JP2017097918A - Image display method and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce visually induced motion sickness caused by receiving a visual effect not synchronized with movement of an HMD.SOLUTION: An image display method includes: generating a virtual camera space image captured by a virtual camera in a virtual space, which has a third person perspective different from a first person perspective associated with a user; generating a field-of-view image in which a display object, into which the virtual camera space image is rendered, is arranged in the virtual space; and outputting the generated field-of-view image to the head-mounted display. If there exist a plurality of display objects in a field-of-view image region, update timing is set for each display object, and each display object is updated with the set update timing.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image display method and a program. More specifically, the present invention relates to a method and program for displaying an image of a virtual space on a head-mounted display (HMD).

  Patent Document 1 discloses that an HMD is provided with a head tracking function and an image linked to the movement of the head of a user wearing the HMD is displayed on the HMD.

Japanese Patent Laying-Open No. 2015-95045

  However, the HMD system disclosed in Patent Document 1 may cause video sickness (so-called VR (Virtual Reality) sickness) when HMD tracking and the display image displayed on the HMD are not good.

In order to solve the above-described problem, one aspect of the present invention is a method of displaying a virtual space image on a head-mounted display worn by a user,
Generating a virtual camera space image captured by a virtual camera (virtual camera 2) in a virtual space having a third person viewpoint different from the first person viewpoint associated with the user;
Drawing the virtual camera space image on a display object in the virtual space;
Generating a visual field image in which the display object is arranged in the virtual space;
Outputting the field-of-view image to a head mounted display.

  According to the present invention, it is possible to reduce video sickness caused by receiving a visual effect that is not linked to the movement of the HMD.

1 is a diagram illustrating an HMD system according to an embodiment of the present invention. It is a figure which shows the hardware-like structure of a control circuit part. An orthogonal coordinate system in a three-dimensional space defined around the head of a user wearing an HMD is shown. It is an XYZ space figure which shows an example of virtual space. It is a block diagram which shows the function of the control circuit part for implement | achieving the function of a HMD system. It is a flowchart which shows the process for implement | achieving the function of a HMD system. It is a flowchart which shows the process for implement | achieving the function of a HMD system. It is a figure which shows the relationship between the some display object arrange | positioned in virtual space, and a visual field area | region. It is a figure which illustrates the update timing set for every display object. It is an example of the image displayed on the visual field area of the user wearing HMD.

First, the contents of one embodiment of the present invention will be listed and described. An image display method and program according to an embodiment of the present invention have the following configuration.
(Item 1)
A method of displaying a virtual space image on a head mounted display worn by a user,
Generating a virtual camera space image captured by a virtual camera in a virtual space having a third person viewpoint different from the first person viewpoint associated with the user;
Drawing the virtual camera space image on a display object in the virtual space;
Generating a visual field image in which the display object is arranged in the virtual space;
Outputting the field-of-view image to a head-mounted display;
Including methods.
(Item 2)
Determining a predetermined timing for rendering the virtual camera space image;
Drawing the virtual camera space image on a display object at the predetermined timing;
The method according to item 1, further comprising:
(Item 3)
The method according to item 1, wherein when it is determined that the virtual camera space image corresponds to a predetermined scene to be drawn on the display object, the virtual camera space image is drawn on the display object.
(Item 4)
There are a plurality of the virtual cameras, each arranged at different positions in the virtual space, the plurality of display objects are present, each arranged at a different position in the virtual space,
The method according to item 1, wherein each of the plurality of display objects corresponds to one virtual camera of the plurality of virtual cameras.
(Item 5)
Identifying a reference line of sight of a user wearing the head mounted display;
Determining a field of view of the user based on the reference line of sight;
5. The method according to item 4, wherein the virtual camera space image is drawn on a display object in the view field among the plurality of display objects, and is not drawn on a display object outside the view field among the plurality of display objects.
(Item 6)
6. The method according to item 5, further comprising a step of changing a timing of updating each of the plurality of display objects in the view field area when there are a plurality of display objects in the view field area.
(Item 7)
6. The method according to item 5, wherein the user's reference line-of-sight is specified based on one or both of an inclination of the head-mounted display and a user's line-of-sight direction specified by a gaze sensor included in the head-mounted display.
(Item 8)
A program for causing a computer to execute the method according to any one of items 1 to 7.
[Details of One Embodiment of the Present Invention]
A specific example of a program for controlling a head mounted display system according to an embodiment of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to the claim are included. In the following description, the same reference numerals are given to the same elements in the description of the drawings, and redundant descriptions are omitted.
[Overview of HMD system 100]
FIG. 1 shows an HMD system 100 including an HMD 110 according to the present embodiment. The HMD system 100 includes an HMD 110 that is worn on the head of a user 150, a control circuit unit 120, and a tracking sensor 132.

  The HMD 110 includes a display 112 that is a non-transmissive display device, a sensor unit 114, and a gaze sensor 116. The control circuit unit 120 displays a right-eye image and a left-eye image on the display 112, thereby providing a three-dimensional image using parallax between both eyes as a virtual space. By arranging the display 112 in front of the user 150, the user 150 can be immersed in the virtual space. The virtual space includes a background, various objects that can be operated by the user, menu images, and the like.

  The display 112 may include a right-eye sub-display that provides a right-eye image and a left-eye sub-display that provides a left-eye image. Moreover, as long as the image for right eyes and the image for left eyes can be provided, you may be comprised by one display apparatus. For example, the right-eye image and the left-eye image can be provided independently by switching the shutter so that the display image can be recognized by only one eye at high speed.

  The control circuit unit 120 is a computer connected to the HMD 110. As shown in FIG. 2, the control circuit unit 120 includes a processing circuit 202, a memory 204, an input / output interface 206, and a communication interface 208. The processing circuit 202, the memory 204, the input / output interface 206, and the communication interface 208 are connected to each other via a bus used as a data transmission path.

  The processing circuit 202 includes various processing circuits such as a CPU (Central Processing Unit), an MPU (Micro-processing unit), and a GPU (Graphics Processing Unit), and functions to control the control circuit unit 120 and the entire HMD system 100. Have

The memory 204 is a ROM (Read Only Memory) or a RAM (Random).
(Access Memory) and the like, and temporarily store control data such as programs and calculation parameters used by the processing circuit. The memory also includes a non-volatile storage device such as a flash memory or HDD (Hard Disc Drive), and stores data related to various images and objects, simulation programs, and user authentication programs, and manages various data. A database including a table for doing this may be constructed.

  The input / output interface 206 includes various wired connection terminals such as a USB (Universal Serial Bus) terminal, a DVI (Digital Visual Interface) terminal, an HDMI (registered trademark) (High-Definition Multimedia Interface) terminal, and various processes for wireless connection. A circuit is included, and various sensors including the HMD 110 and the motion sensor 130, an external controller, and the like are connected.

  The communication interface 208 includes various wired connection terminals for communicating with an external device via the network NW and various processing circuits for wireless connection, and communicates via a LAN (Local Area Network) or the Internet. It is configured to conform to various communication standards and protocols.

  The control circuit unit 120 presents a virtual space on the display 112 by executing a predetermined application stored in the memory. The memory stores programs for operating various objects displayed in the virtual space and displaying / controlling various menu images and the like. The control circuit unit 120 may not be mounted on the HMD 110 and may be configured as another hardware (for example, a known personal computer or a server computer via a network). In addition, the control circuit unit 120 may implement only a part of functions in the HMD 110 and implement the remaining functions in different hardware.

  The motion sensor 130 detects information related to the position and inclination of the HMD 110. The motion sensor 130 includes a sensor unit 114 and a tracking sensor 132. The sensor unit 114 may include a plurality of light sources. The light source is, for example, an LED that emits infrared rays. The tracking sensor 132 is, for example, an infrared sensor, and detects information about the position and angle of the HMD 110 in the real space according to the movement of the user 150 by detecting infrared rays from the light source as detection points of the HMD 110 over time. Then, based on the change over time of the information detected by the tracking sensor 132, it is possible to determine the time change of the position and angle of the HMD 110 and detect information related to the movement of the HMD 110.

  Information regarding the position and tilt acquired by the motion sensor 130 will be described with reference to FIG. A three-dimensional coordinate system is defined with the head of the user 150 wearing the HMD 110 as the center. The vertical direction in which the user stands upright is the yaw direction, the front-rear direction orthogonal to the yaw direction and connecting the center of the display 112 and the user is the roll direction, and the lateral direction orthogonal to the yaw direction and the roll direction is the pitch direction. Thereby, the temporal change of the position of the user in the three-dimensional space is acquired. Further, the pitch angle as the tilt angle of the HMD 110 around the pitch direction, the yaw angle as the tilt angle of the HMD 110 around the yaw direction, and the roll angle as the tilt angle of the HMD 110 around the roll direction are acquired.

  The motion sensor 130 may be configured by one of a sensor unit 114 fixed near the display 112 and a tracking sensor 132. The sensor unit 114 may be a geomagnetic sensor, an acceleration sensor, or a gyro sensor, and detects the position and inclination of the HMD 110 (particularly, the display 112) mounted on the head of the user 150 using at least one of them. To do. Thereby, the information regarding the motion of the HMD 110 can be detected. For example, the angular velocity sensor can detect the angular velocities around the three axes of the HMD 110 over time according to the movement of the HMD 110 and determine temporal changes in the angles around the respective axes. In this case, the tracking sensor 132 may not be provided. The tracking sensor 132 may include an optical camera. In this case, information related to the movement of the HMD 110 can be detected based on the image information, and the sensor unit 114 is not necessary.

  The gaze sensor 116 has an eye tracking function that detects the direction in which the eyes of the right eye and the left eye of the user 150 are directed. The gaze sensor 116 preferably includes a right eye sensor and a left eye sensor, and detects the gaze direction that the user 150 gazes by detecting the direction in which the gaze of the right eye and the left eye is directed. As the gaze sensor 116, a known sensor having an eye tracking function can be adopted. For example, the right eye and the left eye are irradiated with infrared light, and reflected light from the cornea and the iris is acquired, whereby the rotation angle of the eyeball is adjusted. It may be asking.

The gaze sensor 116 detects the gaze direction of the right eye and the left eye of the user 150, and specifies the gaze point that is the intersection of both. The gazing point specified when the user is looking near is closer to the user than the gazing point specified when the user is looking far. When the gazing point is specified, the line-of-sight direction of the user 150 is specified. The line-of-sight direction is the direction in which the user 150 is actually directed by both eyes. The line-of-sight direction is defined as, for example, the direction in which a straight line passes through the center of the user's right eye and left eye and the point of gaze.
[Description of One Embodiment]
FIG. 4 is an XYZ space diagram showing an example of a virtual space according to an embodiment of the present invention. In FIG. 4, the XZ plane represents the ground surface, and the Y-axis extends in the height direction. As shown in FIG. 4, the virtual space 6 is formed in a spherical shape with the center 3 as the center. A virtual camera 1 and a virtual camera 2 are arranged in the virtual space 6 and each captures an image in the virtual space 6. The motion of the virtual camera 1 is associated with the motion of the motion sensor 130, and the image captured by the virtual camera 1 changes as the motion sensor 130 moves. The virtual camera 1 has a first-person viewpoint of the user 150 or a viewpoint associated with the user's avatar. On the other hand, the virtual camera 2 is arranged at an arbitrary position in a virtual space different from the virtual camera 1 and has a third person viewpoint different from the first person viewpoint associated with the user.

  In FIG. 4, the virtual camera 1 captures a view image 8 in the view field 5 based on the reference line of sight 4, and the virtual camera 2 captures a second virtual camera space image 7. The second virtual camera space image 7 is an image including a character 9 playing a soccer game in the virtual space 6 and a ball, for example. A display object 10 in which the second virtual camera space image 7 is drawn is arranged in the view field image 8. A specific method for generating the view field image 8 will be described later.

  Next, the relationship between the motion sensor 130 and the virtual camera 1 arranged in the virtual space 6 will be described with reference to FIG. In order to explain the positional relationship between the virtual camera 1 and the motion sensor 130, the position of the motion sensor 130 is the position of the tracking sensor 132 when the tracking sensor 132 is provided, and the sensor when the tracking sensor 132 is not provided. The position of the portion 114 is assumed.

  In the present embodiment, it is preferable to adjust so that the center 3 of the celestial sphere is always arranged on the line connecting the virtual camera 1 and the sensor 130 in the XZ plane. For example, the virtual camera 1 may always be arranged at the center 3. In addition, when the user 150 wearing the HMD 110 moves and the position of the virtual camera 1 moves in the X direction, the region of the virtual space 6 so that the center 3 is located on the extension line of the virtual camera 1 and the motion sensor 130. May be changed. In these cases, the position of the virtual camera 1 in the virtual space 6 is fixed, and only the tilt changes. On the other hand, if the position of the virtual camera 1 is moved in conjunction with the movement of the motion sensor 130 in an arbitrary direction, the position of the virtual camera 1 in the virtual space 6 is variably set.

  FIG. 5 is a block diagram illustrating functions of the control circuit unit 120 for performing display processing of the virtual space 6 in the HMD system 100 and drawing of display objects displayed in the virtual space 6. The control circuit unit 120 controls an output image to the display 112 mainly based on inputs from the motion sensor 130 and the gaze sensor 116.

  The control circuit unit 120 includes a display control unit 200, an object control unit 300, and a storage unit 400. The display control unit 200 includes an HMD motion detection unit 210, a line-of-sight detection unit 220, a reference line-of-sight specification unit 230, a field-of-view area determination unit 240, and a field-of-view image generation unit 250. The object control unit 300 includes a second virtual camera image acquisition unit 310, an object specification unit 320, an update timing setting unit 330, a drawing timing determination unit 340, and an object drawing unit 350. The storage unit 400 includes a spatial information storage unit 410 and an image storage unit 420. Furthermore, the storage unit 400 includes various data necessary for calculation for providing output information corresponding to inputs from the motion sensor 130 and the gaze sensor 116 to the display 112. Note that the storage unit 400 corresponds to the memory 204 shown in FIG.

  First, with reference to FIGS. 4 to 6, a processing flow for determining the view area 5 corresponding to the virtual camera 1 will be described. The virtual space 6 can be provided by the interaction of the HMD 110, the gaze sensor 116, the motion sensor 130, and the control circuit unit 120.

  First, the HMD motion detection unit 210 acquires the position information and tilt information of the HMD 110 detected by the motion sensor 130, and determines the viewing direction of the user 150 based on the position information and tilt information of the HMD 110 (step S1). ).

  Next, the line-of-sight detection unit 220 acquires information regarding the movement of the left and right eyeballs of the user 150 detected by the gaze sensor 116, and specifies the user's line-of-sight direction based on the information regarding the movement of the user's eyeballs (step) S2).

  Next, the reference line-of-sight specifying unit 230 specifies the reference line of sight 4 based on the field-of-view direction specified by the inclination of the HMD 110 and / or the line-of-sight direction of the user 150 (step S3). For example, the straight line connecting the center of the right eye and the left eye of the user 150 and the center of the display 112 positioned in the viewing direction is specified as the reference line of sight 4. Further, for example, the user's line-of-sight direction specified by the gaze sensor 140 is specified as the reference line-of-sight 4.

  Next, the view area determination unit 240 determines the view area 5 of the virtual camera 1 in the virtual space 6 based on the reference line of sight 4 (step S4). The field-of-view area 5 includes a first area defined by the reference line of sight 4 and the YZ section of the virtual space 6, and a second area defined by the reference line of sight 4 and the XZ section of the virtual space 6. The first region is set as a range including a predetermined polar angle with the reference line of sight 4 as the center, and the second region is set as a range including a predetermined azimuth with the reference line of sight 4 as the center. As shown in FIG. 4, the visual field area 5 of the virtual camera 1 is a part of the virtual space 6 that constitutes the visual field of the user 150, and is associated with the inclination of the HMD 110 and / or the visual line direction of the user 150.

Next, a processing flow for generating the visual field image 8 in the visual field region 5 using the image captured by the virtual camera 2 will be described with reference to FIGS. 4, 5, and 7.
First, the second virtual camera image generation unit 310 generates the second virtual camera space image 7 captured by the virtual camera 2. (Step S5). The virtual camera 2 captures an image / video of an application (for example, a soccer game) being performed in a virtual space from a third person viewpoint.

  Then, the image storage unit 420 stores the generated second virtual camera space image 7. In order to reduce the memory load, the image storage unit 420 may store the captured second virtual camera space image only for a predetermined period, and sequentially delete the image after the predetermined period has elapsed. When there are a plurality of virtual cameras 2, the image storage unit 420 stores images taken for each of the plurality of virtual cameras.

  The image storage unit 420 also replays a second virtual camera space image of a predetermined scene, for example, a scene corresponding to a period from just before the character 9 determines a goal in the soccer game until the character 9 returns to the position. Only images may be stored. If the object drawing unit 350 described later does not determine that a replay image stored is to be drawn, the image storage unit 420 may sequentially delete the images from the memory.

  Next, the object specifying unit 320 specifies the display object 10 in the visual field area 5 based on the visual field area 5 determined in step S4 (step S6). The display object 10 is a display object in a virtual space for drawing an image photographed by the virtual camera 2, and is arranged at an arbitrary position in the virtual space 6.

  Next, the object specifying unit 320 determines whether or not there are a plurality of display objects 10 in the view area 5 (step S7). If a plurality of display objects do not exist in the view area 5, the process proceeds to step S9. On the other hand, when there are a plurality of display objects in the view field 5, the update timing setting unit 330 sets the update timing for each display object (step S8).

  FIG. 8 shows the relationship between a plurality of display objects 10 (1) to (5) arranged at different positions in the virtual space 6 and the visual field region 5. The plurality of display objects are respectively associated with one virtual camera 2 among the plurality of virtual cameras 2 arranged at different positions in the virtual space. As shown in FIG. 8, the entire display object 10 (1) is arranged in the view area 5, and only a part (a) of the display objects 10 (2) and (3) is arranged in the view area 5. The The display objects 10 (4) and (5) exist outside the field of view. One virtual camera 2 may be associated with a plurality of display objects, or a plurality of virtual cameras 2 may be associated with one display object.

  FIG. 9 is a diagram illustrating the update timing set for each display object. The update timing setting unit 330 sets the update timing for the display object as an update target if at least a part of the display object is within the view field area 5. For example, the update timing setting unit 330 sets the update timing with the display objects 10 (1) to (3) in the field-of-view area 5 being updated. Then, for example, the update timing setting unit 330 sets the update timing of the display object 10 (1) to frames 1 and 4, the update timing of the display object 10 (2) to frames 2 and 5, and the display object 10 (3). Update timings are set for frames 3 and 6, respectively. The plurality of display objects 10 (1) to (3) in the view area 5 are not updated at the same time, but the update timing is set so that, for example, one display object is updated for each frame. The update timing of the display objects 10 (4) and (5) existing outside the visual field area 5 is not set.

  Next, the drawing timing determination unit 340 determines a predetermined timing for drawing the second virtual camera space image 7 on the display object 10 (step S9). For example, when a soccer game is in progress, the drawing timing determination unit 340 displays the second virtual camera space image 7 when the player character returns to the position after the player character has determined a goal. The predetermined timing for drawing on the object 10 is determined. The second virtual camera space image 7 is drawn on the display object 10 from a predetermined timing. Thereby, the period during which the second virtual camera space image 7 is drawn on the display object 10 is shortened, and the load required for the drawing process of the display object 10 can be reduced. Note that the drawing timing determination unit 340 may always draw the second virtual camera space image 7 on the display object 10.

  When the drawing timing determination unit 340 determines a predetermined timing for drawing the second virtual camera space image 7, the object drawing unit 350 reads the second virtual camera space image 7 from the image storage unit 420, and The second virtual camera space image 7 is drawn on the display object 10 at the timing (step S10). At this time, the object drawing unit 350 draws only the display object 10 in the visual field area 5 specified in step S6, and does not draw the display object outside the visual field area 5.

  Further, the object drawing unit 350 may determine a predetermined scene to be drawn on the display object. The object drawing unit 350 corresponds to a predetermined scene to be drawn in the second camera virtual space image read from the image storage unit 420, for example, a scene from a predetermined time before the goal scene to a predetermined time after determining the goal. Draw an image.

  When there are a plurality of display objects 10 in the view field 5, the object drawing unit 350 draws the second virtual camera space images photographed by the virtual camera 2 on the corresponding display objects 10, respectively. At this time, as illustrated in FIG. 8, the object drawing unit 350 may draw the second virtual camera space image on a part (a) of the display objects 10 (2) and (3) in the field-of-view area 5. . As a result, the load required for the drawing process can be reduced. Further, the object drawing unit 350 updates each of the plurality of display objects in the view area 5 at the update timing set in step S8. By reducing the number of display objects updated in one frame and changing the timing of updating each of the plurality of display objects, the drawing processing load required for one update can be reduced.

  Then, the visual field image generation unit 250 refers to the spatial information storage unit 410 and generates a visual field image 8 corresponding to the visual field region 5 (step S11). The spatial information storage unit 410 stores a three-dimensional spatial image of the virtual space 6. The visual field image generation unit 250 generates a visual field image 8 in which display objects existing in the visual field region 5 are arranged in the virtual space.

  And HMD110 receives the information regarding the visual field image 8 from the control circuit part 120, outputs the visual field image 8 on the display 112, and displays it (step S12). The field-of-view image includes two two-dimensional images for the left eye and right eye, and these are superimposed on the display 112 to provide the user 150 with a virtual space 6 as a three-dimensional image.

  FIG. 10 is an example of an image displayed in the field of view of the user 150 wearing the HMD. In FIG. 10, a visual field image 8 is included in the visual field region 5 set based on the reference visual line. One display object 10 is arranged in the view field image 8. The display object 10 draws the second virtual camera space image 7 taken by the third-person viewpoint virtual camera 2. In the present embodiment, when the user 150 wearing the HMD moves his / her head or line of sight, the view field image 8 is changed in conjunction with the movement of the user's head or line of sight. The field-of-view image 8 is not composed of the second virtual camera space image 7 but includes a display object 10 that draws the second virtual camera space image 7. In the present invention, the second virtual camera 2 space image is drawn on the display object 10 arranged in the virtual space. Therefore, the user wearing the HMD does not visually recognize an image composed entirely of the second virtual camera space image 7 of the third person viewpoint that is not linked to the movement of the HMD. According to the present invention, it is possible to reduce video sickness caused by visually recognizing a virtual space image that is not linked to the movement of the HMD.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. Those skilled in the art will appreciate that various modifications of the embodiments can be made without departing from the spirit and scope of the invention as set forth in the appended claims.

DESCRIPTION OF SYMBOLS 100 ... Head mounted display (HMD) system, 110 ... HMD, 112 ... Display, 114 ... Sensor part, 116 ... Gaze sensor, 120 ... Control circuit part, 130 ... Motion sensor, 132 ... Tracking sensor, 200 ... Display control part, DESCRIPTION OF SYMBOLS 300 ... Object control part, 400 ... Memory | storage part, 1 ... Virtual camera 1, 2 ... Virtual camera 2, 4 ... Reference | standard line of sight, 5 ... View field area, 6 ... Virtual space, 7 ... 2nd virtual camera space image, 8 ... View field Image, 9 ... character, 10 ... display object

Claims (8)

A method of displaying a virtual space image on a head mounted display worn by a user,
Generating a virtual camera space image captured by a virtual camera in a virtual space having a third person viewpoint different from the first person viewpoint associated with the user;
Drawing the virtual camera space image on a display object in the virtual space;
Generating a visual field image in which the display object is arranged in the virtual space;
Outputting the field-of-view image to a head-mounted display;
Including methods. Determining a predetermined timing for rendering the virtual camera space image;
Drawing the virtual camera space image on a display object at the predetermined timing;
The method of claim 1, further comprising:   The method according to claim 1, wherein, when it is determined that the virtual camera space image corresponds to a predetermined scene to be drawn on the display object, the virtual camera space image is drawn on the display object. There are a plurality of the virtual cameras, each arranged at different positions in the virtual space, the plurality of display objects are present, each arranged at a different position in the virtual space,
The method of claim 1, wherein each of the plurality of display objects corresponds to one virtual camera of the plurality of virtual cameras. Identifying a reference line of sight of a user wearing the head mounted display;
Determining a field of view of the user based on the reference line of sight;
5. The method according to claim 4, wherein the virtual camera space image is drawn on a display object in the view field among the plurality of display objects, and is not drawn on a display object outside the view field among the plurality of display objects.   The method according to claim 5, further comprising a step of changing a timing of updating each of the plurality of display objects in the viewing area when there are a plurality of display objects in the viewing area.   The method according to claim 5, wherein the user's reference line-of-sight is specified based on one or both of an inclination of the head-mounted display and a user's line-of-sight direction specified by a gaze sensor included in the head-mounted display. .   The program for making a computer perform the method of any one of Claims 1-7.