in the following , preferred embodiments of the present invention are described with reference to the accompanying drawings . fig1 is a block diagram showing a configuration of a video game apparatus according to an embodiment of the present invention . the video game apparatus shown in fig1 includes a video game apparatus main body 1 and a camera 2 , a display monitor 3 such as a television monitor , and a speaker 4 that are connected to the video game apparatus main body 1 via a usb ( universal serial bus ) cable , for example . it is noted that the camera 2 does not necessarily have to be directly connected to the video game apparatus main body 1 , and may be connected thereto via a network , for example . the video game apparatus main body 1 includes a program storage device 11 such as a cd - rom or a dvd in which game software ( program ) is stored , a cpu 12 that executes overall control processes of the video game apparatus 1 , a main memory 13 that temporarily stores programs and data used in executing control processes , a graphics memory ( storage ) 14 that stores image data , an image processing unit 15 that generates and controls an image according to game content , an audio processing unit 16 that generates and controls audio , an image output unit 17 that outputs an image signal to the display monitor 3 , and an audio output unit 18 that outputs audio to the speaker 4 . fig2 is a flowchart illustrating steps of a background image acquisition process for acquiring a background image that is required for calculating a background difference in the video game apparatus shown in fig1 . it is noted that the illustrated background image acquisition process may be executed by the cpu 12 of the video game apparatus main body 1 using relevant programs . in the illustrated example of fig2 , when a game is started , a game selection menu is displayed on the display monitor 3 shown in fig1 ( step s 1 ), and a game to be played is determined according to a selection made by a player ( step s 2 ). it is noted that in the present example , it is assumed that plural mini games are included in one set of game software ; however , in a case where the game software includes only one game , steps s 1 and s 2 may be omitted . then , guidance for acquiring a left side background image is displayed on the display monitor 3 ( step s 3 ). fig3 is a diagram showing an exemplary screen that may be displayed upon performing a left side background image acquisition process . in fig3 , a frame 303 indicating a standing position outline for a player 501 and a button item 304 that is to be operated by the player 501 are displayed at a right side operation region 302 on the right side of a monitor screen 301 , and a message 306 indicating , “ making preparations for starting the game . please stand within the indicated frame and wave your hand over the ok button after making sure that no moving objects are shown in this region .” is displayed at a left side background acquisition region 305 on the left side of the monitor screen 301 . it is noted that the left side background acquisition region 305 is arranged to be slightly larger than half the monitor screen 301 so that an overlapping portion with a right side background acquisition region 311 ( see fig5 ) may be provided as is described below . in this way , influence from noise generated at the dividing portion between the left and right background acquisition regions 305 and 311 may be reduced , and a determination may be made as to whether the camera has been moved during acquisition of the background image based on the image difference between the overlapping portions of the left and right background acquisition regions 305 and 311 . referring back to fig2 , a determination is made as to whether the ok button is activated ( step s 4 ). if the ok button is not activated , the guidance for left side background image acquisition continues to be displayed ( step s 3 ). if the ok button is activated , a determination is made as to whether there is any moving object included in the left side background acquisition region 305 based on the frame difference ( i . e ., difference in pixel information between pixels of frames ) detected within the region 305 ( step s 5 ). it is noted that if a moving object is included in a background acquisition region , its corresponding image cannot be used as a background image for calculating the background difference ; therefore , the existence of a moving object is checked for before acquisition of the background image in order to improve efficiency . when it is determined in step s 5 that a moving object exists , an error screen is displayed on the monitor careen 301 for a predetermined period of time ( step s 6 ). fig4 is a diagram showing an exemplary error screen that may be displayed when the left side background image acquisition process fails . in fig4 , a message indicating , “ a moving object is detected in the left side region of the screen . the moving object is indicated by blue dots . please avoid showing any moving objects and try again .” is displayed on the monitor screen 301 . referring back to fig2 , when it is determined in step s 5 that a moving object is not included in the left side background acquisition region 305 , the image of the background acquisition region 305 is acquired as a background image ( step s 7 ). then , guidance for acquiring a right side background image is displayed on the display monitor 3 ( step s 8 ). fig5 is a diagram showing an exemplary screen that may be displayed upon performing a right side background image acquisition process . in fig5 , a frame 309 indicating a standing position outline for the player 501 and a button item 310 to be operated by the player 501 are displayed at a left side operation region on the left side of the monitor screen 301 , and a message 312 indicating , “ please stand within the indicated frame and wave your hand over the ok button after making sure that no moving objects are shown in this region .” is displayed at a right side background acquisition region 311 on the right side of the monitor screen 301 . then , referring back to fig2 , a determination is made as to whether the ok button is activated ( step s 9 ). if the ok button is not activated , the guidance for right side background image acquisition continues to be displayed ( step s 8 ). if the ok button is activated , a determination is made as to whether a moving object is included in the right side background acquisition region 311 based on a frame difference detected in this region 311 ( step s 10 ). when it is determined in step s 10 that a moving object is included in the right side background acquisition region 311 , an error screen is displayed for a predetermined period of time ( step s 11 ). fig6 is a diagram showing an exemplary error screen that may be displayed when the right side background image acquisition process fails . in fig6 , a message 313 indicating , “ a moving object is detected in the right side region of the screen . the moving object is indicated by blue dots . please avoid showing any moving objects and try again .” is displayed on the monitor screen 301 . referring back to fig2 , when it is determined in step s 10 that no moving objects are included in the right side back ground acquisition region 311 , the image of the right side background acquisition region 311 is acquired as a background image ( step s 12 ). then , a determination is made as to whether a color difference above a predetermine level is detected between overlapping portions of the left side background image and the right side background image ( step s 13 ). if such a color difference above the predetermined level is detected , an error screen is displayed ( step s 14 ), and the operation goes back to the step of displaying the guidance for left side background image acquisition ( step s 3 ). fig7 is a diagram showing an exemplary screen that may be displayed in a case where the camera is moved from the time of acquiring the left side background image to the time of acquiring the right side background image . in fig7 , a message 314 indicating , “ camera has been moved . please reinitiate preparations for starting the game .” is displayed on the monitor screen 301 . referring back to fig2 , in a case where a color difference exceeding a predetermined level is not detected between the overlapping regions of the left side background image and the right side background image , the background image acquisition process is ended and the game is started ( step s 15 ). fig8 is a diagram showing an exemplary screen that may be displayed when only images of left and right side edge regions are acquired in the background image acquisition process . in the process described with reference to fig2 through 7 , a background image of the entire region that is captured by the camera is acquired ; however , in some games , the background difference may only be required for certain portions of the region being captured . in this case , the background images of such portions may be acquired . in the example shown in fig8 , background images of background acquisition regions 315 and 316 corresponding to left and right side edge regions with predetermined widths are arranged to be acquired . in this case , a frame 318 indicating a standing position outline for the user 501 and a button item 319 to be operated by the player 501 are displayed at a central operation region 317 , and a message 320 indicating , “ making preparations for starting the game . please wave your hand over the ok button after making sure that no moving objects are shown in the left and right side regions .” is displayed . it is noted that the messages for providing information and guidance to the user as is described in the above examples do not necessarily have to be output in the form of a display screen , and for example , one or more messages may alternatively be output in the form of a voice message . according to an aspect of the present invention , in acquiring a background image necessary for calculating a background difference , a button item that responds to an operation of a player is composited with a portion of an image captured by a camera which captured image includes an image of the posture of a player to generate a composite image to be displayed on a monitor screen , the player is guided so that the player image is accommodated within the portion of the captured image , and an image of a region of the captured image other than the portion is acquired as a background image . in this way a background image may be acquired while enabling command input based on image recognition of the player image , and the player may be relieved of inconveniences such as having to use a game controller . the following descriptions relate to pose detection that is realized using a background image acquired through the background image acquisition process as is described above . fig9 is a block diagram showing a functional configuration for realizing pose detection . according to the illustrated example , pose detection may rely on functional parts pertaining to background difference including a background image storage part 101 for storing a background image acquired from the camera 2 , a background difference calculating part 102 for calculating a difference ( background difference ) between a current image captured by the camera 2 and the background image stored by the background image storage part 101 , and a background difference determining part 103 for determining whether there is a background difference in a predetermined region . also , pose detection according to the present example may rely on functional parts pertaining to frame difference including a previous frame image storage part 104 for storing a previous frame image captured by the camera 2 , a frame difference calculating part 105 for calculating a difference ( frame difference ) between a current image captured by the camera 2 and the previous frame image stored by the previous frame image storage part 104 , a frame difference center calculating part 106 for calculating a frame difference center position within a predetermined region , and an analog stick input conversion part 107 for converting an input command into a signal corresponding to an analog stick signal from a game controller based on the center position of the frame difference calculated by the frame difference center calculating part 106 . further , pose detection according to the present example may rely on a functional part for integrating the background difference detection result and the frame difference detection result which functional part is embodied by an operations information conversion part 108 for generating operations information for a game character based on the determination result of the background difference determining part 103 and the analog stick signal from the analog stick input conversion part 107 . fig1 is a diagram illustrating an exemplary image processing operation for realizing pose detection using the functional parts shown in fig9 . in the illustrated example of fig1 , the frame difference calculating part 105 calculates the frame difference for a captured image 201 captured by the camera 2 , and the frame difference center calculating part 106 calculates frame difference centers 204 and 205 corresponding to the positions of the hands ( arms ) of the player 501 detected within regions 202 and 203 corresponding to left and right side regions , respectively , with predetermined widths . specifically , center positions 206 and 207 corresponding to height positions of the frame difference centers 204 and 205 of the regions 202 and 203 are calculated , respectively , and a difference ( height difference ) 208 between the left and right side center positions 206 and 207 and an average value ( average height value ) 209 of the calculated left and right side center positions 206 and 207 are calculated . the analog stick input conversion part 107 outputs a value obtained by multiplying the difference 208 between the centers 206 and 207 by a predetermined coefficient as a left / right direction signal of an analog stick signal , and outputs a value obtained by multiplying the average value 209 of the centers 206 and 207 by a predetermined coefficient as an up / down direction signal of the analog stick signal . also , the background difference calculating part 102 calculates the background difference for the captured image 201 . the background difference determining part 103 determines whether there are background differences in the left and right side regions 202 and 203 as well as a region 210 . it is noted that in a case where pose detection is limited to detecting poses based on the positions of centers 204 and 205 for the left and right hands of the player 501 ( e . g ., spreading arms ( hands ) in left - right directions and simultaneously raising / lowering arms , or alternatingly raising / lowering left and right arms ), the determination of whether a background difference is detected in the region 210 may be unnecessary . however , when the pose detection is arranged to detect a pose with both hands ( arms ) directed downward to signal a “ halt ” command , or a pose with both hands directed upward to signal an “ abruptly accelerate ” command , for example , the pose detection relies on the existence of background differences in the regions 202 and 203 as well as the region 210 ( it is noted that a detection region for detecting the existence of hands in a lower side region is not provided since such positioning of the hands of the player 501 is difficult to detect owing to the fact that the image of the hands may not be easily distinguished from the image of the body ). specifically , when the left and right hands of the player 501 are directed upward or downward , the centers 204 and 205 for the left and right hands of the player 501 may not be detected in which case no background differences would be detected in the regions 202 and 203 . in this case , when no background difference is detected in the region 210 , it may be determined that the hands of the player 501 are directed downward . also , when no background differences are detected in the regions 202 and 203 , and a background difference is detected in the region 210 , it may be determined that the hands of the player 501 are directed upward . in the above described example , the centers 204 and 205 of the hands of the player 501 are detected based on background difference ; however , in another example , the detection may be realized based on frame difference . in such an example , a frame difference may not be detected when there is no movement in the hands of the player 501 , and a previous detection state is maintained in such a case . the detection based on background difference has an advantage in that a pose may be detected even when there is no movement in the hands of the player 501 . also , it is noted that frame difference may be used in place of background difference in the detection region 210 as well ; however , background difference is preferably used in order to realize more accurate image recognition . fig1 is a diagram showing an exemplary screen that may be displayed on a monitor screen upon performing pose detection . in fig1 , a game screen is displayed on the monitor screen 331 of the display monitor 3 ( see fig1 ), and an operations confirmation screen 335 for realizing pose detection is displayed at a portion of the monitor screen 331 . in the illustrated game screen , a boat 333 that is driven by a game character ( e . g ., monkey ) is maneuvered along a winding course 332 to catch items ( e . g ., bananas 334 ) provided along the course 332 . as is shown in the enlarged operations confirmation screen 335 of fig1 , in regions 336 and 337 for detecting the positions of the hands ( arms ) of the player 501 , the detected positions of the hands are arranged to be indicated by markers 338 and 339 . fig1 a through 12e are diagrams illustrating exemplary poses that may be detected in pose detection for the game shown in fig1 . fig1 a shows a pose with both hands spread out and raised that represents a “ slow down ” command ; fig1 b shows a pose with both hands spread out in horizontal directions that represents a “ slightly accelerate ” command ; fig1 c shows a pose with both hands spread out and lowered that represents an “ accelerate ” command ; fig1 d shows a pose with the left hand being raised and the right hand being lowered that represents a “ turn right ” command ; and fig1 e shows a pose with the right hand being raised and the left hand being lowered that represents a “ turn left ” command . it is noted that in the present example , the intermediate ( transitional ) states of the poses are also detected , and operations information is generated in an analog fashion . fig1 is a diagram showing another exemplary screen that may be displayed on a monitor screen upon performing pose detection . in fig1 , a game screen is displayed on the monitor screen 341 of the display monitor 3 , and an operations confirmation screen 343 for realizing pose detection is displayed at a portion of the monitor screen 341 . in the illustrated game screen , a game character 342 is arranged to fly in the displayed region . as is shown in the enlarged operations confirmation screen of fig1 , in regions 344 and 345 for detecting the positions of the hands ( arms ) of the player 501 , the detected positions of the hands are indicated by markers 346 and 347 . also , in the illustrated operations confirmation screen , a dash button 348 for abruptly increasing the flying speed is displayed at an upper region . it is noted that this dash button 348 is an example of the region 210 shown in fig1 . fig1 a through 14i are diagrams illustrating exemplary poses that may be detected in pose detection for the game shown in fig1 . fig1 a shows a pose with both hands spread out and raised that represents an “ ascend while moving forward ” command ; fig1 b shows a pose with both hands spread out in horizontal directions that represents a “ level off ” command ; fig1 c shows a pose with both hands spread out and lowered that represents a “ descend while moving forward ” command ; fig1 d shows a pose with the left hand being raised and the right hand being lowered that represents a “ turn right while moving forward ” command ; fig1 e shows a pose with the right hand being raised and the left hand being lowered that represents a “ turn left while moving forward ” command ; fig1 f shows a pose with both hands raised that represents an “ abruptly accelerate ” command ; fig1 g shows a pose with the left hand being directed downward and the right hand being directed horizontally that represents a “ turn right in halt state ” command ; fig1 h shows a pose with the right hand being directed downward and the left hand being directed horizontally that represents a “ turn left in halt state ” command ; and fig1 i shows a pose with a “ halt ” command . it is noted that in the present example , intermediate ( transitional ) states of the poses are also detected , and operations information is generated in an analog fashion . according to an aspect of the present invention , a player does not have to move out of the view of a camera in acquiring a background image that is required for calculating a background difference , and the background image may be acquired while enabling command input through image recognition of the player image so that a game controller does not have to be used in the background image acquisition . it is noted that embodiments within the scope of the present invention include a background image acquisition method , a video game apparatus , a background image acquisition program , and computer - readable medium containing a computer program . the background image acquisition program may be embodied in any computer - readable medium for carrying or having computer - executable instructions or data structures stored thereon . such a computer - readable medium can be any available medium which can be accessed by a general purpose or a special purpose computer . by way of example , and not limitation , such a computer - readable medium can comprise a physical storage medium such as a ram , a rom , an eeprom , a cd - rom , other optical disk storage devices , other magnetic storage devices , or any other medium which can be used to carry or store desired program code means in the form of computer - executable instructions or data structures and which can be accessed by a general purpose or special purpose computer . such a medium may include a wireless carrier signal , for example . when information is transferred or provided over a network or other communications connection ( either hard - wired , wireless , or combinations thereof ) to a computer , the computer properly views the connection as a computer - readable medium . thus , any such connection is properly termed a computer - readable medium . combinations of the above should also be included within the scope of the computer - readable medium . computer - executable instructions comprise , for example , instructions and data which cause a general purpose computer , a special purpose computer , or a processing device to perform a certain function or a group of functions . although the present invention is shown and described with respect to certain preferred embodiments , it is obvious that equivalents and modifications will occur to others skilled in the art upon reading and understanding the specification . the present invention includes all such equivalents and modifications , and is limited only by the scope of the claims . the present application is based on and claims the benefit of the earlier filing date of japanese patent application no . 2004 - 255947 filed on sep . 2 , 2004 , the entire contents of which are hereby incorporated by reference .
a background image acquisition technique is disclosed for acquiring a background image that is required for calculating a background difference in a video game that uses an input image supplied by a camera while enabling command input through image recognition of a player image . the background image acquisition technique involves compositing a button item for responding to an operation of a player with a portion of an image captured by the camera which captured image includes a player image representing the posture of the player , displaying the composite image on a monitor screen , guiding the player into position such that the player image is accommodated within said portion of the captured image , and acquiring an image of a region of the captured image other than said portion as the background image .