Patent Publication Number: US-2022232202-A1

Title: Head-up display system and movable object

Description:
FIELD 
     The present disclosure relates to a head-up display system and a movable object. 
     BACKGROUND 
     A known head-up display apparatus allows a user aboard a vehicle to view a virtual image plane with image light entering the eyeballs of the user after the image light is transmitted through, for example, a liquid crystal panel in a projection unit and reflected from an optical member, such as a vehicle windshield. The head-up display apparatus detects the positions of the user&#39;s eyes and controls the mirror angle that defines the traveling direction of the image light based on the eye positions to allow image light to reach the user&#39;s eyes (Patent Literature 1). 
     However, a movable object incorporating such a head-up display apparatus may vibrate during traveling, causing vibration of a virtual image plane formed by the head-up display apparatus. A virtual image plane, which may form within the field of view of the user, appears vibrating to the user. To accurately see the virtual image plane, the user moves the viewpoint in accordance with the vibration and may view the virtual image plane in an uncomfortable manner. An apparatus is awaited to allow the user to view a virtual image plane in a comfortable manner. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2018/042898 
     BRIEF SUMMARY 
     A head-up display system according to an aspect of the present disclosure includes a three-dimensional display device, an optical member, and an accelerometer. The three-dimensional display device includes a display panel, an optical element, and a controller. The display panel displays an image. The optical element defines a traveling direction of image light emitted from the display panel. The optical member reflects the image light from the three-dimensional display device toward a user&#39;s eye. The optical member is at a fixed position relative to the three-dimensional display device. The accelerometer detects acceleration of the three-dimensional display device. The controller controls a position of the image on the display panel based on the acceleration. 
     A movable object according to another aspect of the present disclosure includes a head-up display system. The head-up display system includes a three-dimensional display device, an optical member, and an accelerometer. The three-dimensional display device includes a display panel, an optical element, and a controller. The display panel displays an image. The optical element defines a traveling direction of image light emitted from the display panel. The optical member reflects the image light from the three-dimensional display device toward a user&#39;s eye. The optical member is at a fixed position relative to the three-dimensional display device. The accelerometer detects acceleration of the three-dimensional display device. The controller controls a position of the image on the display panel based on the acceleration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The objects, features, and advantages of the present disclosure will become more apparent from the following detailed description and the drawings. 
         FIG. 1  is a diagram of an example movable object incorporating a head-up display (HUD) according to a first embodiment. 
         FIG. 2  is a schematic diagram of a three-dimensional (3D) display device shown in  FIG. 1 . 
         FIG. 3  is a diagram of an example display panel shown in  FIG. 2  viewed in the depth direction. 
         FIG. 4  is a diagram of an example parallax barrier shown in  FIG. 2  viewed in the depth direction. 
         FIG. 5  is a diagram describing the relationship between a virtual image and a user&#39;s eyes shown in  FIG. 1 . 
         FIG. 6  is a graph showing the displacements of the 3D display device and the user&#39;s eye with time. 
         FIG. 7  is a diagram describing the positional relationship between the user&#39;s eye, a virtual image plane, an optical member, and the display device in v-direction. 
         FIG. 8  is a diagram describing the positional relationship between the user&#39;s eye, the virtual image plane, the optical member, and the display device in w-direction. 
         FIG. 9  is a diagram of the user&#39;s eye, the virtual image plane, and the optical member shown in  FIG. 8  as viewed in u-direction. 
         FIG. 10  is a diagram of a modification of the 3D display device shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will now be described with reference to the drawings. The drawings referred to hereafter are schematic and are not drawn to scale relative to the actual size of each component. 
     A head-up display system and a movable object according to one or more embodiments of the present disclosure allow a user to view a virtual image plane in a comfortable manner. 
     As shown in  FIG. 1 , a head-up display (HUD) system  100  according to an embodiment of the present disclosure includes an accelerometer  1 , at least one reflector  2 , an optical member  3 , and a three-dimensional (3D) display device  4 . The HUD system  100  is mounted on a movable object  10 . The reflector  2  and the optical member  3  together may also be referred to as an optical system. 
     The movable object according to one or more embodiments of the present disclosure includes a vehicle, a vessel, or an aircraft. The vehicle according to one or more embodiments of the present disclosure includes, but is not limited to, an automobile or an industrial vehicle, and may also include a railroad vehicle, a community vehicle, or a fixed-wing aircraft traveling on a runway. The automobile includes, but is not limited to, a passenger vehicle, a truck, a bus, a motorcycle, or a trolley bus, and may also include another vehicle traveling on a road. The industrial vehicle includes an agricultural vehicle or a construction vehicle. The industrial vehicle includes, but is not limited to, a forklift or a golf cart. The agricultural vehicle includes, but is not limited to, a tractor, a cultivator, a transplanter, a binder, a combine, or a lawn mower. The construction vehicle includes, but is not limited to, a bulldozer, a scraper, a power shovel, a crane vehicle, a dump truck, or a road roller. The vehicle includes a man-powered vehicle. The classification of the vehicle is not limited to the above. For example, the automobile may include an industrial vehicle traveling on a road, and one type of vehicle may fall within a plurality of classes. The vessel according to one or more embodiments of the present disclosure includes a jet ski, a boat, or a tanker. The aircraft according to one or more embodiments of the present disclosure includes a fixed-wing aircraft or a rotary-wing aircraft. 
     The accelerometer  1  detects the acceleration of the 3D display device  4  incorporating the accelerometer  1  and transmits the detected acceleration to the 3D display device  4 . The accelerometer  1  may be fixed to a housing for the 3D display device  4 . The accelerometer  1  may be fixed to any component of the movable object  10  that fixes and incorporates the 3D display device  4 . The accelerometer includes, but is not limited to, a mechanical accelerometer, an optical accelerometer, or a semiconductor accelerometer. The semiconductor accelerometer includes, for example, a piezoresistive accelerometer, a capacitive accelerometer, or a thermal accelerometer based on a gas temperature profile. 
     The reflector  2  is at a fixed position relative to the 3D display device  4 . For example, the 3D display device  4  is at a fixed position relative to the movable object  10 , and the reflector  2  is at a fixed position relative to the movable object  10 . The reflector  2  is thus at a fixed position relative to the 3D display device  4 . 
     The reflector  2  reflects image light emitted from the 3D display device  4  toward a predetermined area on the optical member  3 . The predetermined area reflects image light toward the user&#39;s eyes. The predetermined area may be defined by the direction in which the user&#39;s eyes are located relative to the optical member  3  and the direction in which image light is incident on the optical member  3 . The reflector  2  includes at least one reflective element. 
     The reflective element may be a mirror, such as a concave mirror. The reflective element may be one mirror as shown in  FIG. 1 , or may be a combination of two or more mirrors. 
     The optical member  3  is at a fixed position relative to the 3D display device  4 . For example, the 3D display device  4  is at a fixed position relative to the movable object  10 , and the optical member  3  is at a fixed position relative to the movable object  10 . The optical member  3  is thus at a fixed position relative to the 3D display device  4 . 
     The optical member  3  reflects image light emitted from the 3D display device  4  and reflected from the reflector(s)  2  toward the user&#39;s left eye (first eye) and the user&#39;s right eye (second eye). For example, the movable object  10  may include a windshield as the optical member  3 . The HUD system  100  thus directs light emitted from the 3D display device  4  to the user&#39;s left and right eyes along an optical path L. The user views light reaching the eyes along the optical path L as a virtual image plane V. 
     As shown in  FIG. 2 , the 3D display device  4  may include a communicator  41 , a controller  42 , a display panel  43 , an illuminator  44 , and a parallax barrier  45 . 
     The communicator  41  can communicate with the accelerometer  1 . The communicator  41  may communicate with the accelerometer  1  using a wireless communication standard, a mobile network wireless communication standard, or a wired communication standard. A short-range wireless communication standard may include, for example, Wi-Fi (registered trademark), Bluetooth (registered trademark), infrared communication, or near field communication (NFC). The mobile network wireless communication standard may include, for example, long term evolution (LTE), a fourth generation mobile communication system, or a fifth generation mobile communication system. 
     The communicator  41  receives the acceleration detected by the accelerometer  1 . 
     The controller  42  may be connected to the components of the 3D display device  4  to control these components. The components controlled by the controller  42  include the communicator  41  and the display panel  43 . The controller  42  may be, for example, a processor. The controller  42  may include one or more processors. The processors may include a general-purpose processor that reads a specific program and performs a specific function, and a processor dedicated to specific processing. The dedicated processor may include an application-specific integrated circuit (ASIC). The processor may include a programmable logic device (PLD). The PLD may include a field-programmable gate array (FPGA). The controller  42  may be either a system on a chip (SoC) or be a system in a package (SiP) in which one or more processors cooperate with other components. 
     The controller  42  performs various control operations based on the acceleration detected by the accelerometer  1  and received by the communicator  41 . The control performed by the controller  42  will be described in detail later. 
     As shown in  FIG. 3 , the display panel  43  includes a two-dimensional (2D) active area A including multiple divisional areas. The active area A is an area on which a parallax image appears. The parallax image includes a left-eye image (first image) and a right-eye image (second image) having parallax with the left-eye image. The left-eye image is viewed with the user&#39;s left eye (first eye). The right-eye image is viewed with the user&#39;s right eye (second eye). The divisional areas are defined in a first direction and in a second direction perpendicular to the first direction. The direction perpendicular to the first and second directions is referred to as a third direction. In the present embodiment, the first direction is defined as a horizontal direction, the second direction is defined as a vertical direction, and the third direction is defined as a depth direction. However, the first, second, and third directions are not limited to the directions referred to above. In the drawings, the first direction is written as x-direction, the second direction as y-direction, and the third direction as z-direction. 
     However, the first, second, and third directions are not limited to the directions referred to above. In  FIGS. 2 to 4 and 10 , the first direction is written as x-direction, the second direction as y-direction, and the third direction as z-direction. In  FIGS. 1, 5, and 7 to 9  and hereafter, an interocular direction or the direction of a line passing through the user&#39;s right and left eyes is written as u-direction, and the front-rear direction as viewed from the user is written as w-direction. The height direction perpendicular to u-direction and w-direction is written as v-direction. 
     Each divisional area corresponds to a subpixel. Thus, the active area A includes multiple subpixels arranged in a grid in the horizontal and vertical directions. 
     Each subpixel may correspond to one of red (R), green (G), and blue (B). A set of three subpixels colored R, G, and B may form a pixel. A pixel may be referred to as a picture element. The display panel  43  is not limited to a transmissive liquid crystal panel but may be another display panel such as an organic electroluminescent (EL) display. For the display panel  43  being a self-luminous display panel, the 3D display device  4  may eliminate the illuminator  44 . 
     As described above, multiple subpixels arranged in the active area A form subpixel groups Pg. The subpixel groups Pg each include a predetermined number of subpixels in rows and columns. In the example shown in  FIG. 3 , the subpixel groups Pg each include (2×n×b) subpixels P 1  to P(2×n×b), which are consecutively arranged in b row(s) in the vertical direction and in 2×n columns in the horizontal direction. In the example shown in  FIG. 3 , the subpixel groups Pg are repeatedly arranged in the horizontal direction. The subpixel groups Pg are repeatedly arranged in the vertical direction at positions shifted by one subpixel in the horizontal direction from the corresponding subpixels. In the present embodiment, n=5 and b=1 are satisfied, for example. As shown in  FIG. 3 , the active area A includes the subpixel groups Pg each including ten consecutive subpixels P 1  to P 10  arranged in one row in the vertical direction and in ten columns in the horizontal direction. Each of symbols P 1  to P 10  is identification information for the corresponding subpixel. In  FIG. 3 , some of the subpixel groups Pg are denoted by reference signs. 
     Each subpixel group Pg is the smallest unit controllable by the controller  42  (described later) to display an image. The subpixels included in each subpixel group Pg are identified using the identification information P 1  to P(2×n×b). The subpixels P 1  to P(2×n×b) included in each subpixel group Pg with the same identification information are controlled by the controller  42  at the same time. For example, the controller  42  switches the image to be displayed by the subpixels P 1  from the left-eye image to the right-eye image at the same time in all the subpixel groups Pg. 
     The illuminator  44  may illuminate a surface of the display panel  43 . The illuminator  44  may include a light source, a light guide plate, a diffuser plate, and a diffuser sheet. The illuminator  44  emits illumination light using the light source, and spreads the illumination light uniformly for illuminating the surface of the display panel  43  using, for example, the light guide plate, the diffuser plate, and the diffusion sheet. The illuminator  44  may emit uniform light toward the display panel  43 . The display panel  43  being a liquid crystal panel will now be described. 
     As shown in  FIG. 2 , the parallax barrier  45  is planar along the active area A, and is arranged at a predetermined distance (gap) g from the active area A. The parallax barrier  45  may be located opposite to the illuminator  44  from the display panel  43 . The parallax barrier  45  may be located between the display panel  43  and the illuminator  44 . 
     As shown in  FIG. 4 , the parallax barrier  45  includes multiple barrier areas s arranged in a grid in the horizontal and vertical directions. The barrier areas s included in the parallax barrier  45  form barrier area groups sg. The barrier area groups sg each include a predetermined number of barrier areas in the horizontal and vertical directions. More specifically, the barrier area groups sg each include (2×n×b) barrier areas s 1  to s(2×n×b), which are consecutively arranged in b row(s) in the vertical direction and in 2×n columns in the horizontal direction. The barrier area groups sg are arranged to correspond to the arrangement of the subpixels in the subpixel groups Pg. The barrier area groups sg are repeatedly arranged in the horizontal direction. The barrier area groups sg are repeatedly arranged in the vertical direction at positions shifted by one barrier area in the horizontal direction from the corresponding barrier areas. 
     For n=5 and b=1 as described above, each barrier area group sg includes ten consecutive barrier areas s 1  to s 10  arranged in one row in the vertical direction and in ten columns in the horizontal direction. In  FIG. 4 , some of the barrier area groups sg are denoted with reference numerals. Each barrier area group sg may include (2×n′×b′) barrier areas s 1  to s(2×n′×b′), which are consecutively arranged in b′ (b′ b) row(s) in the vertical direction and in 2×n′ (n′ n) columns in the horizontal direction. 
     As shown in  FIG. 4 , the parallax barrier  45  includes, for example, multiple attenuation areas  452  in which image light is attenuated. Each attenuation area  452  includes multiple barrier area groups sg. The attenuation areas  452  define transmissive areas  451  between adjacent attenuation areas  452 . The transmissive areas  451  have a higher light transmittance than the attenuation areas  452 . 
     A virtual image plane barrier pitch, VBp, which is the barrier pitch of a second virtual image plane V 2  corresponding to the interval between the transmissive areas  451  in the horizontal direction, and a virtual image plane gap, Vg, which is the gap or distance between the second virtual image plane V 2  and a first virtual image plane V 1 , are defined to satisfy 
     Formulas 1 and 2 below. 
         E:Vd =( n×VHp ): Vg    (1)
 
         Vd:VBp =( Vd+Vg ):(2 ×n×VHp )  (2)
 
     where Vd is the proper viewing distance between the second virtual image plane V 2  corresponding to the parallax barrier  45  and the user&#39;s right and left eyes, and E is the interocular distance between the user&#39;s right and left eyes. The interocular distance E may be, for example, a distance of 61.1 to 64.4 mm, which is calculated through studies performed by the National Institute of Advanced Industrial Science and Technology. In the formulas, VHp is the dimension of the virtual image for the subpixels on the first virtual image plane V 1  in the direction (u-direction) corresponding to the first direction. 
     Image light emitted from the active area A on the display panel  43  is thus partly transmitted through the transmissive areas  451 , reflected from the reflector(s)  2  to reach the optical member  3 , reflected from the optical member  3 , and then reaches the user&#39;s eyes as shown in  FIG. 1 . 
     As shown in  FIG. 5 , the parallax barrier  45  defines the traveling direction of image light to define the area of the first virtual image plane V 1  corresponding to the active area A viewable with the user&#39;s eyes. The user thus sees the parallax image appearing as the first virtual image plane V 1  corresponding to the display panel  43  through the second virtual image plane V 2 , which is a virtual image corresponding to the parallax barrier  45 . In reality, the user does not see the second virtual image plane V 2  corresponding to the parallax barrier  45 . 
     The areas in the first virtual image plane V 1  viewable by the user with image light reaching the positions of the user&#39;s eyes are hereafter referred to as viewable areas Va. The areas in the first virtual image plane V 1  viewable by the user with image light reaching the position of the user&#39;s left eye are referred to as left viewable areas VaL (first viewable areas). The areas in the first virtual image plane V 1  viewable by the user with image light reaching the position of the user&#39;s right eye are referred to as right viewable areas VaR (second viewable areas). 
     The parallax barrier  45  may include, for example, a liquid crystal shutter. The liquid crystal shutter may control the light transmittance in accordance with a voltage applied. The liquid crystal shutter may include multiple pixels and control the light transmittance for each pixel. 
     The control performed by the controller  42  will now be described in detail. 
     The controller  42  causes the display panel  43  to display an image. More specifically, the controller  42  may cause the display panel  43  to display an image based on image data received by the communicator  41 . The controller  42  may cause the display panel  43  to display an image based on image data stored in a memory included in the 3D display device  4 . The controller  42  may cause the display panel  43  to display an image based on image data input with an input interface included in the 3D display device  4 . 
     The controller  42  causes the display panel  43  to display an image by controlling the position of the image appearing on the display panel  43  based on the acceleration detected by the accelerometer  1 . 
     More specifically, the controller  42  first calculates a distance L 1 (t) by which the 3D display device  4  is displaced from time t 0  to time t based on acceleration α(t) received by the communicator  41  from the accelerometer  1 . For example, the controller  42  may calculate the distance L 1 (t) using Formula  3  by double integration of the acceleration α(t) from time t 0  to time t. 
         L   1 ( t )=∫∫ t     0     t α( t ) d   2   t    (3)
 
     The controller  42  estimates a distance L 2 (t) by which the user&#39;s eye is displaced from time t 0  to time t based on the acceleration α(t) received by the communicator  41  from the accelerometer  1 . The inventor and others have noticed that the user aboard the movable object  10  incorporating the 3D display device  4  may be affected by, for example, cushioning of the seat, and displaced at a time delayed by a predetermined period Δt from when the movable object  10  is displaced and at a predetermined ratio r relative to the displacement of the movable object  10 . The 3D display device  4  is at a fixed position relative to the movable object  10  as described above. As shown in  FIG. 6 , the user&#39;s eye is thus displaced at a time delayed by the predetermined period Δt from when the 3D display device  4  is displaced. The user&#39;s eye is displaced by the distance L 2 (t) at the predetermined ratio r to the distance L 1 (t) by which the 3D display device  4  is displaced. The controller  42  can use, for example, Formula 4 to estimate the distance L 2 (t) by which the user&#39;s eye is displaced based on the acceleration α(t) received by the communicator  41  from the accelerometer  1 . 
         L   2 ( t )= r ∫∫ t     0     t α( t−Δt ) d   2   t    (4)
 
     The predetermined period Δt and the predetermined ratio r may be preset as appropriate based on, for example, the characteristics of the seat on the movable object  10  or the physical characteristics (e.g., the weight) of the user. For example, a shorter predetermined period Δt may be set for a harder seat (with less cushioning). The predetermined ratio r may be set lower for a user with a greater weight. For an input from the user indicating the weight being in a first range, the predetermined ratio r may be set to r1. For an input indicating the weight being in a second range heavier than the first range, the predetermined ratio r may be set to r2 less than r1. The predetermined period At and the predetermined ratio r may be set based on actual displacement measured for each user in response to displacement of the movable object  10 . 
     The controller  42  may calculate the distance L 1 (t) by which the 3D display device  4  is displaced, estimate the distance L 2 (t) by which the user&#39;s eye is displaced, and then calculate a distance ΔL(t) by which the relative position of the user&#39;s eye relative to the 3D display device  4  is shifted. The distance ΔL(t) by which the relative position of the user&#39;s eye is shifted is the difference between the distance L 1 (t) and the distance L 2 (t) as shown in Formula 5. 
       Δ L ( t )= L   2 ( t )− L   1 ( t )  (5)
 
     Control Over Displacement in u- and v-Directions 
     The control over displacement of the virtual image plane V along the virtual image plane V (in u- and v-directions) performed by the controller  42  will now be described with reference to  FIG. 7 . The controller  42  may control the position of the image appearing on the display panel  43  to reduce the positional change of the virtual image plane V as viewed from the user. For example, the controller  42  may change the position of the parallax image on the display panel  43  to change the position of the virtual image in a direction opposite to the relative displacement direction of the virtual image plane V relative to the user&#39;s eye along the virtual image plane V. The control over the image in v-direction will now be described for displacement of the 3D display device  4  and the user&#39;s eye in v-direction. The distances by which the 3D display device  4  and the user&#39;s eye are displaced in v-direction are respectively denoted by L 1v (t) and L 2v (t). The control for u-direction can be performed similarly to that for v-direction and will not be described repeatedly. 
     As described above, the reflector  2  and the optical member  3  are each at a fixed position relative to the 3D display device  4 . As shown in  FIG. 7 , for the 3D display device  4  displaced by the distance L 1v (t) in v-direction from the position at time t 0  to the position at time t, the reflector  2  and the optical member  3  are each displaced by the distance L 1v (t) in v-direction. In  FIG. 7 , Ey(t 0 ), 4(t 0 ), 2(t 0 ), 3(t 0 ), and V(t 0 ) respectively denote the user&#39;s eye Ey, the 3D display device  4 , the reflector  2 , the optical member  3 , and the virtual image plane V at time t 0 . Ey(t), 4(t), 2(t), 3(t), and V(t) respectively denote the user&#39;s eye Ey, the 3D display device  4 , the reflector  2 , the optical member  3 , and the virtual image plane V at time t. 
     For the user&#39;s eye Ey displaced by the distance L 2v (t) from the position at time t 0  to the position at time t, the virtual image plane V is displaced by a distance L 3v (t). As expressed by Formula 6, the distance L 3v (t) is calculated using a distance Z between the optical member  3  and the user and a distance D between the virtual image plane V (first virtual image plane V 1 ) and the user&#39;s eye. The distance D satisfies D=Vg+Vd, where Vg is the gap between the virtual image planes and Vd is the proper viewing distance as described with reference to  FIG. 5 . The distances Z and D are changed in response to a change in the relative position of the eye relative to the optical member  3 , but the changes are negligible as compared with the distances Z and D. The distances Z and D are thus approximated to be constant for calculating the distance L 3v (t). 
     
       
         
           
             
               
                 
                   
                     
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     The relative position of the virtual image plane V(t) relative to the user at time t is shifted from the relative position of the virtual image plane V(t 0 ) relative to the user at time to by a distance of L 3v (t)−L 2v (t) in v-direction. The controller  42  changes the position of the image appearing on the display panel  43  to cause the virtual image on the virtual image plane V to appear at a position shifted by a distance of L 3v (t)−L 2v (t) in a direction opposite to the direction in which the virtual image plane V is displaced as viewed from the user. In  FIG. 7 , OB(t 0 ) denotes a virtual image OB of an object included in the virtual image plane V at time t 0 . OB(t) denotes the virtual image OB of the object included in the virtual image plane V at time t for an image position unchanged by the controller  42 . OB(t)′ denotes the virtual image OB of the object included in the virtual image plane V at time t for an image position changed by the controller  42 . 
     This structure reduces the change in the relative position of the virtual image relative to the user&#39;s eye along the virtual image plane in response to a change in the relative position of the 3D display device  4  relative to the user&#39;s eye due to the vibration of the movable object  10 . The user can thus view the virtual image in a comfortable manner without largely moving the viewpoint along the virtual image plane to accurately see the image. 
     Control Over Displacement in w-Direction 
     The control over displacement of the virtual image plane V in a direction normal to the virtual image plane V (in w-direction) performed by the controller  42  will now be described with reference to  FIGS. 8 and 9 . The controller  42  controls the position of the image appearing on the display panel  43  to reduce the positional change of a stereoscopic image  3 DI formed by the virtual image plane V as viewed from the user. For example, the controller  42  changes the parallax of the parallax image to change the position of the stereoscopic image  3 DI formed by the virtual image plane V in the same direction as the relative displacement direction of the eye relative to the virtual image plane V in a direction normal to the virtual image plane V. The control over the image will now be described for displacement of the 3D display device  4  and the user&#39;s eye in w-direction. The distances by which the 3D display device  4  and the user&#39;s eye are displaced in w-direction are respectively denoted by L 1w (t) and L 2w (t). 
     As described above, the reflector  2  and the optical member  3  are each at a fixed position relative to the 3D display device  4 . As shown in  FIG. 8 , for the 3D display device  4  displaced by the distance L 1w (t) in w-direction from the position at time t 0  to the position at time t, the reflector  2 , the optical member  3 , and the virtual image plane V are each displaced by the distance L 1w (t) in w-direction. The position of the virtual image plane V is determined by the positions of the 3D display device  4 , the reflector  2 , and the optical member  3 . For the 3D display device  4 , the reflector  2 , and the optical member  3  each displaced by the distance L 1w (t) in w-direction, the virtual image plane V is displaced by the distance L 1w (t) in w-direction. 
     In  FIG. 8 , Ey(to), 4(t 0 ), 2(t 0 ), 3(t 0 ), and V(t 0 ) respectively denote the user&#39;s eye Ey, the 3D display device  4 , the reflector  2 , the optical member  3 , and the virtual image plane V at time to. Ey(t), 4(t), 2(t), 3(t), and V(t) respectively denote the user&#39;s eye Ey, the 3D display device  4 , the reflector  2 , the optical member  3 , and the virtual image plane V at time t. 
     For the user&#39;s eye Ey displaced by the distance L 2w (t) and the virtual image plane V displaced by the distance L 1w (t) from time t 0  to time t, the relative position of the user&#39;s eye Ey relative to the virtual image plane V is shifted by L 2w (t)−L 1w (t). A system in which the virtual image plane V and the fixed optical member  3  are at rest will now be described with reference to  FIG. 9 . In  FIG. 9, 3DI (t 0 ) denotes the stereoscopic image  3 DI formed by the virtual image plane V(t 0 ) at time to,  3 DI(t) denotes the stereoscopic image  3 DI formed by the virtual image plane V(t) at time t for an image position unchanged by the controller  42 , and  3 DI(t)′ denotes the stereoscopic image  3 DI formed by the virtual image plane V(t) at time t for an image position changed by the controller  42 . In  FIG. 9 , Ey(t 0 ) denotes the user&#39;s eye Ey at time t 0 , and Ey(t) denotes the user&#39;s eye Ey at time t, similarly to  FIG. 8 . 
     In the system shown in  FIG. 9 , the virtual image plane V and the optical member  3  are at rest, and the user&#39;s eye is displaced by a distance of L 2w (t)−L 1w (t). For a virtual parallax image on the virtual image plane V having parallax unchanged between time to and t, the user may be displaced relative to the virtual image plane V in a direction (w-direction) normal to the virtual image plane V. The user displacement then causes the stereoscopic image  3 DI formed by the virtual image plane V to be displaced by L 4w (t). More specifically, the user&#39;s eye displaced away from the virtual image plane V causes the stereoscopic image  3 DI to be displaced away from the virtual image plane V. The user&#39;s eye displaced toward the virtual image plane V causes the stereoscopic image  3 DI to be displaced toward the virtual image plane V. 
     The controller  42  may change the parallax of the parallax image on the display panel  43  to form a stereoscopic image  3 DI(t)′ resulting from displacement of the stereoscopic image  3 DI(t) by a distance of L 2w (t)−L 1w (t)+L 4w (t) in a direction opposite to the displacement direction of the stereoscopic image  3 DI as viewed from the user. The controller  42  may use any known method to calculate the parallax based on the distance between the user&#39;s eye and the virtual image plane V and the distance between the user&#39;s eye and the stereoscopic image  3 DI. The controller  42  may change the position of the parallax image on the display panel  43  to cause the parallax image to have the calculated parallax. This structure reduces the change from the relative position of the stereoscopic image  3 DI(t 0 ) relative to the user&#39;s eye Ey(t 0 ) to the relative position of the stereoscopic image  3 DI(t) relative to the user&#39;s eye Ey(t). The user thus perceives a smaller displacement of the stereoscopic image  3 DI in w-direction. 
     This structure reduces the change in the relative position of the virtual image relative to the user&#39;s eye in a direction normal to the virtual image plane in response to a change in the relative position of the 3D display device  4  relative to the user&#39;s eye due to the vibration of the movable object  10 . The user can thus view the virtual image in a comfortable manner. 
     The controller  42  in the present embodiment may be an information processing apparatus such as a computer. The information processing apparatus includes a memory that stores a program describing the processes for implementing the functions of the controller  42  in the embodiment, and a processor that reads the program for execution. 
     As described above, the controller  42  in the present embodiment controls the position of the image on the display panel  43  based on the acceleration. The controller  42  can thus control the position of the image in accordance with a change in the relative position of the image relative to the user. This reduces the change in the relative positions of the virtual image plane V and the stereoscopic image  3 DI relative to the user. The user can thus view the virtual image in a comfortable manner with less movement of the viewpoint in accordance with changes in the relative positions of the virtual image plane V and the stereoscopic image  3 DI. 
     The structure according to one embodiment of the present disclosure allows the user to view the virtual image plane in a comfortable manner. 
     Although the above embodiments are described as typical examples, various modifications and substitutions to the embodiments are apparent to those skilled in the art without departing from the spirit and scope of the present disclosure. Thus, the above embodiments should not be construed to be restrictive, but may be variously modified or altered within the scope of the present disclosure. For example, multiple structural blocks described in the above embodiments may be combined into a structural block, or each structural block may be divided. 
     The optical element is not limited to the parallax barrier  45  as in the above embodiments. As shown in  FIG. 10 , the parallax barrier  45  may be replaced with, for example, a lenticular lens  46  as the optical element included in the 3D display device  4 . The lenticular lens  46  is a horizontal array of cylindrical lenses  47  each extending vertically. The lenticular lens  46  defines the traveling direction of image light emitted from subpixels arranged in the active area A, similarly to the parallax barrier  45 . 
     The elements in the present disclosure implement operations that are implementable. The operations implemented by the elements in the present disclosure can thus refer to the elements operable to implement the operations. The elements implementing operations in the present disclosure can be expressed as the elements operable to implement the operations. The operations implementable by the elements in the present disclosure can be expressed as elements including or having the elements operable to implement the operations. A first element causing a second element to implement an operation in the present disclosure can refer to the first element operable to cause the second element to perform the operation. A first element causing a second element to perform an operation in the present disclosure can be expressed as the first element operable to control the second element to perform the operation. Operations implemented by the elements in the present disclosure that are not described in the claims are understood as being optional operations. 
     REFERENCE SIGNS LIST 
     
         
           1  accelerometer 
           2  reflector 
           3  optical member 
           4  3D display device 
           10  movable object 
           41  communicator 
           42  controller 
           43  display panel 
           44  illuminator 
           45  parallax barrier 
         VaL left viewable area 
         VaR right viewable area 
           100  head-up display system 
           451  transmissive area 
           452  attenuation area 
         A active area 
         L optical path 
         V virtual image plane 
         V 1  first virtual image plane 
         V 2  second virtual image plane 
         Pg subpixel group 
         P 1  to P 10  subpixel 
         s barrier area 
         sg barrier area group