 
     As illustrated in  FIGS. 4, 5A, and 5B , the HMD  20  is configured to cover (mask) the entirety of each eye of the player P, and allow the player P to view the simulation image that is viewed from the player P, and represents the simulation space that is linked to the real space within the structure  10  in synchronization with the detected player&#39;s state. 
     A marker unit (head detection marker unit)  60   a  that is used to detect the direction and the position of the head of the player P is provided to the upper part of the HMD  20 . The HMD  20  has a display size of 200×1080 pixels and a refresh rate of 90 fPS, for example. 
     The HMD  20  includes a headphone jack (not illustrated in the drawings), and a headphone  61  is connected to the headphone jack. The player P wears the headphone  61  together with the HMD  20 . The headphone  61  outputs an ambient sound (stereophonic sound) in the simulation space that is generated by the simulation control device  100 . 
     The hanging unit  30  is configured to connect the HMD  20  and the hanging control unit  50  that is disposed above the player P (e.g., provided to the ceiling  15  of the structure  10 ) so that the hanging unit  30  hangs the HMD  20  while being hung from the structure  10 . 
     The hanging unit  30  is provided to the upper part (i.e., ceiling  15 ) of the structure  10  so as to be situated over the head of the player P such that the hanging unit  30  can hang the HMD  20  to be able to follow the movement and the motion of the player P in each direction within the play area  12 . 
     The hanging unit  30  includes a line (hereinafter referred to as “cable”) that connects the HMD  20  and the simulation control device  100  through a cable communication channel. 
     As illustrated in  FIGS. 4, 5A, and 5B , the hanging unit  30  includes a link member  31  that is used to link the hanging unit  30  to the hanging control unit  50 , a string-like member (cable)  32  that has an end (hereinafter referred to as “first end”) that is shaped to be attached to the link member  31 , and a connection member  33  that is used to connect a second end of the string-like member  32  that differs from the first end, to the HMD  20 , for example. 
     The string-like member  32  has a structure that prevents a situation in which the HMD  20  reaches the floor of the structure  10  when the player P has made a large motion (e.g., when the player P is about to fall down), and the HMD  20  has been removed from the player P. 
     The string-like member  32  includes a cable that is stretchable, and transfers a predetermined signal and data transmitted from the simulation control device to the HMD  20 . 
     For example, the string-like member  32  has a length that prevents a situation in which the HMD  20  reaches the floor of the structure  10  when the HMD  20  has been removed from the player P, or is formed in a stretchable spiral shape that prevents a situation in which the HMD  20  reaches the floor of the structure  10  when the HMD  20  has been removed from the player P, or is configured so that the cable can be wound to adjust the length of the string-like member  32  and prevent a situation in which the HMD  20  reaches the floor of the structure  10  when the HMD  20  has been removed from the player P. 
     Note that it suffices that the HMD  20  be configured to be worn by the player P, and display an image so as to be viewable by the player P. The HMD  20  may be a see-through HMD as long as the simulation process can be accurately performed. 
     When a signal or data is transferred between the HMD  20  and the simulation control device  100  through a wireless communication channel, the string-like member  32  need not necessarily be a cable, but may be a string formed of a predetermined material, or may be a band-like member having a certain width. 
     The link member  31  and the string-like member  32  included in the hanging unit  30  are used in common by the hanging unit  30  and the fall prevention unit  40  (as described below). 
     2-3. Fall Prevention Unit 
     The fall prevention unit  40  included in the game system  1  is described below with reference to  FIG. 4 . 
     The fall prevention unit  40  is used to support the player P and prevent the player P from falling down when the player P has lost his/her balance, or when the player P has lost his/her balance and fallen down since the player P wears the HMD  20 , or since the moving path R in which the player P can walk is narrow, or since the player P wears the HMD  20 , and the moving path R in which the player P can walk is narrow. 
     The fall prevention unit  40  is configured so that it is possible to prevent a situation in which the HMD  20  breaks, or the player P is injured, when the player P has lost his/her balance, and also prevent a situation in which the player P falls down (e.g., the player P loses his/her balance during movement, and falls down) due to the HMD  20  that is worn by the player P during the game. 
     As illustrated in  FIG. 4 , the fall prevention unit  40  includes a holder member  41  that holds the player P, and a hanging member that hangs the player P from the structure  10 , and the hanging member is implemented by the link member  31  and the string-like member  32  included in the hanging unit  30 , for example. 
     The holder member  41  is implemented by a vest-type jacket, for example. The holder member  41  is worn by the player P during the game, and holds the player P during the game. The holder member  41  is connected to one end of the string-like member  32 , and supports the body of the player P through the link member  31  that is provided to the ceiling  15  of the structure  10 . 
     2-4. Hanging Control Unit 
     The hanging control unit  50  that is included in the game system  1  is described below with reference to  FIGS. 1, 3, and 4 . 
     The hanging control unit  50  is a unit that changes the hanging position of the player P and the hanging position of the HMD  20  corresponding to the movement of the player P in the real space that is defined by the structure  10 . The hanging control unit  50  is configured to necessarily hang the HMD  20  and the player P from above (i.e., from the ceiling of the structure  10 ). 
     The hanging control unit  50  is configured to hang the HMD  20  and the player P at an appropriate position in the real space so as to follow the movement and the motion of the player P in the moving direction. The hanging control unit  50  is configured to prevent an unexpected situation in which the player P is injured during the simulation, or the HMD  20  breaks or malfunctions while ensuring safety with respect to the player P, for example. 
     Therefore, the hanging control unit  50  can appropriately prevent a situation in which the movement of the player P is limited, or the player P feels uncomfortable (due to a cable that supplies a signal and data to the HMD  20 , and a member that holds the player P and is provided on the side of the player P or under the player P) even when the player P arbitrarily moves in the real space, and can appropriately and necessarily hang the HMD  20  and the player P even when the player P moves or makes a motion. 
     More specifically, the hanging control unit  50  is integrally formed from the standby area  11  to the play area  12 , and is configured so that the HMD  20  and the fall prevention unit  40  follow the player P who moves in the real space, or the player P who changes in attitude. 
     For example, the hanging control unit  50  includes a rail  51  that is provided corresponding to each player P who moves in the real space, and formed along the moving direction of the player P from the standby area  11  (i.e., a point at which the player P wears the HMD  20  and the fall prevention unit) to the play area  12 , and a sliding member  52  that is connected to the link member  31  of the hanging unit  30 , and slides along the rail  51  (see  FIGS. 1, 3, and 4 ). 
     Each rail  51  is provided to the ceiling over the moving path R in the movement experience zone  14  in which the player P moves along the moving path R (i.e., a zone in which the player P linearly moves in the forward-backward direction). 
     In an area in the real space other than the movement experience zone  14 , each rail  51  is provided along path (hereinafter referred to as “guiding path”) S along which the player P is guided to the start position and the like when the player P plays the simulation after the player P has worn the HMD  20  and the fall prevention unit  40 . 
     Note that the rail  51  is not limited as to the shape, the material, and the like as long as the position of the hanging unit  30  can be changed corresponding to the movement of the player P. 
     The sliding member  52  slides along the rail  51  according to the tension produced corresponding to the state (e.g., movement or change in attitude) of the player P, and changes the hanging position of the HMD  20  and the player P through the hanging unit  30 . 
     As illustrated in  FIGS. 1, 3, and 4 , the sliding member  52  is configured so that the simulation control device  100  is secured thereon in order to reduce the length of the cable that electrically connects the HMD  20  and the simulation control device  100 , and appropriately transfer a signal and data, and slides together with the simulation control device  100 . 
     The sliding member  52  is not particularly limited as long as the sliding member  52  slides along the rail  51 , changes the hanging position of the HMD  20  and the player P through the hanging unit  30  corresponding to the state (e.g., movement or change in attitude) of the player P. 
     2-5. Simulation Control Device 
     The simulation control device  100  included in the game system  1  is described below with reference to  FIG. 6 . 
       FIG. 6  is a configuration diagram illustrating the block configuration of the simulation control device  100 . Note that the configuration of the simulation control device  100  is not limited to the configuration illustrated in  FIG. 6 . Various modifications and variations may be made, such as omitting some of the elements illustrated in  FIG. 6 , or providing an additional element. 
     The simulation control device  100  is implemented by a computer-controllable device (computer) such as a personal computer. An operation section (e.g., keyboard) (not illustrated in  FIG. 6 ) that is operated by the administrator is removably provided to the simulation control device  100 . 
     The simulation control device  100  that provides the simulation space to the player P is configured to generate an image that corresponds to the state of the game while proceeding with the game corresponding to the player&#39;s state and the elapsed time, and controls the effect devices  90  to  93  in synchronization with the effect control device. 
     The simulation control device  100  acquires an image output from the imaging camera  70 , detects a marker due to the marker unit  60  from the acquired image, and detects the player&#39;s state based on the positional relationship between the area to which the marker belongs, and another marker, and the time of stay in the area to which the marker belongs. 
     The simulation control device  100  includes a storage section  170  that stores various types of data, an information storage medium  180  that stores data (e.g., simulation application), a processing section  101  that performs various processes that implement the game and generate a simulation environment, and a communication section  196 . 
     The storage section  170  serves as a work area for the processing section  101  and the communication section  196 . The function of the storage section  170  may be implemented by a RAM (DRAM or VRAM) or the like. The storage section  170  includes a main storage section  172  that mainly stores (records) a game program, an image buffer  174 , and a data buffer  176 . 
     The main storage section  172  mainly stores (records) the game program. The data buffer  176  is a storage area that stores game data. For example, the data buffer  176  may be included in a main storage, and may be controlled (with respect to a read-out process and a writing process) by means of software. 
     The game program is software that includes an instruction code for executing the game process. The game data includes data for determining whether or not the player is in the specific state, data required when executing the game program, data with respect to the effect object  80 , a control program for controlling the effect devices  90  to  93 , and the like. 
     The information storage medium  180  (computer-readable medium) stores a program, data, and the like. The function of the information storage medium  180  may be implemented by an optical disk (CD or DVD), a hard disk drive (HDD), a memory (e.g., ROM), or the like. 
     The processing section  101  performs various processes according to one embodiment of the invention based on a program (data) stored in the information storage medium  180 . Specifically, a program that causes a computer (i.e., a device that includes an operation section, a processing section, a storage section, and an output section) to function as each section according to one embodiment of the invention (i.e., a program that causes a computer to perform the process of each section) is stored in the information storage medium  180 . 
     The communication section  196  communicates with the HMD  20  through a cable, and communicates with the imaging camera  70  and the effect devices  90  to  93  through a cable or wireless network. The function of the communication section  196  may be implemented by hardware such as a communication ASIC or a communication processor, and communication firmware. 
     A program (data) that causes a computer to function as each section according to one embodiment of the invention may be distributed to the information storage medium  180  (or the storage section  170 ) from an information storage medium included in a host device (server system) (not illustrated in the drawings) through a network and the communication section  196 . Use of the information storage medium included in the host device is also intended to be included within the scope of the invention. 
     The processing section  101  (processor) performs a game calculation process, an image generation process, a sound generation process, an effect control process, and the like based on the timing with respect to the game start timing, data of an image output from the imaging camera  70  (hereinafter referred to as “image data”), a program, and the like. 
     The processing section  101  performs various processes using the storage section  170  as a work area. The function of the processing section  101  may be implemented by hardware such as a processor (e.g., CPU or GPU) or an ASIC (e.g., gate array), and a program. 
     The processing section  101  includes a game calculation section  110 , an object space setting section  111 , a state detection processing section  112 , a movement-motion processing section  113 , an effect control processing section  114 , a communication control section  120 , an image generation section  130 , and a sound generation section  140 . Note that the processing section  101  may have a configuration in which some of these sections are omitted. 
     The game calculation section  110  performs a process that starts the game when a game start condition has been satisfied, a process that proceeds with the game, a process that places an object (including the effect object  80 ) necessary for forming the simulation space, a process that displays an object, a process that terminates the game when a game termination condition has been satisfied, a determination process that determines whether or not the player&#39;s state is the specific state (i.e., the corresponding state during the game), and the like. 
     The game calculation section  110  detects the line-of-sight direction of the player P and an area that intersects the line of sight of the player P (hereinafter referred to as “line-of-sight area”) corresponding to the detected player&#39;s state (i.e., the position of the player P in the real space, and the attitude of the player P), and sets a space that is viewed from the player P within the three-dimensional space corresponding to the detected line-of-sight direction, the detected line-of-sight area, the current game environment, and the state of the game. 
     The game calculation section  110  determines whether or not the game termination condition has been satisfied corresponding to the detected player&#39;s state, or based on a predetermined elapsed time from the start of the game, and terminates the game when it has been determined that the game termination condition has been satisfied. 
     The game calculation section  110  determines whether or not the player P is in the specific state during the game corresponding to the detected player&#39;s state based on the data stored in advance in the data buffer  176 , and proceeds with the game corresponding to the determination result. The game calculation section  110  also instructs the effect control processing section  114 , the image generation section  130 , and the sound generation section  140  to implement the corresponding effect. 
     The object space setting section  111  places an object (i.e., an object formed by a primitive such as a polygon, a free-form surface, or a subdivision surface) (e.g., effect object  80 , building, moving path R, pillar, wall, and map (geographical feature)) that is used to form a predetermined simulation space in an object space (i.e., virtual three-dimensional space). 
     Specifically, the object space setting section  111  determines the position and the rotational angle (synonymous with orientation or direction) of the object in a world coordinate system, and places the object at the determined position (X, Y, Z) and the determined rotational angle (rotational angles around X, Y, and Z-axes). 
     In one embodiment of the invention, the effect object  80  includes a moving object (e.g., animal) that appears in the simulation space (hereinafter referred to as “effect moving object”), and an object that is placed in the real space so that the player P can determine a stationary object placed in the simulation space (hereinafter referred to as “effect stationary object”), and the object space setting section  111  places these effect objects  80  in the simulation space. 
     In one embodiment of the invention, the effect moving object includes an object that moves in the real space, and also moves in the simulation space (when imaged) (hereinafter referred to as “true moving object”), and an object that does not move in the real space, and moves only in the simulation space (when imaged) (hereinafter referred to as “pseudo-moving object”). 
     The state detection processing section  112  determines the positions of the marker units  60  provided to both hands and both feet of the player P and the upper part of the HMD  20  at an identical timing within the image (hereinafter referred to as “player&#39;s image”) of the player P output from a plurality of imaging cameras  70  that capture the player P. 
     The state detection processing section  112  detects the player&#39;s state that represents the position and the attitude of the player P in the real space based on the position of each marker unit  60  within each image, the positional relationship between the marker units  60 , and the time of stay of each marker unit  60  at each position. 
     A plurality of predetermined areas in the real space are set to the image output from each imaging camera  70 . The state detection processing section  112  detects the position of each marker unit  60  in the real space on an image frame basis by detecting the position of each marker unit  60  included in each player&#39;s image at an identical timing as to an area to which each marker unit  60  belongs. 
     The state detection processing section  112  compares the detected position of each marker unit  60  with the position of each marker unit  60  in the previous frame while detecting the position of each marker unit  60  in the real space on a frame basis, and detects the time of stay of each marker unit  60  at an identical position based on the number of frames in which each marker unit  60  has been detected to be situated at an identical position. 
     The state detection processing section  112  detects the attitude of the player P in the real space based on the position of each marker unit  60  in the real space at an identical timing, and the time of stay of each marker unit  60 . 
     For example, the state detection processing section  112  (1) determines the position of the player P in the real space (i.e., the coordinates of the center position (center-of-gravity position) of the player P in the real space) based on information (hereinafter referred to as “part information”) about at least one of the position, the height, and the time of a given part (e.g., head, both hands, or both feet) of the player P, (2) determines the attitude of the player P represented by the positional relationship between each part (e.g., head, body, hands, and feet) of the player P based on the part information about the player P, or (3) detects the player&#39;s state based either or both of the position and the attitude of the player P. 
     The state detection processing section  112  (A) detects the viewpoint position and the viewpoint direction of the player P in the real space based on the position of the head of the player P, (B) detects the position and the attitude of the player P in the real space based on the position of the hand or the foot of the player P, and (C) models the player P (forms bones) based on the position and the attitude of the player P. 
     For example, the parts of the user include the head, the hand, or the foot of the user, and the part information includes information about the position (position coordinates in the user movement space), the direction, the shape (planar shape and three-dimensional shape), the color (including a grayscale), and the like about each part. 
     The state detection processing section  112  determines the position of the marker unit  60  provided to the effect moving object in the real space in the same manner as described above, and detects the position (i.e., the position of the center or the center of gravity) of the effect moving object in the real space based on the position of the marker unit  60  in the real space optionally together with the state of the effect moving object. 
     The movement-motion processing section  113  calculates the positional relationship between the player P and the effect object  80  based on the detected player&#39;s state, the current game environment, the state of the game, or information about two or more items among the player&#39;s state, the current game environment, and the state of the game, and performs a movement-motion calculation process (movement-motion simulation process) on the effect object  80  based on the calculated positional relationship between the player P and the effect object  80 . 
     More specifically, the movement-motion processing section  113  causes various objects to move or make a motion (animation) in the object space based on the detected player&#39;s state and the like. 
     More specifically, the movement-motion processing section  113  performs a simulation process that sequentially calculates movement information (position, rotational angle, speed, or acceleration) and motion information (i.e., the position or the rotational angle of each part that forms the object) about each object every frame ( 1/60th of a second). 
     Note that the term “frame” refers to a time unit used when the object movement-motion process (simulation process) and the image generation process are performed. 
     The movement-motion processing section  113  calculates the movement information and the motion information about the effect moving object based on the position of the effect moving object in the real space, the positional relationship between the effect moving object and the player P in the real space, the state of the effect moving object (i.e., the moving direction and the attitude of the effect moving object), the current game environment, and the state of the game. 
     The movement-motion processing section  113  calculates the movement information and the motion information about the pseudo-moving object (effect moving object) in the simulation space based on either or both of the position and the state of the pseudo-moving object in the real space corresponding to the user&#39;s state in synchronization with the position and the state in the real space, or so that an image can be formed in a seamless manner with respect to the position and the state in the real space. 
     For example, when the pseudo-moving object is an animal object (e.g., cat), the movement-motion processing section  113  calculates the movement information and the motion information about the pseudo-moving object so that the pseudo-moving object makes a motion at a position that differs from the position at which the pseudo-moving object is placed in the real space, or moves over different areas, returns to the position at which the pseudo-moving object is placed in the real space at a predetermined timing, and is placed in the same manner as in the real space. 
     In such a case, the movement-motion processing section  113  calculates the movement information and the motion information so that the pseudo-moving object is visualized (imaged) at the position at which the pseudo-moving object is placed in the real space, and the motion (e.g., attitude and gesture) of the pseudo-moving object is visualized. 
     More specifically, the movement-motion processing section  113  calculates the movement information and the motion information so as to visualize (1) the pseudo-motion of a cat object (pseudo-moving object) around the player P even when the cat object is placed at the end point of the moving path R in the real space when the game has started, (2) the motion of the cat object that changes from the pseudo-motion to the actual state when the cat object moves toward the position at which the cat object is placed in the real space when the player P has advanced by a constant distance toward the end point of the moving path R, and is set to the same state as that in the real space, (3) the pseudo-motion of the cat object (e.g., various gestures when the player P has hold the cat object) that does not change in the real space, and (4) the motion of the cat object that is synchronized with the movement in the real space when the player P has hold the cat object, and released the cat object (when the cat object falls). 
     The effect control processing section  114  performs a process that controls the effect devices  90  to  93  corresponding to the player&#39;s state (including the specific state), the state of the effect moving object, the current game environment, and the state of the game. More specifically, the effect control processing section  114  performs an ON/OFF control process on the effect devices  90  to  93 , a process that changes the ability of the effect devices  90  to  93 , or a control process based on a program set in advance. 
     For example, when the blower  90  is used as the effect devices  90  to  93 , the effect control processing section  114  performs a drive control process (blow control process) and a stop control process. When a temperature control device is used as the effect devices  90  to  93 , the effect control processing section  114  performs a temperature control process. When the moving path R is used as the effect devices  90  to  93 , the effect control processing section  114  controls the effect devices  90  to  93  that should be changed corresponding to the state of the player P (e.g., controls a swing unit provided to the moving path R, or controls a vibration unit). 
     The communication control section  120  performs a process that generates data (mainly image data for presenting the simulation space to the player P) that is transmitted to the HMD  20 . The communication control section  120  transmits and receives a control signal that controls the effect devices  90  to  93 . 
     The image generation section  130  performs a drawing process based on the results of various processes (game process) performed by the processing section  101 , and various types of information such as the player&#39;s state (including the specific state) to generate an image (particularly an image that presents the simulation space to the player P), and outputs the generated image to the HMD  20  through the communication control section  120 . 
     The image generation section  130  acquires object data (model data) that includes vertex data (e.g., vertex position coordinates, texture coordinates, color data, normal vector, or alpha value) with respect to each vertex of the object (model), and performs a vertex process (i.e., a shading process using a vertex shader) based on the vertex data included in the acquired object data (model data). 
     The image generation section  130  may optionally perform a vertex generation process (tessellation, curved surface division, or polygon division) for subdividing the polygon when performing the vertex process. 
     When the image generation section  130  performs the vertex process, the image generation section  130  performs a vertex movement process and a geometric process such as a coordinate transformation process (world coordinate transformation process and camera coordinate transformation process), a clipping process, or a perspective transformation process, according to a vertex processing program (vertex shader program or first shader program), and changes (updates or adjusts) the vertex data with respect to each vertex of the object based on the processing results. 
     The image generation section  130  performs a rasterization process (scan conversion process) based on the vertex data subjected to the vertex process to link the surface of the polygon (primitive) to pixels. 
     The image generation section  130  then performs a pixel process (i.e., a shading process using a pixel shader, or a fragment process) that draws the pixels that form the image (fragments that form the display screen). 
     The image generation section  130  implements the pixel process by determining the final drawing color of each pixel that forms the image by performing various processes such as a texture readout (texture mapping) process, a color data setting/change process, a translucent blending process, and an anti-aliasing process, according to a pixel processing program (pixel shader program or second shader program), and outputting (drawing) the drawing color of the object subjected to a perspective transformation process to (in) the storage section  170  (i.e., a buffer that can store image information on a pixel basis (VRAM or rendering target)). 
     The image generation section  130  also performs a per-pixel process that sets or changes the image information (e.g., color, normal, luminance, and alpha-value) on a pixel basis. The image generation section  130  thus generates an image viewed from the player P in the object space. 
     The vertex process and the pixel process are implemented by hardware that enables a programmable polygon (primitive) drawing process (i.e., a programmable shader (vertex shader and pixel shader)) based on a shader program written in shading language. 
     The programmable shader enables a programmable per-vertex process and a programmable per-pixel process to increase the degree of freedom with respect to the drawing process so that the representation capability can be significantly improved as compared with a fixed drawing process using hardware. 
     The image generation section  130  performs a geometric process, a texture mapping process, a hidden surface removal process, an alpha-blending process, and the like when drawing the object. 
     The geometric process includes performing a coordinate transformation process, a clipping process, a perspective projection transformation process, a light source calculation process, and the like on the object. 
     The image generation section  130  stores the object data (e.g., object&#39;s vertex position coordinates, texture coordinates, color data (brightness data), normal vector, or alpha-value) subjected to the geometric process (perspective transformation process) in the storage section  170 . 
     The image generation section  130  performs a texture mapping process that maps a texture (texel value) stored in the storage section  170  onto the object. More specifically, the image generation section  130  reads a texture (surface properties such as color (RGB) and alpha-value) from the storage section  170  using the texture coordinates set (assigned) to the vertices of the object, and the like. 
     The image generation section  130  maps the texture (two-dimensional image) onto the object. In this case, the image generation section  130  performs a pixel-texel link process, a bilinear interpolation process (texel interpolation process), and the like. 
     The image generation section  130  performs the hidden surface removal process using a Z-buffer method (depth comparison method or Z-test) that utilizes a Z-buffer (depth buffer) that stores the Z-value (depth information) of the drawing pixel. 
     Specifically, the image generation section  130  refers to the Z-value stored in the Z-buffer when drawing the drawing pixel that corresponds to the primitive of the object. The image generation section  130  compares the Z-value stored in the Z-buffer with the Z-value of the drawing pixel of the primitive. When the Z-value of the drawing pixel is the Z-value in front of the player P (e.g., a small Z-value), the image generation section  130  draws the drawing pixel, and updates the Z-value stored in the Z-buffer with a new Z-value. 
     The image generation section  130  performs a translucent blending process (e.g., normal alpha-blending process, additive alpha-blending process, or subtractive alpha-blending process) based on the alpha-value (A value). 
     Note that the alpha-value is information that can be stored so as to be linked to each pixel (texel or dot), such as additional information other than the color information. The alpha-value can be output as mask information, translucency (equivalent to transparency or opacity), bump information, or the like. 
     The sound generation section  140  performs a sound process based on the results of various processes performed by the processing section  101  with respect to the player&#39;s state (including the specific state) and the like to generate a game sound (i.e., an ambient sound (stereophonic sound) in the simulation space) (e.g., background music (BGM), effect sound, or voice), and outputs the generated game sound to the headphone  61  through the HMD  20 . 
     2-6. Player Marker Unit and Imaging Camera 
     The player marker unit  60  and the imaging camera  70  included in the game system  1  are described below with reference to  FIGS. 1 and 4 . 
     In one embodiment of the invention, the marker units  60  are provided to a plurality of parts of each player P in order to detect the player&#39;s state (see above). More specifically, the marker units  60  are provided to the head, both hands, and both feet of each player P (see  FIGS. 1 and 4 ). 
     Each marker unit  60  is formed using a material (e.g., reflecting sheet) having a reflecting surface. Each marker unit  60  is formed by a spherical marker. For example, each marker unit  60  reflects the applied light, and emits white light or light in a specific color. 
     More specifically, the marker units  60  include a head detection marker unit  60   a , a right hand or left hand detection marker unit  60   b , and a right foot or left foot detection marker unit  60   c.    
     A light source unit (not illustrated in the drawings) that applies light to each marker unit  60  is provided in a movement experience area within the structure  10 . 
     The emission color of the marker unit  60  is not particularly limited. When a plurality of players P are simultaneously present in the movement experience area, the emission color of the marker unit  60  may be changed on a player (P) basis, or may be changed on a part basis. 
     Each imaging camera  70  is placed at a predetermined position within the structure  10  (see  FIG. 1 , for example). Each imaging camera  70  forms an image of an area captured within the angle of view, and sequentially outputs the image data to the simulation control device  100 . 
     Each imaging camera  70  is provided outside the moving range (i.e., moving path R) of the player P in the real space. 
     Each imaging camera  70  is placed so as to be able to image the play area  12 , and image each player P who moves within the play area  12 , or changes the attitude, or moves within the play area  12  and changes the attitude. 
     Each imaging camera  70  includes a predetermined image sensor (e.g., CCD), and a lens that has a predetermined focal length. Each imaging camera  70  images an area within a predetermined angle of view at a predetermined focal length, and sequentially outputs the image data to the simulation control device  100 . 
     When the player P can freely move within the play area  12 , it is necessary to image the entire play area  12 , and each imaging camera  70  is placed so that the entire play area  12  can be imaged. 
     Each imaging camera  70  must be a color camera when it is desired to detect a colored marker unit  60 . 
     The number and the placement positions of marker units  60  are not limited to those described above. The number and the placement positions of marker units  60  are not basically limited as long as the marker units  60  can be captured using the imaging cameras  70 . 
     2-7. Effect Object and Effect Device 
     2-7-1. Configuration 
     The effect object  80  and the effect devices  90  to  93  included in the game system  1  are described below with reference to  FIGS. 1, 3, 4, 7A and 7B .  FIGS. 7A and 7B  illustrate an example of the moving path member  93  (i.e., effect device). 
     The effect object  80  and the effect devices  90  to  93  are placed in the real space defined by the structure  10 , and are configured to allow the player P to experience a given effect in synchronization with the simulation image under control of the simulation control device  100 . 
     The effect object  80  and the effect devices  90  to  93  are used to allow the player P to experience the simulation space (e.g., outdoor space, indoor space, height (high place), closed place, dangerous place, special space, hot place, or cold place), and allow the player P to experience the simulation situation in synchronization with the simulation image to provide an effective simulation. 
     More specifically, the effect object  80  is classified as the effect moving object or the effect stationary object, and the effect moving object is classified as the true moving object or the pseudo-moving object (see above). 
     The pseudo-moving object is visualized in a state that differs from the state of the effect object  80  in the real space. 
     The effect moving object is provided with a marker unit  60   d  for detecting the position of the effect moving object in the real space, the positional relationship between the effect moving object and the player P in the real space, and the state of the effect moving object (i.e., the moving direction and the attitude of the effect moving object). 
     The simulation control device  100  determines either or both of the position and the state of the effect moving object in the real space by capturing the marker unit  60   d  using the imaging camera  70 . 
     For example, the marker unit  60   d  provided to the effect object  80  may be the same as the marker units  60   a ,  60   b , and  60   c  provided to each part of the player P. Note that it is desirable that the marker unit differ between a plurality of players P and the effect object  80  that appears in the game so as to distinguish the color of the marker units  60   a ,  60   b , and  60   c  provided to each part of the player P and the stored information. 
     Note that a vibration unit may be provided in the effect object  80 , and the simulation control device  100  may control the vibration of the vibration unit to produce an effect that surprises the player P in synchronization with the simulation image, or independently of the simulation image. 
     The effect devices  90  to  93  are devices that are used reliably generate the simulation space as a more realistic space, and provide a given effect directly to the player P. 
     The effect devices  90  to  93  include a structure-type effect device that is placed in the real space, and provides a given effect to the player P due to the placement and the structure in synchronization with the simulation image, and a synchronization-type effect device that provides a given effect corresponding to the detected player&#39;s state, the game environment, and the state of the game. 
     Examples of the synchronization-type effect device include the blower  90  illustrated in  FIG. 1 , and an effect device (e.g., temperature control device, illumination device, or vibration device) (not illustrated in the drawings) that forms the environment of the simulation space. 
     Examples of the structure-type effect device include the moving path member  93  that forms the moving path R, a start block (not illustrated in the drawings) that provides the start point, and a member that allows the player P to experience a touch feeling (e.g., convexities and concavities or a material for a wall surface and a floor) (e.g., the spring floor panel  92  that provides an elevator). 
     For example, the blower  90  blows air toward the front side of the player P when the player P has entered the movement experience zone  14  from the start zone  13 , and blows air toward the front side of the player P from the lower side of the player P when the player P has moved to the moving path R (see  FIGS. 1 and 4 ). 
     The moving path member  93  includes an effect area  93   a  that is provided under the moving path R in which the player P moves, and is formed to vibrate or swing corresponding to the simulation image (see  FIGS. 3, 7A, and 7B ). 
     Specifically, the moving path member  93  in the effect area  93   a  is formed to have a different height (height in the direction toward the ceiling) with respect to the floor (non-moving path NR) of the structure  10  (see  FIG. 7A ). 
     The moving path member  93  in the effect area  93   a  includes a plurality of driver units  95  that vibrates or swings the effect area based on a given condition (e.g., a condition whereby the player P has started walking on the moving path member  93 ). 
     Each driver unit  95  includes a wheel  96  that rotates in the direction orthogonal to the moving path direction (travel direction of the player P) (the moving path member  93  is horizontal to the floor), and a support plate  97  that has a gap D having a predetermined height (e.g., 5 mm) from the surface with which the wheel  96  comes in contact, and supports the moving path member  93 . 
     The driver units  95  are adjacently disposed in the effect area  93   a  along the moving path R. 
     The moving path member  93  in the effect area  93   a  may be configured so that the entire moving path R is formed by the driver unit  95 , or may be configured so that the moving path R is formed by the driver unit  95  and the moving path member  93  that is not driven.  FIG. 7B  illustrates an example in which the driver units  95  continuously form the moving path R. 
     The driver unit may be configured to vibrate or swing independently (see  FIGS. 7A and 7B ), or may be configured so that the vibration motion and the swing motion are mechanically controlled by the effect control processing section  114 . 
     The structure or the form of the effect device may be changed only within the simulation space. Specifically, the structure or the form of the effect object and the effect device may not be changed in the real space, and may be changed only within the simulation space. 
     For example, when a predetermined event has occurred, the image generation section  130  may change the structure or the form of the effect object and the effect device (i.e., reduce the width of the moving path member  93 , or move the wall  17  toward the player P, or move the ceiling  15  downward) only within the simulation space. 
     2-7-2. Operation of Game System 
     The operation of the game system  1  is described below with reference to  FIGS. 8 and 9 .  FIGS. 8 and 9  are flowcharts illustrating the operation of the game system  1 . 
     Note that the operation of the game system  1  is described below taking an example in which the game system  1  implements a fear of heights experience game that allows the player P to experience a fear of heights. The fear of heights experience game is a game in which the player P starts from the start zone  13 , moves along the moving path member  93  having a predetermined width, holds (rescues) the effect object  80  (e.g., cat) that is situated at the end point (i.e., a point situated away from the start zone  13 ) of the moving path member  93 , and returns to the start zone  13  within the time limit. 
     The player P wears the HMD  20  and the fall prevention unit  40  (i.e., necessary hardware) before starting the game. 
     The game calculation section  110  detects whether or not a button (not illustrated in the drawings) has been pressed (i.e., whether or not the game has been started) based on an operation performed by the administrator on condition that the player P who wears the HMD  20  and the fall prevention unit  40  is situated at a predetermined position (i.e., within the start zone  13 ) (step S 101 ). 
     In the step S 101 , the game calculation section  110  may detect whether or not the game has been started by detecting the player&#39;s state using the state detection processing section  112 , and detecting whether or not the player P is situated at the predetermined position. 
     The game calculation section  110  performs various calculations with respect to the fear of heights experience game, and the object space setting section  111 , the state detection processing section  112 , the movement-motion processing section  113 , the effect control processing section  114 , the image generation section  130 , and the sound generation section  140  perform the simulation process with respect to the fear of heights experience game (step S 102 ). 
     More specifically, (1) the state detection processing section  112  starts to detect the marker units  60  provided to each part of the player P and the effect object  80 , and starts to detect the player&#39;s state and the state of the effect object  80 , (2) the object space setting section  111  and the movement-motion processing section  113  start to generate the simulation space viewed from the player P corresponding to the player&#39;s state, the state of the effect object  80 , the game environment, and the state of the game, (3) the effect control processing section  114  starts to control the corresponding effect device (blower  90 , automatic door  91 , and spring floor panel  92 ) corresponding to the player&#39;s state, the state of the effect object  80 , the game environment, and the state of the game, and (4) the image generation section  130  and the sound generation section  140  respectively generate the simulation image and the sound corresponding to the player&#39;s state, the state of the effect object  80 , the game environment, and the state of the game. 
     Note that each process in the step S 102  is continuously performed until it is determined that the game has ended. 
     The image generation section  130  displays the image within the elevator on the HMD  20 , and the effect control processing section  114  controls the spring floor panel  92  in the start zone  13 , and performs the simulation process with respect to the start zone  13  (step S 103 ). 
     When the effect control processing section  114  has detected a predetermined timing (i.e., the end of the simulation process in the start zone  13 ) (step S 104 ), the effect control processing section  114  performs the game start process (step S 105 ), and the game calculation section  110  starts the game start countdown process (step S 106 ). 
     For example, the effect control processing section  114  (1) stops controlling the spring floor panel  92 , (2) sets the automatic door  91  (i.e., elevator door) provided between the start zone  13  and the movement experience zone  14  from the closed state to the open state, and (3) performs the blowing process using the blower  90  (i.e., controls the corresponding effect device). 
     The game calculation section  110  detects whether or not the countdown process has ended (step S 107 ), and the state detection processing section  112  performs a process that determines whether or not the player P has moved to the movement experience zone  14  from the start zone  13  (hereinafter referred to as “start error determination process”) (step S 108 ). 
     When the state detection processing section  112  has determined that the player P has moved to the movement experience zone  14  from the start zone  13  before the countdown process has ended, the state detection processing section  112  issues a warning using the HMD  20  (step S 109 ), and performs the step S 107 . 
     Note that the state detection processing section  112  may performs the step S 105  again after producing an effect (e.g., suspending the game start motion), or may suspend the game. 
     When the game calculation section  110  has detected that the countdown process has ended, the game calculation section  110  starts various calculations for implementing the fear of heights experience game (step S 110 ). Specifically, the game calculation section  110  starts to perform the time limit count process, and starts the determination process with respect to the game termination process. 
     The game calculation section  110  determines whether or not the game termination condition has been satisfied (step S 111 ). More specifically, the game calculation section  110  determines whether or not the player&#39;s state or the state of the effect object  80  has satisfied the termination condition in cooperation with the state detection processing section  112 , and determines whether or not the time limit has been reached. 
     For example, the game calculation section  110  determines whether or not the specific state that satisfies the termination condition has occurred (e.g., whether or not the player P has run off the moving path R, or whether or not the effect object  80  to be rescued has fallen) based on the detected player&#39;s state. 
     When the game calculation section  110  has determined that the game termination condition has not been satisfied, the game calculation section  110  determines whether or not an event start condition has been satisfied in cooperation with the state detection processing section  112 , or corresponding to the state of the game (step S 112 ). 
     More specifically, the game calculation section  110  determines the event start condition (including whether or not the player is in the specific state) (e.g., whether or not the player has moved to the moving path member  93 , whether or not the player has reached a first position (i.e., the position at which the cat object (effect object  80 ) was initially placed) of the moving path member  93 , or whether or not the player has reached the position in the real space at which the cat object (effect object  80 ) was placed (i.e., the end point of the moving path member  93 )). 
     When the game calculation section  110  has determined that the event start condition has been satisfied, the effect control processing section  114 , the image generation section  130 , and the sound generation section  140  perform a process that corresponds to the detected event (step S 113 ). When the game calculation section  110  has determined that the event start condition has not been satisfied, the step S 111  is performed. 
     More specifically, (1) when it has been determined that the player P has moved to the moving path member  93  (i.e., the player P is in the specific state), the effect control processing section  114  operates the blower  90  disposed under the player P, and blows air toward the player P from the lower side. (2) When it has been determined that the player P has reached the first position (i.e., the position at which the cat object (effect object  80 ) was initially placed) of the moving path member  93  (i.e., the player P is in the specific state), the image generation section  130  and the sound generation section  140  produce an effect in which the cat object runs away to the end point of the moving path R. (3) When it has been determined that the player P has reached the position at which the cat object was placed (i.e., the player P is in the specific state), the image generation section  130  and the sound generation section  140  produce an effect in which the cat object is rescued. (4) When it has been determined that the player P has rescued the cat object (i.e., the player P is in the specific state), the image generation section  130  and the sound generation section  140  produce an effect in which the width of the moving path member  93  is reduced when the moving direction coincides with the direction toward the start zone  13 . (5) When it has been determined that the player P has reached the start zone  13  in a state in which the player P holds the cat object (i.e., the player P is in the specific state), the image generation section  130  and the sound generation section  140  produce a game-clear effect in which the player P escapes from the elevator. 
     When the game calculation section  110  has determined that the game termination condition has been satisfied, the image generation section  130  and the sound generation section  140  generate and output an image and a sound for producing a game termination effect (step S 114 ), and the game calculation section  110  terminates the process. A different effect is produced as the game termination effect corresponding to the game termination condition. 
     2-8. Eye Input 
     2-8-1. Outline of Eye Input 
     The game calculation section  110  included in the simulation control device  100  appropriately operates as a display processing section  110 A, a measurement section  110 B, a reception section  110 C, a notification section  110 D, and an execution section  110 E according to the game program (i.e., simulation control program) stored (recorded) in the main storage section  172 , and functions as a user interface that utilizes an eye input. The term “eye input” used herein refers to an input performed by the player P when the player P has input an instruction to the simulation control device  100  by gazing at the object. An outline of the operation performed by the display processing section  110 A, the measurement section  110 B, the reception section  110 C, the notification section  110 D, and the execution section  110 E with respect to the eye input is described below. 
     (1) Display Processing Section  110 A 
     The display processing section  110 A displays the virtual three-dimensional space (reference sign OBS in  FIG. 10 ) on the HMD  20 . For example, the display processing section  110 A controls the object space setting section  111  and the image generation section  130 , and places various objects for forming the simulation space in the virtual three-dimensional space (reference sign OBS in  FIG. 10 ). The objects include an icon (push button objects OB 1  to OB 4  described later), a virtual controller, a virtual mechanical switch, a character, an item, and the like. The display processing section  110 A performs a process that controls the image generation section  130 , sets virtual camera within the virtual three-dimensional space OBS, and generates image data that allows the player P to view an image within the field-of-view range (reference sign DA in  FIG. 10 ) of the virtual camera. The display processing section  110 A performs a process that controls the communication control section  120  and the communication section  196 , and outputs the image data to the HMD  20 . The display processing section  110 A also performs a process that changes the position and the attitude of the virtual camera in the virtual three-dimensional space (reference sign OBS in  FIG. 10 ) corresponding to a change in the position and the attitude of the head of the player P that has been detected by the state detection processing section  112 . 
     (2) Measurement Section  110 B 
     The measurement section  110 B measures the gaze time of the player P with respect to the object (reference sign OB 1  in  FIG. 11 , for example) that is placed in the virtual three-dimensional space (reference sign OBS in  FIG. 11 ). When the line of sight (reference sign LSA in  FIG. 10 ) of the player P has been moved so as not to intersect the object (reference sign OB 1  in  FIG. 11 ), the measurement section  110 B decreases the measured value (hereinafter referred to as “charge amount”) corresponding to the time in which the line of sight (reference sign LSA in  FIG. 10 ) of the player P does not intersect the object. The measurement section  110 B performs a process (gaze determination process) that determines whether or not the player P is gazing at the push button object OB 1  based on the positional relationship between a hit area OBHA (i.e., an area that is considered to include an object) with respect to the push button object OB 1  that is situated in the display area DA, and a line-of-sight area LSA (i.e., an area that is considered to intersect a line of sight) that is set to the display area DA, for example. 
     (3) Reception Section  110 C 
     The reception section  110 C determines that the object (reference sign OB 1  in  FIG. 11 ) has been selected (locked) when the charge amount has reached a first threshold value Th1 (e.g., 2 seconds), determines that the lock with respect to the object (reference sign OB 1  in  FIG. 11 ) has been confirmed when the charge amount has reached a second threshold value Th2 (e.g., 5 seconds) that is larger than the first threshold value Th1, and determines that the lock with respect to the object (reference sign OB 1  in  FIG. 11 ) has been canceled when the charge amount has decreased to a third threshold value Th3 (e.g., 0 seconds) before the lock with respect to the object is confirmed. Note that the reception section  110 C may determine that the lock with respect to the object has been confirmed even when the charge amount has not reached the second threshold value Th2 (e.g., 5 seconds), on condition that the charge amount has exceeded the first threshold value Th1, and the player P has made a predetermined action. The predetermined action may be an action that presses a button provided in the real space, or may be an action that gazes at a confirmation object. Note that an example in which a button provided in the real space and a confirmation object are not used, is described below. 
     (4) Notification Section  110 D 
     The notification section  110 D notifies the player P of the magnitude of the charge amount. The notification section  110 D controls the object space setting section  111 , and notifies the player P of the magnitude of the charge amount by providing a visual effect that changes corresponding to the magnitude of the charge amount ( FIG. 14 ) to the object (reference sign OB 1  in  FIG. 11 ), for example. Note that the notification section  110 D may notify the player P of the magnitude of the charge amount by causing the sound generation section  140  to generate an effect sound, and outputting the effect sound to the headphone  61  of the HMD  20  through the communication control section  120  and the communication section  196 . The notification section  110 D may notify the player P of the magnitude of the charge amount by utilizing at least one of the sense of sight, the sense of hearing, the sense of touch, and the sense of smell of the player P. Note that an example in which the notification section  110 D notifies the player P of the magnitude of the charge amount by utilizing the sense of sight of the player P is mainly described below. 
     (5) Execution Section  110 E 
     The game calculation section  110  that functions as the execution section  110 E performs a predetermined process that is linked to the object (reference sign OB 1  in  FIG. 11 ) when the lock with respect to the object (reference sign OB 1  in  FIG. 11 ) has been confirmed. The predetermined process that is linked to the object (reference sign OB 1  in  FIG. 11 ) is a process that sets a game parameter, a process that attacks the object, a process that moves the object, a process that is performed on another object, or a process that executes a predetermined program, for example. The execution section  110 E may perform the predetermined process at a timing immediately after the lock with respect to the object has been confirmed, or may perform the predetermined process when a predetermined time has elapsed after the lock with respect to the object has been confirmed. Note that the following description is given taking an example in which the execution section  110 E performs the predetermined process at a timing immediately after the lock with respect to the object has been confirmed. 
     The following description is given taking an example in which the operation of the display processing section  110 A, the operation of the measurement section  110 B, the operation of the reception section  110 C, the operation of the notification section  110 D, and the operation of the execution section  110 E, are implemented by the game calculation section  110 . 
     2-8-2. Line-of-Sight Area 
       FIG. 10  illustrates the relationship with respect to the virtual three-dimensional space OBS, a space (area) that is viewed from the player P who wears the HMD  20  (hereinafter referred to as “display area DA”), and the line-of-sight area LSA with respect to the player P who wears the HMD  20 . 
     The display area DA corresponds to the field-of-view range of the virtual camera that is placed in the virtual three-dimensional space OS (see above), and is an area that is displayed to the player P through the HMD  20 . Since the game calculation section  110  causes the position and the attitude of the virtual camera to follow the position and the attitude of the head of the player P (see above), the position and the attitude of the display area DA in the virtual three-dimensional space OBS also follow the position and the attitude of the head of the player P. 
     The line-of-sight area LSA is an area having a predetermined size that has been set by the game calculation section  110  at the center of the display area DA. The line-of-sight area LS corresponds to a range (gaze range or line-of-sight range) that is considered to be gazed at by the player P during the gaze determination process described later. The ratio of the size of the line-of-sight area LSA with respect to the size of the display area DA is basically fixed, and the position of the line-of-sight area LSA within the display area DA is also basically fixed. For example, the line-of-sight area LSA is provided at the center of the display area DA, and has a circular shape (see  FIG. 10 ). 
     Note that the ratio of the size of the line-of-sight area LSA with respect to the size of the display area DA may be set to be variable. The position of the line-of-sight area LSA within the display area DA may also set to be variable. When the position of the line-of-sight area LSA within the display area DA is set to be variable, the position of the line-of-sight area LSA may be changed to follow the direction of the line of sight of the player P (see the line of sight input described later). Although  FIG. 10  illustrates an example in which the contour and the center coordinates of the line-of-sight area LSA are displayed, the contour and the center coordinates of the line-of-sight area LSA may not be displayed. 
       FIG. 10  illustrates an example in which a selection screen is displayed as the virtual three-dimensional space OBS when the fear of heights experience game has been started (step S 101 ). The selection screen is a screen for the player P to perform several settings with respect to the game process. 
       FIG. 10  illustrates an example in which push button objects OB 1  and OB 2  for selecting the number of players, and push button objects OB 3  and OB 4  for selecting the difficulty level, are displayed within the selection screen at specific intervals. Each of the push button objects OB 1 , OB 2 , OB 3 , and OB 4  is an object that represents a virtual operation button, and may be referred to as “icon”. An additional object (e.g., a scenery that is viewed from the start point of the fear of heights experience game) (not illustrated in  FIG. 10 ) that is situated in the virtual three-dimensional space OBS may be displayed behind the push button objects OB 1 , OB 2 , OB 3 , and OB 4  with respect to the player P. 
     A function of setting the mode of the game process that is performed by the processing section  101  (mainly the game calculation section  110 ) to “single-player mode” is assigned to the push button object OB 1 . The single-player mode is a mode that is suitable when a single player P who wears the HMD  20  moves within the structure  10 . 
     A function of setting the mode of the game process that is performed by the processing section  101  (mainly the game calculation section  110 ) to “two-player mode” is assigned to the push button object OB 2 . The two-player mode is a mode that is suitable when two players P who wear the HMD  20  move within an identical structure  10  (i.e., a mode that allows two players P to share the experience of fear). When the mode has been set to the two-player mode, the game calculation section  110  sets two virtual cameras that respectively correspond to two players P within the virtual three-dimensional space OBS, and places two characters (avatars) that respectively correspond to the two players P within the virtual three-dimensional space OBS, for example. The game calculation section  110  links the two virtual cameras to the heads of the two players P, and links each part of the two avatars to each part of the two players P. 
     A function of setting the mode of the game process that is performed by the processing section  101  (mainly the game calculation section  110 ) to “beginner mode” is assigned to the push button object OB 3 . The beginner mode is a mode in which the difficulty level of the game process (e.g., the probability that the player P falls) is set to be lower than that of the advanced mode described below. The difficulty level of the game process is adjusted by adjusting the parameter of the game process, for example. 
     A function of setting the mode of the game process that is performed by the processing section  101  (mainly the game calculation section  110 ) to “advanced mode” is assigned to the push button object OB 4 . The advanced mode is a mode in which the difficulty level of the game process (e.g., the probability that the player P falls) is set to be higher than that of the beginner mode (see above). The difficulty level of the game process is adjusted by adjusting the parameter of the game process, for example. 
     The following description is given on the assumption that the player P plays the game in the single-player mode and the beginner mode (i.e., the player P locks the push button objects OB 1  and OB 3  by performing an eye input, and confirms the lock with respect to the push button objects OB 1  and OB 3  by performing an eye input). 
     As illustrated in  FIG. 10 , the push button objects OB 1 , OB 2 , OB 3 , and OB 4  are placed in a 2×2 matrix, and the display area DA and the line-of-sight area LSA are situated at the center of the matrix when the selection screen has been displayed, for example. 
     When the player P has turned his/her head to the upper left, the display area DA and the line-of-sight area LSA move to the upper left area of the selection screen (with respect to the player P) (see  FIG. 11 ). 
       FIG. 11  illustrates a state in which the line-of-sight area LSA is situated at the center of the push button object OB 1 , and the display area DA covers the entirety of the push button object OB 1 . In this case, the game calculation section  110  determines that the player P is gazing at the push button object OB 1 . Note that the details of the gaze determination process are described later. 
     When the player P has moved backward, the size of the display area DA and the line-of-sight area LSA within the virtual three-dimensional space OBS increases, and the entirety of the push button objects OB 1 , OB 2 , OB 3 , and OB 4  can be displayed within the display area DA (not illustrated in  FIGS. 10 and 11 ), for example. 
     2-8-3. Gaze Determination Process 
     The game calculation section  110  sets a hit area to each of the push button objects OB 1 , OB 2 , OB 3 , and OB 4  that are displayed (placed) within the selection screen.  FIG. 12A  illustrates an example of a hit area OBHA that is set to the push button object OB 1 . 
     As illustrated in  FIG. 12A , the hit area OBHA that is set to the push button object OB 1  is an area that has a predetermined shape and a predetermined size, and has the same center coordinates as those of the push button object OB 1 . The shape of the contour of the hit area OBHA is similar to that of the push button object OB 1 , for example. The size of the hit area OBHA that is set to the push button object OB 1  is almost equal to the size of the push button object OB 1 , for example. 
     Note that the size of the hit area OBHA may be larger than the size of the push button object OB 1 , or may be smaller than the size of the push button object OB 1 . The size of the hit area OBHA may be adjustable. 
     The game calculation section  110  determines whether or not the player P is gazing at the push button object OB 1  based on the positional relationship between the hit area OBHA that is set to the push button object OB 1  and the line-of-sight area LSA (gaze determination process). 
     For example, the game calculation section  110  determines that the player P is gazing at the push button object OB 1  when at least part of the hit area OBHA and at least part of the line-of-sight area LSA overlap each other, and determines that the player P is not gazing at the push button object OB 1  (i.e., the line of sight of the player P does not intersect the push button object OB 1 ) when the hit area OBHA and the line-of-sight area LSA do not overlap each other.  FIG. 12B  illustrates a state in which at least part of the hit area OBHA and at least part of the line-of-sight area LSA overlap each other. The game calculation section  110  determines that the player P is gazing at the push button object OB 1  when the state illustrated in  FIG. 12B  has occurred. 
     Although an example in which the game calculation section  110  determines that the player P is gazing at the push button object OB 1  when at least part of the hit area OBHA and at least part of the line-of-sight area LSA overlap each other, has been described above, the game calculation section  110  may determine that the player P is gazing at the push button object OB 1  when one of the hit area OBHA and the line-of-sight area LSA is included in the other of the hit area OBHA and the line-of-sight area LSA, or when the interval between the hit area OBHA and the line-of-sight area LSA has become less than a threshold value, for example. 
     Although  FIGS. 12A and 12B  illustrate an example in which the line-of-sight area LSA is smaller than the hit area OBHA, the line-of-sight area LSA may be larger than the hit area OBHA. 
     The player P can change the position of the line-of-sight area LSA within the virtual three-dimensional space OBS, and gaze at the push button object OB 1 , or move the line of sight so as not intersect the push button object OB 1 , by changing the attitude and the position of the head on which the HMD  20  is worn, for example. 
     A state in which it has been determined by the gaze determination process that the player P is gazing at the push button object may be referred to as “gaze” or the like, and a state in which it has been determined by the gaze determination process that the player P is not gazing at the push button object may be referred to as “gaze has been removed” or the like. The term “line of sight” used herein may not refer to the actual line of sight (i.e., the visual axis of the eyeball) of the player P in the real space, but may refer to the line-of-sight area LSA within the virtual three-dimensional space OBS. 
     The line-of-sight area LSA and the hit area OBHA basically may not be displayed on the HMD  20 . Specifically, since the game calculation section  110  enhances the gazed object when the player P has gazed at an arbitrary object within the virtual three-dimensional space OBS (see  FIG. 14  described later, for example), it is considered that the player P does not lose the position of the line of sight within the virtual three-dimensional space OBS even when the line-of-sight area LSA and the hit area OBHA are not displayed. 
     2-9. Charge Amount with Respect to Gaze Time 
     The game calculation section  110  basically measures (charges) the gaze time of the player P with respect to each object placed in the virtual three-dimensional space OBS, and controls the state of the object between the locked state, the unlocked state, and the confirmed state corresponding to the measured amount (charge amount) with respect to each object. The game calculation section  110  manages the charge amount with respect to the gaze time with respect to each object using a read-write memory (e.g., data buffer  176 ). 
       FIGS. 13A, 13B, 13C, and 13D  are graphs illustrating the relationship between the elapsed time (from the start of gaze) and the charge amount with respect to one object. The horizontal axis indicates time, and the vertical axis indicates the charge amount. 
       FIG. 13A  illustrates the change pattern of the charge amount when the player P gazed at the object for a short time (i.e., when the player P removed his/her gaze from the object before the charge amount reaches the first threshold value Th1). 
       FIG. 13B  illustrates the change pattern of the charge amount when the player P gazed at the object for a certain time, and then removed his/her gaze from the object (i.e., when the player P removed his/her gaze from the object after the charge amount had reached the first threshold value Th1). 
       FIG. 13C  illustrates the change pattern of the charge amount when the player P continuously gazed at the object for a sufficient time (i.e., when the charge amount reached the second threshold value Th2). 
       FIG. 13D  illustrates the change pattern of the charge amount when the player P gazed at the object for a certain time, temporarily removed his/her gaze from the object, and gazed at the object again (i.e., when the player P removed his/her gaze from the object after the charge amount had exceeded the first threshold value Th1, but before the charge amount reaches the second threshold value Th2, and gazed at the object again before the charge amount reaches “0”). 
     When the player P has gazed at an arbitrary object (see the charge start time is in  FIGS. 13A to 13D ), the game calculation section  110  charges the gaze time with respect to the object. For example, the game calculation section  110  increases the charge amount by “1” each time the gaze time has increased by 1 second. 
     When the player P has removed his/her gaze from the object (see the gaze removal time tO in  FIGS. 13B and 13D ), the game calculation section  110  decreases the charge amount with respect to the gaze time corresponding to the time in which the player P does not gaze at the object. For example, the game calculation section  110  increases the charge amount by “0.5” each time the gaze time has increased by 1 second. 
     When the charge amount has reached the first threshold value Th1 (e.g., Th1=2), the game calculation section  110  determines that the object has been locked (see the lock start time tL in  FIGS. 13B, 13C, and 13D ). 
     Specifically, the player P can lock the object by continuously gazing at the object for a time equal to or longer than a specific time (e.g., 2 seconds). 
     When the charge amount has reached the second threshold value Th2 (e.g., Th2=5) that is larger than the first threshold value Th1 (see the confirmation time tC in  FIGS. 13C and 13D ), the game calculation section  110  determines that the lock with respect to the object has been confirmed, and resets the charge amount to “0”. 
     Specifically, the player P can confirm the lock with respect to the object by continuously (or intermittently) gazing at the locked object for a time equal to or longer than a specific time. 
     When the charge amount has decreased to the third threshold value Th3 (e.g., Th3=0) after the object has been locked, but before the lock with respect to the object is confirmed, the game calculation section  110  determines that the lock with respect to the object has been canceled (see the lock cancelation time tU in  FIG. 13B ). 
     Specifically, the player P can cancel the lock with respect to the object by removing his/her gaze from the locked object for a time equal to or longer than a specific time. Moreover, the lock with respect to the object can be canceled at an earlier timing as the gaze time with respect to the object decreases. On the other hand, the player P can increase the time in which the lock with respect to the object is maintained after the player P has removed his/her gaze from the locked object by gazing at the locked object for a long time. In other words, the degree by which the player P desires to gaze at the object is reflected in the degree of lock (i.e., the difficulty level with respect to cancelation of the lock). 
     When the player P has removed his/her gaze from the object before the charge amount reaches the first threshold value Th1 (see the gaze removal time tO in  FIG. 13A ), the game calculation section  110  (immediately) resets the charge amount to “0”. 
     Specifically, the object is not locked when the player P has merely glanced at the object (e.g., when the player P has not decided the object to be locked), and the object is locked only when the player P has intentionally gazed at the object to be locked (for a long time). 
     The game calculation section  110  decreases the speed at which the charge amount approaches the third threshold value Th3 after the object has been locked, to be lower than the speed at which the charge amount approaches the second threshold value Th2. For example, the game calculation section  110  increases the charge amount by “1” each time the gaze time has increased by 1 second, and decreases the charge amount by “0.5” each time the time in which the player P does not gaze at the locked object has increased by 1 second. As illustrated in  FIGS. 13B and 13D , the slope of the curve along which the charge amount decreases is less steep than that of the curve along which the charge amount increases. 
     Specifically, the player P can easily maintain the lock with respect to the object after the object has been locked without completely maintaining the position and the attitude of the head. 
     The player P can confirm the lock with respect to the desired object within a short time by continuously gazing at the desired object without hesitation (see  FIG. 13C ). When the player P hesitates to confirm the lock with respect to the object, the player P can increase the remaining time until the lock with respect to the object is confirmed, by removing his/her gaze from the object, and gazing at the object again (see  FIG. 13D ). 
     As described above, the player P can arbitrarily switch the state of the object between the locked state, the unlocked state, and the confirmed state. 
     Although an example in which the game calculation section  110  adjusts the speed at which the charge amount is increased or decreased in order to adjust the speed at which the charge amount approaches the threshold value, has been described above, the game calculation section  110  may adjust the threshold value instead of (or in addition to) adjusting the speed at which the charge amount is increased or decreased. Specifically, since the charge amount and the threshold value are relative values, the adjustment of the speed at which the charge amount is increased or decreased is considered to be equivalent to the adjustment of the threshold value with respect to the charge amount (hereinafter the same). 
     Although an example in which the game calculation section  110  determines that the lock with respect to the object has been canceled at a timing at which the charge amount with respect to the object has reached the threshold value Th3, has been described above, the game calculation section  110  may determine that the lock with respect to the object has been canceled when the player P has made a predetermined action even when the charge amount with respect to the object has not reached the threshold value Th3. The predetermined action may be an action that presses a button provided in the real space, or may be an action that gazes at a cancelation object, or may be an action that gazes at the object again (described later). Note that an example in which a button provided in the real space for cancelling a lock, a cancelation object, and an action that gazes at the object again are not used, is described below. 
     2-10. Visual Effect with Respect to Object 
       FIG. 14  illustrates an example of a visual effect applied to the object. 
     The game calculation section  110  visualizes a temporal change in the charge amount with respect to the object (i.e., a state in which the charge amount increases and decreases) by providing a visual effect to the object, and changing the visual effect corresponding to the charge amount with respect to the object (see  FIG. 14 ). 
     When the charge amount with respect to each object is visualized, the player P can determine the object at which the player P gazes, and determine whether or not the player P is gazing at the desired object. The player P can also determine the remaining time until the object at which the player P gazes is locked, the remaining time until the lock with respect to the object is canceled, and the gaze time required for the lock with respect to the object to be confirmed. 
     The visual effect is implemented by an animation, for example. It is possible to notify the player P of a temporal change in charge amount in real time (successively) by utilizing an animation. In the example illustrated in  FIG. 14 , the animation is designed so that a line of marks (i.e., a line of neon marks) is placed around the outer edge of the object, and the number of marks that form a line is changed corresponding to the charge amount. 
     When the charge amount is thus reflected in the number of marks that form a line, the player P can intuitively determine the magnitude of the charge amount, whether the charge amount has increased or decreased, the speed at which the charge amount increases, the speed at which the charge amount decreases, and the like. 
     (a) in  FIG. 14  illustrates a line of marks when the charge amount has reached “0.5” after the player P has started gazing at the object (immediately after the player P has started gazing at the object, or immediately before the lock with respect to the object is canceled). 
     (b) in  FIG. 14  illustrates a line of marks when the charge amount has reached “1” after the player P has started gazing at the object. 
     (c) in  FIG. 14  illustrates a line of marks when the charge amount has reached “1.5” after the player P has started gazing at the object. 
     (d) in  FIG. 14  illustrates a line of marks when the charge amount has reached “2” (i.e., when the object has been locked) after the player P has started gazing at the object. 
     (e) in  FIG. 14  illustrates a line of marks when the charge amount has reached “2.5” after the object has been locked. 
     (f) in  FIG. 14  illustrates a line of marks when the charge amount has reached “3” after the object has been locked. 
     (g) in  FIG. 14  illustrates a line of marks when the charge amount has reached “3.5” after the object has been locked. 
     (h) in  FIG. 14  illustrates a line of marks when the charge amount has reached “4” after the object has been locked. 
     (i) in  FIG. 14  illustrates a line of marks when the charge amount has reached “4.5” after the object has been locked. 
     (j) in  FIG. 14  illustrates a line of marks when the charge amount has reached “5” (i.e., when the lock with respect to the object has been confirmed) after the object has been locked. 
     Specifically, the animation illustrated in  FIG. 14  represents the charge amount using the number of marks that form a line. Each white arrow illustrated in  FIG. 14  indicates the direction in which a change in line of marks occurs when the player P has continuously gazed at the object for 5 seconds. When the player P has removed his/her gaze from the object, a change in the line of marks occurs in the direction opposite to the direction indicated by each white arrow since the charge amount decreases. In one embodiment of the invention, since the speed at which the charge amount decreases is lower than the speed at which the charge amount increases (see above), the speed at which the animation changes in the direction opposite to the direction indicated by each white arrow is lower than the speed at which the animation changes in the direction indicated by each white arrow (see  FIG. 14 ). 
     The game calculation section  110  distinguishes the object that has been locked from the object that is not locked. For example, the game calculation section  110  sets the enhancement level with respect to the object that has been locked ((d) to (j) in  FIG. 14 ) to be higher than the enhancement level with respect to the object that is not locked ((a) to (c) in  FIG. 14 ). In the example illustrated in  FIG. 14 , the density of the object that has been locked ((d) to (j) in  FIG. 14 ) is set to be higher than the density of the object that is not locked ((a) to (c) in  FIG. 14 ). 
     Therefore, the player P can determine whether or not the object has been locked based on the enhancement level (e.g., density) with respect to the object. 
     Note that the enhancement level with respect to the object may be adjusted using at least one of the following parameters (1) to (14), for example. 
     (1) Density of a line of marks 
     (2) Brightness of a line of marks 
     (3) Color of a line of marks 
     (4) Opacity of a line of marks 
     (5) Saturation of a line of marks 
     (6) Shape of a line of marks 
     (7) Change pattern of at least one of density, brightness, color, opacity, saturation, and shape of a line of marks 
     (8) Density of object 
     (9) Brightness of object 
     (10) Color of object 
     (11) Opacity of object 
     (12) Saturation of object 
     (13) Shape of object 
     (14) Change pattern of at least one of density, brightness, color, opacity, saturation, and shape of object 
     Although an example in which the animation that notifies the player P of the charge amount is designed so that a line of marks is placed around the outer edge of the object, and the number of marks that form a line is changed corresponding to the charge amount, has been described above, the animation may be designed so that a ring-shaped or subring-shaped gauge (indicator) (tubular gauge) is placed around the outer edge of the object, and the length of the gauge (corresponding to the position of the pointer of the gauge) is changed corresponding to the charge amount (see  FIG. 26 ). 
     When the charge amount is thus reflected in the length of the gauge, the player P can intuitively determine the magnitude of the charge amount, whether the charge amount has increased or decreased, the speed at which the charge amount increases, the speed at which the charge amount decreases, and the like. 
     The animation that notifies the player P of the charge amount may be designed so that the enhancement level with respect to the object is changed corresponding to the charge amount instead of using a line of marks or a gauge. The enhancement level with respect to the object may be adjusted using at least one of the parameters (7) to (12) (see above), for example.  FIG. 19  illustrates an example in which the density of the object is changed corresponding to the charge amount. 
     When the player P is notified of the charge amount using a change in the enhancement level with respect to the object, it is possible to provide a sufficient space around the object, and improve the degree of freedom with respect to the layout of the object. This configuration is effective when a number of objects are closely placed, for example. 
     It is desirable that the enhancement level increase as the charge amount increases. An increase in enhancement level refers to an increase in density, an increase in saturation, an increase in opacity, an increase in color change cycle (color change), and the like. 
     2-11. Combo Process 
     2-11-1. Outline of Combo Process 
     The game calculation section  110  performs (implements) a combo process when a predetermined condition has been satisfied in a state in which two or more objects have been locked. The term “combo process” used herein refers to a process that receives an eye input (i.e., an eye input that confirms a lock, or an eye input that cancels a lock) with respect to all of two or more objects that have been locked based on a common charge amount. 
     The term “common charge amount” refers to a charge amount (integrated charge amount) that is obtained by integrating the charge amounts with respect to two or more objects, the charge amount (representative charge amount) with respect to an object that represents two or more objects, a new charge amount (separate charge amount) that is set separately from the charge amounts with respect to two or more objects, or the like. The term “integrated charge amount” refers to the average value of the charge amounts with respect to two or more objects, the weighted average value of the charge amounts with respect to two or more objects, the sum of the charge amounts with respect to two or more objects, a product of the charge amounts with respect to two or more objects, or the like. An example in which the sum of the charge amounts with respect to two or more objects is used as the integrated charge amount is mainly described below (the details thereof are described later). 
     When the player P desires to implement the combo process, the player P locks one object, locks another object before the lock with respect to the one object is canceled so that two or more objects are locked at the same time, and satisfies a predetermined condition (hereinafter referred to as “combo implementation condition”). The details of the combo implementation condition are described later. 
     When the combo process has started, the player P can confirm the lock with respect to two or more objects by performing one eye input. Therefore, it suffices for the player P to perform an eye input (mainly a head turn motion in one embodiment of the invention) a reduced number of times as compared with the case where the player P performs an eye input corresponding to each of two or more objects. 
     For example, when the combo process has started in a state in which the push button objects OB 1  and OB 2  (see  FIG. 10 ) have been locked, the player P can input an instruction that confirms the lock with respect to the push button objects OB 1  and OB 2  (i.e., an instruction that designates the single-player mode and the beginner mode) to the simulation control device  100  by performing an eye input with respect to only one of the push button objects OB 1  and OB 2 . 
     2-11-2. Combo Implementation Condition 
     The combo implementation condition is described in detail below. 
     The game calculation section  110  uses a condition whereby an object included in two or more locked objects that has been locked earlier than the other(s) has been gazed at by the player P again, as the combo implementation condition. 
     An example in which the combo process is performed with respect to the push button objects OB 1  and OB 3  (see (a) in  FIG. 15 ), is described below. Although an example in which the combo process is performed with respect to two objects is described below, the combo process may be performed with respect to three or more objects. 
     The player P gazes at one (e.g., push button object OB 1 ) of the push button objects OB 1  and OB 3  for a sufficient time equal to or longer than 2 seconds to lock the push button object OB 1 . The sufficient time is longer than 2 seconds and shorter than 5 seconds, for example. 
     The player P then removes his/her gaze from the locked push button object OB 1 , and gazes at the push button object OB 3  for a time equal to or longer than 2 seconds before the lock with respect to the push button object OB 1  is canceled to lock the push button object OB 3 . The push button objects OB 1  and OB 3  are thus locked. 
     The player P then removes his/her gaze from the locked push button object OB 3 , and gazes at the push button object OB 1  again before the lock with respect to the push button objects OB 1  and OB 3  is canceled. The curve indicated by the thick dotted arrow illustrated in  FIG. 15  (see (a)) indicates an example of the moving path of the line of sight of the player P. The combo implementation condition is thus satisfied with respect to the push button objects OB 1  and OB 3 , and the combo process is performed. 
     When one push button object has been locked, the lock with respect to the one push button object is not immediately canceled even when the player P has removed his/her gaze from the one push button object. Therefore, the player P can lock another object before the lock with respect to the one push button object is canceled. Accordingly, the player P can easily satisfy the combo implementation condition. 
     Note that the game calculation section  110  may decrease the speed at which the charge amount with respect to the object that is not locked approaches the first threshold value Th1, as the charge amount (or the total charge amount) with respect to the object that has been locked increases. 
     In this case, since the object that is not locked is not easily locked when the player P has gazed at the object that has been locked for a long time, the player P can determine whether or not to implement the combo process, and determine whether or not to lock two or more objects by gazing at the object to be locked for a long time when it is desired not to implement the combo process, or increase the number of objects to be locked, and gazing at the object to be locked for a short time when it is desired to implement the combo process, or increase the number of objects to be locked, for example. 
     2-11-3. Visual Effect Before and after Implementation of Combo Process 
     A visual effect before and after the implementation of the combo process is described below. 
     The game calculation section  110  changes the visual effect with respect to two or more locked objects (OB 1  and OB 3 ) depending on whether or not the combo process has been started. 
     (a) in  FIG. 15  illustrates an example of the visual effect before the combo process is started, and (b) in  FIG. 15  illustrates an example of the visual effect after the combo process has been started.  FIG. 15  ((a) and (b)) illustrates an example in which a line of marks is used as the visual effect. Note that marks forming a line that are being displayed are indicated by the solid line, and marks forming a line that are not being displayed are indicated by the dotted line. The dotted lines need not necessarily displayed. The curve indicated by the thick dotted arrow illustrated in  FIG. 15  (see (a)) indicates an example of the moving path of the line of sight of the player P. 
     For example, the game calculation section  110  displays two lines of marks OB 11  and OB 13  that respectively enclose the push button objects OB 1  and OB 3  that have been locked (see (a) in  FIG. 15 ) before the combo process is started, and displays one integrated line of marks OB 20  obtained by integrating the line of marks OB 11  and OB 13  (see (b) in  FIG. 15 ) after the combo process has been started. 
     The integrated line of marks OB 20  is a line of marks that encloses the push button objects OB 1  and OB 3  that are subjected to the combo process. The number of displayed marks included in the integrated line of marks OB 20  represents the charge amount used during the combo process (hereinafter referred to as “integrated charge amount” (i.e., measured value after integration)). The details of the integrated charge amount are described later. 
     Therefore, the player P can determine whether or not the combo process with respect to the locked push button objects OB 1  and OB 3  has been started based on the visual effect displayed with respect to the locked push button objects OB 1  and OB 3  (e.g., whether or not the lines of marks OB 11  and OB 13  have been integrated into one line of marks OB 20  (see  FIG. 15 )). 
     2-11-4. Integration of Charge Amounts During Combo Process 
     The combo process (integration of charge amounts) performed by the game calculation section  110  is described below. 
     The game calculation section  110  performs the combo process that integrates the charge amounts of two or more objects (e.g., push button objects OB 1  and OB 3 ) that are subjected to the combo process. In this case, the remaining charge amounts are also integrated. The term “remaining charge amount” used herein refers to the charge amount required until the lock with respect to the object is confirmed (threshold value Th2). The remaining charge amount decreases as the charge amount increases, and increases as the charge amount decreases. The charge amount obtained by integration is hereinafter referred to as “integrated charge amount”, and the remaining charge amount obtained by integration is hereinafter referred to as “integrated remaining charge amount”. 
     (a) in  FIG. 16  is a conceptual diagram illustrating the charge amount immediately before integration, and (b) in  FIG. 16  is a conceptual diagram illustrating the integrated charge amount. In  FIG. 16  ((a) and (b)), the charge amount is indicated by a solid line (block), and the remaining charge amount is indicated by a dotted line (block). 
     For example, the game calculation section  110  applies the charge amounts (and the remaining charge amounts) of the push button objects OB 1  and OB 3  that have been locked, to the push button objects OB 1  and OB 3  before the combo process is started (see (a) in  FIG. 16 ). The game calculation section  110  applies the integrated charge amount (and the integrated remaining charge amount) obtained by integrating the charge amounts (and the remaining charge amounts) of the push button objects OB 1  and OB 3 , to the push button objects OB 1  and OB 3  after the combo process has been started (see (b) in  FIG. 16 ). 
     The expression “application of the charge amount” and the like mean that the state (“locked state”, “unlocked state”, and “confirmed state”) is controlled based on the charge amount (and the remaining charge amount). 
     The ratio of the integrated charge amount to the integrated remaining charge amount ((b) in  FIG. 16 ) is the same as the ratio of the total charge amount immediately before integration to the total remaining charge amount immediately before integration ((a) in  FIG. 16 ). 
     The game calculation section  110  increases or decreases the integrated charge amount corresponding to the gaze time of the player P with respect to the push button object OB 1  that is one of the push button objects OB 1  and OB 3  that have been subjected to the combo process, and has been locked earlier than the other. 
     The game calculation section  110  does not reflect the gaze time of the player P with respect to the push button object OB 3  that is one of the push button objects OB 1  and OB 3  that have been subjected to the combo process, and has been locked later than the other, in the integrated charge amount. 
     Therefore, the player P can control the state of the push button objects OB 1  and OB 3  that have been subjected to the combo process between the locked state, the unlocked state, and the confirmed state by merely controlling the gaze time with respect to the push button object OB 1  that has been locked earlier than the push button object OB 3 , after the combo process has been started. Since the charge amount before the combo process is started is used as the charge amount after the combo process has been started, it is possible to effectively utilize the gaze motion of the player P performed before the combo process is started. 
     In one embodiment of the invention, the integrated charge amount and the integrated remaining charge amount are set to have a size corresponding to the number of objects that have been subjected to the combo process (see (b) in  FIG. 16 ). In this case, the gaze time required for the lock with respect to the object to be confirmed increases, and the gaze time required for the lock with respect to the object to be canceled increases as the number of objects that have been subjected to the combo process increases. The scale of the charge amount can be increased by increasing the second threshold value Th2, for example. Note that the scale of the charge amount can also be increased by decreasing the speed at which the charge amount increases and the speed at which the charge amount decreases instead of increasing the second threshold value Th2. 
     Although an example in which the game calculation section  110  reflects only the gaze time with respect to the push button object OB 1  that is one of the push button objects OB 1  and OB 3  that have been subjected to the combo process, and has been locked earlier than the other, in the integrated charge amount (and the remaining charge amount), has been described above, the gaze time with respect to two or more push button objects that have been subjected to the combo process may be reflected in the integrated charge amount (and the remaining charge amount). Specifically, the game calculation section  110  may increase the integrated charge amount when the player P has gazed at at least one of two or more objects that have been subjected to the combo process, and may decrease the integrated charge amount when the player P has removed his/her gaze from all of two or more objects that have been subjected to the combo process. In such a case, the player P can increase the integrated charge amount by gazing at an arbitrary object among two or more objects that have been subjected to the combo process, and decrease the integrated charge amount by removing his/her gaze from all of the two or more objects. 
     2-11-5. Partial Cancelation Process During Combo Process 
     When the player P has performed the following actions (a) to (c) on a specific object that is included in two or more objects that have been subjected to the combo process within a sufficiently short predetermined time, the game calculation section  110  excludes the specific object from the target of the combo process. The sufficiently short predetermined time is equal to or shorter than 2 seconds, for example. 
     (a) The player P gazes at the specific object. 
     (b) The player P removes his/her gaze from the specific object. 
     (c) The player P gazes at the specific object. 
     The process that excludes an object that is included in two or more objects that have been subjected to the combo process from the target of the combo process is hereinafter referred to as “partial cancelation process”. The process that excludes an object from the target of the combo process is equivalent to a process that cancels the lock with respect to the object, for example. 
     An example in which the combo process is performed with respect to the push button objects OB 1  and OB 3  (see (a) in  FIG. 17 ), and the player P cancels the lock with respect to the push button object OB 3 , is described below. Although an example in which the combo process is performed with respect to two objects is described below, the following description is similarly applied to the case where the combo process is performed with respect to three or more objects. 
     The player P can cancel the lock with respect to the push button object OB 3  while maintaining the lock with respect to the push button object OB 1  by gazing at the push button object OB 3 , removing his/her gaze from the push button object OB 3 , and gazing at the push button object OB 1  again within the sufficiently short predetermined time. 
     2-11-6. Visual Effect Before and after Implementation of Partial Cancelation Process 
     A visual effect before and after the implementation of the partial cancelation process is described below. 
     The game calculation section  110  changes the visual effect applied to the objects OB 1  and OB 3  that have been subjected to the combo process depending on whether or not the partial cancelation process has been performed. 
     (a) in  FIG. 17  illustrates an example of the visual effect before the partial cancelation process is performed, and (b) in  FIG. 17  illustrates an example of the visual effect after the partial cancelation process has been performed.  FIG. 17  ((a) and (b)) illustrates an example in which a line of marks is used as the visual effect. Note that marks forming a line that are being displayed are indicated by the solid line, and marks forming a line that are not being displayed are indicated by the dotted line. The dotted lines need not necessarily displayed. The curve indicated by the thick dotted arrow illustrated in  FIG. 17  (see (a)) indicates an example of the moving path of the line of sight of the player P. 
     For example, the game calculation section  110  displays an integrated line of marks OB 20  that encloses the push button objects OB 1  and OB 3  that have been subjected to the combo process locked (see (a) in  FIG. 17 ) before the partial cancelation process is performed, and displays two line of marks OB 11  and OB 12  obtained by dividing the integrated line of marks OB 20  (see (b) in  FIG. 17 ) after the partial cancelation process has been performed. 
     The line of marks OB 11  is a line of marks that encloses the push button object OB 1  that has been locked, and the number of marks of the line of marks OB 11  that are being displayed represents the charge amount applied to the push button object OB 1  that has been locked. 
     The line of marks OB 13  is a line of marks that encloses the push button object OB 3  that has been unlocked, and the number of marks of the line of marks OB 13  that are being displayed represents the charge amount applied to the push button object OB 3  that has been unlocked. 
     Therefore, the player P can determine whether or not the partial cancelation process has been performed with respect to the push button objects OB 1  and OB 3  (that have been subjected to the combo process) based on the visual effect displayed with respect to the push button objects OB 1  and OB 3  (e.g., whether or not the integrated line of marks OB 20  has been divided into the lines of marks OB 11  and OB 13  (see  FIG. 15 )). 
     2-11-7. Separation of Charge Amount During Partial Cancelation Process 
     The partial cancelation process (separation of charge amount) performed by the game calculation section  110  is described below. 
     When the game calculation section  110  has started the partial cancelation process, the game calculation section  110  separates the charge amount with respect to the object that has been unlocked from the integrated charge amount. In this case, the remaining charge amount is also separated. The term “remaining charge amount” used herein refers to the charge amount required until the lock with respect to the object is confirmed (threshold value Th2). The remaining charge amount decreases as the charge amount increases, and increases as the charge amount decreases. 
     (a) in  FIG. 18  is a conceptual diagram illustrating the charge amount immediately before separation, and (b) in  FIG. 18  is a conceptual diagram illustrating the charge amount immediately after separation. In  FIG. 18  ((a) and (b)), the charge amount is indicated by a solid line (block), and the remaining charge amount is indicated by a dotted line (block). 
     For example, the game calculation section  110  applies the integrated charge amount (and the integrated remaining charge amount) to the push button objects OB 1  and OB 3  before the partial cancelation process is started (see (a) in  FIG. 18 ). After the partial cancelation process has been started, the game calculation section  110  applies the integrated charge amount (and the integrated remaining charge amount) to the push button object OB 1  that has been locked, and applies the charge amount (and the remaining charge amount) of the push button object OB 3  to the push button object OB 3  that has been unlocked (see (b) in  FIG. 18 ). 
     The expression “application of the charge amount” and the like mean that the state (“locked state”, “unlocked state”, and “confirmed state”) is controlled based on the charge amount (and the remaining charge amount). 
     The integrated charge amount and the integrated remaining charge amount immediately before the partial cancelation process is performed ((a) in  FIG. 18 ) is applied to the charge amount and the remaining charge amount of the push button object OB 1  that has been locked (see the upper side of (b) in  FIG. 18 ). In the example illustrated in (b) in  FIG. 18  in which the number of objects that have been locked is “1”, the combo process is automatically terminated. When the number of objects that have been locked is “2” or more, the combo process is continued, and the integrated charge amount and the integrated remaining charge amount are applied during the combo process. 
     The integrated charge amount and the integrated remaining charge amount immediately before the partial cancelation process is performed ((a) in  FIG. 18 ) is not applied to the charge amount and the remaining charge amount of the push button object OB 3  that has been unlocked (see the lower side of (b) in  FIG. 18 ). The charge amount of the push button object OB 3  that has been unlocked (see the lower side of (b) in  FIG. 18 ) is set to “0” at the cancelation timing. 
     The game calculation section  110  increases or decreases the charge amount of the push button object OB 1  corresponding to the gaze time of the player P with respect to the push button object OB 1  that has been locked. When the number of objects that have been locked is “2” or more, the combo process is continued, and the integrated charge amount is increased or decreased corresponding to the gaze time with respect to the object that has been locked at the earliest timing. 
     The game calculation section  110  increases or decreases the charge amount of the push button object OB 3  corresponding to the gaze time of the player P with respect to the push button object OB 3  that has been unlocked. 
     Therefore, the player P can cancel only the lock with respect to the push button object OB 3  while maintaining the lock with respect to the push button object OB 1  by gazing at the specific push button object OB 3  again after the combo process has been started. Since the charge amount before cancelation is applied to the charge amount after cancelation, it is possible to effectively utilize the gaze motion of the player P performed before cancelation. 
     In one embodiment of the invention, the charge amount and the remaining charge amount are set to have a size corresponding to the number of objects to which the charge amount is applied (see (b) in  FIG. 18 ). In this case, the gaze time required for the lock with respect to the object to be confirmed decreases, and the gaze time required for the lock with respect to the object to be canceled decreases as the number of objects to which the charge amount is applied decreases. The scale of the charge amount can be decreased by decreasing the second threshold value Th2, for example. Note that the scale of the charge amount can also be decreased by increasing the speed at which the charge amount increases and the speed at which the charge amount decreases instead of decreasing the second threshold value Th2. 
     Although an example in which the partial cancelation process is performed during the combo process has been described above, the partial cancelation process may be performed when the combo process is not performed. Specifically, when the player P has performed the actions (a) to (c) on one object that has been locked within the sufficiently short predetermined time, the game calculation section  110  may determine that the lock with respect to the object has been canceled, and reset the charge amount with respect to the object to “0”. 
     2-12. Control of Degree of Lock 
     The game calculation section  110  sets the determination standard for determining that the player P is gazing at the object that has been locked (determination standard for the gaze determination process on the object that has been locked) to be less severe than the determination standard for determining that the player P is gazing at the object that is not locked (determination standard for the gaze determination process on the object that is not locked). The degree of lock with respect to the object can be increased by setting the determination standard to be less severe. Note that the term “degree of lock” used herein refers to the possibility that the lock is canceled. 
     Specifically, the player P cannot lock the object that is not locked without gazing at the object, but can easily maintain the lock with respect to the object (that has been locked) without gazing at the object. 
     The following description is given taking the push button object OB 1  (see  FIGS. 12A and 12B ) as an example. Note that the following description is similarly applied to other objects. 
     The game calculation section  110  determines whether or not the player P is gazing at the push button object OB 1  based on whether or not at least part of the hit area OBHA that is set to the push button object OB 1  and at least part of the line-of-sight area LSA overlap each other (see above). The game calculation section  110  determines that the player P is gazing at the push button object OB 1  when at least part of the hit area OBHA that is set to the push button object OB 1  and at least part of the line-of-sight area LSA overlap each other, and determines that the player P is not gazing at the push button object OB 1  when at least part of the hit area OBHA that is set to the push button object OB 1  and at least part of the line-of-sight area LSA do not overlap each other. 
     The game calculation section  110  increases the size of at least one of the hit area OBHA that is set to the push button object OB 1  and the line-of-sight area LSA so that the determination standard for the gaze determination process on the push button object OB 1  becomes less severe. 
     An example in which the size of the hit area OBHA is variable, and the size of the line-of-sight area LSA is fixed, is described below for convenience of explanation taking account of a situation in which the determination standard is controlled on an object basis. The size adjustment may be performed continuously, or may be performed stepwise as described below. 
     The game calculation section  110  sets the determination standard for the gaze determination process on the push button object OB 1  to be less severe as the charge amount with respect to the push button object OB 1  increases. Specifically, the game calculation section  110  increases the size of the hit area OBHA that is set to the push button object OB 1  as the charge amount with respect to the push button object OB 1  increases. 
     For example, the game calculation section  110  sets the size of the hit area OBHA that is set to the push button object OB 1  to a normal size illustrated in  FIG. 20A  when the push button object OB 1  is not locked, sets the size of the hit area OBHA that is set to the push button object OB 1  to a medium size illustrated in  FIG. 20B  when the charge amount with respect to the push button object OB 1  is less than a predetermined value (e.g., “3”) between the threshold values Th1 and Th2, and sets the size of the hit area OBHA that is set to the push button object OB 1  to a large size illustrated in  FIG. 20C  when the charge amount with respect to the push button object OB 1  is equal to or larger than the predetermined value (e.g., “3”). 
     In this case, the degree of lock with respect to the push button object OB 1  increases as the gaze time of the player P with respect to the push button object OB 1  increases. 
     The game calculation section  110  sets the enlargement ratio of the size of the hit area OBHA in the rightward-leftward direction with respect to the player P to be larger than the enlargement ratio of the size of the hit area OBHA in the upward-downward direction with respect to the player P (see  FIGS. 20A, 20B, and 20C ). 
     According to this configuration, the degree of lock with respect to the push button object OB 1  in the rightward-leftward direction is relatively higher than the degree of lock with respect to the push button object OB 1  in the upward-downward direction. 
     Since the line-of-sight direction of the player P (that is determined by the attitude of the head, for example) is normally unstable in the rightward-leftward direction as compared with the upward-downward direction, it is considered that an erroneous eye input (e.g., a situation in which the player P unintentionally cancels the lock with respect to the object) can be reduced without impairing operability when the degree of lock in the rightward-leftward direction is set to be relatively higher than the degree of lock in the upward-downward direction. 
     2-13. Supplementation with Respect to Control of Degree of Lock 
     Although an example in which the game calculation section  110  controls the degree of lock by adjusting (spatially adjusting) the size of the line-of-sight area LSA, has been described above, the degree of lock may be controlled by adjusting (temporally adjusting) the speed at which the charge amount decreases, or may be controlled by combining the spatial adjustment and the temporal adjustment. 
     For example, the game calculation section  110  may decrease the speed at which the charge amount approaches the third threshold value Th3 as the charge amount with respect to the push button object OB 1  increases. Note that the speed at which the charge amount approaches the third threshold value Th3 may be adjusted by adjusting the threshold value, or adjusting the speed at which the charge amount increases or decreases (see above). 
     In this case, the player P cannot lock the object without gazing at the object for a time equal to or longer than a specific time (e.g., when the player P hesitates to lock the object). On the other hand, the lock with respect to the object is maintained unless the player P removes his/her gaze from the locked object for a long time. 
     Although an example in which the game calculation section  110  adjusts (spatially adjusts) the size of the hit area OBHA within the virtual three-dimensional space OBS, has been described above, the game calculation section  110  may adjust the size of the line-of-sight area LSA within the virtual three-dimensional space OBS, or may adjust the size of both the hit area OBHA and the line-of-sight area LSA. In this case, the game calculation section  110  may control the size of the line-of-sight area LSA corresponding to the charge amount with respect to the object that is situated nearest to the line-of-sight area LSA, for example. 
     2-14. Flow of Eye Input Reception Process 
     The flow of the eye input reception process that is performed by the game calculation section  110  is described below with reference to  FIG. 21 . 
     The flow of the eye input reception process is appropriately performed during, before, after the fear of heights experience game ( FIGS. 8 and 9 ), when it is necessary for the game system  1  to receive one or more instructions from the player P, for example. The flow of the eye input reception process is performed corresponding to each object (object that can be designated by the player P) that is subjected to the eye input reception process. The selection screen ( FIG. 10 ) is used as an example. 
     For example, when the mode of the game process has been set to the single-player mode or the two-player mode, the push button objects OB 1  and OB 2  displayed within the selection screen ( FIG. 10 ) are not subjected to the eye input reception process, and the push button objects OB 3  and OB 4  displayed within the selection screen ( FIG. 10 ) are subjected to the eye input reception process. 
     When the mode of the game process has been set to the beginner mode or the advanced mode, the push button objects OB 3  and OB 4  displayed within the selection screen ( FIG. 10 ) are not subjected to the eye input reception process, and the push button objects OB 1  and OB 2  displayed within the selection screen ( FIG. 10 ) are subjected to the eye input reception process. 
     The flow illustrated in  FIG. 21  is described below. 
     The game calculation section  110  displays the object that is subjected to the eye input reception process (step S 11 ). The object is displayed by placing the object in the virtual three-dimensional space. 
     The game calculation section  110  performs the gaze determination process with respect to the object (step S 13 ). When the player P is gazing at the object (step S 13 Y), the game calculation section  110  measures the gaze time with respect to the object (step S 15 ). When the player P is not gazing at the object (step S 13 N), the game calculation section  110  repeats the gaze determination process (step S 13 ). When the gaze time is measured, the charge amount is increased corresponding to the time in which the player P gazes at the object, and is decreased corresponding to the time in which the player P removes his/her gaze from the object. When the game calculation section  110  has started to measure the gaze time (step S 15 ), the game calculation section  110  performs the gaze determination process, and cyclically repeats the process that increases or decreases the charge amount corresponding to the result of the gaze determination process until the charge amount is reset. The repetition cycle is set to be identical to the frame cycle, for example. 
     The game calculation section  110  repeats the process that determines whether or not the charge amount has reached the first threshold value Th1 (=2) (step S 17 ), and the process that determines whether or not the player P has removed his/her gaze from the object (step S 19 ) as long as the charge amount does not exceed the first threshold value Th1 (=2) (step S 17 N), and the player P has not removed his/her gaze from the object (step S 19 N). 
     When the player P has removed his/her gaze from the object (step S 19 Y) before the charge amount exceeds the first threshold value Th1 (=2) (step S 17 N), the game calculation section  110  resets the charge amount with respect to the object to “0” (step S 20 ), and performs the gaze determination process (step S 13 ). 
     When the charge amount has exceeded the first threshold value Th1 (=2) (step S 17 Y), the game calculation section  110  determines that the object has been locked (step S 21 ). The object is thus locked. 
     When the object has been locked (step S 21 ), the game calculation section  110  repeats the process that determines whether or not the charge amount has reached the second threshold value Th2 (=5) (step S 23 ), and the process that determines whether or not the charge amount has decreased to the third threshold value Th3 (=0) (step S 25 ) as long as the charge amount has not reached the second threshold value Th2 (=5) (step S 23 N), and the charge amount has not decreased to the third threshold value Th3 (=0) (step S 25 N). 
     When the charge amount has decreased to the third threshold value Th3 (=0) (step S 25 Y), the game calculation section  110  determines that the lock with respect to the object has been canceled (step S 29 ), and performs the gaze determination process (step S 13 ). 
     When the charge amount has reached the second threshold value Th2 (=5) (step S 23 Y), the game calculation section  110  determines that the lock with respect to the object has been confirmed (step S 27 ), resets the charge amount to “0” (step S 30 ), and terminates the process (flow). The lock with respect to the object is thus confirmed. 
     When the charge amount with respect to the object has exceeded the first threshold value Th1 (step S 17 Y), or when the player P has removed his/her gaze from the object (step S 19 Y) before the charge amount reaches the second threshold value, or when the charge amount has reached the second threshold value Th2 (step S 23 Y), or when the charge amount has decreased to the third threshold value Th3 (step S 25 Y), the game calculation section  110  performs the process that changes the visual effect applied to the object (see  FIG. 14 , for example). 
     2-15. Flow of Combo Process 
     The flow of the combo process is described below with reference to  FIG. 22 . 
     The flow of the combo process is performed in parallel with the flow of the eye input reception process ( FIG. 21 ). The flow of the combo process is not performed on an object basis, but is performed corresponding to a plurality of objects that can be designated by the player P. 
     The game calculation section  110  repeats the process that determines whether or not two or more objects have been locked (step S 31 ) as long as two or more objects have not been locked (step S 31 N). 
     When two or more objects have been locked (step S 31 Y), the game calculation section  110  performs a step S 33 . 
     The game calculation section  110  determines whether or not the player P has gazed at the object that is one of the two or more objects have been locked, and has been locked earlier than the other (i.e., whether or not the combo implementation condition has been satisfied) (step S 33 ). When the combo implementation condition has not been satisfied (step S 33 N), the game calculation section  110  performs the lock determination process (step S 31 ). When the combo implementation condition has been satisfied (step S 33 Y), the game calculation section  110  performs the combo process (steps S 35  to S 43 ). 
     During the combo process (steps S 35  to S 43 ), the game calculation section  110  performs an integration process that integrates the charge amounts with respect to the two or more objects that have been locked (step S 35 ). The charge amounts with respect to the two or more objects are thus integrated into the integrated charge amount. 
     The game calculation section  110  repeats the process that determines whether or not the integrated charge amount has reached “0” (step S 37 ), and the start condition determination process (step S 41 ) as long as the integrated charge amount has not reached “0” (step S 37 N), and the start condition with respect to the partial cancelation process (i.e., a condition whereby the player P has gazed at one of the two or more objects (that have been locked) again) has not been satisfied (step S 41 N). 
     When the start condition has been satisfied (step S 41 Y), the game calculation section  110  performs the partial cancelation process that separates the charge amount with respect to the object that has been gazed at again from the integrated charge amount (step S 43 ). The partial cancelation process (step S 43 ) cancels the lock with respect to the object that has been gazed at again, and sets the charge amount with respect to the object that has been gazed at again to “0”. 
     When the integrated charge amount has reached “0” (step S 37 Y), the game calculation section  110  performs a separation process that separates the charge amounts with respect to all of the objects that have been locked (step S 39 ). The separation process (step S 39 ) sets the charge amounts with respect to all of the objects that have been locked to “0”. After the game calculation section  110  has performed the separation process (step S 39 ), the game calculation section  110  terminates the process (flow). 
     When the combo process has been started (step S 53 ), or when the partial cancelation process has been performed (step S 43 ), the game calculation section  110  performs the process that changes the visual effect applied to the object (see  FIGS. 15 and 17 , for example). 
     2-16. Flow of Degree-of-Lock Control Process 
     The flow of the degree-of-lock control process is described below with reference to  FIG. 23 . 
     The flow of the degree-of-lock control process is performed in parallel with the flow of the eye input reception process ( FIG. 21 ). The flow of the degree-of-lock control process is repeatedly performed corresponding to each object that has been subjected to the eye input reception process. 
     The game calculation section  110  determines whether or not the object has been locked (step S 51 ). 
     When the object has not been locked (step S 51 N), the game calculation section  110  sets the size of the hit area to “normal” (step S 53 ), sets the speed at which the charge amount decreases when the player P has removed his/her gaze from the object to “high” (step S 55 ), and terminates the process (flow). Note that the speed at which the charge amount decreases is lower than the speed at which the charge amount increases. 
     When the object has been locked (step S 51 Y), the game calculation section  110  determines whether or not the charge amount with respect to the object is equal to or larger than “3” (step S 57 ). 
     When the charge amount with respect to the object is not equal to or larger than “3” (step S 57 N), the game calculation section  110  sets the size of the hit area to “medium” (step S 59 ), sets the speed at which the charge amount decreases when the player P has removed his/her gaze from the object to “medium” (step S 61 ), and terminates the process (flow). 
     When the charge amount with respect to the object is equal to or larger than “3” (step S 57 Y), the game calculation section  110  sets the size of the hit area to “large” (step S 65 ), sets the speed at which the charge amount decreases when the player P has removed his/her gaze from the object to “low” (step S 67 ), and terminates the process (flow). 
     Note that the game calculation section  110  sets the enlargement ratio of the size of the hit area in the rightward-leftward direction with respect to the player P to be larger than the enlargement ratio of the size of the hit area in the upward-downward direction with respect to the player P (see  FIGS. 20A to 20C ). The above steps may be appropriately changed in order. 
     2-17. Application Example of Eye Input 
     For example, a mode in which the player P is given a mission to catch (rescue) a cat effect object  80 , and a mode in which the player P is not given a mission to catch (rescue) the cat effect object  80 , are provided to the fear of heights experience game according to one embodiment of the invention. 
     In this case, the game calculation section  110  places a character image “Do you catch the cat?”, a cat object, and push button objects OB 5  and OB 6  within the virtual three-dimensional space OBS (see  FIG. 24 ) when the game starts, for example. 
     The push button object OB 5  is a push button object for the player P to input “YES” to the simulation control device  100 , and the push button object OB 6  is a push button object for the player P to input “NO” to the simulation control device  100 . 
     For example, when the player P has locked the push button object OB 6  that corresponds to “NO”, and confirmed the lock with respect to the push button object OB 6  by performing an eye input (see  FIG. 25 ), the game calculation section  110  displays an animation in which the cat object moves away from the virtual moving path within the virtual three-dimensional space OBS through the object space setting section  111 . The game calculation section  110  causes the cat effect object  80  to move away from the end point of the actual moving path R through the effect control processing section  114 . 
     For example, when the player P has locked the push button object OB 5  that corresponds to “YES”, and confirmed the lock with respect to the push button object OB 5  by performing an eye input, the game calculation section  110  displays an animation in which the cat object runs away toward the end point of the virtual moving path within the virtual three-dimensional space OBS through the object space setting section  111 . The game calculation section  110  places the cat effect object  80  at the end point of the actual moving path R through the effect control processing section  114 . 
     Although an example in which the object that can be designated by the player P by means of an eye input is the push button object (icon), has been described above, the object may be a character (enemy character, ally character, animal character, or player&#39;s avatar), an item (treasure box or card), or the like, or may be part of a character or an item. 
     2-18. Advantageous Effects of Embodiments (1) 
     As described above, the simulation control device  100  includes the display processing section  110 A that displays the virtual three-dimensional space on the HMD  20 , the measurement section  110 B that measures the gaze time of the player P with respect to the object placed within the virtual three-dimensional space, and decreases the charge amount corresponding to the time in which the player P does not gaze at the object when the player P has removed his/her gaze from the object, the reception section  110 C that determines that the object has been locked when the charge amount has reached the first threshold value Th1, determines that the lock with respect to the object has been confirmed when the charge amount has reached the second threshold value Th2 that is larger than the first threshold value Th1, and determines that the lock with respect to the object has been canceled when the charge amount has decreased to the third threshold value Th3, the notification section  110 D that notifies the player P of the magnitude of the charge amount, and the execution section  110 E that performs a predetermined process linked to the object when it has been determined that the lock with respect to the object has been confirmed. 
     Therefore, the player P can lock the object by gazing at the object until the charge amount with respect to the object reaches the first threshold value Th1. On the other hand, the player P can maintain the object to be in an unlocked state by gazing at (glancing) the object so that the charge amount with respect to the object does not exceed the first threshold value Th1. 
     When the object has been locked, the lock with respect to the object is maintained until the charge amount decreases to the third threshold value Th3 even when the player P has removed his/her gaze from the object. Therefore, the player P need not fix the line of sight in order to maintain the lock with respect to the object. The player P can cancel the lock with respect to the object by removing his/her gaze from the object until the charge amount decreases to the third threshold value Th3. 
     After the object has been locked, the player P can input an instruction that confirms the lock with respect to the object to the simulation control device  100  by merely gazing at the object until the charge amount reaches the second threshold value Th2. On the other hand, the player P can maintain the lock with respect to the object by gazing at the object, or removing his/her gaze from the object so that the charge amount fall within the range between the second threshold value Th2 and the third threshold value Th3. 
     Therefore, the player P can arbitrarily control the state of the object between the locked state, the unlocked state, and the confirmed state by performing an eye input. The term “eye input” refers to an input performed by the player P when the player P has input an instruction to the simulation control device  100  by gazing at the object. 
     Since the game calculation section  110  that functions as the notification section  110 D notifies the player P of the charge amount, the player P can determine the object at which the player P gazes, and determine whether or not the player P is gazing at the desired object. The player P can also determine the remaining time until the object is locked, the remaining time until the lock with respect to the object is canceled, and the gaze time required for the lock with respect to the object to be confirmed, based on the charge amount. 
     Therefore, the simulation control device  100  allows the player P to comfortably perform an eye input. 
     2-19. Advantageous Effects of Embodiments (2) 
     When the combo implementation condition has been satisfied in a state in which two or more objects have been selected, the game calculation section  110  that functions as the reception section  110 C performs the combo process that determines at least whether or not the lock with respect to the two or more objects that have been selected has been confirmed or canceled based on the common charge amount. 
     When the combo process has started, the player P can perform an input with respect to two or more objects that have been locked by performing a common eye input. Therefore, the player P can conveniently perform an eye input (mainly a head turn motion in one embodiment of the invention) as compared with the case where the player P performs an eye input corresponding to each of two or more objects. 
     2-20. Advantageous Effects of Embodiments (3) 
     The game calculation section  110  that functions as the measurement section  110 B sets the determination standard for determining that the player P is gazing at the object that has been locked to be less severe than the determination standard for determining that the player P is gazing at the object that is not locked. 
     Therefore, the player P cannot lock the object that is not locked without gazing at the object, but can easily maintain the lock with respect to the object (that has been locked) without gazing at the object. 
     For example, when the determination standard is adjusted by spatial adjustment (see above), the player P cannot lock the object without gazing at the object at a position around the center of the object (e.g., when the player P hesitates to lock the object). On the other hand, the lock with respect to the object is maintained unless the player P moves his/her gaze to a position away from the center of the object. 
     For example, when the determination standard is adjusted by temporal adjustment (see above), the player P cannot lock the object without gazing at the object for a time equal to or longer than a specific time (e.g., when the player P hesitates to lock the object). On the other hand, the lock with respect to the object is maintained unless the player P removes his/her gaze from the locked object for a long time. 
     Therefore, the simulation control device  100  can reduce a situation in which the player P performs an erroneous eye input (e.g., cancellation of the lock with respect to the object due to unintentional head shake) after the object has been locked. 
     3. Modifications 
     3-1. Motion Sensor 
     Although the game system  1  has been described above taking an example in which the imaging camera  70  is used to detect the position and the attitude of the head of the player P, a motion sensor provided to the HMD  20  may be used, or the imaging camera  70  and a motion sensor may be used in combination. An acceleration sensor, an angular velocity sensor (gyro sensor), or the like may be used as the motion sensor. A motion sensor is suitable for accurately detecting the motion of a moving object (e.g., the head of the player P) that changes in attitude. 
     3-2. Detection of Line of Sight 
     Although the embodiments have been described above taking an example in which the game calculation section  110  sets the motion (position and attitude) of the display area DS and the line-of-sight area LSA within the virtual three-dimensional space OBS to follow the motion (position and attitude) of the head of the player P, the game calculation section  110  sets the motion (position and attitude) of the display area DS and the line-of-sight area LSA within the virtual three-dimensional space OBS to follow the motion (position and attitude) of the eyeball of the player P. 
     Alternatively, the game calculation section  110  may set the motion (position and attitude) of the display area DS and the line-of-sight area LSA within the virtual three-dimensional space OBS to follow the motion of the head and the eyeball of the player P. 
     In this case, the game calculation section  110  may set the motion of the display area DS within the virtual three-dimensional space OBS to follow the motion of the head of player P, and set the motion of the line-of-sight area LSA within the display area DS to follow the motion of the eyeball of player P. 
     The motion of the eyeball of the player P may be detected by providing a line-of-sight sensor that detects the line-of-sight direction of the player P to the HMD  20 , and causing the processing section  101  (game calculation section  110 ) to receive the output from the line-of-sight sensor through the communication section  196  and the communication control section  120 , for example. 
     At least one of the following line-of-sight sensors (1) and (2) may be used as the line-of-sight sensor, for example. 
     (1) A line-of-sight sensor that includes a camera that captures at least one eye of the player P, and a processing section that detects the position of the pupil of the player P (i.e., information that represents the direction of the visual axis) based on the image captured by the camera. 
     (2) A line-of-sight sensor that includes a light-emitting section that emits detection light (e.g., infrared light) toward at least one eye of the player P, a detection section that detects the quantity of reflected light (detection light) from the retina of the eyeball, and a processing section that detects the direction of the visual axis of the eyeball based on the output from the detection section. 
     When the line-of-sight sensor is used, it is desirable to perform a calibration process on the line-of-sight sensor corresponding to each player P. The calibration process performed on the line-of-sight sensor is a process for accurately detecting the line-of-sight direction of the player P independently of an individual variation in the position of the eyeball of each player P, an individual variation in the size of the pupil of each player P, a variation in the attitude of the HMD  20 , and the like. For example, the calibration process allows the player P who wears the HMD  20  to gaze at several known directions, and adjusts the parameters of the processing section based on the output from the line-of-sight sensor. 
     3-3. Virtual Mechanical Switch 
     Although the embodiments have been described above taking an example in which the game calculation section  110  changes the enhancement level with respect to the object in order to notify the player P of the charge amount (see  FIGS. 14 and 19 , for example), the enhancement level may be changed using another method. For example, the outward appearance of the object may be changed. 
     For example, the game calculation section  110  may utilize a virtual push button (virtual mechanical switch) as an object that can be designated by the player P, and change the amount of depression of the virtual mechanical switch corresponding to the charge amount. In this case, the game calculation section  110  increases the amount of depression of the virtual mechanical switch as the charge amount increases so that the player P can press the virtual mechanical switch by gazing at the virtual mechanical switch. The virtual mechanical switch is locked when the virtual mechanical switch has been pressed (gazed at), and the lock with respect to the virtual mechanical switch is confirmed when the virtual mechanical switch has been further pressed (gazed at). 
     3-4. Selection 
     The embodiments have been described above taking an example in which the game calculation section  110  starts measurement at a timing at which the player has gazed at one object, determines that the object has been selected (locked) at a timing at which the charge amount has exceeded the first threshold value Th1, determines that the lock with respect to the object has been confirmed at a timing at which the charge amount has reached the second threshold value Th1, and determines that the lock with respect to the object has been canceled at a timing at which the charge amount has decreased to the third threshold value Th3. Note that the term (instruction) used in connection with the game calculation section  110  at each timing may be replaced by another term. 
     For example, the terms (instructions) may be replaced by the following terms. Specifically, the game calculation section  110  may determine that one object has been selected at a timing at which the player has gazed at the object, determine that the object is continuously selected (locked) at a timing at which the charge amount has exceeded the first threshold value Th1, determine that the selection of the object has been confirmed at a timing at which the charge amount has reached the second threshold value Th1, and determine that the selection of the object has been canceled at a timing at which the charge amount has decreased to the third threshold value Th3. 
     3-5. Assignment of Functions 
     The functions of the elements of the game system  1  according to the embodiments may be appropriately modified as long as the effects are not impaired. 
     For example, some of the functions of the HMD  20  may be implemented by the simulation control device  100 , and some or all of the functions of the simulation control device  100  may be implemented by the HMD  20 . The function of each element included in the processing section  101  of the simulation control device  100  may also be appropriately modified. 
     For example, some or all of the functions of the display processing section  110 A may be implemented by the object space setting section  111 , and some or all of the functions of the object space setting section  111  may be implemented by the display processing section  110 A. 
     Some or all of the functions of the measurement section  110 B may be implemented by the state detection processing section  112 , and some or all of the functions of the state detection processing section  112  may be implemented by the measurement section  110 B. 
     The simulation control device  100  may include a dedicated circuit (hardware) that implements some or all of the functions of the processing section  101 . Specifically, some or all of the functions of the processing section  101  may be implemented by means of software, or may be implemented by means of hardware. 
     3-6. Game Device 
     Although the embodiments have been described above taking an example in which an eye input performed using the HMD  20  is applied to the game system  1  (simulation control device  100  included in the game system  1 ) that includes the structure that defines the real space in which the player P can move, an eye input performed using the HMD  20  may also be applied to another game system. 
     For example, an eye input performed using the HMD  20  may be applied to a system that is configured so that a game is provided from a server device to a terminal device through a network (e.g., Internet). In this case, the game program may be executed by the terminal device, or may be executed by the server device. The terminal device may implement the game by means of an operation input and streaming image display. 
     An eye input performed using the HMD  20  may be applied to a stand-alone game device that is not connected to a network, and may be applied to a terminal device that allows the user to perform an operation input using a touch panel (e.g., smartphone, tablet-type information terminal device, personal computer, monitor, or TV). 
     3-7. Eye Input Device 
     Although the embodiments have been described above taking an example in which the non-see-through HMD  20  is used to perform an eye input, another wearable image display device such as a see-through HMD or a monocular HMD may be used to perform an eye input. The HMD may be worn by the player P using various methods (e.g., cap-type HMD, eyeglass-type HMD, helmet-type HMD, sun visor-type HMD, and headband-type HMD). 
     4. Other 
     The invention is not limited to the embodiments described above. Various modifications and variations may be made of the embodiments described above. Any term cited with a different term having a broader meaning or the same meaning at least once in the specification and the drawings may be replaced by the different term in any place in the specification and the drawings. 
     The invention includes various other configurations substantially the same as the configurations described above in connection with the embodiments (e.g., a configuration having the same function, method, and results, or a configuration having the same objective and effects). The invention also includes a configuration in which an unsubstantial element described above in connection with the embodiments is replaced by another element. The invention also includes a configuration having the same effects as those of the configurations described above in connection with the embodiments, or a configuration capable of achieving the same objective as that of the configurations described above in connection with the embodiments. The invention further includes a configuration in which a known technique is added to the configurations described above in connection with the embodiments. 
     Although only some embodiments of the invention have been described in detail above, those skilled in the art would readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of the invention. Accordingly, all such modifications are intended to be included within the scope of the invention.