Patent Publication Number: US-11030771-B2

Title: Information processing apparatus and image generating method

Description:
TECHNICAL FIELD 
     The present invention relates to technology for generating an image to be displayed on a head-mounted display device. 
     BACKGROUND ART 
     A head-mounted display device (HMD) worn on the head of a user provides the user with a virtual reality (VR) world. Recently, applications have been introduced which allow the user to play games while watching the screen displayed on the HMD. Existing non-portable display devices such as TV sets let the user&#39;s visual field range stray outside the screen. This may prevent the user from concentrating on the screen or having the sense of immersion in the ongoing game. In contrast, wearing the HMD allows the user to watch only images displayed on the HMD. This enhances the sense of immersion in the game and also boosts the sense of entertainment for the user. When the HMD is provided with a head tracking function that updates the display screen in keeping with the attitude of the user&#39;s head, the sense of immersion in the visual word is further enhanced. 
     Recent years have witnessed widespread use of an omnidirectional camera (omniazimuth camera) for capturing an omnidirectional, 360-degree panoramic image. Also, development has been underway of remotely controllable unmanned flying objects. Mounting multiple cameras on such a flying object makes it possible to capture an omnidirectional panoramic image from the air. If the HMD is caused to display the omnidirectional panoramic image captured in this manner and to let the head tracking function update the display screen in keeping with the attitude of the user&#39;s head, the user is expected to feel as if he or she is in the real world. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     JP 2015-95045A 
     SUMMARY 
     Technical Problem 
     In a recent trend of improving the realistic sensation of imagery, development is actively underway of techniques for widening the viewing angle of the HMD. HMDs with their viewing angle exceeding 80 degrees have already been commercialized. The wide-viewing-angle HMD offers the user the world of video imagery comparable to the real world. 
     The inventors experimented with various kinds of image display on the wide-viewing-angle HMD. The experiments revealed the possibility that the user may feel sick depending on how the image is presented. This sensation will be referred to as “motion sickness” or “simulator sickness” hereunder in this description. In some experiments, the same image did not provoke motion sickness when being viewed on the TV screen but triggered the unpleasant sensation while being watched on a HMD screen. Where information elements such as a menu were included in the display image, the user had a feeling of discomfort depending on how the information elements were presented. The findings from such diverse experiments have led the inventors to devise a manner of display control suitable for the HMD. 
     The present invention has been made in view of the above circumstances. An object of the invention is therefore to provide technology for implementing display control suitable for the HMD. 
     Solution to Problem 
     In solving the above problem and according to one aspect of the present invention, there is provided an information processing apparatus including: a detecting section configured to detect the attitude of a head-mounted display device worn on the head of a user; a visual line direction determining section configured to determine a visual line direction in accordance with the attitude of the head-mounted display device detected by the detecting section; an image generating section configured to generate an image based on the determined visual line direction; an image providing section configured to provide the head-mounted display device with the generated image; and an instruction acquiring section configured to acquire from an input device an instruction to switch the visual line direction. When the instruction acquiring section acquires the switching instruction, the visual line direction determining section changes the visual line direction by a predetermined angle. 
     According to another aspect of the present invention, there is provided an image generating method including the steps of: detecting the attitude of a head-mounted display device worn on the head of a user; determining a visual line direction in accordance with the detected attitude of the head-mounted display device; generating an image based on the determined visual line direction; and acquiring from an input device an instruction to switch the visual line direction. When the switching instruction is acquired, the visual line direction determining step changes the visual line direction by a predetermined angle. 
     Incidentally, if other combinations of the above-outlined composing elements or the above expressions of the present invention are converted between different forms such as a method, a device, a system, a computer program, and a recording medium having the computer program recorded readably thereon, they still constitute effective embodiments of this invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a typical configuration of an information processing system as one embodiment of the present invention. 
         FIG. 2  is an explanatory diagram explaining a rendering process performed by an information processing apparatus. 
         FIG. 3  is a schematic diagram illustrating a typical external shape of the HMD. 
         FIG. 4  is a block diagram illustrating functional blocks of the HMD. 
         FIG. 5  is a block diagram illustrating functional blocks of the information processing apparatus. 
         FIG. 6  is an explanatory diagram explaining panoramic image data to be stored in an image storing section. 
         FIG. 7  is a schematic diagram illustrating a display image. 
         FIG. 8  is a schematic diagram illustrating another display image. 
         FIG. 9  is a schematic diagram illustrating another display image. 
         FIG. 10  is a set of schematic diagrams illustrating the top and the rear side of an input device. 
         FIG. 11  is a schematic diagram illustrating another display image. 
         FIG. 12  is a set of schematic diagrams illustrating display images given before and after a switch. 
         FIG. 13  is a set of schematic diagrams illustrating how information elements are typically superimposed on display images. 
         FIG. 14  is a set of schematic diagrams illustrating how information elements are moved along with panoramic images. 
         FIG. 15  is a set of schematic diagrams illustrating how information elements are superimposed on panoramic images. 
         FIG. 16  is a schematic diagram illustrating how information elements are superimposed on a panoramic image. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
       FIG. 1  illustrates a typical configuration of an information processing system  1  as one embodiment of the present invention. The information processing system  1  includes an information processing apparatus  10 , a head-mounted display device (HMD)  100  worn on the head of the user, an input device  6  operated by the user with fingertips, an imaging device  7  for imaging the user wearing the HMD  100 , and an output device  4  for displaying images. 
     In this embodiment, the information processing apparatus  10  includes a processing device  12  and an output control device  14 . The processing device  12  is a terminal device that executes diverse applications including games in response to operation information coming from the input device  6  operated by the user. The processing device  12  and the input device  6  may be connected with each other by cable or by known wireless communication technology. The output control device  14  is a processing unit that outputs image data to the HMD  100 . The output control device  14  and the HMD  100  may be connected with each other by cable or by known wireless communication technology. 
     The imaging device  7  captures an image of the user wearing the HMD  100  and feeds the captured image to the processing device  12 . The imaging device  7  may be a stereo camera. As will be discussed later, the HMD  100  is furnished with markers (light-emitting diodes (LEDs) for tracking) permitting tracking of the user&#39;s head. In accordance with the captured marker positions, the processing device  12  detects the movement of the HMD  100 . The HMD  100  is further provided with attitude sensors (an acceleration sensor and a gyro sensor). The processing device  12  acquires from the HMD  100  sensor information detected by the attitude sensors, implementing a highly precise tracking process with the joint use of the sensor information and captured marker images. 
     Installation of the output device  4  is not mandatory for the information processing system  1  because the user views images displayed on the HMD  100 . Still, the output control device  14  or the processing device  12  may cause the output device  4  to output the same image as that displayed on the HMD  100 . This allows another user to view on the output device  4  the image being watched by the user on the HMD  100 . As will be described later, the image displayed on the HMD  100  is corrected for the distortion of optical lenses. This requires getting the output device  4  to output an image not corrected for the distortion. 
     In the information processing system  1 , the processing device  12 , output device  4 , input device  6 , and imaging device  7  may constitute a conventional game system. In this case, the processing device  12  may be a game device that executes applications such as video games, and the input device  6  may be a game controller, a keyboard, a mouse, a joystick, or some other suitable device for supplying the processing device  12  with the operation information from the user. Supplementing this game system with the output control device  14  and the HMD  100  as additional components makes up the information processing system  1  that executes virtual reality (VR) applications. 
     The function of the output control device  14  may be incorporated in the processing device  12  as part of the functions of VR applications. That is, the information processing apparatus  10  may be formed by the processing device  12  alone or by the processing device  12  and the output control device  14 . In the description that follows, the function of the processing device  12  and that of the output control device  14  necessary for implementing VR applications will be explained collectively as functions of the information processing apparatus  10 . 
     The information processing apparatus  10  generates image data to be displayed on the HMD  100 . In this embodiment, the information processing apparatus  10  prepares an omnidirectional, 360-degree panoramic image captured by an omnidirectional camera, and causes the HMD  100  to display an image generated on the basis of a visual line direction determined by the attitude of the HMD  100  worn on the user&#39;s head. The content of display may be a still image or a moving image. The display content is not limited to actually captured images; the content may also be images rendered in real time by a game application. 
     The HMD  100  is a display device that is worn on the user&#39;s head and displays images through optical lenses on a display panel positioned in front of the user&#39;s eyes. The HMD  100  displays a left-eye image on the left half of the display panel and a right-eye image on the right half of the display panel in a manner independent of each other. These images constitute parallax images viewed from the right and left viewpoints. When displayed on the right-half and left-half portions of the display panel, the parallax images make up a stereoscopic image. Because the user views the display panel through optical lenses, the information processing apparatus  10  supplies the HMD  100  with the image data corrected beforehand for the optical distortion of the lenses. In the information processing apparatus  10 , the optical distortion correcting process may be performed by the processing device  12  or by the output control device  14 . 
       FIG. 2  is an explanatory diagram explaining a rendering process performed by the information processing apparatus  10 . For VR application of this embodiment, a virtual environment is provided in such a manner that the user feels as if he or she is at the center of a sphere and that the image visible to the user is varied when his or her visual line direction is changed. A content image constituting the image material is pasted on the inner circumferential surface of a virtual sphere centering on a center point  9  at which the user is positioned. The content image is an omnidirectional, 360-degree panoramic image captured by an omnidirectional camera. This content image is pasted on the inner circumferential surface of the virtual sphere in such a manner that the zenith and nadir positions of the image coincide with those of the virtual sphere. When the zenith and nadir positions of the user&#39;s real world are thus aligned with those of the visual world offered to the HMD  100 , it permits implementation of a VR application that reproduces the realistic visual world. 
     The information processing apparatus  10  detects the rotation angle and inclination of the user&#39;s head (HMD  100  in reality) by performing a tracking process on the user&#39;s head position. The rotation angle of the HMD  100  is defined relative to a horizontal reference direction. For example, the direction in which the HMD  100  is oriented when switched on may be established as the reference direction. The inclination of the HMD  100  is defined as an inclination angle relative to a horizontal plane. Existing techniques may be used to implement the head tracking process. The information processing apparatus  10  may detect the rotation angle and inclination of the HMD  100  solely from the sensor information detected by the attitude sensors of the HMD  100 . The information processing apparatus  10  may also detect the rotation angle and inclination of the HMD  100  with higher accuracy by analyzing images of the markers (tracking LEDs) on the HMD  100  captured by the imaging device  7 . 
     The information processing apparatus  10  determines the attitude of a virtual camera  8  in the virtual sphere in accordance with the detected rotation angle and inclination of the HMD  100 . The virtual camera  8  is arranged to image the inner circumferential surface of the virtual sphere from its center point  9 . The information processing apparatus  10  aligns the detected rotation angle and inclination with the rotation angle and inclination of the optical axis of the virtual camera  8  in the virtual sphere. The information processing apparatus  10  acquires a captured image  5  from the virtual camera  8 , i.e., performs a rendering process, corrects the image for the optical distortion of the optical lenses, and feeds the image data to the HMD  100 . Whereas  FIG. 2  illustrates a single virtual camera  8 , a right-eye virtual camera  8  and a left-eye virtual camera  8  are in reality provided to generate their respective image data. 
       FIG. 3  illustrates a typical external shape of the HMD  100 . In this example, the HMD  100  is made up of an output mechanism section  102  and a wearing mechanism section  104 . The wearing mechanism section  104  includes a wearing band  106  worn by the user to fasten the HMD  100  around the head. The wearing band  106  uses a material or a structure that allows the band to be adjusted in length to fit around the user&#39;s head. 
     The output mechanism section  102  includes an enclosure  108  that covers both eyes of the user wearing the HMD  100 . Inside the enclosure  108  is a display panel positioned straight against the eyes when the HMD  100  is worn. The display panel may be formed by a liquid crystal panel or by an organic electroluminescent (EL) panel, for example. Also inside the enclose  108  are a pair of right and left optical lenses positioned between the display panel and the user&#39;s eyes when the HMD  100  is worn, the optical lenses serving to widen the viewing angle of the user. The HMD  100  may be further equipped with speakers or earphones positioned against the user&#39;s ears when the HMD  100  is worn. 
     The external surface of the enclosure  108  is equipped with light-emitting markers  110   a ,  110   b ,  110   c  and  110   d . Although tracking LEDs make up the light-emitting markers  110  in this example, other suitable markers may be used instead. In any case, the markers need only be imaged by the imaging device  7  in a manner permitting image analysis by the information processing apparatus  10 . Whereas the light-emitting markers  110  are not limited in number and in arrangement, their number and their arrangement need to be such as to permit detection of the attitude of the HMD  100  (its rotation angle and inclination). In the illustration, the markers are arranged at four corners on the front of the enclosure  108 . The light-emitting markers  110  may be additionally arranged on the sides and the rear of the wearing band  106 . These additional markers may be imaged if the user turns his or her back to the imaging device  7 . 
     The HMD  100  may be connected with the information processing apparatus  10  by cable or by known wireless communication technology. The HMD  100  transmits sensor information detected by the attitude sensors to the information processing apparatus  10 . The HMD  100  also receives image data generated by the information processing apparatus  10  and displays the received data on the display panel. 
     Whereas the HMD  100  illustrated in  FIG. 3  is an immersive (non-transmissive) display device that fully covers both eyes, the HMD  100  may also be a transmissive display device. In terms of shape, the HMD  100  may be of hat type as illustrated but may also be of spectacle type. 
       FIG. 4  illustrates functional blocks of the HMD  100 . A control section  120  is a main processor which processes diverse data such as image data, audio data, sensor information, and instructions and which outputs what is processed. A storage section  122  temporarily stores the data and instructions processed by the control section  120 . Attitude sensors  124  detect attitude information about the HMD  100  such as its rotation angle and inclination. The attitude sensors  124  include at least a three-axis acceleration sensor and a three-axis gyro sensor. A microphone  126  converts the user&#39;s voice to an electrical signal. The light-emitting markers  110  are multiple LEDs attached to the wearing band  106  and enclosure  108  of the HMD  100 . 
     A communication control section  128  transmits the data input from the control section  120  to an external information processing apparatus  10  by wired or wireless communication via a network adapter or an antenna. The communication control section  128  further receives data from the information processing apparatus  10  by wired or wireless communication via the network adapter or the antenna, and outputs the received data to the control section  120 . 
     Upon receipt of image data and audio data from the information processing apparatus  10 , the control section  120  feeds the received data to the display panel  130  for display and to an audio output section  132  for audio output. The control section  120  further causes the communication control section  128  to transmit the sensor information from the attitude sensors  124  and the audio data form the microphone  126  to the information processing apparatus  10 . 
       FIG. 5  illustrates functional blocks of the information processing apparatus  10 . The information processing apparatus  10  includes a sensor information acquiring section  20 , a captured image acquiring section  22 , and an instruction acquiring section  24 , the sections serving as an input interface to the outside. The sensor information acquiring section  20  acquires the sensor information at predetermined intervals from the attitude sensors  124  of the HMD  100 . The captured image acquiring section  22  acquires captured images from the imaging device  7  imaging the HMD  100  at predetermined intervals. For example, the imaging device  7  captures images at intervals of 1/60 second, while the captured image acquiring section  22  acquires captured images at intervals of 1/60 second. The instruction acquiring section  24  acquires instructions input by the user from the input device  6 . 
     The information processing apparatus  10  further includes a motion detecting section  30 , a visual line direction determining section  32 , an image generating section  34 , and an image providing section  36 . The motion detecting section  30  detects the attitude of the HMD  100  worn on the user&#39;s head. The visual line direction determining section  32  determines the visual line direction in accordance with the attitude of the HMD  100  detected by the motion detecting section  30 . The image generating section  34  generates images in accordance with the detected attitude of the HMD  100 . Specifically, the image generating section  34  generates images based on the visual line direction determined by the visual line direction determining section  32 . The image providing section  36  provides the generated image to the HMD  100 . 
     The components indicated in  FIG. 5  as functional blocks for carrying out diverse processes may each be configured by a circuit block, a memory, or some other large-scale integrated (LSI) circuit in terms of hardware, or by a program loaded into a memory in terms of software. Thus it will be understood by those skilled in the art that these functional blocks are realized by hardware alone, by software alone, or by a combination of both in diverse forms and are not limited to any of such forms. 
     An image storing section  40  stores 360-degree panoramic image data captured beforehand. The image storing section  40  may store multiple content images. The content images may be still images or moving images. The image storing section  40  in the embodiment stores omnidirectional panoramic image data. The information processing apparatus  10  provides the user wearing the HMD  100  with an omnidirectional panoramic image. When the user turns his or her head to the right or to the left (the user may turn his or her entire body right or left) to turn his or her horizontal visual line right or left, the display panel  130  of the HMD  100  displays a panoramic image in the right or left direction. When the user tilts his or her head upward or downward to incline the visual line vertically, the display panel  130  of the HMD  100  displays a panoramic image in the upward or downward direction. 
     An information element storing section  42  stores information elements to be included in panoramic images. For example, the information elements may be menu items for changing the luminance of content images or for selecting a content image as an image material. When the user inputs to the input device  6  an instruction to display information elements, the image generating section  34  includes the information elements into the image to be generated. The information elements superimposed on the panoramic image are then displayed on the display panel  130  of the HMD  100 . The information elements may also be notification information destined for the user. In this case, the information elements may be superimposed on the panoramic image without being invoked by the user operating the input device  6 . 
       FIG. 6  is an explanatory diagram explaining panoramic image data to be stored in the image storing section  40 . For purpose of explanation,  FIG. 6  illustrates part of an omnidirectional panoramic image, and omits portions of the image from the horizontal plane downward as well as portions of the image in the right and left directions. As discussed above with reference to  FIG. 2 , the display panel  130  of the HMD  100  displays an image formed by rendering the panoramic image pasted on the inner circumferential surface of the virtual sphere. When the user changes his or her visual line direction by moving the rotation angle and inclination of the HMD  100 , the panoramic image being displayed is moved in keeping with the visual line direction. 
     The communication control section  128  of the HMD  100  transmits to the information processing apparatus  10  the sensor information acquired at predetermined intervals by the attitude sensors  124 . The imaging device  7  images the HMD  100  at predetermined intervals and transmits the captured images to the information processing apparatus  10 . Referring to  FIG. 5 , the sensor information acquiring section  20  acquires the sensor information from the attitude sensors  124  and feeds the acquired information to the motion detecting section  30 . The captured image acquiring section  22  acquires a captured image and feeds it to the motion detecting section  30 . 
     In detecting the attitude of the HMD  100 , the motion detecting section  30  performs the head tracking process to detect the attitude of the user&#39;s head wearing the HMD  100 . The head tracking process is carried out to synchronize the field of view displayed on the display panel  130  of the HMD  100  with the attitude of the user&#39;s head. The head tracking process of the embodiment involves detecting the rotation angle of the HMD  100  relative to a horizontal reference direction and the inclination angle of the HMD  100  relative to a horizontal plane. The horizontal reference direction may be established as the direction in which the HMD  100  is oriented when switched on, for example. 
     The head tracking process may be performed using existing techniques, with the motion detecting section  30  detecting the rotation angle of the HMD  100  relative to the horizontal reference direction and the inclination angle of the HMD  100  relative to the horizontal plane solely from the sensor information given by the attitude sensors  124 . Preferably, the accuracy of the detection may be enhanced by the additional use of captured images of the light-emitting markers  110  for tracking purposes. The motion detecting section  30  detects the rotation angle and the inclination angle at predetermined intervals. For example, if the image fed to the HMD  100  is captured at 60 frames per second (fps), the motion detecting section  30  may preferably perform its detecting process at intervals of 1/60 second. 
     The visual line direction determining section  32  determines the visual line direction in accordance with the attitude of the HMD  100  detected by the motion detecting section  30 . The visual line direction thus determined is both the visual line direction of the user and that (optical axis direction) of the virtual camera  8  arranged at the center point  9  of the virtual sphere (see  FIG. 2 ). Here, the visual line direction determining section  32  may determine the rotation angle and inclination angle detected by the motion detecting section  30  directly as representative of the visual line direction (optical axis direction) of the virtual camera  8 . The visual line direction determining section  32  may also determine the visual line direction of the virtual camera  8  by carrying out some suitable correcting process. If the motion detecting section  30  is not supplied with stable sensor information, as when the sensor information is contaminated with noise, the motion detecting section  30  might detect vibrating movements despite the user&#39;s head being stationary. In such a case, the visual line direction determining section  32  may determine the visual line direction by smoothing out the movements detected by the motion detecting section  30 . 
     Incidentally, the field of view of the humans is vertically asymmetrical; the view under the visual line is slightly wider than the view thereabove. Thus the visual line direction determining section  32  may determine the visual line direction of the virtual camera  8  by slightly tilting down the inclination angle detected by the motion detecting section  30 . 
     The image generating section  34  generates an image in accordance with the attitude of the HMD  100  detected by the motion detecting section  30 . Specifically, the image generating section  34  generates an image based on the visual line direction of the virtual camera  8  determined by the visual line direction determining section  32 . The image generating section  34  determines the right-eye and left-eye visual fields defined by the visual line direction, before generating a right-eye image and a left-eye image through rendering. At this point, the image generating section  34  generates a panoramic image corrected for the distortion of the image light coming from the display panel and passing through the optical lenses. 
     The HMD  100  of the embodiment provides the user with the visual field at an angle of approximately 100 degrees to the horizontal direction and at an angle of approximately 100 degrees to the vertical direction. Referring to  FIG. 2 , the captured image  5  is obtained at an angle of view of approximately 100 degrees to the horizontal direction and at an angle of view of approximately 100 degrees to the vertical direction inside the virtual sphere. The captured image  5  is displayed on the display panel  130  of the HMD  100 . As mentioned above, the visual field of the humans is slightly wider under the visual line than thereabove. For this reason, the optical lenses and the display panel  130  of the HMD  100  may be tilted 5 degrees in a direction opposite to the eye position. The optical lenses and the display panel  130  may thus be arranged to implement a vertical field of view at an upward angle of 45 degrees and at a downward angle of 55 degrees. 
       FIG. 7  illustrates a display image  200   a  generated by the image generating section  34 . In the drawings referenced below, the display image will be presented as an image cut out of the panoramic image for the ease of understanding the positional relation of the display image within the entire panoramic image. 
     The image generating section  34  generates an image based on a visual line direction  202   a  determined by the visual line direction determining section  32 . In practice, the image generating section  34  generates a right-eye display image and a left-eye display separately through rendering as different images each containing a parallax. In the description that follows, however, no mention will be made of the images being generated separately for both eyes. The image providing section  36  provides the HMD  100  with the display image  200   a  generated by the image generating section  34 . The control section  120  in the HMD  100  causes the display panel  130  to display the display image  200   a . The user is then able to view the display image  200   a  displayed on the display panel  130 . 
       FIG. 8  illustrates a display image  200   b  generated by the image generating section  34 . The visual line direction determining section  32  determines the visual line direction in accordance with the attitude of the HMD  100 . The image generating section  34  generates the image based on the visual line direction thus determined. This example indicates that the user has turned his or her head to the left, with the visual line changed continuously from a visual line direction  202   a  to a visual line direction  202   b . Here, the user has turned his or her head approximately 60 degrees to the left. The turning motion causes the image generating section  34  to generate images at intervals of 1/60 second in a manner turning the panoramic image continuously to the left starting from the display image  200   a . The image providing section  36  provides the HMD  100  with the generated images at intervals of 1/60 second. 
       FIG. 9  illustrates a display image  200   c  generated by the image generating section  34 . The visual line direction determining section  32  determines the visual line direction in accordance with the attitude of the HMD  100 . The image generating section  34  generates the image based on the visual line direction thus determined. This example indicates that the user has tilted up his or her head from the state in which the display image  200   a  is displayed on the display panel  130 , with the visual line changed continuously from the visual line direction  202   a  to a visual line direction  202   c . Here, the user has tilted up his or her head approximately 30 degrees. The tilting motion causes the image generating section  34  to generate images at intervals of 1/60 second in a manner moving the panoramic image continuously upward starting from the display image  200   a . The image providing section  36  provides the HMD  100  with the generated images at intervals of 1/60 second. 
     As described above, the user varies the visual line direction by moving his or her head. The information processing apparatus  10  provides the HMD  100  with a panoramic image given in a desired direction so that the HMD  100  will display the image on the display panel  130 . The manner in which the visual line direction is varied by moving the head is the same as in the real world. What is virtually brought about thus accords with the user&#39;s sensation of the real world. Where the HMD  100  provides the user with a wide viewing angle, the user&#39;s sense of immersion in the panoramic image is further enhanced. 
     Meanwhile, if the user wants to view a panoramic image immediately behind his or her back, the user obviously has to turn back fully. Just turning the head is not enough for turning back fully; the user needs to change the orientation of the body. If the user is sitting on a non-rotating chair such as a sofa, the user is required to stand up to change the orientation of the body. If the HMD  100  is a non-transmissive display device, the user has both eyes covered with the enclosure  108  and is unable to see the surroundings. This might make the user slightly hesitate to move his or her legs to change the orientation of the body. 
     With this embodiment, the user is allowed to input through the input device  6  instructions to switch the visual line direction; the user can change the visual line direction by operating the input device  6  without moving his or her head. Whereas the embodiment presents the input device  6  as a device different from the HMD  100 , the input device  6  may alternatively be integrated with the HMD  100 . That is, the input device  6  may be provided as an operation input section attached to the wearing band  106  or the enclosure  108  of the HMD  100 , for example. 
     Subfigure (a) in  FIG. 10  illustrates the top of the input device  6 . The user operates the input device  6  by holding a left-side grip  78   b  with the left hand and a right-side grip  78   a  with the right hand. The enclosure top of the input device  6  is equipped with arrow buttons  71 , analog sticks  77   a  and  77   b , and operation buttons  76  making up the input section. The arrow buttons  71  include an upward button  71   a , a leftward button  71   b , a downward button  71   c , and a rightward button  71   d . The right analog stick  77   a  and the left analog stick  77   b  are tilted to input directions and tilting amounts. The right analog stick  77   a  and the left analog stick  77   b  also function as depressable buttons that are depressed when pushed by the user and return to the initial position when released. On the enclosure top, a touch pad  79  is arranged in a flat region between the arrow buttons  71  and the operation buttons  76 . The touch pad  79  also functions as a depressable button that is depressed when pushed by the user and returns to the initial position when released. 
     A home button  80  is arranged between the right analog stick  77   a  and the left analog stick  77   b . The home button  80  is used to turn on the power supply of the input device  6  and to activate a communication function for wirelessly communicating with the information processing apparatus  10  at the same time. A SHARE button  81  is arranged to the left of the touch pad  79 . The SHARE button  81  is used to input the user&#39;s instructions to the operating system (OS) of the information processing apparatus  10  or to its software. An OPTIONS button  82  is arranged to the right of the touch pad  79 . The OPTIONS button  82  is used to input the user&#39;s instructions to an application (game) executed by the information processing apparatus  10 . The SHARE button  81  and the OPTIONS button  82  may each be formed as a push-button. 
     Subfigure (b) in  FIG. 10  illustrates the rear side of the input device  6 . At the upper part of the rear side of the enclosure of the input device  6  is a bent touch pad  79  extending from the top of the enclosure. At the lower part of the rear side of the enclosure is a horizontally elongated light-emitting section  85 . The light-emitting section  85  has red (R), green (G), and blue (B) LEDs that are lit in accordance with luminescent color information sent from the information processing apparatus  10 . An upper-right R1 button  83   a  and an upper-left L1 button  83   b  are each configured as a push-button. A lower-right R2 button  84   a  and a lower-left L2 button  84   b  are each configured as a pivotally-supported trigger type button. 
     Of the operating members of the input device  6  in this embodiment, the left analog stick  77   b  is used to input an instruction to switch the visual line direction. Tilting the left analog stick  77   b  left moves the visual line direction to the left, and tilting it right moves the visual line direction to the right. The left analog stick  77   b  is thus fit for the user to intuitively change the visual line direction. Other operating members such as the right analog stick  77   a  or the arrow buttons  71  may be used alternatively to input the instruction to switch the visual line direction. 
     The inventors experimented with various techniques of the process for changing the visual line direction by operation of the left analog stick  77   b . One technique of the changing process involved continuously moving the visual line direction in accordance with the direction in which the left analog stick  77   b  is tilted to generate images in such a manner that the panoramic image would be moved in a flowing manner. This is a commonly-practiced method of changing the visual line direction in games displayed on the TV screen and is thought to well accord with the user&#39;s senses. Diverse experiments were also made with the speed at which the image was moved. 
     The experiments revealed that moving the panoramic image continuously by operation of the left analog stick  77   b  gives motion sickness to the user watching the image. Motion sickness was provoked regardless of the speed at which the image was moved. People experienced motion sickness, to a greater or lesser extent, regardless of whether the image was moved quickly or slowly. 
     On the TV screen, the display image is often moved continuously in one direction for dramatic impact. At such a time, the user usually does not experience motion sickness. Looking into this point, the inventors found that a difference in viewing angle is one of the causes of the unpleasant sensation. 
     The user views the TV screen from a certain distance. That means the viewing angle of the user in the horizontal direction of the TV screen is several dozen degrees at most. In contrast, the user&#39;s viewing angle on a wide-viewing-angle HMD is close to 90 degrees or equal to or greater than 90 degrees (the viewing angle in the horizontal direction of the HMD  100  is assumed to be approximately 100 degrees hereunder). That is, the HMD  100  tends to give motion sickness to the user because the image is continuously moved within the range of approximately 100 degrees (or over the entire field of view). This turned out to be a problem unique to the HMD  100 . The inventors then experimented with moving the panoramic image not continuously but discontinuously by operation of the left analog stick  77   b . The discontinuous movement of the image was found to prevent the onset of motion sickness. Given these findings, upon acquiring the operation of the left analog stick  77   b  as an instruction to switch the visual line direction, the information processing apparatus  10  of the embodiment performs display control in such a manner as to move the panoramic image at intermittent times (i.e., in steps), the image being moved at a predetermined angle per step. 
     When the user tilts the left analog stick  77   b  on the input device  6 , the instruction acquiring section  24  acquires from the input device  6  the instruction to switch the visual line direction in the direction in which the stick is tilted. For example, if the left analog stick  77   b  is tilted once and allowed to return immediately to the initial position, the instruction acquiring section  24  acquires an instruction to switch the visual line direction in one step in the direction of the tilt. If the left analog stick  77   b  is tilted continuously, the instruction acquiring section  24  continuously acquires instructions to switch the visual line direction in the direction of the tilt. 
     Whether the instruction acquiring section  24  acquires the tilting operation of the left analog stick  77   b  as a one-time switching instruction or as a continuous switching instruction is determined depending on whether the tilting time (i.e., a time period during which the stick is tilted and is allowed to return to the initial position) exceeds a predetermined time period. Specifically, if the tilting time is less than the predetermined time period, the instruction acquiring section  24  acquires the tilting operation as a one-time switching instruction; if the tilting time reaches the predetermined time period, the instruction acquiring section  24  acquires at that time the tilting operation as another switching instruction. If the tilting operation is further continued, the switching instruction may be acquired every time a time period shorter than the predetermined time period elapses. 
     When the instruction acquiring section  24  acquires the instruction to switch the visual line direction, the visual line direction determining section  32  changes the visual line direction by a predetermined angle. For example, if the left analog stick  77   b  is tilted to the left, the instruction acquiring section  24  acquires a switching instruction to turn the visual line direction to the left. The visual line direction determining section  32  then changes the visual line direction to the left by the predetermined angle. Explained below is an example in which, with the display panel  130  of the HMD  100  displaying the display image  200   a  illustrated in  FIG. 7 , the user inputs to the input device  6  a switching instruction to turn the visual line direction to the left. 
       FIG. 11  illustrates a display image  200   d  generated by the image generating section  34 . When the instruction acquiring section  24  acquires the switching instruction to turn the visual line direction to the left, the visual line direction determining section  32  turns the visual line direction to the left by the predetermined angle. The predetermined angle is assumed to be larger than 10 degrees and smaller than the viewing angle (100 degrees) of the HMD  100  in the horizontal direction. 
     If the angle to be changed in one step is set to be smaller than 10 degrees, at least 18 steps are needed by the user to view the image immediately behind his or her back (i.e., to make a 180-degree turn), for example. The numerous steps required might make the user a little impatient. On the other hand, if the angle to be changed in one step is larger than the viewing angle of the HMD  100 , then a totally new image (an image not overlapping with the unswitched image) would suddenly appear on the display panel  130 ; the continuity with the unswitched image is not guaranteed. Preferably, the angle to be changed in one step is set to be larger than 10 degrees and smaller than the viewing angle of the HMD  100 . 
     More preferably, the angle to be changed in one step may be set to be less than half the viewing angle of the HMD  100 . This causes at least half of the unswitched image to be included in the newly switched image. The user is then able to recognize image continuity before and after the switch. To let the user better recognize image continuity, the angle to be changed in one step may preferably be smaller. After experimenting with diverse angles to be changed per step, the inventors concluded that an angle larger than 15 degrees and smaller than 30 degrees is most preferred as the angle to be changed per step in view of image continuity and the time to reach the desired angle through continuous switches.  FIG. 11  illustrates an example in which the viewing angle is changed by 22.5 degrees in one step. 
     Generally, the central visual field of the humans is said to be 40 to 45 degrees and the peripheral visual field to be approximately 200 degrees. With the characteristic of the central visual field taken into consideration and with the changing angle per step set to be smaller than the central visual field, the same image may be arranged to partially remain in the central visual field before and after the switch. In view of this, it makes sense to set the angle to be changed in one step at 30 degrees or less. Preferably, the angle to be changed in one step is set to be a value obtained by dividing 360 degrees by an integer N so that the user can return to the initial display position by inputting through the input device  6  the integer number of switching instructions. 
       FIG. 12  illustrates display images displayed on the display panel  130  before and after a switch. Subfigure (a) in  FIG. 12  illustrates the display image  200   a , and Subfigure (b) in  FIG. 12  indicates the display image  200   d . The display image  200   d  is obtained by turning the display image  200   a  to the left by 22.5 degrees in one step. Given the viewing angle of 100 degrees, the angle to be changed in one step is set to be 22.5 degrees. Thus making one switch changes 22.5 percent of the image, which leaves 77.5 percent of the image being the same before and after the switch. This allows the user to easily recognize image continuity. 
     If the instruction acquiring section  24  acquires switching instructions continuously, the visual line direction determining section  32  changes the visual line direction at intervals of a predetermined angle. The changing interval may be a fixed value such as 1 second or defined dynamically depending on the tilting amount of the left analog stick  77   b . That is, the larger the tilting amount, the shorter the changing interval may be set to be. 
     With this embodiment, the switching instructions given by the use of the input device  6  are effective only in the horizontal direction and not in the vertical direction. That is because people, when sitting or standing, usually look in the horizontal direction and not in the upward or downward direction continuously and because the actual senses of people are respected in VR applications. It is possible to let the switching instructions be effective in the vertical direction as well. Whether or not to enable switching instructions in the vertical direction may be determined depending on the content image. The switching instructions in the vertical direction may be enabled for content images such as captured images of a starry sky in a celestial sphere where the zenith and nadir positions need not be taken into account. 
     Whereas  FIG. 12  illustrates the display images  200   a  and  200   d  displayed before and after a switch made with the angle of 22.5 degrees in one step, the user may wish to make the changing angle larger or smaller than 22.5 degrees. In such a case, the display panel  130  may be caused to display alternatives for selecting the angle to be changed per step. The user may then operate the input device  6  to select one of the alternatives displayed on the display panel  130 . This embodiment provides the user with the alternatives of 15 degrees and 30 degrees in addition to the choice of 22.5 degrees. 
     Explained below is a procedure for causing the display panel  130  of the HMD  100  to display the alternatives for selecting the angle to be changed per step, with the display panel  130  displaying the display image  200   a  given in Subfigure (a) of  FIG. 12 . 
     The user may cause the display panel  130  to display menu items by operating a predetermined input section of the input device  6 . For example, a menu display operation may be assigned to a triangle (A) button  75  (see Subfigure (a) in  FIG. 10 ). When the user pushes the A button  75 , the input device  6  transmits to the information processing apparatus  10  operation information indicating that the A button  75  is pushed. The instruction acquiring section  24  in the information processing apparatus  10  acquires the pushing information from the A button  75  as an instruction to display the menu items. 
     When the instruction acquiring section  24  acquires the instruction to display the menu items, the image generating section  34  performs a process of including the menu items into the panoramic image being generated. The menu items are an example the information elements to be presented to the user. Alternatively, the information elements may be other items or notification information. When the image generating section  34  includes the information elements such as the menu into the panoramic image, the user may view the displayed information elements on the display panel  130  of the HMD  100  while looking at the panoramic image in the visual line direction on the display panel  130 . 
     The information element storing section  42  stores the information elements to be included in the panoramic image. The image generating section  34  reads from the information element storing section  42  the information elements corresponding to the display instruction, and superimposes a window listing the information elements onto the panoramic image. The image providing section  36  provides the HMD  100  with the panoramic image including the menu items. 
     Subfigure (a) in  FIG. 13  illustrates an example in which a menu window  204  is superimposed on the display image  200   a . In Subfigure (a) of  FIG. 13 , the panoramic image behind the menu window  204  is hidden thereby. Preferably, the menu window  204  in practice may be displayed transparently so that the panoramic image behind the menu window  204  can be seen through. 
     In the menu window  204 , a selection rectangle enclosing one item is displayed. The user may move the selection rectangle by pushing the upward button  71   a  or the downward button  71   c  on the input device  6 . When the selection rectangle is positioned to the desired item and an enter button (e.g., a circle (○) button  72 ) is pushed, the display panel  130  displays the information elements subordinate to the selected item. 
     In the information processing apparatus  10 , the instruction acquiring section  24  acquires the operation information from the arrow buttons  71  as instructions to select the item and the operation information from the ○ button  72  as the instruction to enter the item. The image generating section  34  moves the selection rectangle in accordance with the instructions to select the item, reads from the information element storing section  42  the corresponding information elements in accordance with the instruction to enter the item, and includes the retrieved information elements into the panoramic image. 
     Subfigure (b) in  FIG. 13  illustrates an example in which a selection window  206  is superimposed on the display image  200   a . If the item “Change the angle to be changed in one step” is selected in the menu window  204  of Subfigure (a) in  FIG. 13 , the selection window  206  is displayed to present alternatives for selecting the angle to be changed per step. Preferably, as with the menu window  204 , the selection window  206  may be displayed transparently to let the user view the panoramic image behind the window. When the user selects one of the angle-selecting alternatives and enters the selection on the selection window  206 , the new angle to be changed in one step is selected. 
     The information elements are superimposed on the panoramic image in the manner described above and displayed on the display panel  130 . In the information processing system  1 , the panoramic image on the display panel  130  is changed in accordance with the changing attitude of the HMD  100  (i.e., movement of the visual line direction) and with operations on the left analog stick  77   b . The inventors examined how the HMD  100  would control the manner in which the information elements are displayed along with the panoramic image. 
     The inventors first experimented with a technique for always displaying the information elements at the center of the screen on the display panel  130 . In this case, the position where the information elements are displayed on the screen remains unchanged regardless of the panoramic image behind the information elements being changed in keeping with the changing visual line direction. The information elements are always displayed in a fixed position at the center of the screen. This displaying technique turned out to give the impression as if the information elements were moving within the panoramic image, thereby provoking a feeling of discomfort. 
     The inventors then experimented with a technique by which the information elements, once included into the panoramic image, are kept fixed where they are arranged. That is, the position where the information elements are arranged in the panoramic image is fixed. When the user changes his or her visual line direction, the information elements are moved integrally with the panoramic image. This technique turned out not to provoke a feeling of discomfort in the user because the relative positional relation between the information elements and the panoramic image is fixed so that the information elements appear to be part of the panoramic image. 
     In this case, however, a large movement of the visual line direction can cause the information elements to disappear from the display screen because they move along with the panoramic image. The user then loses sight of the information elements. In particular, because the input device  6  is assigned to the menu operations while the menu is being displayed, the user is required to change his or her visual line direction to find the menu window that has disappeared, regardless of whether the menu operations are to be continued or terminated. 
     To solve that problem, the inventors came up with a technique for basically fixing the relative positional relation between information elements and the panoramic image so that the information elements will move together with the panoramic image. When the information elements are about to disappear from the screen, this technique causes the relative positional relation between the information elements and the panoramic image to be changed. 
     According to this technique, the point at which information elements are initially displayed is determined as a reference point. The attitude of the HMD  100  is then changed from the reference point. If the amount of change in the attitude is smaller than a predetermined first angle, the image generating section  34  displays the information elements in a manner that they move along with the image. If the amount of change in the attitude reaches the predetermined first angle, the image is generated in such a manner that the information elements are displayed in a position moved by a predetermined second angle within the image. The amount of change in the attitude of the HMD  100  corresponds to the amount of change in the movement of the visual line direction. 
     Specifically, the visual line direction determining section  32  monitors changes in the movement of the visual line direction starting from the point at which the information elements are initially displayed. If the movement of the visual line direction is smaller than the predetermined first angle, the image generating section  34  displays the information elements in a manner that they move along with the panoramic image. If the movement of the visual line direction reaches the predetermined first angle, the image generating section  34  displays the information elements in a position moved by the predetermined second angle within the panoramic image. Explained below is an example in which the visual line direction determining section  32  monitors the rotation angle of the visual line in the horizontal direction, with the image generating section  34  performing display control of the information elements accordingly. 
     For example, suppose that starting from the point at which the display of information elements is started, the user turns his or her head to the left in the horizontal direction so that the user&#39;s visual line direction is changed to the left. In this case, the panoramic image is displayed as if it were flowing to the right. Thus the displayed information elements also appear to be flowing rightward on the display panel  130 . When the traveling angle of the information elements, i.e., the rotation angle of the visual line in the horizontal direction, reaches the predetermined first angle, the image generating section  34  displays the information elements after moving them to a position reached by a left turn at the predetermined second angle within the panoramic image. The first angle is set to be such that the information elements will not disappear completely from the user&#39;s visual field when moved. The information elements are thus kept displayed at least partially on the display panel  130 . In the description that follows, the first angle may be referred to as “position change reference angle” and the second angle as “return angle” where appropriate. 
     Explained below is an example in which the user turns his or her visual line direction to the left from an initial display state where the menu window  204  is initially displayed as illustrated in Subfigure (a) of  FIG. 13 .  FIG. 14  illustrates how the menu window  204  listing the information elements is moved along with the panoramic image. When the user turns his or her head to the left in the horizontal direction, the motion detecting section  30  detects the attitude of the HMD  100 . The visual line direction determining section  32  turns the visual line direction to the left in accordance with the detected attitude of the HMD  100 . Subfigure (a) in  FIG. 14  illustrates the display image  200   d  as it is turned to the left by 22.5 degrees from the initial display state. As discussed above, the 22.5-degree turn is made alternatively by operation of the left analog stick  77   b . Without moving the head, the user may tilt the left analog stick  77  to the left to have the display image  200   d  displayed on the display panel  130 . 
     Because the relative positional relation between the menu window  204  and the panoramic image is fixed, the menu window  204  is moved along with the panoramic image. On the display panel  130 , the user thus watches the menu window  204  moving to the right. 
     Subfigure (b) in  FIG. 14  illustrates a display image  200   e  as it is turned 45 degrees to the left from the initial display state. In order to display the display image  200   e , the user may turn his or her head 45 degrees to the left. Alternatively, the 45-degree left turn may be accomplished using the left analog stick  77   b  being tilted twice to the left or by the user turning the head 22.5 degrees to the left followed by the left analog stick  77   b  being tilted once to the left. Compared with Subfigure (a) in  FIG. 14 , the menu window  204  appears to be moved further to the right. 
     Subfigure (a) in  FIG. 15  illustrates a display image  200   f  as it is turned to the left from the initial display state until the position change reference angle is just about to be reached. The position change reference angle is set here to be such that the information elements do not entirely disappear from the screen. 
     This requires that the position change reference angle be set to be smaller than the viewing angle of the HMD  100 . If the position change reference angle is equal to or larger than the viewing angle, the information elements could entirely disappear from the screen. The position change reference angle in the horizontal direction should thus be set to be smaller than 100 degrees because the viewing angle of the HMD  100  in the horizontal direction is 100 degrees. 
     Experiments with various settings of the position change reference angle led the inventors to conclude that a position change reference angle set to be extremely small compared with the viewing angle of the HMD  100  gives the impression of the display elements more or less tracking the visual line. After the experiments, the position change reference angle was set to be approximately half the viewing angle of the HMD  100 . This angle setting was found to allow both display elements and the panoramic image in the background to be integrally recognized and to provide a user interface enabling the display elements to be visible. In the embodiment, the position change reference angle (first angle) is set to be 60 degrees, which is close to half (50 degrees) the viewing angle of the HMD  100 . 
     Subfigure (b) in  FIG. 15  illustrates a display image  200   g  with the information elements turned to the left from the initial display state within the panoramic image by the second angle (60 degrees), which is the return angle. When the amount of change in the visual line in this direction reaches the first angle (60 degrees) starting from the initial display state, the image generating section  34  rearranges the relative positional relation between the information elements and the panoramic image to a position moved by the second angle in the current direction. At this point, the second angle is set to be the same as the first angle. After the rearrangement, the visual line direction determining section  32  monitors changes in the movement of the visual line direction in reference to the visual line direction determined at the point of the rearrangement. 
     Whereas Subfigure (a) in  FIG. 15  illustrates only part of the menu window  204 , Subfigure (b) in  FIG. 15  illustrates the entire menu window  204  being displayed again in its entirety. This facilitates menu operation of the menu window  204  by the user. And as illustrated in Subfigure (a) of  FIG. 15 , display control is performed in such a manner that the menu window  204  will not disappear completely from the screen. This allows the user to operate when desired the menu window  204  without losing sight of it while enjoying the panoramic image. 
     The foregoing paragraphs explained the example in which the first angle as the position change reference angle is the same as the second angle as the return angle. If the first angle and the second angle are identical and if the visual line direction is changed by the first angle from the initial display state, the information elements are rearranged to the center position at which the information elements were displayed in the initial display state as illustrated in Subfigure (b) of  FIG. 15 . Thus if the user turns his or her head further to the left in the horizontal direction, the menu window  204  moves from the center to the right. That means the menu window  204  is displayed somewhat to the right, which is slightly uncomfortable to look at. 
     Explained below is an example in which the first angle as the position change reference angle is set to be larger than the second angle as the return angle. The second angle is set to be 60 degrees, and the first angle is set to be 45 degrees. 
     Suppose now that Subfigure (b) in  FIG. 14  illustrates the display image  200   e  as it is turned to the left from the initial display state until the first angle (45 degrees) is just about to be reached. If the visual line in this direction is further moved so that the amount of its change reaches the first angle (45 degrees), the image generating section  34  rearranges the relation positional relation between the information elements and the panoramic image to a position moved by the second angle (60 degrees) in the current direction. Because the second angle is set to be larger than the first angle, the rearranged information elements are returned past the center position of the initial display state and thus displayed to the left of the center position. Following the rearrangement, the visual line direction determining section  32  monitors changes in the movement of the visual line direction in reference to the visual line direction determined at the point of the rearrangement. 
       FIG. 16  illustrates a display image  200   h  with the information elements turned to the left at the second angle (60 degrees) within the panoramic image. When the amount of change in the visual line in this direction reaches the first angle (45 degrees) from the initial display state, the image generating section  34  rearranges the information elements to a position returned in the current direction by the second angle (60 degrees) within the panoramic image. 
     With the second angle set to be larger than the first angle, if the user keeps moving his or her visual line direction further to the left, the menu window  204  gradually moves to the right toward the center. This makes it possible to enhance the visibility of the information elements while allowing the user to recognize the integrity of the display elements and the panoramic image in the background. If the first angle is set to be larger than half the second angle, the amount of movement of the display elements in the panoramic image is prevented from increasing when the display elements are rearranged. 
     It was explained above that when the change of the visual line in the horizontal direction reaches the position change reference angle (first angle), the arrangement of the information elements in the horizontal direction is changed by the return angle (second angle). The same applies to the vertical direction. When the change of the visual line in the vertical direction reaches a position change reference angle in that direction, display control may be performed in such a manner that the arrangement of the information elements in the vertical direction is changed by a return angle in that direction. The position change reference angle in the vertical direction may be set to be smaller than the position change reference angle in the horizontal direction. For example, the position change reference angle in the vertical direction may be set to be 30 degrees. 
     Examined next is where information elements are to be initially displayed. One technique involves positioning the information elements near the center of the currently displayed image whenever the elements are displayed as illustrated in Subfigure (a) of  FIG. 13 . This technique is effective because it is easy to understand intuitively for the user. 
     Another technique may involve setting beforehand candidate display positions for the information elements in the virtual sphere. In the coordinate system of the virtual sphere, the horizontal reference direction on the horizontal plane is assumed to be zero degree. On that assumption, candidate rotation angles are set to be 60 degrees, 120 degrees, 180 degrees, 240 degrees, and 300 degrees in the horizontal direction; and candidate inclination angles are set to be 0 degree, ±30 degrees, and ±60 degrees in the vertical direction. When the information elements are displayed, the candidate rotation angle and the candidate inclination angle closest to the visual line direction may be extracted, and the information elements may be displayed in a manner centering on the extracted rotation angle and inclination angle. Where the initial position candidates are determined in advance, the image generating section  34  need only select the appropriate initial position from among the candidate positions. This technique simplifies display control. 
     While the present invention has been described in conjunction with a specific embodiment given as an example, it should be understood by those skilled in the art that the above-described composing elements and various processes may be combined in diverse ways and that such combinations, variations and modifications also fall within the scope of this invention. 
     It was explained above with reference to Subfigure (a) of  FIG. 14  that the display image  200   d  and menu window  204  are displayed turned 22.5 degrees to the left from the initial display state and that the 22.5-degree left turn may be achieved either by the user turning his or her head or by the left analog stick  77   b  being operated. In a variation, the left analog stick  77   b  may be operated to cancel the relative positional relation between the information elements and the panoramic image. In this case, the information elements may be displayed at the center as illustrated in Subfigure (b) of  FIG. 13  in the image following the witch by operation of the left analog stick  77   b . This is made possible because image continuity is broken by a jump of the display image when it is switched by operation of the left analog stick  77   b . In such a case, display control of the information elements is available following the unlocking of the relative positional relation between the information elements and the panoramic image. 
     It was also explained in connection with the embodiment that display control of the information elements is performed using the position change reference angle and the return angle. In a variation, display control of the information elements may be carried out in accordance with the information element type. For example, if the system issues an important warning to the user, the image generating section  34  may always arrange the warning at the center of the display image as the information element. 
     The image generating section  34  may implement a mirror function as a VR application function. The mirror function in this context is the function of including an image opposite to the visual line direction into the display image. What is included into the display image is not an image that is the exact opposite to the visual line direction but an image obtained by reversing only the rotation angle of the visual line direction, i.e., by turning the rotation angle of the visual line direction 180 degrees while leaving the inclination angle intact. This mirror function allows the user to view the image behind his or her back at the same height. 
     REFERENCE SIGNS LIST 
     
         
           1  Information processing system 
           6  Input device 
           8  Virtual camera 
           10  Information processing apparatus 
           12  Processing device 
           14  Output control device 
           20  Sensor information acquiring section 
           22  Captured image acquiring section 
           24  Instruction acquiring section 
           30  Motion detecting section 
           32  Visual line direction determining section 
           34  Image generating section 
           36  Image providing section 
           40  Image storing section 
           42  Information element storing section 
           100  Head-mounted display device (HMD) 
           102  Output mechanism section 
           104  Wearing mechanism section 
           106  Wearing band 
           108  Enclosure 
           110  Light-emitting marker 
           120  Control section 
           122  Storage section 
           124  Attitude sensor 
           126  Microphone 
           128  Communication control section 
           130  Display panel 
           132  Audio output section 
       
    
     INDUSTRIAL APPLICABILITY 
     The present invention may be implemented in technical fields where the head-mounted display device is used.