Patent Publication Number: US-2023161169-A1

Title: Image display system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-189237 filed on Nov. 22, 2021, which is incorporated herein by reference in its entirety including the specification, claims, drawings, and abstract. 
     TECHNICAL FIELD 
     The present description discloses an image display system that displays a subject image representing a predetermined subject to be superimposed on a field of vision of a user or an occupant of a mobile body using an augmented reality (hereinafter abbreviated as “AR”) technique. 
     BACKGROUND 
     A technique for displaying an image that augments reality on a field of vision of a user or an occupant has been proposed. Patent Document 1, for example, discloses a technique of displaying, on smart glasses or a glasses-type display worn by a user or a driver of a vehicle, an image of a leading vehicle that the user&#39;s vehicle is following. In Patent Document 1, the leading vehicle expressed by the image moves to guide the driver&#39;s vehicle to a destination. This technique enables the driver to travel to the destination by operating the vehicle so as to follow the leading vehicle. 
     In the AR technique, it is necessary to estimate the position and attitude (hereinafter referred to as the “location”) of a display in a real space and determine, based on the obtained location of the display, a display position of a subject image representing a subject in the display. Known techniques for estimating the location of the display or localizing the display include visual Simultaneous Localization and Mapping (SLAM). The visual SLAM is a technique for estimating, based on a SLAM image captured by a camera, environmental three-dimensional information and the position and attitude of the camera. This technique also enables estimation of behavior of the display from a change of the SLAM image. 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Document 1] JP 2017-129406 A 
       
    
     SUMMARY 
     When the user is in the mobile body, it is difficult to judge whether a change of the SLAM image results from movement of the user&#39;s head (and therefore movement of the display) or movement of the mobile body. This may reduce the accuracy of estimation of the location of the display, failing to display the subject image at an appropriate position. 
     While it may be possible to receive, from the mobile body, information regarding its movements and estimate the location of the display based on the information, this configuration requires that the mobile body should have a communication function with a wearable device including the display. This limits mobile bodies to those that can utilize the AR technique described above. Further, a mobile body configured to provide information regarding movements of the mobile body uses some amount of calculation and communication resources for calculation and communication to collect the information, which may adversely affect the calculation and communication for moving control of the mobile body. 
     An aspect of the disclosure is therefore aimed toward an image display system that enables more appropriate display of a subject image without increasing the communication load or the like of a mobile body. 
     In accordance with an aspect of the disclosure, an image display system includes a wearable device to be attached to the head of a user that is an occupant of a mobile body and including a display configured to display an image on a field of vision of the user and a first sensor configured to detect an absolute movement of the display with respect to an external stationary object, a mobile terminal including a second sensor configure to detect an absolute movement of the mobile body, and an image controller including a computer installed in at least one of the wearable device or the mobile terminal and configured to enable the display to display a subject image representing an object in such a manner that the object that is represented by the subject image is actually present. The image controller is configured to determine a display position of the subject image on the display based at least on the absolute movement of the display detected by the first sensor and the absolute movement of the mobile body detected by the second sensor. 
     This configuration enables separate acquisition of the absolute movement of the display and t of the mobile body, to thereby enable more appropriate display of the subject image. Further, detection of the absolute movement of the mobile body by the mobile terminal prevents an increase in the load of communication required of, for example, the mobile body. 
     In this configuration, the image controller may be configured to estimate a relative movement of the display with respect to the mobile body based on a difference between the absolute movement of the display detected by the first sensor and the absolute movement of the mobile body detected by the second sensor and determine the display position of the subject image associated with the mobile body on the display based at least on the relative movement of the display. 
     This configuration enables more appropriate display of the subject image associated with the mobile body. 
     The wearable device may further include a SLAM camera configured to capture an image of surroundings of the wearable device as a SLAM image, and the image controller may be configured to determine the display position of the subject image on the display, based on a movement of the display that is estimated from the SLAM image and detection results of the first sensor and the second sensor. 
     Determining the display position in consideration of the SLAM image in addition to the detection results from the first sensor and the second sensor enables more appropriate display of the subject image. 
     Each of the first sensor and the second sensor may include at least one of an accelerometer or a gyroscope sensor. 
     The use of at least one of the accelerometer or the gyroscope sensor enables more accurate detection of the absolute movements of the display and the mobile body. 
     The second sensor may be configured to detect the absolute movement with reference to mutually orthogonal three detection reference axes, and the image display system may further include an on-vehicle holder fixed to the mobile body and configured to hold the mobile terminal such that the detection reference axes of the second sensor are parallel to a longitudinal axis, a vertical axis, and a lateral axis of the mobile body, respectively. 
     The use of the on-vehicle holder reduces a moving amount to be input to the two detection reference axes orthogonal to the longitudinal direction of the mobile body, of the three detection reference axes, thereby enabling reduction in the volume of calculation required to obtain the absolute movement. 
     The mobile terminal may be fixed to a surface of a rigid body, within a vehicle cabin, secured to the mobile body. 
     This configuration can reduce the movement of the second sensor with respect to the mobile body, to thereby enable more accurate detection of absolute movement of the mobile body. 
     The image controller may be configured to exclude a detection value of the second sensor which is equal to or smaller than a predetermined reference value from calculation of the display position. 
     This configuration can remove effects of slight oscillation of the mobile terminal on the mobile body, to thereby enable more appropriate display of the subject image. 
     The technique disclosed in the disclosure enables more appropriate display of the subject image without increasing the load of calculation on the mobile body. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present disclosure will be described based on the following figures, wherein: 
         FIG.  1    is a block diagram illustrating a configuration of an image display system; 
         FIG.  2    illustrates a user wearing a wearable device; 
         FIG.  3    schematically illustrates a field of view of a driver or a user; 
         FIG.  4    schematically illustrates the field of view of a user with the subject image being displayed; 
         FIG.  5    illustrates an example SLAM image; 
         FIG.  6    is a flowchart illustrating a flow of calculation of a display position of the subject image; 
         FIG.  7    illustrates an example on-vehicle holder; and 
         FIG.  8    illustrates another example on-vehicle holder. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The configuration of an image display system  10  will be described with reference to the drawings. While in the following specific embodiments are described for the ease of understanding, they are only examples and may be modified as appropriate.  FIG.  1    is a block diagram illustrating the configuration of the image display system  10 . The image display system  10  includes a wearable device  12  and a mobile terminal  28 . 
     The wearable device  12  is a device an occupant of the vehicle, such as a driver, wears on their head, and may be an eyeglasses or goggles shaped device. The wearable device  12  includes a display  14 , a SLAM camera  16 , a pupil position sensor  18 , a first sensor  19 , and a device controller  20 . 
     The wearable device  12  will be described in detail with reference to  FIG.  2   .  FIG.  2    illustrates a user  100  that is an occupant of the vehicle wearing the wearable device  12 . The wearable device  12  is in the form of a pair of eyeglasses and is referred to as smart glasses or AR glasses. The wearable device  12  includes temples  26  that are linear frames to be put over respective ears and a rim  24  that is a frame surrounding the eyes and that is to be put across the nose. 
     The display  14  displays an image on the field of vision of the user  100  wearing the wearable device  12 . In this example, the display  14  is an organic EL display or a liquid crystal display having a display area  22  disposed within the rim  24 , and displays an image in part of or over the entire region of the display area  22 . The display area  22  having high transparency allows the user  100  or the occupant to visually recognize a front scenery through the display area  22  when no image is displayed on the display area  22 . The display area  22 , when displaying an image in only part of the display area  22 , allows the user  100  to see both the front scenery in the field of view and the displayed image simultaneously. At this time, the image may be opaque or translucent. In the following description, an image displayed on the display  14  is referred to as a “subject image” for discrimination from other images. Also, an object which is virtually represented by the subject image is referred to as a “subject”. 
     The SLAM camera  16  is fixed to the display  14  to image the surroundings of the display  14 . The SLAM camera  16  is secured, for example, to the vicinity of a front end of the temple  26  so as to face forward, and captures an image within a range similar to that of the field of vision of the user  100 . In the following description, an image captured by the SLAM camera  16  will be referred to as a “SLAM image”. An image controller  30 , which will be described below, extracts a feature point from a scenery in the SLAM image to specify the position and attitude of the display  14  in a real space, as will be described below. 
     The pupil position sensor  18  detects the positions of pupils in right and left eyes of the user  100 , and is fixed to the vicinity of the center of the rim  24 , for example. The pupil position sensor  18  may be formed of a camera, including an infrared camera, for example. 
     The first sensor  19  detects movement of the display  14 , and further movement of an eyepoint of the user  100 , with respect to the earth (hereinafter referred to as “absolute movement”). The first sensor  19  includes an accelerometer and a gyroscope sensor, for example. The accelerometer detects the acceleration along mutually orthogonal three axes (hereinafter referred to “detection reference axes”) as a voltage value. The gyroscope sensor detects the angular velocity about mutually orthogonal three detection reference axes as a voltage value. The detected values from the first sensor  19  are transmitted, via the device controller  20 , to the image controller  30 . The first sensor  19  is fixed to the display  14 , and is contained in the temple  26 , for example. 
     The device controller  20  controls operation of the wearable device  12  in response to an instruction from the image controller  30 . The device controller  20  may be a computer having a processor and a memory, for example. The device controller  20  continuously transmits the images captured by the SLAM camera  16  and the pupil position sensor  18  and the detected values of the first sensor  19  to the image controller  30 . The device controller  20  further displays the subject image on the display  14  in accordance with an instruction from the image controller  30 . 
     Referring again to  FIG.  1   , the mobile terminal  28  will be described. The mobile terminal  28  is a mobile information terminal, such as a smartphone, a tablet terminal, a notebook computer, or a mobile game machine. The mobile terminal  28  is carried with the user  100  into the vehicle cabin. 
     The vehicle includes, within the vehicle cabin, an on-vehicle holder  44  to which the mobile terminal  28  can be temporarily attached. As illustrated in  FIG.  3   , the mobile terminal  28  brought into the vehicle cabin is fixed to the vehicle via the on-vehicle holder  44 . In other words, the mobile terminal  28  is held to move with the vehicle. 
     The mobile terminal  28  includes the image controller  30 , a user interface (hereinafter referred to as a “user I/F”)  38 , and a second sensor  40 . The user I/F  38  includes an input device that receives an operation instruction from the user  100 , and an output device that provides information to the user  100 . The input device includes at least one of a keyboard, a switch, a touch screen, or a microphone, and the output device includes at least one of a display, a lamp, or a speaker. 
     The second sensor  40  detects the movement of the mobile terminal  28  and also of the vehicle with respect to the earth, that is, the absolute movement. The second sensor  40  includes, for example, an accelerometer that detects acceleration along the detection reference axes, and a gyroscope sensor that detects the angular velocity about the detection reference axes. The detected values from the second sensor  40  are transmitted to the image controller  30 . 
     The image controller  30  generates data of a subject image to be displayed on the display  14 . The image controller  30  is physically a computer including a processor  32 , a memory  34 , and a communication I/F  35 . The computer includes a microcontroller composed of a computer system integrated into a single integrated circuit. The processor  32  refers to a processor in a broad sense, and includes a general-purpose processor, such as a Central Processing Unit (CPU), and a special-purpose processor, such as a Graphics Processing Unit (GPU), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), or a programmable logic device. 
     The memory  34  refers to a device that stores digital data to be processed by the computer. The memory  34  includes at least one of a main memory connected with the processor  32  via a memory bus or a secondary memory device that accesses the processor  32  via an input/output channel. The memory  34  may include at least one of a semiconductor memory, such as RAM, ROM or solid state drive, for example, or a magnetic dick, such as a hard disk drive. 
     The communication I/F  35  transmits and receives data, wirelessly or through a wired connection, to and from other electronic devices, or specifically the wearable device  12 . For example, the communication I/F  35  may transmit and receive data to and from the wearable device  12  through short-distance wireless communication such as Bluetooth (registered mark), Wi-Fi (registered mark), or infrared communication. 
     The image controller  30  may include a plurality of computers mechanically spaced from each other, rather than being a single computer. The processing of the image controller  30  which will be described below may be partially or entirely implemented by a computer installed in the wearable device  12 . As such, the image controller  30  may be disposed in the wearable device  12  rather than in the mobile terminal  28 . 
     The image controller  30  causes the display  14  to display subject images  50   a ,  50   b , and  50   c , as if virtual objects represented by the respective subject images  50   a ,  50   b , and  50   c , or subjects, actually exist in the real space, as illustrated in  FIG.  4   . For example, the image controller  30  virtually places the subject in the real space, calculates the view of the subject, which is assumed to be actually present, from the user  100 , generates a subject image in accordance with the view, and displays the subject image on the display area  22  of the display  14 . Hereinafter, a display mode in which a subject is displayed as if the subject actually exists will be referred to as an “AR display” mode. 
     An example sample display of the subject image  50  in the present embodiment will be described.  FIG.  4    schematically illustrates the field of vision of the user  100  (or a driver in this example) with the subject images  50   a ,  50   b , and  50   c  being displayed. In the example illustrated in  FIG.  4   , the subject image  50   a  indicating information on a store outside the vehicle (information indicating a grand opening in the illustrated example), the subject image  50   b  indicating the travelling direction of the vehicle, and the subject image  50   c  indicating a message to attract the driver&#39;s attention, are displayed in the AR display mode. These subject images  50   a ,  50   b , and  50   c  are displayed on the display  14  at positions and in sizes that are similar to the positions and sizes of the subjects represented by the respective images existing in the real world. The subject image  50   a , for example, is displayed in the display area  22  at the position and in the size of a text object represented by the subject image  50   a  existing on the actual store outside the vehicle, assuming that the text object actually exists in the real world. Therefore, the position and the size of the subject image  50   a  within the display area  22  changes with the change of the relative positional relationship between the eye point of the user  100  and the store caused by travelling of the vehicle and the movement of the body of the user  100 . 
     The subject image  50   b  is displayed in the display area  22  at the position and in the size similar to those of an arrow-shape object represented by the subject image  50   b  assuming that the object actually resides on the road surface in front of the vehicle that actually exists. The subject image  50   c  is displayed in the display area  22  at the position and in the size similar to those of a text object represented by the subject image  50   c  assuming that the object actually resides on the upper right portion of the steering wheel  60  in the real world. Thus, the display positions and the sizes of the subject images  50   b  and  50   c  within the display area  22  change with the movement of the eye point of the user  100  relative to the vehicle. 
     As described above, the AR display mode enables display of the subject image  50  in consideration of locations of actual objects, thereby reliably preventing the subject image  50  from obstructing the driving operation. The AR display mode further enables the subject image  50  to be displayed at a position correlated to the actual object, such as a store, thus effectively directing attention of the user  100  toward the object. 
     To perform the AR display, it is necessary to calculate the view of the subject from the user  100  assuming that the subject is actually present, as described above. The view is determined based on a virtual disposing position of the subject, the position and attitude of the wearable device  12  in the real space, and the positions of the pupils with respect to the display  14 . 
     The virtual disposition of the subject is determined by the image controller  30  based on the information content to be provided to the user  100 . The positions of the pupils with respect to the display  14  are detected by the pupil position sensor  18 , as described above. 
     The position and attitude of the wearable device  12  in the real space, or the location of the wearable device  12 , can be estimated by repeating execution of visual SLAM in a predetermined sampling time. The visual SLAM refers to a technique of estimating three-dimensional information of the real space and the location of the display  14  from the SLAM image  52  (see  FIG.  5   ) captured by the SLAM camera  16 . The location of the wearable device  12  can be expressed, for example, with coordinate values of three orthogonal axes or x, y, and z coordinate values, and a rotation amount about the three orthogonal axes, or roll, yaw, and pitch. The image controller  30  extracts feature points from an actual scenery in the SLAM image  52 , and performs space recognition and localization based on the positions of the feature points in the SLAM image  52  and the distance between the feature points, for example. 
     Some subject images are associated with the vehicle. For example, the positions of the subjects represented by the subject images  50   b  and  50   c  in  FIG.  4   , respectively, are determined with reference to the vehicle, and therefore the subject images  50   b  and  50   c  are determined to be associated with the vehicle. To determine the display positions of these subject images  50   b  and  50   c  accurately, it is required that the location of the display  14  with respect to the vehicle should be accurately estimated. However, it is difficult to accurately determine the location of the display  14  with respect to the vehicle only based on the SLAM image  52  described above. 
     More specifically, the movement of the display  14  with respect to an external stationary object appears as a change in the scenery in the SLAM image  52 . It is therefore possible to specify the movement of the display  14  with respect to the external stationary object by specifying a change in the scenery in the SLAM image  52 , or a change in the feature points. When the user  100  is on the vehicle, however, it is difficult to determine whether such a change in the scenery is caused by the movement of the head of the user  100  or by the movement of the vehicle. 
     Assuming that an imaging area  56  of the SLAM camera  16  at a certain sampling timing corresponds to a rectangular area indicated by dashed line in  FIG.  3   , for example, a SLAM image  52   a  shown in the upper portion of  FIG.  5    can be captured. It is further assumed that a SLAM image  52   b  shown in the lower portion of  FIG.  5    is captured at a subsequent different sampling timing. Comparison between these two SLAM images  52   a  and  52   b  shows a change in the scenery, which leads to an assumption that at least one of the user&#39;s head or the vehicle has moved between the two sampling timings. It is, however, difficult to specify how each of the head and the vehicle has moved. As the two SLAM images  52   a  and  52   b  show part of the vehicle cabin, which is part of a room mirror in the illustrated example, it is possible to extract the feature points of the part of the vehicle cabin to thereby estimate the movement of the head with respect to the vehicle cabin. In this case, however, a larger number of feature points of the vehicle cabin are not shown in the SLAM images  52   a  and  52   b , which results in failure to accurately estimate the movement of the head with respect to the vehicle cabin. Also, the SLAM image  52  showing a large number of feature points of the vehicle cabin may often similarly fail to estimate the movement of the head (and further, the display  14 ) with respect to the vehicle cabin at high precision, only based on the SLAM image  52  that is a two-dimensional image. 
     In this embodiment, the first sensor  19  disposed in the wearable device  12  and the second sensor  40  disposed in the mobile terminal  28  are used to respectively detect the movement, the absolute movement, of the display  14  with respect to the external stationary object and the movement, the absolute movement, of the vehicle with respect to the external stationary object individually, and these absolute movements are then used for localization and space recognition of the display  14 . 
     Specifically, the image controller  30  acquires the absolute movement of the display  14  based on the detection result of the first sensor  19 , and acquires the absolute movement of the vehicle based on the detection result of the second sensor  40 , and calculates a relative movement of the display  14  with respect to the vehicle and further a relative movement between the vehicle and the display  14 , based on a difference between the two absolute movements. More specifically, the image controller  30  may calculate a difference between the accelerations detected by the first sensor  19  and the second sensor  40 , respectively, determine the moving speed of the display  14  with respect to the vehicle by applying time integration to the acceleration difference that is determined, and determine a displacement of the display  14  with respect to the vehicle by applying second-order integration to the acceleration difference. 
     Based on the relative movement of the display  14  that is acquired, the image controller  30  may correct the result of localization obtained through visual SLAM. This configuration increases the accuracy of estimation of the location or localization of the display  14 , to thereby enable display of the subject images  50   b  and  50   c  at appropriate positions. 
       FIG.  6    is a flowchart illustrating a flow of calculation of the display positions of the subject images  50   b  and  50   c . As illustrated in  FIG.  6   , the image controller  30  acquires the SLAM image  52  (S 10 ), and extracts feature points from a scenery in the SLAM image  52  (S 12 ). The image controller  30  then estimates the movement of the display  14  based on a change of the extracted feature points (S 14 ). Specifically, the image controller  30  estimates the movement of the display  14  with respect to the external space based on a change of the feature point outside the vehicle and estimates the movement of the display  14  with respect to the vehicle based on a change of the feature point within the vehicle. In the following description, the movement of the display  14  estimated from the SLAM image  52  will be referred to as an “image-based behavior”. 
     The image controller  30  further estimates the absolute movement of the display  14  based on a detection value of the first sensor  19  (S 16 ), and estimates the absolute movement of the vehicle based on a detection value of the second sensor  40  (S 18 ). The image controller  30  then estimates the relative movement of the display  14  with respect to the vehicle based on the absolute movement of the vehicle and the absolute movement of the display  14  (S 20 ). In the following description, the relative movement of the display  14  estimated from the detection values of the first sensor  19  and the second sensor  40  will be referred to as a sensor-based behavior. 
     After acquiring both the image-based behavior and the sensor-based behavior, the image controller  30  corrects the image-based behavior based on the sensor-based behavior, to estimate the accurate movement of the display  14  with respect to the vehicle (S 22 ). The image controller  30 , based on the estimation result, determines the display positions of the subject images  50   b  and  50   c  (S 24 ). Similar procedure will be repeated in a predetermined sampling cycle. 
     As described above, modifying the image-based behavior in consideration of the sensor-based behavior increases accuracy of localization of the display  14  to enable more appropriate display of the subject images  50   b  and  50   c . In the present embodiment, the movement of the vehicle is detected by the second sensor  40  disposed in the mobile terminal  28 , and the image controller  30  installed in the mobile terminal  28  executes image processing. As such, the present embodiment eliminates the need for communication between the wearable device  12  and the vehicle. This configuration enables achieving AR display of the subject image  50  at appropriate position on a vehicle that is not equipped with a function of communicating with the wearable device  12 . A vehicle capable of communication with the wearable device  12  may also suffer from deterioration of travelling control quality due to a reduction of operation and communication resources available for vehicle travelling control operations when providing information on the absolute movement of the vehicle, from the vehicle to the wearable device  12 . In the present embodiment, execution of the operation necessary for AR display by the mobile terminal  28  and the wearable device  12  does not adversely affect the operation and communication regarding the vehicle travelling control. 
     In some embodiments, the vehicle may include a function to communicate with the wearable device  12  to enable the computer and the sensor mounted on the vehicle to execute processing similar to that executed by the mobile terminal  28  when the mobile terminal  28  is not in the vehicle. This configuration allows the user  100  to utilize the AR function even when the user  100  fails to bring the mobile terminal  28  in the vehicle. In this configuration, the vehicle may be further configured to automatically interrupt communication with the wearable device  12  when the mobile terminal  28  is in the vehicle, thereby reducing communication load of the vehicle. 
     While in the example illustrated in  FIG.  6   , the image-based behavior is calculated in the same cycle as the sensor-based behavior, the image-based behavior may be calculated in a cycle differing from that the sensor-based behavior. For example, after the image-based behavior is calculated first, the display positions of the subject images  50   b  and  50   c  may be calculated only based on the sensor-based behavior for a predetermined period of time, and, after elapse of the predetermined period of time, the image-based behavior may be calculated once again. This configuration reduces an operation amount required for determining the display positions of the subject images  50   b  and  50   c.    
     While in the above example, determination of the display positions of only the subject images  50   b  and  50   c  associated with the vehicle has been described, the detection results of the first sensor  19  and the second sensor  40  may also be used for determination of the display position of the subject image associated with the external space, such as the subject image  50   a  illustrated in  FIG.  4   , for example. To appropriately determine the display position of the subject image  50   a  in  FIG.  4    associated with a store outside the vehicle, it is necessary to accurately detect the relative positional relationship between the display  14  and the store. However, extraction of sufficient feature points surrounding the store may fail depending on the content of the SLAM image  52 , which may further lead to failure of accurate detection of the relative positional relationship between the display  14  and the store. In this case, the relative positional relationship between the display  14  and the store determined from the SLAM image  52  may be corrected based on the detection result of the first sensor  19 . Specifically, the image controller  30  may determine the relative movement of the display  14  with respect to the store based on the absolute movement of the display  14  detected by the first sensor  19  and correct the relative positional relationship between the display  14  and the store determined from the SLAM image  52 , based on the determined relative movement. 
     As described above, in the present embodiment, the second sensor  40  mounted in the mobile terminal  28  is used to detect the absolute movement of the vehicle. To accurately detect the absolute movement of the vehicle, it is necessary to fix the mobile terminal  28  so as to prevent movement of the mobile terminal  28  with respect to the vehicle. The present embodiment therefore includes the on-vehicle holder  44  that holds the mobile terminal  28 . The on-vehicle holder  44  that holds the mobile terminal  28  will be described. 
     In this embodiment, the on-vehicle holder  44  holds the mobile terminal  28  such that three detection reference axes of the second sensor  40  are parallel to the longitudinal axis, vertical axis, and horizontal axis of the vehicle (hereinafter referred to as “vehicle reference axes”), respectively. Typically, the second sensor  40  is mounted in the mobile terminal  28  in such an attitude that the detection reference axes correspond to the widthwise axis, vertical axis, thickness-wise axis of the mobile terminal  28 , respectively. Therefore, the on-vehicle holder  44  typically holds the mobile terminal  28  such that the widthwise axis, vertical axis, thickness-wise axis of the mobile terminal  28  are parallel to the vehicle reference axes, respectively. 
     Maintaining the detection reference axes of the second sensor  40  in parallel to the vehicle reference axes results in a reduction of volume of operation regarding the absolute movement of the vehicle. Specifically, vehicles often move frontward and backward, and the amount of movement in the widthwise direction and the vertical direction is small. Setting one of the detection reference axes of the second sensor  40  in parallel to the longitudinal axis of the vehicle reduces the movements to be input to the remaining two axes of the detection reference axes, thereby further reducing the volume of operation required to specify the absolute movement of the vehicle. 
     The on-vehicle holder  44  is attached to a surface of a rigid body fixed to the vehicle body, such as an instrument panel, door trims, or console within the vehicle cabin. The on-vehicle holder  44  includes an attachment portion  68  attached to the vehicle and a holder portion  66  that holds the mobile terminal  28 , as illustrated in  FIGS.  7  and  8   , for example. The attachment portion  68  is attached using a suction cup, adhesive tape, clip, screw, engagement, or the like. The holder portion  66  holds the mobile terminal  28  in a stationary state with respect to the attachment portion  68 . The holder portion  66  may be a container that houses part or whole of the mobile terminal  28  so as not to move, or may be a clamp that holds the mobile terminal  28  with a spring or screw force. 
     The on-vehicle holder  44  may be configured to hold the mobile terminal  28  to thereby properly adjust the position and attitude of the mobile terminal  28 . The on-vehicle holder  44 , for example, may include the attachment portion  68  attached to the instrument panel  63  with an adhesive tape  68   a  and the holder portion  66  fixed to the attachment portion  68 , as illustrated in  FIG.  7   . In the example illustrated in  FIG.  7   , the holder portion  66  has a clamp that clamps the mobile terminal  28  vertically with an urging force of a screw (not shown). By clamping and holding the mobile terminal  28  with the clamp, the position and attitude of the mobile terminal  28  with respect to the attachment portion  68  is fixed. The attachment portion  68  is previously positioned to a position and attitude such that the detection reference axes of the second sensor  40  are parallel to the vehicle reference axes and is then attached to the instrument panel  63 . As it can be difficult for the user  100  to perform the positioning of the attachment portion  68 , the positioning of the attachment portion  68  and the attaching operation may be performed by a dealer or repair facility. 
     In another embodiment, the attachment portion  68  of the on-vehicle holder  44  may be configured to be attached to a predetermine portion of the vehicle to allow automatic positioning. For example, as illustrated in  FIG.  8   , the on-vehicle holder  44  may include an attachment portion  68  to be fit in a non-circular recess (that is a beverage holder  64  in the example of  FIG.  8   ) disposed in the vehicle cabin with no space between the attachment portion  68  and the recess, and a holder portion  66  in a shape of a container housing most of the mobile terminal  28  with no space between the container and the mobile terminal  28 . This configuration automatically keeps the position and attitude of the holder portion  66  or further the mobile terminal  28  held in the holder with respect to the vehicle in a proper state, just by insertion of the attachment portion  68  into the non-circular recess or a beverage holder  64 . While in the example illustrated in  FIG.  8   , the attachment portion  68  is inserted into the beverage holder  64 , the attachment portion  68  that is perfectly fittable into a non-circular recess formed within the vehicle cabin without space may be inserted into other portions, such as an inner space of a console box or a door pocket formed in the door trim. This configuration enables the user  100  to position the on-vehicle holder  44  easily. 
     In spite of the modified configuration of the on-vehicle holder  44 , the on-vehicle holder  44 , which is a component independent of the vehicle, and the mobile terminal  28  are likely to oscillate slightly with respect to the vehicle. To eliminate effects of small oscillation of the mobile terminal  28  with respect to the vehicle, the image display system may be configured to exclude the detection value less than a predetermined reference value (that is, acceleration) of the detection values of the second sensor  40 , from calculation of the absolute movement of the vehicle. For example, a low-pass filter or band-pass filter that removes a small acceleration change resulting from the small oscillation may be applied with respect to the detection values of the second sensor  40 . This configuration effectively prevents the display position of the subject image  50  from oscillating due to the small oscillation of the mobile terminal  28 . This further enables more appropriate AR display. 
     As is clear from the above description, according to the present embodiment, the first sensor  19  mounted in the wearable device  12  and the second sensor  40  mounted in the mobile terminal  28  individually detect the absolute movement of the display  14  and the absolute movement of the vehicle, respectively. This configuration enables increasing the accuracy of localization of the display  14  with respect to the vehicle, thereby enabling more appropriate AR display. The configurations described above are only examples and may be modified as appropriate. For example, while in the above description, the mobile body the user  100  boards is a vehicle, the mobile body the user  100  boards may be a train, a motor bike, an airplane, or a scooter, for example. 
     REFERENCE SIGNS LIST 
       10  image display system,  12  wearable device,  14  display,  16  SLAM camera,  18  pupil position sensor,  19  first sensor,  20  device controller,  22  display area,  24  rim,  26  temple,  28  mobile terminal,  30  image controller,  32  processor,  34  memory,  35  communication I/F,  38  user I/F,  40  second sensor,  44  on-vehicle holder,  50  subject image,  52  SLAM image,  56  imaging area,  60  steering wheel,  63  instrument panel,  64  beverage holder,  66  holder portion,  68  attachment portion,  68   a  adhesive tape,  100  user.