Patent Publication Number: US-2015065244-A1

Title: Storage medium having image display program stored therein, image display apparatus, image display system, and image display method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 12/869,311 filed Aug. 26, 2010; which claims priority from Japanese Patent Application No. 2010-112386 filed May 14, 2010. The disclosures of the prior applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a storage medium having stored therein an image display program for displaying an image using an augmented reality technique, an image display apparatus, an image display system, and an image display method. 
     2. Description of the Background Art 
     In the prior art, there are augmented reality techniques in which an image of the real space (image captured by a camera) and an image of a virtual space (CG image) are synthesized and displayed together. Non-Patent Document 1 (Hirokazu Kato, Mark Billinghurst, “Marker Tracking and HMD Calibration for a video-based Augmented Reality Conferencing System”, proceedings of IWAR99 (the 2nd International Workshop on Augmented Reality), the United States, October 1999) describes a basic image generating method of an augmented reality technique. In an augmented reality technique, markers are placed in the real space, and a camera is used to capture an image around a marker. The marker in the captured image is detected by an image recognition process, and the three-dimensional position of the marker is calculated. It is possible to display an image in which a virtual object is synthesized with the captured image of the real space by generating an image (CG image) of the object in the virtual space by using the calculated three-dimensional position, and displaying the generated image while synthesizing it at the position of the marker in the camera image. 
     With the conventional augmented reality technique, the real captured image and the virtual CG image may be displayed with misalignment or the CG image may be displayed with flickering. 
     In an augmented reality technique, it is necessary to calculate the three-dimensional position of a marker by recognizing the marker included in the captured image, and such recognition and calculation processes are time-consuming. Therefore, the CG image generated based on the recognition and calculation process results lags behind the captured image in time. Therefore, when the camera is moved and the captured image changes, a CG image to be synthesized with the captured image lags behind in time and is therefore synthesized at a position before the change, thus resulting in misalignment between the position of the marker in the captured image and the position at which the CG image is synthesized. 
     In practice, even if a marker is captured, the recognition process may fail by failing to successfully recognize the marker from the captured image. If the recognition process fails, a CG image cannot be generated from the captured image which is the subject of the recognition process, and thus only the captured image is displayed. Therefore, if the recognition process fails intermittently, the CG image is displayed with flickering. 
     As described above, in the prior art, a CG image may be displayed with misalignment or displayed with flickering, and therefore a synthesized image obtained by synthesizing together a captured image and a CG image may lack realness. Thus, it is not possible to sufficiently achieve the effect of “giving a feel as if the CG image actually existed”, which is the characteristic of augmented reality techniques. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a storage medium having stored therein an image display program capable of displaying a synthesized image with increased realness with an augmented reality technique, an image display apparatus, an image display system, and an image display method. 
     The present invention employs configurations (1) to (15) below to solve the problems mentioned above. 
     (1) 
     The present invention is directed to a computer-readable storage medium having stored therein an image display program for instructing a computer of an information processing apparatus capable of obtaining a captured image from an image-capturing section to function as a recognition section, a synthesis section, and a display control section. The recognition section is a section for repeatedly executing a recognition process on the obtained captured image for recognizing a predetermined object included in the captured image. The synthesis section is a section for repeatedly generating a synthesized image obtained by synthesizing an image of a virtual object generated by using a process result of the recognition process with a captured image which is a subject of the recognition process. The display control section is a section for displaying the synthesized image on a display section. If the recognition process fails, the synthesis section synthesizes a captured image which is a subject of a recognition process which has succeeded before with an image of a virtual object generated by using a process result of the recognition process. 
     The “information processing apparatus” is a concept that includes any computer that performs an information process by executing a computer program, as well as the game apparatus described in the embodiment below. The “information processing apparatus” is not limited as to whether it is a portable type or not. 
     The “image display program” is a concept including an application program to be executed by a personal computer or a portable terminal while an example thereof is the game program described in the embodiment below. 
     The “predetermined object” may be any object as long as it can be recognized by a recognition process, as well as a marker  52  in the embodiment below. For example, the face, etc., of the user (player) may be recognized as the predetermined object. 
     The “virtual object” may be any object as long as an image thereof is generated for the purpose of being synthesized with a captured image, and the generation method or content of the image of the virtual object may be any method or content. 
     The “a recognition section” and the “a synthesis section” both execute repeated processes, but the frequencies of repetition (cycles) of their processes do not need to be equal to each other. For example, the frequency of the recognition process by the recognition section may be lower than the frequency of the synthesis process by the synthesis section as in the configuration (7) below or in the embodiment below. 
     With the configuration (1), the synthesis section generates a synthesized image between an image of a virtual object, which is generated by using a process result of the recognition process, and a captured image. Here, if the recognition process fails, the synthesis section generates a synthesized image by using a captured image which is a subject of a recognition process having succeeded most recently. Therefore, even if recognition fails, a synthesized image including an image of a virtual object is displayed, and therefore it is possible to prevent the virtual object from being displayed with flickering due to the virtual object disappearing (being not displayed) each time recognition fails. The image of the virtual object to be synthesized when the recognition process fails is generated by using a process result of the recognition process having succeeded most recently so as to match the captured image. Therefore, since the image of the virtual object is synthesized with a captured image based on which the image has been generated, no misalignment occurs between the captured image and the virtual object. As described above, with the configuration (1), there is no image flickering or misalignment, thus adding to the realness as if the virtual object were present in the real world. That is, with the configuration (1), it is possible to display a synthesized image with an augmented reality technique with increased realness. 
     (2) 
     The synthesis section may generate a synthesized image by updating the image of the virtual object irrespective of the process result of the recognition process. 
     The phrase “irrespective of the process result of the recognition process” means that the synthesis section generates a synthesized image by updating the image of the virtual object both when the recognition process succeeds and when it fails. If the synthesized image generating interval is shorter than the process time of the recognition process, the synthesis section generates a synthesized image by updating the image of the virtual object even in a situation where the process result of the recognition process has not been obtained (as the timing for generating a synthesized image arrives). 
     With the configuration (2), the image of the virtual object of the synthesized image is updated with a certain frequency. Therefore, for the virtual object, it is possible to display a smooth motion picture irrespective of the success/failure of the recognition process. Here, with the configuration (1), since the captured image is not updated when the recognition process fails, the motion picture is not displayed smoothly and the user may feel awkward with regard only to the captured image. However, with the configuration (2), since the motion picture of the virtual object, synthesized with the captured image, is displayed smoothly, it is possible to give the user an impression that the synthesized image as a whole is smooth, thus reducing/suppressing the awkwardness the user may feel with the captured image. 
     (3) 
     If the recognition process succeeds, the image display program may instruct the computer to further function as a storage control section for storing, in a storage section that can be accessed by the computer, a captured image which is a subject of the successful recognition process and the process result of the recognition process. Then, the synthesis section synthesizes a captured image stored in the storage section with the image of the virtual object generated based on the process result stored in the storage section. 
     With the configuration (3), if the recognition process succeeds, the captured image which is the subject of the process and the process result are stored in the storage section, and it is therefore possible to easily generate a synthesized image using the captured image. 
     (4) 
     If the recognition process succeeds, the storage control section may update storage contents of the storage section so that a captured image which is a subject of the successful recognition process and the process result of the recognition process are stored. 
     With the configuration (4), the latest captured image for which the recognition process has succeeded and the process result can be stored, and it is therefore possible to reliably generate a synthesized image using the captured image. 
     (5) 
     If the recognition process succeeds, the recognition section may calculate, as the process result, a positional relationship between the predetermined object recognized by the recognition process and the image-capturing section or the information processing apparatus. Then, if the recognition process succeeds, the synthesis section generates the image of the virtual object based on the positional relationship calculated from a captured image which is a subject of the successful recognition process. If the recognition process fails, the synthesis section generates the image of the virtual object based on the positional relationship calculated from a captured image which is a subject of a recognition process which has succeeded before. 
     With the configuration (5), the image of the virtual object is generated based on the positional relationship between the predetermined object and the image-capturing section as the process result of the recognition process. Therefore, it is possible to generate the image of the virtual object as viewed from a direction that reflects the positional relationship between the predetermined object and the image-capturing section, and it is therefore possible to generate/display the image of the virtual object with more realness. With the configuration (5), if the recognition process fails, the image of the virtual object is generated based on the positional relationship calculated from the captured image which is the subject of the recognition process having succeeded most recently, and therefore no misalignment occurs between the captured image and the virtual object. Particularly, if the configuration (2) and the configuration (5) are combined together, the image of the virtual object is displayed with no misalignment of the position at which the virtual object is displayed while updating the action and orientation of the virtual object every time, and it is therefore possible to display the synthesized image with increased realness. 
     (6) 
     The image display program may instruct the computer to further function as a game control process section for executing a predetermined game control process of controlling an action of an object appearing in a virtual space by using, as a game input, the process result of the recognition process. 
     With the configuration (6), an image using an augmented reality technique can be used in a game. With the configuration (6), since the process result of the recognition process is used as a game input, the user (player) can perform the game operation by moving the camera itself. Then, it is possible to provide a novel game using an augmented reality technique, in which the player him/herself actually moves around in the real space that is shown as if a virtual object were present therein. 
     (7) 
     If the recognition process fails, the game control process section may execute the game control process by using a process result of a recognition process which has succeeded before. 
     With the configuration (7), the game control process section can execute the game control process even if the recognition process fails. Therefore, even if the recognition process fails intermittently, it is possible to continuously execute the game control process, and it is possible to smoothly operate an object controlled by the game control process. 
     (8) 
     The image display program may instruct the computer to further function as a game process section for executing a predetermined game process irrespective of the process result of the recognition process. 
     The “game process” may be any process as long as the game progresses, and is a concept including a process in which the process result of the recognition process is not used as a game input as in the process of step S 24  in the embodiment below, for example. 
     With the configuration (8), the progress of the game continues even if the recognition process fails, and therefore the game is prevented from being discontinued frequently when the recognition process fails intermittently. Therefore, a game using an augmented reality technique can be made to progress smoothly without making the player feel unpleasant. 
     (9) 
     The game process section may execute, as the game process, a process of controlling the action of the virtual object. 
     Note that the “process of controlling the action of the virtual object” may be a process of controlling the action of the virtual object according to the input by the user, or a process of controlling the action of the virtual object according to an algorithm predetermined in the game program. 
     With the configuration (9), as a game process result, an image (motion picture) in which the virtual object moves can be displayed by an augmented reality technique. Moreover, since the game process is executed irrespective of the process result of the recognition process, the image of the virtual object is updated with a certain frequency as in the configuration (2). Therefore, since the motion picture of the virtual object is displayed smoothly, it is possible to give the user an impression that the synthesized image (game image) as a whole is smooth. 
     (10) 
     The display control section may display a warning image different from the captured image on the display section in response to a predetermined condition being satisfied as a result of the recognition process having failed successively. 
     With the configuration (10), if the recognition process has failed successively (to such an extent that the predetermined condition is satisfied), the display transitions from the synthesized image to the warning image. Therefore, by appropriately setting the predetermined condition, it is possible to notify the user of the fact that the recognition process is unlikely to succeed. Here, one may consider a method in which only the image of the virtual object is erased while displaying only the captured image when the recognition process has failed successively. With this method, however, it may be difficult to determine whether the image of the virtual object has disappeared because the virtual object itself has changed (e.g., in an example of a game, an enemy object may disappear as a result of being taken down) or the image of the virtual object has disappeared because of the recognition process failure. In contrast, with the configuration (10), the warning image different from the captured image is displayed, and it is therefore possible to clearly notify the user of the fact that the recognition process has failed successively. 
     (11) 
     The display control section may display the warning image on the display section based on the predetermined condition being the recognition process having failed successively over a predetermined period of time. 
     With the configuration (11), the warning image is displayed when the recognition process has failed successively over a predetermined period of time. In such a case, it is assumed that the recognition process is unlikely to succeed again, and therefore it is possible to warn the user at an appropriate timing with the configuration (11). 
     (12) 
     The image display program may instruct the computer to further function as a movement detection section for detecting a movement of the image-capturing section or the information processing apparatus. Then, the display control section may display the warning image on the display section based on the predetermined condition being the movement detection section detecting that the image-capturing section or the information processing apparatus has moved greater than a predetermined criterion. 
     With the configuration (12), the warning image is displayed when the image-capturing section is moved greater than or equal to a predetermined criterion. In such a case, it is assumed that the recognition process is unlikely to succeed again, and therefore it is possible to warn the user at an appropriate timing with the configuration (12). 
     (13) 
     The image display program may instruct the computer to further function as a game control process section for executing a predetermined game control process of controlling the action of the virtual object by using, as a game input, the process result of the recognition process. Then, the game control process section discontinues a progress of the game in response to display of the warning image. 
     With the configuration (13), the game is discontinued when the warning image is displayed. Therefore, it is possible to prevent the game from progressing while the warning image, which is not a game image (synthesized image), is displayed. 
     (14) 
     The display control section may display a captured image obtained at present on the display section in response to a predetermined condition being satisfied as a result of the recognition process having failed successively since the warning image is displayed. 
     With the configuration (14), the captured image is displayed if a predetermined condition is satisfied after the warning image is displayed. Here, if the warning image continues to be displayed, the user cannot grasp the image being captured by the image-capturing section and therefore cannot correctly capture an image of the predetermined object, and it becomes difficult to return to a situation where recognition can succeed. In contrast, with the configuration (14), since the captured image is displayed after the warning image, the user can easily correct the direction of the image-capturing section so as to correctly capture an image of the predetermined object, and thus it is possible to easily return to a state where recognition succeeds. 
     (15) 
     The display control section may display a captured image obtained at present on the display section in response to a predetermined condition being satisfied as a result of the recognition process having failed successively. 
     With the configuration (15), if the recognition process has failed successively (to such an extent that the predetermined condition is satisfied), the display transitions from the synthesized image to the warning image. Therefore, by appropriately setting the predetermined condition, it is possible to notify the user of the fact that the recognition process is unlikely to succeed. 
     The present invention may also be embodied in the form of an image display apparatus including various sections similar to those described above. In such an image display apparatus, the various sections may be implemented by a computer executing an image display program, or some or all of the various sections may be implemented by a dedicated circuit or circuits. The present invention may also be embodied in the form of an image display system including one or more information processing apparatuses each having various sections described above. Then, the one or more information processing apparatuses may communicate with each other directly via a wired or wireless connection, or may communicate with each other via a network. Moreover, the present invention may also be embodied in the form of an image display method performed by the various sections described above. 
     According to the present invention, if a recognition process for a captured image fails, the process displays a synthesized image between a captured image which is the subject of a recognition process having succeeded most recently and an image of a virtual object generated by using the process result of that recognition process, and it is therefore possible to prevent flickering or misalignment of images, thus displaying a synthesized image with increased realness. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view of a game apparatus of the present embodiment; 
         FIG. 2  is a block diagram showing an example of an internal configuration of the game apparatus; 
         FIG. 3  is a view showing an example of how the game apparatus is used; 
         FIG. 4  is a view showing an example of a game image displayed on the game apparatus; 
         FIG. 5  is a diagram illustrating a process of generating a synthesized image; 
         FIG. 6  is a diagram illustrating a process of generating a synthesized image where recognition fails; 
         FIG. 7  is a diagram showing a synthesis process where the captured image obtaining interval and the amount of time required for the recognition process are set to be equal to the CG image generating interval; 
         FIG. 8  is a diagram showing various data to be used in processes performed by a game program; 
         FIG. 9  is a main flow chart showing the flow of a game process executed by the game apparatus; 
         FIG. 10  is a flowchart showing the flow of a recognition management process (step S 3 ) shown in  FIG. 9 ; 
         FIG. 11  is a flowchart showing the flow of a game control process (step S 4 ) shown in  FIG. 9 ; 
         FIG. 12  is a flow chart showing the flow of a display process (step S 5 ) shown in  FIG. 9 ; and 
         FIG. 13  is a flow chart showing a variation of a display process (step S 5 ) in the present embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [Hardware Configuration of Game Apparatus] 
     Referring to the drawings, an image display program and an image display apparatus according to one embodiment of the present invention will be described. While the present invention is realized by executing an image display program by any information processing apparatus (computer) which displays an image on a display apparatus, the present embodiment is directed to a case where a game apparatus  1  shown in  FIG. 1  is used as an example of the information processing apparatus. 
       FIG. 1  is an external view of the game apparatus  1  according to the present embodiment. Here, a hand-held game apparatus is shown as an example of the game apparatus  1 . Note that the game apparatus  1  includes a camera and functions as an image-capturing device for capturing an image with the camera, displaying the captured image on the screen, and storing data for the captured image. 
     In  FIG. 1 , the game apparatus  1  is a foldable hand-held game apparatus, and the game apparatus  1  shown in the figure is unfolded (open state). The game apparatus  1  is structured in a size allowing the user to hold it with both hands, or even one hand, when unfolded. 
     The game apparatus  1  has a lower housing  11  and an upper housing  21 . The lower housing  11  and the upper housing  21  are connected so as to be openable/closable (foldable). In the example of  FIG. 1 , the lower housing  11  and the upper housing  21  are each formed in the shape of a horizontally elongated rectangular plate, and connected so as to be pivotable at their common long side joint. Typically, the user uses the game apparatus  1  in open state. Also, when the user does not use the game apparatus  1 , the game apparatus  1  is stored in closed state. In addition, in the example shown in  FIG. 1 , the game apparatus  1  can maintain not only the closed and open states but also its opening state via friction force generated at the joint at any angle that can be made by the lower housing  11  and the upper housing  21  between the closed and open states. That is, the upper housing  21  can remain stationary at an arbitrary angle with respect to the lower housing  11 . 
     The lower housing  11  is provided with a lower LCD (Liquid Crystal Display)  12 . The lower LCD  12  is horizontally long, and is arranged such that its longitudinal direction coincides with the longitudinal direction of the lower housing  11 . Note that in the present embodiment, LCDs are used as display devices to be included in the game apparatus  1 , but any other display devices, such as EL (Electro Luminescence) display devices, may be used. In addition, display devices of any resolution can be used for the game apparatus  1 . Note that an image being captured by an internal camera  23  or an external camera  25  is displayed in real-time on lower LCD  12 . 
     The lower housing  11  is provided with various operation buttons  14 A to  14 K and a touch panel  13  as input devices. As shown in  FIG. 1 , of all the operation buttons  14 A to  14 K, the direction input button  14 A, the operation button  14 B, the operation button  14 C, the operation button  14 D, the operation button  14 E, the power button  14 F, the start button  14 G, and the select button  14 H are provided at the inner principal surface of the lower housing  11 . The inner principal surface is a surface to be the interior side when the upper housing  21  and the lower housing  11  are folded. In the example shown in  FIG. 1 , the direction input button  14 A and the power button  14 F are provided to the left or right (in  FIG. 1 , to the left) of a lower LCD  12  provided at the center of the inner principal surface of the lower housing  11 . In addition, the operation buttons  14 B to  14 E, the start button  14 G, and the select button  14 H are provided to the opposite side (in  FIG. 1 , to the right) of the lower LCD  12  on the inner principal surface of the lower housing  11 . The direction input button  14 A, the operation buttons  14 B to  14 E, the start button  14 G, and the select button  14 H are used for various operations on the game apparatus  1 . For example, the direction input button  14 A is used for selection operations and so on. The operation buttons  14 B to  14 E are used for setting and cancellation operations and so on. The power button  14 F is used for turning ON/OFF the game apparatus  1 . 
     Note that in  FIG. 1 , the operation buttons  14 I to  14 K are not shown. For example, the L button  14 I is provided at the left edge of the upper surface of the lower housing  11 , and the R button  14 J is provided at the right edge of the upper surface of the lower housing  11 . For example, the L button  14 I and the R button  14 J are used for image-capturing instruction operations (shutter operations) on the game apparatus  1  having an image-capturing function. Furthermore, the volume button  14 K is provided at the left side surface of the lower housing  11 . The volume button  14 K is used for controlling the volume of a speaker included in the game apparatus  1 . 
     Also, in addition to the operation buttons  14 A to  14 K, the game apparatus  1  further includes a touch panel  13  as an example of a pointing device that is an input device allowing designation of an arbitrary position on the screen. The touch panel  13  is attached so as to cover the screen of the lower LCD  12 . Note that in the present embodiment, a touch panel  13  is, for example, of a resistive film type. However, a touch panel  13  is not limited to the resistive film type, and may be of any type. Also, in the present embodiment, for example, the touch panel  13  has the same resolution (detection accuracy) as the lower LCD  12 . However, the touch panel  13  is not necessarily required to be equal in resolution to the lower LCD  12 . In addition, the lower housing  11  has an insertion opening (in  FIG. 1 , indicated by broken lines) provided in its right side surface. The insertion opening can store a touch pen  27  to be used for operating the touch panel  13 . Note that any input to the touch panel  13  (touch input) is normally performed with the touch pen  27 , but the touch pen  27  is not restrictive, and the touch panel  13  can be operated with the user&#39;s finger. 
     Also, the lower housing  11  has provided in the right side surface an insertion opening (in  FIG. 1 , indicated by two-dot chain lines) for storing a memory card  28 . The insertion opening has provided therein a connector (not shown) for electrically connecting the game apparatus  1  and the memory card  28 . The memory card  28  is, for example, an SD (Secure Digital) memory card removably attached to the connector. For example, the memory card  28  is used for storing (saving) images captured by the game apparatus  1  and reading into the game apparatus  1  images generated by other apparatuses. 
     Furthermore, the lower housing  11  has provided in its upper side surface an insertion opening (in  FIG. 1 , indicated by one-dot chain lines) for storing a memory card  29 . This insertion opening also has provided therein a connector (not shown) for electrically connecting the game apparatus  1  and the memory card  29 . The memory card  29  is a storage medium having an information processing program such as a game program stored therein, and is removably loaded into the insertion opening provided in the lower housing  11 . 
     Three LEDs  15 A to  15 C are provided to the left of the joint between the lower housing  11  and the upper housing  21 . Here, the game apparatus  1  is capable of wirelessly communicating with other equipment, and the first LED  15 A is lit up while the game apparatus  1  is ON. The second LED  15 B is lit up while the game apparatus  1  is being charged. The third LED  15 C is lit up when wireless communication is established. Accordingly, the three LEDs  15 A to  15 C allow the user to know the statuses of the game apparatus  1 , regarding ON/OFF of the power supply, battery charge, and communications. 
     On the other hand, the upper housing  21  is provided with an upper LCD  22 . The upper LCD  22  is horizontally long, and is arranged such that its longitudinal direction coincides with the longitudinal direction of the upper housing  12 . Note that as in the case of the lower LCD  12 , any display device of any other type and resolution may be used instead of the upper LCD  22 . Note that a touch panel may be provided over the upper LCD  22 . The upper LCD  22  displays, for example, an operation instruction screen for teaching the user about the roles of the operation buttons  14 A to  14 K and the touch panel  13 . 
     Also, the upper housing  21  is provided with two cameras (an internal camera  23  and an external camera  25 ). As shown in  FIG. 1 , an internal camera  23  is provided at the inner principal surface close to the joint of the upper housing  21 . On the other hand, the external camera  25  is provided opposite to the side of the inner principal surface where the internal camera  23  is provided, i.e., the external principal surface of the upper housing  21  (the exterior surface of the game apparatus  1  in closed state; the back of the upper housing  21  shown in  FIG. 1 ). Note that in  FIG. 1 , the external camera  25  is indicated by a broken circle. As a result, the internal camera  23  can capture images of the direction in which the inner principal surface of the upper housing  21  is oriented, while the external camera  25  can capture images of the direction opposite to the image-capturing direction of the internal camera  23 , i.e., the direction in which the external principal surface of the upper housing  21  is oriented. In this manner, in the present embodiment, the two cameras, i.e., the internal and external cameras  23  and  25 , are provided so as to capture images in their respective directions opposite to each other. For example, the user can use the internal camera  23  to capture images of a view from the game apparatus  1  toward the user, and also can use the external camera  25  to capture images of a view in the opposite direction, i.e., from the user toward the game apparatus  1 . 
     Note that a microphone (microphone  43  shown in  FIG. 2 ) is provided as an audio input device under the inner principal surface close to the joint. In addition, a microphone hole  16  is provided in the inner principal surface close to the joint such that the microphone  43  can sense sound from outside the game apparatus  1 . The microphone  43  and the microphone hole  16  are not necessarily required to be positioned at the joint. For example, the microphone  43  may be accommodated within the lower housing  11 , and the microphone hole  16  may be provided in the lower housing  11  at a position corresponding to the position where the microphone  43  is accommodated. 
     Also, a fourth LED  26  (in  FIG. 1 , indicated by a broken circle) is provided at the external principal surface of the upper housing  21 . The fourth LED  26  is lit up at the time the internal camera  23  or the external camera  25  captures an image (when the shutter button is pressed). Furthermore, the light is on while the internal camera  23  or the external camera  25  is capturing a motion picture. As such, the fourth LED  26  allows any subject and bystander to know the game apparatus  1  captured (or is capturing) a picture. 
     Also, a sound hole  24  is provided to both the left and the right of an upper LCD  22  provided at the center of the inner principal surface of the upper housing  21 . A speaker is accommodated within the upper housing  21  below each sound hole  24 . The sound hole  24  is a hole for emanating the sound from the speaker to the outside of the game apparatus  1 . 
     As described above, the upper housing  21  is provided with the internal and external cameras  23  and  25  which are image-capturing means for capturing images, as well as the upper LCD  22  acting as a display means for mainly displaying the operation instruction screen. On the other hand, the lower housing  11  is provided with the input devices (the touch panel  13  and the operation buttons  14 A to  14 K) for operational inputs to the game apparatus  1 , and the lower LCD  12  acting as a display means for displaying captured images. Accordingly, when using the game apparatus  1 , the user can see a captured image (an image captured by the camera) displayed on the lower LCD  12  and make inputs via the input devices while holding the lower housing  11 . 
     Next, an internal configuration of the game apparatus  1  will be described with reference to  FIG. 2 .  FIG. 2  is a block diagram illustrating an exemplary internal configuration of the game apparatus  1 . 
     In  FIG. 2 , the game apparatus  1  includes electronic parts, such as a CPU  31 , a main memory  32 , a memory control circuit  33 , a saved data memory  34 , a preset data memory  35 , a memory card interface (memory card I/F)  36 , a memory card I/F  37 , a wireless communication module  38 , a local communication module  39 , a real time clock (RTC)  40 , a power circuit  41 , an interface circuit (I/F circuit)  42 , a first GPU (Graphics Processing Unit)  45 , a second GPU  46 , a first VRAM (Video RAM)  47 , a second VRAM  48 , and an LCD controller  49 . These electronic parts are mounted on an electronic circuit board, and accommodated within the lower housing  11  (or may be accommodated within the upper housing  21 ). 
     The CPU  31  is an information processing means for executing a predetermined program (here, an image display program according to the present embodiment). In the present embodiment, the game program is stored, as an example of the image display program, in a memory (e.g., saved data memory  34 ) within the game apparatus  1  as well as in the memory card  28  and/or  29 , and the CPU  31  executes the game program, thereby executing a game process to be described later. Note that the program to be executed by the CPU  31  may be prestored in the memory within the game apparatus  1  or may be acquired from the memory card  28  and/or  29  or from other equipment through communication therewith. 
     The CPU  31  is connected to the main memory  32 , the memory control circuit  33 , and the preset data memory  35 . The memory control circuit  33  is connected to the saved data memory  34 . The main memory  32  is a storage means used as a working area or buffering area for the CPU  31 . Specifically, a main memory  32  stores various data to be used in the game process, and programs acquired from outside (e.g., the memory cards  28  and  29  and other equipment). In the present embodiment, for example, a PSRAM (Pseudo-SRAM) is used as the main memory  32 . The saved data memory  34  is a storage means for storing, for example, the programs to be executed by the CPU  31  and data for images captured by the internal camera  23  and the external camera  25 . The saved data memory  34  is configured by a nonvolatile storage medium, e.g., in the present embodiment, a NAND flash memory. The memory control circuit  33  is a circuit for controlling data reading from/writing to the saved data memory  34  in accordance with an instruction by the CPU  31 . The preset data memory  35  is a storage means for storing data (preset data) such as various present parameters for the game apparatus  1 . As for the preset data memory  35 , a flash memory connected to the CPU  31  via an SPI (Serial Peripheral Interface) bus can be used. The memory card I/Fs  36  and  37  are each connected to the CPU  31 . The memory card I/F  36  reads/writes data from/to the memory card  28  attached to the connector in accordance with an instruction from the CPU  31 . Also, the memory card I/F  37  reads/writes data from/to the memory card  29  attached to the connector in accordance with an instruction from the CPU  31 . In the present embodiment, image data captured by the internal camera  23  and the external camera  25 , as well as image data received from other devices are written into the memory card  28 , and image data stored in the memory card  28  is read from the memory card  28  and stored to the saved data memory  34  or transmitted to other devices. In addition, various programs stored in the memory card  29  are read and executed by the CPU  31 . 
     Note that the game program may be supplied to a computer system not only via an external storage medium, such as the memory card  29 , but also via a wired or wireless communication line. Also, the game program may be pre-recorded to a nonvolatile storage device within the computer system. Note that the information storage medium for storing the game program is not limited to the nonvolatile storage device, and may be a CD-ROM, a DVD, or a similar optical disk storage medium. 
     The wireless communication module  38  has a function of connecting to a wireless LAN in accordance with a system complying with, for example, the IEEE802.11.b/g standard. Also, the local communication module  39  has a function of wirelessly communicating with similar game apparatuses in accordance with a predetermined communication system. The wireless communication module  38  and the local communication module  39  are connected to the CPU  31 . The CPU  31  is capable of transmitting/receiving data to/from other equipment via the Internet using the wireless communication module  38 , as well as transmitting/receiving data to/from other similar game apparatuses via the Internet using the local communication module  39 . 
     The CPU  31  is also connected to the RTC  40  and the power circuit  41 . The RTC  40  counts time and provides an output to the CPU  31 . For example, the CPU  31  can calculate the current time (date) based on the time counted by the RTC  40 . The power circuit  41  controls power supplied from the power supply (typically, a battery accommodated in the lower housing  11 ) provided in the game apparatus  1 , and supplies power to various parts of the game apparatus  1 . 
     The game apparatus  1  is also provided with the microphone  43  and an amplifier  44 . The microphone  43  and the amplifier  44  are each connected to the I/F circuit  42 . The microphone  43  senses the voice of the user speaking to the game apparatus  1 , and outputs an audio signal representing the voice to the I/F circuit  42 . The amplifier  44  amplifies the audio signal from the I/F circuit  42  to provide an output from the speaker (not shown). The I/F circuit  42  is connected to the CPU  31 . 
     Also, the touch panel  13  is connected to the I/F circuit  42 . The I/F circuit  42  includes an audio control circuit for controlling the microphone  43  and the amplifier  44  (speaker), and a touch panel control circuit for controlling the touch panel  13 . The audio control circuit performs A/D conversion and D/A conversion on the audio signal, and also converts the audio signal into audio data of a predetermined format. The touch panel control circuit generates touch position data (detected coordinate data to be described later) of a predetermined format based on a signal from the touch panel  13 , and outputs the generated data to the CPU  31 . The touch position data is data representing coordinates of a position detected by the touch panel  13  as being the position at which an input was made to the input screen of the touch panel  13 . Note that the touch panel control circuit performs reading of a signal from the touch panel  13  and generation of detected coordinate data once every predetermined period of time. 
     The above-described operation buttons  14 A to  14 K constitute an operation button section  14  connected to the CPU  31 . The operation button section  14  outputs to the CPU  31  operation data representing the status of input to the operation buttons  14 A to  14 K (whether or not the buttons have been pressed). The CPU  31  acquires the operation data from the operation button section  14 , and executes a process in accordance with an input to the operation button section  14 . 
     The internal camera  23  and the external camera  25  are each connected to the CPU  31 . The internal camera  23  and the external camera  25  each capture an image in accordance with an instruction from the CPU  31 , and output data for the captured image to the CPU  31 . In the present embodiment, the CPU  31  gives an image-capturing instruction to either the internal camera  23  or the external camera  25 , and the camera receiving the image-capturing instruction captures an image and transmits image data to the CPU  31 . Note that the internal camera  23  and the external camera  25  are also capable of capturing motion pictures. That is, the internal camera  23  and the external camera  25  are also capable of repeatedly capturing images and repeatedly sending the captured data to the CPU  31 . 
     The first GPU  45  is connected to the first VRAM  47 , and the second GPU  46  is connected to the second VRAM  48 . In accordance with an instruction from the CPU  31 , the first GPU  45  generates a first display image based on display image generation data stored in the main memory  32 , and creates an image on the first VRAM  47 . In accordance with an instruction from the CPU  31 , the second GPU  46  generates a second display image, and creates an image on the second VRAM  48 , as in the case of the first GPU  45 . The first VRAM  47  and the second VRAM  48  are connected to the LCD controller  49 . 
     The LCD controller  49  includes a register  491 . The register  491  stores the value of 0 or 1 in accordance with an instruction from the CPU  31 . When the value in the register  491  is 0, the LCD controller  49  outputs the first display image created on the first VRAM  47  to the lower LCD  12 , and also outputs the second display image created on the second VRAM  48  to the upper LCD  22 . Alternatively, when the value in the register  491  is 1, the LCD controller  49  outputs the first display image created on the first VRAM  47  to the upper LCD  22 , and also outputs the second display image created on the second VRAM  48  to the lower LCD  12 . For example, the CPU  31  is capable of causing the lower LCD  12  to display an image acquired from either the internal camera  23  or the external camera  25 , while causing the upper LCD  22  to display an operation instruction screen generated by a predetermined process. 
     [Summary of Image Synthesis Process] 
     Next, referring to  FIGS. 3 to 6 , an image synthesis process to be performed during a game process executed by the game program will be described. The game program allows a player to play a game while displaying a game image by using an augmented reality technique in which an image (CG image) of a virtual space is displayed while being synthesized with an image (captured image) of the real space captured by a camera. 
       FIG. 3  is a view showing an example of how a game apparatus is used. In the present embodiment, the player (user) places a marker  52  in a place (on a table  51  in  FIG. 3 ), and uses the game apparatus  1  to capture an image of the marker  52  and the vicinity thereof, as shown in  FIG. 3 . The camera used for image-capturing may be either the internal camera  23  or the external camera  25 , but the external camera  25  is used in the example described here. The marker  52  in the present embodiment is a thin plate marker with a predetermined pattern drawn thereon. However, the marker is not limited to a dedicated item such as the marker  52  as long as it is recognizable by a recognition process to be described later, and a part (the face, etc.) of the player&#39;s body may be used as the marker, for example. 
       FIG. 4  is a view showing an example of a game image displayed on the game apparatus.  FIG. 4  shows a game image to be displayed on the upper LCD  22  of the game apparatus  1  when the game apparatus  1  captures an image of the marker  52  placed on the table  51  as shown in  FIG. 3 . Note that while the game image is displayed on the upper LCD  22  in the present embodiment, the game image may be displayed on the lower LCD  12 . As shown in  FIG. 4 , an image of a cannon  53 , as an image (CG image) of a virtual object in the virtual space, is displayed on the upper LCD  22  while being synthesized with the captured image. Thus, it is possible to display an image which looks as if the cannon  53  were present on the actual table  51 . Note that as will be described later in detail, the action of the cannon  53  is controlled by the game program so that the cannon  53  changes its direction or launches a cannonball. That is, in the present embodiment, the image of the virtual object is displayed as a motion picture. Note that while  FIG. 4  shows an image of the cannon  53  as an example of the virtual object, the virtual object may be any object. 
     An image (synthesized image) obtained by synthesizing a CG image with a captured image can be generated by the following process, for example. First, the game apparatus  1  executes a recognition process of recognizing the marker  52  included in the captured image obtained by the camera. When the marker  52  is recognized, the game apparatus  1  calculates the positional relationship between the game apparatus  1  (camera) and the marker  52  from the shape, direction, etc., of the recognized marker  52 . The positional relationship is, for example, represented as the three-dimensional position and orientation of one of the game apparatus  1  and the marker  52  with respect to those of the other. Note that when calculating the three-dimensional position, the process may determine the distance therebetween based on, for example, the size (on the captured image), etc., of the recognized marker  52 . The game apparatus  1  calculates the positional relationship as a process result of the recognition process. Moreover, the game apparatus  1  calculates the position and orientation of the virtual camera in the virtual space based on the positional relationship. The position and orientation of the virtual camera are calculated so that the positional relationship between the virtual camera and the virtual object in the virtual space coincides with the positional relationship between the game apparatus  1  and the marker  52  in the real space. Once the position of the virtual camera is determined, the game apparatus  1  generates a CG image of the virtual object as viewed from the position of the virtual camera, and synthesizes the CG image of the virtual object with the captured image. By the process above, the game apparatus  1  can generate and display a synthesized image. Note that the recognition process (the process of calculating the positional relationship) and the process of calculating the position of the virtual camera from the positional relationship may be similar to those of a conventional augmented reality technique. 
     Here, with an augmented reality technique in which a CG image is displayed while being synthesized with a captured image, the captured image and the CG image may be displayed with misalignment or the CG image may be displayed with flickering, thus detracting from the realness of the synthesized image, as described above as a problem to be solved by the present invention. In view of this, the present invention attempts to add to the realness of the synthesized image by the process to be described below. Referring to  FIGS. 5 and 6 , a process of generating a synthesized image will now be described. 
       FIG. 5  is a diagram illustrating a process of generating a synthesized image. In  FIG. 5  (also in  FIG. 6  to be referred to later), the horizontal axis represents time, and the sections “captured image”, “CG image” and “synthesized image” each represent the timing at which the image is obtained (generated). The section “recognition process” represents the timing at which the recognition process for a captured image is completed. Note that it is assumed in  FIG. 5  that CG images are generated at an interval of a predetermined period of time (one frame period), whereas captured images are obtained at an interval longer than the CG image generating interval, and that the amount of time required for the recognition process is longer than the CG image generating interval. 
     In the present embodiment, a recognition process is executed for a captured image, a CG image is generated based on the process result, and the captured image and the CG image are displayed while being synthesized together, as described above. Note that a CG image is generated based on the process result of a recognition process, and a recognition process is time-consuming. Therefore, at a point in time when a CG image is generated, the captured image based on which the CG image is generated is not the latest image, and the latest captured image is a different image from the captured image based on which the CG image has been generated (see, for example, time T1, T4 and T7 shown in  FIG. 5 ). 
     In the present embodiment, when generating a synthesized image, the game apparatus  1  synthesizes a CG image with a captured image for which the recognition process has been completed, but not with the latest captured image obtained. For example, in  FIG. 5 , at time T1, the recognition process is completed for a captured image R0 which has been obtained prior to time T1, and the recognition process is not completed for a captured image R1 obtained at the current time T1. Therefore, at time T1, a CG image V1 is synthesized with the captured image R0, but not with the captured image R1. Here, the CG image V1 is an image generated based on the process result of the recognition process for the captured image R0. Therefore, at time T1, the CG image V1 is synthesized with the captured image R0 based on which the CG image V1 is generated, and thus there will be no such misalignment as described above. 
     At time T2 and T3, the recognition process for the captured image R1 is unfinished, and CG images V2 and V3 are generated based on the result of the recognition process for the captured image R0 like at time T1 since the next recognition process has not been completed. Therefore, a synthesized image is generated using the captured image R0 also at time T2 and T3 like at time T1. That is, since the CG images V2 and V3 are synthesized with the captured image R0 based on which the CG images V2 and V3 are generated, it is possible also at time T2 and T3 to prevent such misalignment as described above like at time T1. Thus, the game apparatus  1  generates a synthesized image by using a captured image for which the recognition process has been completed until the next recognition process is completed. 
     Note that in  FIG. 5 , also at and after time T4, a CG image is synthesized with a captured image based on which the CG image has been generated, like at time T1 to T3. That is, since the recognition process for the captured image R1 is completed at time T4, the CG images V4 to V6 which are generated based on the captured image R1 are synthesized with the captured image R1 at time T4 to T6, respectively. Since the recognition process for a captured image R2 is completed at time T7, a CG image V7 generated based on the captured image R2 is synthesized with the captured image R2 at time T7. 
     As described above, the game apparatus  1  can prevent such misalignment as described above because the captured image based on which the CG image is generated coincides with the captured image used for generating a synthesized image. 
     Next, consider a case where the recognition process fails. Even if the camera captures an image of the marker  52 , the recognition thereof may fail by failing to correctly recognize the marker  52  in the recognition process for reasons such as reflection light entering the marker  52 , a change of the color of the marker  52  within the captured image due to light, etc. In the present embodiment, the game apparatus  1  appropriately synthesizes a captured image and a CG image together even considering cases where recognition fails. 
       FIG. 6  is a diagram illustrating a process of generating a synthesized image where recognition fails. In  FIG. 6 , a case is assumed where the recognition process for the captured image R1 fails at time T4. In this case, the game apparatus  1  cannot generate a CG image based on the captured image R1. If it is so configured that no CG image is displayed (because a CG image cannot be generated due to recognition failure), the CG image will be displayed with flickering when recognition fails intermittently. Therefore, in the present embodiment, the game apparatus  1  generates a CG image V4 based on the result of the recognition process for the latest captured image R0 successfully recognized. That is, since the game apparatus  1  does not immediately stop generating a CG image in response to recognition failure, it is possible to prevent the CG image from being displayed with flickering. 
     Moreover, in the present embodiment, the CG image V4 at time T4 is synthesized with the last captured image R0 successfully recognized (see  FIG. 6 ) but not with the latest captured image R1 for which the recognition process is completed at time T4. If it is so configured that a synthesized image is generated by using the latest captured image R1 for which the recognition process is completed, as in a case where recognition succeeds (the case at time T4 in  FIG. 5 ), the captured image based on which the CG image V4 has been generated will be different from the captured image to be used for synthesis, thus resulting in misalignment described above. In view of this, in the present embodiment, if the recognition fails, the game apparatus  1  generates a synthesized image using the latest captured image successfully recognized. Then, even if the recognition fails, it is possible to synthesize a CG image with a captured image based on which the CG image has been generated, thereby preventing misalignment. 
     As described above, in the present embodiment, if a recognition process fails, the game apparatus  1  synthesizes a captured image which is the subject of the recognition process having succeeded most recently with a CG image generated by using the process result of that recognition process. Therefore, it is possible to prevent a CG image from being displayed with flickering and to prevent a CG image from being displayed while being misaligned with the captured image, thus improving the realness of the synthesized image. That is, it is possible to sufficiently achieve the effect of “giving a feel as if the CG image actually existed”, which is the characteristic of augmented reality techniques. 
     Note that if a recognition process fails, “a captured image which is the subject of the recognition process having succeeded most recently” is used for generating the synthesized image in the present embodiment. In other embodiments, the captured image to be used for generating the synthesized image when the recognition process fails is not limited to a captured image which is the subject of the recognition process having succeeded most recently (i.e., the last one to have succeeded), but may be a captured image which is the subject of any recognition process which has succeeded before. 
     Note that in the present embodiment, the last captured image for which the recognition process has succeeded is used for a synthesized image, and therefore if recognition fails, one captured image is displayed over a long period of time, and apparently the motion picture of the displayed synthesized image may not be smooth. However, in the present embodiment, the CG image of the synthesized image is updated every time irrespective of success/failure of the recognition process, and therefore the motion picture of the CG image is displayed smoothly. Thus, it is believed that the user will not feel unnatural about the motion picture of the synthesized image as a whole, and will not feel awkwardness. 
     Note that while the present embodiment assumes a case where the amount of time required for the recognition process is longer than the CG image generating interval, the present invention is effective also in a case where the amount of time required for the recognition process is equal to (or shorter than) the CG image generating interval. How the present invention is effective also in such a case will now be described with reference to  FIG. 7 . 
       FIG. 7  is a diagram showing a synthesis process where the captured image obtaining interval and the amount of time required for the recognition process are set to be equal to the CG image generating interval.  FIG. 7  assumes a case where a captured image is obtained and a recognition process for the captured image is completed every frame period (the unit time in which a CG image is generated). 
     In  FIG. 7 , it is assumed that recognition of the captured image R2 failed at time T2. If no CG image is displayed at time T2 (because of the recognition process failure), the CG image will be displayed with flickering. If the CG image V1 based on the captured image R1 and the captured image R2 are to be synthesized together at time T2, there may occur misalignment between the captured image R2 and the CG image V1. In contrast, according to the present invention, the game apparatus  1  synthesizes the captured image R1 which is the subject of the recognition process having succeeded most recently with the CG image V2 generated by using the process result of that recognition process, like at time T4 shown in  FIG. 6 . Thus, the game apparatus  1  can prevent the flickering of a CG image and the misalignment between a captured image and a CG image, as in the above embodiment. 
     [Details of Game Process] 
     Next, referring to  FIGS. 8 to 12 , the details of the game process executed by the game program will be described. Hereinafter, a game in which a player character controlled by the player attempts to hit the cannon  53  with a cannonball as shown in  FIG. 4  will be described, as an example of a game performed by the game process. Specifically, in the present game, the cannon  53 , whose operation is controlled by the game program so that the cannon  53  changes its direction or launches a cannonball, is displayed as a game image. The present game is a so-called “first-person shooter game”, and the position of the virtual camera in the virtual space is used also as the position of the player character. That is, the player can move the player character in the virtual space by changing the positional relationship between the game apparatus  1  and the marker  52  by moving the game apparatus  1  around. The player can make the player character launch a cannonball by performing a predetermined operation. In the present game, the player plays the game by attempting to hit the cannon  53  with a cannonball while moving the player character around so as not to be hit with a cannonball of the cannon  53 . 
     First, various data used in the game process will be described.  FIG. 8  is a diagram showing various data to be used in processes performed by the game program. In  FIG. 8 , a game program area  60 , a captured image buffer area  62 , a recognized image buffer area  65  and a game process data area  67  are set in a main memory  32  of the game apparatus  1 . 
     The game program area  60  is an area for storing a game program  61  for instructing the CPU  31  of the game apparatus  1  to execute a game process ( FIG. 9 ) to be described later. A part or whole of the game program  61  is read out from the memory card  29  at an appropriate timing so as to be stored in the main memory  32 . The game program  61  includes a program for executing the image synthesis process described above, and a program for executing the recognition process. 
     The captured image buffer area  62  stores image data of the image captured by the camera (captured image data). The captured image buffer area  62  can store a predetermined number of (e.g., 3) pieces of captured image data. The captured image data stored in the captured image buffer area  62  is managed so that the oldest piece of the captured image data, except for the one currently under the recognition process, is deleted and updated to new captured image data. Specifically, status data  63  and captured image data  64  are stored, while being associated with each other, in the captured image buffer area  62 . The status data  63  is data representing the status of the captured image data  64  associated therewith, and specifically indicates one of the statuses “to be overwritten”, “under recognition process”, and “available for process”. 
     The recognized image buffer area  65  stores, among other captured image data, captured image data for which the recognition process has been successfully completed (referred to as the recognized image data)  66 . That is, the recognition process is performed on the captured image data  64  stored in the captured image buffer area  62 , and if the recognition process succeeds, the captured image data  64  is stored in the recognized image buffer area  65 . Note that the recognized image buffer area  65  only needs to store only the latest recognized image data  66 . As will be described later in detail, captured image data stored in the recognized image buffer area  65  is the subject of the synthesis process to be synthesized with a CG image. 
     The game process data area  67  is an area for storing various data used in the game process. The game process data area  67  stores recognition result data  68 , player position data  69 , enemy object data  70 , display image data  71 , and failure counter data  72 . Note that in addition to those described above, the game process data area  67  stores various data necessary for the game, such as data of various objects (e.g., the cannon  53 ) appearing in the game, and sound data such as BGM. 
     The recognition result data  68  is data representing the process result of the recognition process described above, and is specifically data representing the positional relationship described above. The positional relationship is represented as a three-dimensional position of one of the game apparatus  1  and the marker  52  with respect to that of the other, or as a three-dimensional direction from one to the other, and is therefore represented by three-dimensional coordinates, a three-dimensional vector, or the like. 
     The player position data  69  is data representing the position of the player character in the virtual space (game space). As described above, in the present embodiment, the position of the virtual camera in the virtual space is also used as the position of the player character. That is, it can be said that the player position data  69  also represents the position of the virtual camera in the virtual space. The player position data  69  is calculated based on the recognition result data  68 . 
     The enemy object data  70  is data representing various parameters of the enemy object (i.e., the cannon  53 ). The various parameters include parameters representing the direction of the cannon  53 , the number of times the player has hit it with a cannonball, etc. 
     The display image data  71  is data of the game image displayed on the display apparatus (the upper LCD  22 ). As will be described later in detail, data of a warning image to be described later or a captured image may be displayed, in addition to a synthesized image described above, in the present game process. 
     The failure counter data  72  is data representing the failure counter. The failure counter represents the amount of time elapsed since the first failure in a case where the recognition process fails successively. That is, if the recognition process fails, the CPU  31  starts counting the failure counter, and continues to count the failure counter while the recognition process keeps failing. 
     Next, the details of the game process performed by the game apparatus  1  will be described with reference to  FIGS. 9 to 12 .  FIG. 9  is a main flow chart showing the flow of a game process executed by the game apparatus  1 . When the power button  14 F is pressed down and the power of the game apparatus  1  is turned ON, the CPU  31  of the game apparatus  1  displays a menu screen (e.g., an image including images (icons) representing various applications) used for instructing to start various applications. On the menu screen, the user instructs to start the game program  61 , e.g., instructs to select the icon of the game program  61 . When the start of the game program  61  is instructed on the menu screen, the CPU  31  initializes the main memory  32 , etc., and starts executing the game program  61  for performing the process shown in  FIG. 4 . In the present embodiment, as the game program  61  is executed, the CPU  31  functions as various sections recited in the claims. That is, the game program  61  instructs the CPU  31  to function as various sections recited in the claims. 
     First, in step S 1 , the CPU  31  determines whether a captured image has been obtained. That is, it determines whether data of a captured image has been sent from a camera (the external camera  25  in the present embodiment). Note that it is assumed in the present embodiment that while the process loop through steps S 1  to S 6  is executed once every frame period ( 1/60 sec), the camera sends a captured image with a lower frequency (once every about a few frame periods). If the determination result of step S 1  is affirmative, the process of step S 2  is executed. On the other hand, if the determination result of step S 1  is negative, the process of step S 2  is skipped, and the process of step S 3  is executed. 
     In step S 2 , the CPU  31  stores the captured image data obtained in step S 1  in the captured image buffer area  62  of the main memory  32 . Note that if the maximum number of pieces of the captured image data  64  that can be stored are already stored in the captured image buffer area  62 , the CPU  31  deletes one piece and newly stores (updates) the captured image data obtained. The captured image data  64  to be deleted is the captured image data  64  associated with the status data  63  which indicates “to be overwritten”. When the writing of the new captured image data  64  is completed, the CPU  31  changes the status data  63  associated with the captured image data  64  so as to indicate “available for process”. It also changes the status data  63  associated with the oldest one of pieces of the captured image data  64  whose status data  63  indicate “available for process” so as to indicate “to be overwritten”. As described above, the oldest piece of the captured image data  64 , except for the one currently under the recognition process, is updated to new captured image data. The process of step S 3  is executed, following step S 2 . 
     In step S 3 , the CPU  31  executes the recognition management process. In the recognition management process, the recognition process for a captured image is executed, and a data updating process, etc., are executed based on the result of the recognition process. The details of the recognition management process will now be described with reference to  FIG. 10 . 
       FIG. 10  is a flowchart showing the flow of a recognition management process (step S 3 ) shown in  FIG. 9 . In the recognition management process, first, in step S 11 , the CPU  31  determines whether there is a captured image under the recognition process. That is, the CPU  31  determines whether there is an image, among the captured images stored in the captured image buffer area  62 , which is being the subject of the recognition process. This determination can be made based on whether there is a piece of the captured image data  64  whose status data  63  indicates “under recognition process”. If the determination result of step S 11  is affirmative, the process of step S 12  is executed. On the other hand, if the determination result of step S 11  is negative, the process of step S 19  to be described later is executed. 
     In step S 12 , the CPU  31  executes the recognition process for the captured image. In the present embodiment, a series of recognition process operations (one iteration of the recognition process) until the recognition result for one captured image is calculated may not always be executed with the same cycle as the display process which is executed with a cycle of one frame period. Typically, it may be difficult to complete one iteration of the recognition process within one frame period, and in such a case, only a part of the entire recognition process is executed in one iteration of the process of step S 12 . In this case, the process to be executed in one iteration of step S 12  is adjusted to such an amount of process that a series of processes through steps S 1  to S 6  can be completed within one frame period. 
     If the recognition process is completed (i.e., if the recognition result is calculated) in the process of step S 12 , the CPU  31  stores the data of the recognition result in the main memory  32 . Specifically, if the recognition process succeeds, data of the positional relationship described above is stored in the main memory  32  as the recognition result. On the other hand, if the recognition process fails (if the marker  52  cannot be recognized from the captured image), data representing a recognition failure is stored in the main memory  32  as the recognition result. 
     On the other hand, if the recognition process is not completed in the process of step S 12 , the CPU  31  stores data necessary for executing the remaining recognition process in the main memory  32  and ends the process of step S 12 . When the process of step S 12  is executed next, the remaining process is executed by using the data stored in the main memory  32 . The process of step S 13  is executed, following step S 12  described above. 
     In step S 13 , the CPU  31  determines whether the recognition process for one captured image has been completed. That is, the determination result of step S 13  is affirmative if the process result of the recognition process is obtained in step S 12 , and the determination result of step S 13  is negative if the process result of the recognition process is not obtained in step S 12 . If the determination result of step S 13  is affirmative, the process of step S 14  is executed. On the other hand, if the determination result of step S 13  is negative, the CPU  31  ends the recognition management process. 
     In step S 14 , the CPU  31  determines whether the process result of the recognition process executed in step S 12  is successful. The determination of step S 14  can be made by referencing data of the recognition result stored in the main memory  32  at the end of step S 12 . If the determination result of step S 14  is negative, the process of step S 15  is executed. On the other hand, if the determination result of step S 14  is affirmative, the process of step S 16  to be described later is executed. 
     In step S 15 , the CPU  31  counts the failure counter. That is, the value of the failure counter represented by the failure counter data  72  is incremented by one, and data representing the incremented value is stored in the main memory  32  as new failure counter data  72 . Thus, it is possible to measure the amount of time elapsed since the recognition process failed. The process of step S 19  to be described later is executed, following step S 15 . 
     In steps S 16  to S 18 , a data updating process, etc., which are necessary when the recognition process succeeds are executed. In step S 16 , the CPU  31  sets the captured image for which the recognition process has been completed in step S 12  as the subject of the synthesis process. Specifically, the CPU  31  newly stores the captured image data  64  for which the recognition process has been completed in the recognized image buffer area  65  as the recognized image data  66 . Thus, the recognized image data  66  is updated. Note that the captured image data  64  for which the recognition process has been completed is one of the pieces of the captured image data  64  stored in the captured image buffer area  62  whose status data  63  is “under recognition process”. As will be described later in detail, the recognized image data  66  stored in the recognized image buffer area  65  is the subject to be synthesized with a CG image. The process of step S 17  is executed, following step S 16 . 
     In step S 17 , the CPU  31  updates the recognition result data  68  stored in the main memory  32 . That is, data representing the recognition result stored in step S 12  is stored in the main memory  32  as new recognition result data  68 . The process of step S 18  is executed, following step S 17 . 
     In step S 18 , the CPU  31  resets the failure counter. That is, the value of the failure counter data  72  stored in the main memory  32  is set to 0. The process of step S 19  is executed, following step S 18 . 
     In step S 19 , the CPU  31  determines a captured image to be the subject for which the recognition process is executed next. Here, the captured image data to be the new subject of the recognition process is the latest one of the pieces of the captured image data  64  stored in the captured image buffer area  62  whose status data  63  indicates “available for process”. The CPU  31  changes the status data  63  associated with the captured image data  64  to be the subject of the recognition process so that it indicates “under recognition process”. Thus, in the next iteration of step S 12 , the recognition process is executed for the piece of the captured image data  64  whose status data  63  has been changed to “under recognition process”. The CPU  31  changes the status data  63 , which had indicated “under recognition process” since before step S 19 , to “to be overwritten”, so as to avoid a case where there are a plurality of pieces of the status data  63  that indicate “under recognition process”. After step S 19 , the CPU  31  ends the recognition management process. 
     Note that with step S 19  described above, when one iteration of the recognition process ends, the next recognition process is executed for the latest one of the captured images obtained. Therefore, a captured image that is not the latest one may be discarded without being subjected to a recognition process. Here, if it is so configured that the recognition process is executed successively for all of the captured images obtained, and if the amount of time required for one iteration of the recognition process is longer than the captured image obtaining interval, there will gradually be more and more captured images to be displayed for which the recognition process has been completed. That is, the captured images displayed may gradually lag behind the real time. In contrast, according to the present embodiment, the recognition process is not performed for a captured image that is not the latest one, and it is therefore possible to prevent the accumulation of delays of captured images to be displayed. 
     With the recognition management process described above, when a recognition process ends for a captured image and if the recognition is successful (YES in steps S 13  and S 14 ), the captured image is set to be the subject of the synthesis process (step S 35  to be described later) (step S 16 ). That is, a synthesized image is generated by using a captured image for which the recognition process has succeeded. On the other hand, if the recognition fails (NO in step S 14 ), the process of step S 16  is not executed, and therefore the captured image is not set to be the subject of the synthesis process. Therefore, if the recognition fails, the captured image to be the subject of the synthesis process remains unchanged, and the same captured image as that of the previous iteration is used in the synthesis process. That is, if the recognition fails, the captured image which is the subject of the recognition process having succeeded most recently continues to be used in the synthesis process. In the recognition management process, also when the recognition process fails to end, the process of step S 16  is not executed, and therefore the captured image to be the subject of the synthesis process remains unchanged and the same captured image as that of the previous iteration is used in the synthesis process. Therefore, in the synthesis process, the same captured image as that of the previous iteration (i.e., the captured image which is the subject of the recognition process having succeeded most recently) is used until the recognition process ends. 
     Referring back to  FIG. 9 , in step S 4 , following the recognition management process (step S 3 ), the CPU  31  executes the game control process. In the game process, the action of an object in the virtual space is controlled based on the game input from the player, etc. The details of the game control process will now be described with reference to  FIG. 11 . 
       FIG. 11  is a flow chart showing the flow of the game control process (step S 4 ) shown in  FIG. 9 . In the game control process, first, in step S 21 , the CPU  31  determines whether the value of the failure counter is greater than a predetermined first threshold N1. That is, the CPU  31  reads out the failure counter data  72  stored in the main memory  32 , and compares the value of the failure counter data  72  with the first threshold N1. Note that the first threshold N1 is predetermined, and is set to an appropriate value depending on the content of the game, etc. Here, the value of the failure counter represents the amount of time elapsed since the first failure in a case where the recognition process fails successively. That is, the determination process of step S 21  is a process for determining whether the recognition process has failed successively over a predetermined period of time. If the determination result of step S 21  is negative, the process of step S 22  is executed. On the other hand, if the determination result of step S 21  is affirmative, the process of step S 25  to be described later is executed. 
     In step S 22 , the CPU  31  accepts a game operation input from the player. Specifically, the CPU  31  obtains touch position data representing the position at which an input has been made on the input surface of the touch panel  13 , and obtains operation data representing the status of input to the operation buttons  14 A to  14 K (whether or not the buttons have been pressed). The process of step S 23  is executed, following step S 22 . 
     In step S 23 , the CPU  31  performs a game control based on the input. Here, in the present embodiment, not only the input to the input devices (the touch panel  13  and the operation buttons  14 A to  14 K) accepted in step S 22  described above, but also the recognition result data  68  are used as game inputs. Specifically, the CPU  31  calculates the position (and orientation) of the player character based on the recognition result data  68 . In the present embodiment, the position and orientation of the player character are the same as the position and orientation of the virtual camera. That is, the position of the player character is calculated so that the position of the player character with respect to the cannon  53  is a position according to the positional relationship represented by the recognition result data  68  (the positional relationship between the game apparatus  1  and the marker  52  in the real space). The orientation of the player character (the orientation of the virtual camera) is calculated so that the cannon  53  is displayed at the position of the marker  52  on the display screen. More specifically, in the process, the CPU  31  reads out the recognition result data  68  from the main memory  32 , calculates the position of the player character based on the recognition result data  68 , and stores data representing the calculated position in the main memory  32  as the player position data  69 . As an action in response to an input on the input device, the CPU  31  performs, for example, an operation of launching a cannonball from the player character in response to the depression of a predetermined button. The process of step S 24  is executed, following step S 23 . 
     In step S 24 , the CPU  31  performs other game controls other than the process executed in step S 23 . The process executed in step S 24  may be, for example, controlling the action of the cannon  53  (the action of changing the direction of the cannon  53  or launching a cannonball), determining whether objects are hit by a cannonball, and calculating the score, etc. Note that data representing parameters of the cannon  53  (parameters representing the direction, etc., of the cannon  53 ) are stored in the main memory  32  as the enemy object data  70 . After step S 24 , the CPU  31  ends the game control process shown in  FIG. 11 . 
     On the other hand, in step S 25 , the CPU  31  discontinues the game. Here, a case where step S 25  is executed is a case where the recognition process fails successively over a predetermined period of time. As will be described later in detail, if the recognition process fails successively over a predetermined period of time (if the value of the failure counter becomes greater than or equal to the first threshold N1), a warning image indicating that the recognition has not been successful is displayed instead of a synthesized image (step S 34  to be described later). If a warning image is displayed, the game cannot proceed, and therefore the CPU  31  discontinues the progress of the game in step S 25 . After step S 25 , the CPU  31  ends the game control process shown in  FIG. 11 . 
     With the game control process described above, the process result of the recognition process is used as a game input. That is, the action of the object (player character) appearing in the virtual space is controlled based on the recognition result data  68  (step S 23 ). Then, the player can move the player character by moving the game apparatus  1  (or by the player him/herself moving). Therefore, in the present embodiment, there is displayed a game image obtained by synthesizing an object (the cannon  53 ) in the virtual space with a captured image representing the real space ( FIG. 4 ), and the position of the game apparatus  1  in the real space is reflected on the position of the object (player character) in the virtual space. Thus, it is possible to provide a novel game using an augmented reality technique, in which the player him/herself actually moves around in the real space that is shown as if a virtual object were present therein. 
     Referring back to  FIG. 9 , the CPU  31  executes a display process of generating and displaying the synthesized image, the warning image, etc., described above in step S 5 , following the game control process (step S 4 ). The details of the display process will now be described with reference to  FIG. 12 . 
       FIG. 12  is a flow chart showing the flow of the display process (step S 5 ) shown in  FIG. 9 . In the display process, first, in step S 31 , the CPU  31  determines whether the value of the failure counter is greater than a predetermined second threshold N2. That is, the CPU  31  reads out the failure counter data  72  stored in the main memory  32 , and compares the value of the failure counter data  72  with the second threshold N2. Note that the second threshold N2 is greater than the first threshold N1, and is pre-set to an appropriate value depending on the content of the game, etc. The determination process of step S 31  is a process for determining whether the recognition process has failed successively over a predetermined period of time corresponding to the second threshold N2. If the determination result of step S 31  is affirmative, the process of step S 32  is executed. On the other hand, if the determination result of step S 31  is negative, the process of step S 33  to be described later is executed. 
     In step S 32 , the CPU  31  displays the captured image on the display apparatus (the upper LCD  22 ). Specifically, the CPU  31  displays the captured image data sent from the camera on the upper LCD  22 . Thus, if the failure counter is greater than or equal to the second threshold N2, only the captured image is displayed, and the CG image is not synthesized therewith. After step S 32 , the CPU  31  ends the display process. 
     On the other hand, in step S 33 , the CPU  31  determines whether the value of the failure counter is greater than the predetermined first threshold N1. The process of step S 33  is the same as the process of step S 21  described above. If the determination result of step S 33  is affirmative, the process of step S 34  is executed. On the other hand, if the determination result of step S 33  is negative, the process of step S 35  to be described later is executed. 
     In step S 34 , the CPU  31  displays a warning image on the display apparatus (the upper LCD  22 ). The warning image is an image for warning the player of the fact that the recognition of the marker  52  has failed successively over a predetermined period. The warning image may be any image, but is preferably an image different from the captured image or an image that does not include the captured image. In the present embodiment, so-called “snow noise image” (which may include text such as “Marker not recognized”) is displayed as the warning image. The CPU  31  may use, as the warning image, a motion picture or a still picture provided in advance in the game program  61 . After step S 34 , the CPU  31  ends the display process. 
     On the other hand, in step S 35 , the CPU  31  generates a synthesized image obtained by synthesizing together the captured image and the CG image. Here, the captured image data used for the synthesis is captured image data (the recognized image data  66 ) stored in the recognized image buffer area  65 . That is, in the present embodiment, the latest captured image for which the recognition process has succeeded (a captured image for which the recognition process has succeeded most recently) is used for the generation of the synthesized image. The data of the synthesized image generated is stored in the main memory  32  as the display image data  71 . 
     Note that in the present embodiment, a synthesized image is generated by a method in which a captured image is used as the background for the image of the virtual space. That is, the CPU  31  places a background object behind the cannon  53  (as viewed from the virtual camera) in the virtual space, and the CPU  31  uses the captured image as the texture of the background object when generating an image of the virtual space as the cannon  53  is viewed from the position of the virtual camera in the direction of the virtual camera. Thus, it is possible to generate a synthesized image in which a CG image of the cannon  53  is synthesized on the captured image. Note that the position and orientation of the virtual camera are calculated as the position and orientation of the player character in step S 23  described above. The action of the cannon  53  (the direction, etc., of the cannon  53 ) is calculated in step S 24  described above. Note that in other embodiments, the CPU  31  may generate a synthesized image by first generating a CG image and then synthesizing the CG image with a captured image. The process of step S 36  is executed, following step S 35 . 
     In step S 36 , the CPU  31  determines whether the value of the failure counter is greater than a predetermined third threshold N3. That is, the CPU  31  reads out the failure counter data  72  stored in the main memory  32 , and compares the value of the failure counter data  72  with the third threshold N3. The third threshold N3 is smaller than the first threshold N1, and is pre-set to an appropriate value depending on the content of the game, etc. The determination process of step S 36  is a process for determining whether the recognition process has failed successively over a predetermined period of time corresponding to the third threshold N3. If the determination result of step S 36  is affirmative, the process of step S 37  is executed. On the other hand, if the determination result of step S 36  is negative, the process of step S 37  is skipped, and the process of step S 38  is executed. 
     In step S 37 , the CPU  31  further synthesizes a warning image with a synthesized image in a proportion according to the value of the failure counter. The method for synthesizing together the synthesized image and the warning image may be any method, and may be a method based on a typical alpha blending process. Then, the CPU  31  determines the alpha value, which represents the transparency of the warning image, so that the transparency is 100% when the value of the failure counter is “N3”, 0% when the value of the failure counter is “N1”, and the transparency decreases as the value of the failure counter increases from “N3” to “N1”. Then, the synthesized image and the warning image are synthesized together according to the determined alpha value. By the process of step S 37  described above, it is possible to generate an image in which the warning image gradually grows thicker while the synthesized image grows paler in the course of time (as the value of the failure counter increases). The image data generated in step S 37  is stored in the main memory  32  as the display image data  71 . The process of step S 38  is executed, following step S 37 . 
     In step S 38 , the CPU  31  displays the synthesized image on the display apparatus (the upper LCD  22 ). Specifically, the CPU  31  reads out the display image data  71  from the main memory  32  and displays it on the upper LCD  22 . Thus, a game image ( FIG. 4 ) is displayed in which a CG image of a game object (the cannon  53 ) is synthesized with a captured image. 
     Note that in step S 38 , the synthesized image generated in step S 35  is displayed as it is, when the recognition process has succeeded or when the value of the failure counter is smaller than the third threshold N3. On the other hand, when the value of the failure counter is greater than or equal to the third threshold N3, an image is displayed in which the synthesized image and the warning image are synthesized together. Since the transparency of the warning image varies depending on the value of the failure counter, when the value of the failure counter is greater than or equal to the third threshold N3, a game image is displayed in which the warning image gradually grows thicker (the synthesized image grows paler) in the course of time. After step S 38 , the CPU  31  ends the display process. 
     By the display process described above, a synthesized image is displayed when the recognition process has succeeded (No in steps S 31 , S 33  and S 36 ). Even when the recognition process has failed, a synthesized image is displayed until the failure continues over a period of time corresponding to the third threshold N3 because the determination result of steps S 31 , S 33  and S 36  is negative. Thereafter, when the recognition process failure time exceeds a period of time corresponding to the third threshold N3, the determination result of step S 36  becomes affirmative, and therefore an image is displayed in which a warning image (a snow noise image) is synthesized with a synthesized image. Then, as the time passes, the warning image grows thicker (step S 37 ). Moreover, when the recognition process failure time exceeds a period of time corresponding to the first threshold N1, the determination result of step S 33  becomes affirmative, and therefore only the warning image is displayed. At the timing when the warning image is displayed, the game is discontinued (step S 25 ). Therefore, with the display process described above, even if the recognition process fails, the display of the CG image does not stop instantly, and therefore a CG image will not be displayed with flickering. Moreover, when the recognition process failure continues successively over a predetermined period of time or more, a warning image is displayed, and it is thus possible to notify the player of the fact that the recognition process has failed. 
     Note that one may consider displaying the captured image (containing no CG image) as the warning image. If the captured image is displayed as the warning image, it appears to the player as if the virtual object had disappeared. However, there may possibly be a case where the virtual object is no longer displayed as the game advances (e.g., where the virtual object has been destroyed), and in such a case, it is difficult for the player to determine whether the recognition has failed. In contrast, according to the present embodiment, an image different from the captured image is displayed as the warning image, and it is therefore possible to more reliably notify the player of the fact that the recognition process has failed. Moreover, in the present embodiment, the warning image is displayed so that it gradually grows thicker in the course of time, and therefore it is possible to give a notification with reduced awkwardness to the player as compared with a case where a warning image is displayed abruptly. Note that in other embodiments, the CPU  31  may display the captured image without displaying the warning image when the recognition process has failed successively. That is, in the display process shown in  FIG. 12 , the CPU  31  may omit the processes of steps S 33  and S 34  described above. Still, it is possible to notify the player of the fact that the recognition process has failed, as in the above embodiment. 
     Moreover, with the display process described above, if the recognition process failure time exceeds a period of time corresponding to the second threshold N2 after the warning image is displayed, the determination result of step S 31  becomes affirmative, thereby displaying only the captured image. This is because if the process continues to display the warning image, the player can no longer grasp the image being captured by the game apparatus  1 , thus failing to correctly capture an image of the marker  52 . That is, in the present embodiment, after the warning image is displayed and the game is discontinued, the captured image is displayed so that the player can easily correct the direction of the game apparatus  1  so as to correctly capture an image of the marker  52 . 
     Referring back to  FIG. 9 , the CPU  31  determines whether the game should be ended in step S 6  following the display process (step S 5 ). The determination of step S 6  is made based on, for example, whether the game has been cleared, the game has been over, the player has given an instruction to quit the game, etc. If the determination result of step S 6  is negative, the process of step S 1  is executed again. The process loop through steps S 1  to S 6  is repeatedly executed until it is determined in step S 6  that the game should be ended. On the other hand, if the determination result of step S 6  is affirmative, the CPU  31  ends the game process shown in  FIG. 9 . The game process is as described above. 
     As described above, with the game process described above, if the recognition process for a captured image succeeds, the captured image is stored in the recognized image buffer area  65  (step S 16 ) and used in the synthesized image (step S 35 ). Then, the CG image to be synthesized is generated based on the successful recognition result. On the other hand, if the recognition process fails, the captured image is not used in the synthesized image, and the previous captured image is maintained in the recognized image buffer area  65 . Thus, if the recognition process fails, a synthesized image is generated and displayed by using the captured image for which the recognition process has succeeded most recently. The CG image to be synthesized is also generated based on the recognition result obtained when the recognition process succeeded most recently. Then, even if recognition failure occurs intermittently, the CG image is not displayed with flickering and there will be no misalignment between the captured image and the CG image. Therefore, according to the present embodiment, it is possible to generate and display a synthesized image with higher realness. 
     In the game process, the process (step S 24 ) of controlling the action of the displayed virtual object (the cannon  53 ) is executed every frame. Not only when the recognition succeeds, but also when it fails, the image of the virtual object is displayed while being synthesized with the captured image over a certain period of time (while step S 33  returns No). Therefore, the action of the virtual object is updated every frame, and the virtual object is displayed as a motion picture. Therefore, even if the captured image is not updated every frame, it is possible to display a smooth motion picture as a synthesized image without giving the player an impression that the number of frames of the motion picture is low. 
     Note that in the game process described above, while the action of the virtual object is updated every frame (step S 24 ), the process (step S 23 ) of determining the display position of the virtual object (the position and orientation of the virtual camera) is performed by using the recognition result data updated when the recognition succeeds (step S 17 ). Therefore, according to the present embodiment, it is possible to prevent the misalignment between the captured image and the image of the virtual object while displaying the action of the virtual object as a smooth motion picture that is updated every frame. 
     With the game process described above, if the recognition process fails successively over a predetermined period of time corresponding to the first threshold N1, a warning image is displayed, and the game is discontinued (step S 25 ). That is, in the present embodiment, the progress of the game is discontinued in response to the display of the warning image. Here, in the present embodiment, the CG image is displayed even if the recognition fails, and it is therefore not necessary to discontinue the game immediately even if the recognition fails. If it is so configured that the game is discontinued in response to a recognition failure, the game may be discontinued frequently. Thus, in the present embodiment, the game is allowed to continue even if the recognition fails. Note however that since the recognition result cannot be used as a game input while the recognition is failing, if the game is allowed to continue long after the recognition fails, there will be a problem in the game because the player cannot perform a game operation. In view of this, in the present embodiment, the game is discontinued when a certain period of time elapses since the recognition fails. Since the game is discontinued in response to the display of the warning image, the game can be discontinued at a timing when the synthesized image is no longer displayed, and it is therefore possible to discontinue the game without giving awkwardness to the player. 
     Note that in other embodiments, the CPU  31  may discontinue the game at a timing when the recognition fails. For example, in a game where the recognition result is used as a game input and where the game operation requirements are severe, one may consider discontinuing the game in response to a recognition failure. In a case where the warning image is displayed so that it gradually grows thicker as in the game process described above, the CPU  31  may discontinue the game in response to the warning image being synthesized at a predetermined proportion with respect to the synthesized image. That is, in the game process described above, the CPU  31  may discontinue the game when the value of the failure counter is equal to a predetermined value between the first threshold N1 and the third threshold N3. 
     [Variation] 
     While the above embodiment is an example of how the present invention can be carried out, the present invention can be carried out with a configuration to be described below, for example, in other embodiments. 
     (Variation Regarding Image Processing Program) 
     While the above embodiment has been described with respect to a game program as an example of the image display program, the present invention can be used with any image display program using an augmented reality technique other than game programs. 
     (Variation Regarding Game) 
     While the above embodiment has been described with respect to a first-person shooter game in which an enemy object (the cannon  53 ) is displayed, as an example of the game executed by the game program, the present invention can be used with a game program of any game in which the game image is displayed using an augmented reality technique. For example, in the above embodiment, the action of the virtual object (player character) not displayed on the screen is controlled based on the recognition result. Here, in other embodiments, the game apparatus  1  may control the action of the virtual object displayed on the screen based on the recognition result. For example, games executed by a game program may include the following examples.
         A game in which the position of the virtual object on the screen is determined based on the recognition result.       

     A crane object whose position in the virtual space is determined according to the positional relationship (recognition result) between the game apparatus  1  and the marker  52  appears, and the player controls the crane object to catch other objects placed in the virtual space.
         A game in which the movement direction of the virtual object on the screen is determined based on the process result.       

     A billiard game in which a billiard ball is placed as the virtual object, wherein the player can strike the ball in the viewing direction of the virtual camera determined based on the recognition result (i.e., the movement direction of the ball is controlled by the positional relationship between the game apparatus  1  and the marker  52 ). 
     (Variation Regarding Condition Under which Warning Image is Displayed) 
     In the above embodiment, the game apparatus  1  displays the warning image on the condition that the recognition process has failed successively over a predetermined period of time. Here, in other embodiments, the game apparatus  1  may detect the movement of the camera (the game apparatus  1 ) and display the warning image on the condition that it is detected that the camera has moved greater than a predetermined criterion. The details of the variation where the warning image is displayed by detecting the movement of the camera will now be described. 
       FIG. 13  is a flow chart showing a variation of a display process (step S 5 ) in the present embodiment. Note that in  FIG. 13 , like processes to those of  FIG. 12  are denoted by like step numbers and will not be described in detail. 
     In  FIG. 13 , if the determination result of step S 33  is affirmative, the process of step S 41  is executed. In step S 41 , the CPU  31  detects the movement of the camera (the game apparatus  1 ). While the movement detection method may be any method, if the game apparatus  1  includes an acceleration sensor or a gyro-sensor, for example, the CPU  31  obtains the detection results from these sensors. The CPU  31  may recognize a predetermined object (which may be a different object from the marker) from the captured image, and calculate the movement of the camera based on the movement of the predetermined object in the captured image. The process of step S 42  is executed, following step S 41 . 
     In step S 42 , the CPU  31  determines whether the movement of the camera (the game apparatus  1 ) is greater than or equal to a predetermined criterion. The predetermined criterion may be, for example, the acceleration (angular velocity) of the camera being greater than or equal to a predetermined value, or the amount of movement of the camera (per unit time) being greater than or equal to a predetermined value. If the determination result of step S 42  is affirmative, the process of step S 34  is executed. On the other hand, if the determination result of step S 42  is negative, the process of step S 35  is executed. 
     As described above, in the variation shown in  FIG. 13 , a warning image is displayed (step S 34 ) when the movement of the camera is more violent than a predetermined criterion, and the synthesized image continues to be displayed when the movement of the camera is less violent than the predetermined criterion. Here, one can assume it is likely that the marker  52  is outside the image-capturing range and can no longer be recognized if the camera has moved violently, and one can assume it is likely that the marker  52  is within the image-capturing range and the recognition will succeed again if the camera has not moved much. Therefore, according to this variation, in a case where recognition fails, the synthesized image continues to be displayed when it is assumed that the recognition will succeed again (that the recognition failure is temporary), and a warning image is displayed when it is unlikely that the recognition will succeed again. Thus, it is possible to display the warning image in more appropriate situations. 
     Note that although not shown, in the variation described above, the game discontinuation process (step S 25 ) in the game control process (step S 4 ) is preferably performed in response to the display of the warning image. That is, in the game control process, the CPU  31  may execute steps S 41  and S 42 , as in  FIG. 13 , when the determination result of step S 21  is affirmative, and execute step S 25  when the determination result of step S 42  is affirmative. 
     In the variation described above, the process of steps S 41  and S 42  may be executed immediately when recognition fails. That is, the first threshold N1 may be set to “1” or a very small value. In the variation described above, the process of displaying the captured image in step S 32  may be executed on the condition that a predetermined period of time has elapsed from the display of the warning image. 
     As described above, according to the variation described above, it is possible to determine whether the recognition failure is temporary by detecting the movement of the camera (the game apparatus  1 ), and it is possible to appropriately determine the timing at which the warning image is displayed. 
     (Variation Regarding Game System) 
     While the above embodiment has been described with respect to a case where a series of processes for generating and displaying a synthesized image are executed by a single apparatus (the game apparatus  1 ), the series of processes may be executed by an information processing system including a plurality of information processing apparatuses in other embodiments. For example, in an information processing system including a terminal-side apparatus, and a server-side apparatus capable of communicating with the terminal-side apparatus via a network, the terminal-side apparatus obtains a captured image using a camera, and transmits the data of the captured image obtained to the server-side apparatus. The server-side apparatus, which has received the captured image data, executes the recognition process (corresponding to the recognition management process in step S 3  in the above embodiment) and the game process (corresponding to the game control process in step S 4  in the above embodiment), and transmits data of the game process result to the terminal-side apparatus. The data of the game process result is, for example, data of the game image, or data necessary for generating the game image. The terminal-side apparatus receives the data of the game process result, and displays the game image such as the synthesized image on the display apparatus based on this data. Thus, the present invention may be implemented as an information processing system including a plurality of information processing apparatuses. 
     As described above, the present invention can be used as a game program or a game apparatus, for example, for the purpose of, for example, displaying a virtual image with an augmented reality technique with increased realness. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.