Patent Publication Number: US-2023161161-A1

Title: Image display controller and image display controlling method for controlling image display on wearable terminal, and image displaying system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-189146 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 disclosure relates to a configuration of an image display controller and an image display controlling method for operating a wearable terminal incorporating a display arranged in a field of view of a wearer, to display an image using augmented reality technology (hereinafter, referred to as AR technology), and to an image displaying system including the wearable terminal and the image display controller. 
     BACKGROUND 
     There have been suggested apparatuses which display on a display of smart glasses, using AR technology, driving operation information indicating driving operations which should be performed on a vehicle to arrive at a destination. Such apparatuses employ a method for detecting, from an image captured by a camera, an object that can be used as a marker of a predetermined display position and displaying an image at the display position (see, for example, JP 2017-129406 A). 
     Meanwhile, an image displaying system has been suggested in which the AR technology is used for giving a wearer the impression that appearances of interior parts of a vehicle are changed (see, for example, JP 2021-64906 A). 
     In the above-described apparatuses, when image displaying operation is performed within a vehicle compartment by means of the AR technology, a display position of an image is determined by employing a method for detecting a marker, for example, from colors or shapes of interior parts present in the vehicle compartment. However, in a case where an appropriate level of brightness of the image is not obtained due to a backlighted situation or during the nighttime, for example, the apparatus may, in some cases, misidentify the marker and accordingly display the image at a position which is not intended by a user. 
     Given these circumstances, the present disclosure is directed to reducing the occurrence of misalignment of an image when the image is displayed within a vehicle compartment using AR technology. 
     SUMMARY 
     An image display controller according to this disclosure is an apparatus for displaying an image on a display of a wearable terminal. The image display controller is configured to estimate a self-location of the wearable terminal, determine a display position of an image on the display based the estimated self-location, display the image at the determined display position, detect whether an actual location of the wearable terminal is in a vehicle compartment, and when the actual location is detected in the vehicle compartment, decrease a frequency of estimating the self-location to a frequency lower than a frequency of estimating the self-location when the actual location is detected outside the vehicle compartment. 
     In this way, the occurrence of misalignment of the image on the display can be reduced in a state where the wearable terminal is used within the vehicle compartment. 
     In the image display controller according to this disclosure, when the actual location is in the vehicle compartment, subsequent estimation of the self-location and a change of the display position may be suspended during a predetermined time period after the self-location is estimated. 
     This can prevent frequent occurrence of misalignment of the image on the display in the state where the wearable terminal is used in the vehicle compartment. 
     In the image display controller according to this disclosure, the image may be displayed at a fixed position, which has been previously defined on the display, until the self-location is initially estimated when the actual location is in the vehicle compartment, or until subsequent estimation of the self-location is successfully completed after a failure in estimation of the self-location when the actual location is in the vehicle compartment. 
     In the state where the wearable terminal is used in the vehicle compartment, because the display position of the image is set to the fixed position in a case where initial estimation of the self-location has not yet been performed or estimation of the self-location has failed, confusion of a wearer of the wearable terminal can be prevented. 
     In the image display controller according to this disclosure, the self-location of the wearable terminal may be estimated based on an image captured by an image pickup device installed in the wearable terminal, and whether the actual location of the wearable terminal is in the vehicle compartment may be detected through communication with the vehicle. 
     With the above-described configuration, both estimation of the self-location of the wearable terminal and detection of the actual location of the wearable terminal can be performed in a simple way. 
     In the image display controller according to this disclosure, it may be further detected through communication with the vehicle whether the vehicle is traveling or a wearer of the wearable terminal is driving, and when it is detected that the vehicle is traveling or the wearer is driving in addition to the actual location being detected in the vehicle compartment, the frequency of estimating the self-location may be decreased to a frequency lower than a frequency of estimating the self-location when it is solely detected that the actual location is in the vehicle compartment. 
     With the above-described configuration, in the state where the wearable terminal is used in the vehicle compartment, the occurrence of misalignment of the image on the display can be further reduced when the vehicle is traveling, or when the wearer is driving the vehicle. 
     In the image display controller according to this disclosure, the frequency of estimating the self-location may be decreased only in a case where the image pickup device captures the image during the nighttime or against the sun. 
     In this way, misalignment of the image on the display can be prevented when the self-location is estimated at a low accuracy. 
     An image display controlling method according to an aspect of this disclosure is a method for displaying an image on a display of a wearable terminal, the method including estimating a self-location of the wearable terminal, determining a display position of an image on the display based on the estimated self-location, displaying the image at the determined display position, detecting whether an actual location of the wearable terminal is in a vehicle compartment, and when the actual location is detected in the vehicle compartment, decreasing a frequency of estimating the self-location to a frequency lower than a frequency of estimating the self-location when the actual location is detected outside the vehicle compartment. 
     In the image display controlling method according to this disclosure, when the actual location is in the vehicle compartment, subsequent estimation of the self-location and a change of the display position may be suspended during a predetermined time period after the self-location is estimated. 
     In the image display controlling method according to this disclosure, when the actual location is in the vehicle compartment, the image may be displayed at a fixed position, which has been previously defined on the display, until the self-location is initially estimated, or until subsequent estimation of the self-location is successfully completed after a failure in estimation of the self-location. 
     In the image display controlling method according to this disclosure, the self-location of the wearable terminal may be estimated based on an image captured by an image pickup device installed in the wearable terminal, and communication with a vehicle can be used to detect whether the actual location of the wearable terminal is in the vehicle compartment. 
     In the image display controlling method according to this disclosure, communication with the vehicle can be used to further detect whether the vehicle is traveling or a wearer of the wearable terminal is driving, and when it is further detected that the vehicle is traveling or the wearer is driving in addition to the actual location being detected in the vehicle compartment, the frequency of estimating the self-location may be decreased to a frequency lower than a frequency of estimating the self-location when it is solely detected that the actual location is in the vehicle compartment. 
     In the image display controlling method according to this disclosure, the frequency of estimating the self-location may be decreased only in a case where the image pickup device captures the image during the nighttime or against the sun. 
     An image displaying system according to an aspect of this disclosure includes a wearable terminal, which incorporates an image pickup device and a display, and a controller configured to estimate a self-location of the wearable terminal based on an image captured by the image pickup device of the wearable terminal, determine a display position of the image on the display based on the estimated self-location, and display the image at the determined display position, in which the controller is further configured to detect, through communication with a vehicle, whether an actual location of the wearable terminal is in the vehicle compartment, and when the actual location is detected in the vehicle compartment, decrease a frequency of estimating the self-location to a frequency lower than a frequency of estimating the self-location when the actual location is detected outside the vehicle compartment. 
     In the image displaying system according to this disclosure, the controller may be further configured to suspend subsequent estimation of the self-location and a change of the display position during a predetermined time period after the self-location is estimated when the actual location is in the vehicle compartment. 
     According to this disclosure, the occurrence of misalignment of the image can be reduced in a case where the image is displayed within the vehicle compartment using AR technology. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present disclosure will be described based on the following figures, wherein: 
         FIG.  1    is a functional block diagram showing a configuration of an image display controller according to an embodiment; 
         FIG.  2    is a perspective view showing a driver wearing a wearable terminal; 
         FIG.  3    is a diagram showing a hardware configuration in the image display controller illustrated in  FIG.  1   ; 
         FIG.  4    is a flowchart showing basic operations of the image display controller illustrated in  FIG.  1   ; 
         FIG.  5    shows a visual field of the driver wearing the wearable terminal on which an image is displayed by the image display controller illustrated in  FIG.  1   ; 
         FIG.  6    shows a visual field of the driver wearing the wearable terminal that is obtained when the driver moves his/her face to the left in a state where a self-location of the wearable terminal has been estimated by the image display controller illustrated in  FIG.  1   ; 
         FIG.  7    is a flowchart showing operations of the image display controller illustrated in  FIG.  1   ; 
         FIG.  8    is a flowchart of operations following those in the flowchart of  FIG.  7   ; and 
         FIG.  9    shows a field of view of the driver wearing the wearable terminal when the driver&#39;s face is turned left in a state where the self-location of the wearable terminal is not estimated by the image display controller illustrated in  FIG.  1   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an image display controller  10  according to an embodiment is explained with reference to the drawings. As shown in  FIG.  1   , the image display controller  10  estimates, based on a captured image taken by a camera  28  which is an image pickup device installed in a wearable terminal  20 , a self-location of the wearable terminal  20  and displays an image on a display  26  of the wearable terminal  20 . The image display controller  10  and the wearable terminal  20  constitute an image displaying system  100 . 
     The wearable terminal  20  is a device which is worn by a driver  200  or a passenger of a vehicle in a like manner with eyeglasses or goggles. The wearable terminal  20  is equipped with the camera  28  being the image pickup device, the display  26 , and a communication device  27 . 
     Referring now to  FIG.  2   , the wearable terminal  20  is explained.  FIG.  2    shows a state of the driver  200  wearing the wearable terminal  20 . The wearable terminal  20  is formed in a shape of a pair of glasses and may be in some cases referred to as smart glasses. The wearable terminal  20  includes temples  22 , and a rim  24  connected to the temples  22 , in which each of the temples  22  constitutes a linear part of a frame that is designed to be put on an ear, and the rim  24  constitutes a part of the frame that is designed to surround the eyes of the wearer  20  and to be put on the nose of the wearer  20 . 
     The display  26  which is a display apparatus is fitted in the rim  24 . The display  26 , which is placed within a visual field of the driver  200  so as to block a forward view in front of the eyes of the driver  200 , is designed to become highly transparent (exhibit a high degree of light transparency) when no image is formed on the display  26 , for allowing the driver  200  to view forward through the display  26  displaying no image. The display  26  can form an image on its partial region or the entire region under control of the image display controller  10 . 
     The camera  28  is installed in the vicinity of a joint between the rim  24  and a left temple  22  on a left eye side of the driver  200 . The camera  28  captures an image of the interior of a vehicle compartment when the driver  200  wearing the wearable terminal  20  enters the vehicle compartment, and outputs the captured image. The temple  22  on a right eye side of the driver  200  incorporates the communication device  27  configured to communicate with a vehicle  30 . The communication device  27  outputs, when communication with a communication device  36  mounted on the vehicle  30  is established, actual location information indicating that the wearable terminal  20  is actually located in the vehicle compartment of the vehicle  30  (i.e., an actual location of the wearable terminal  20  is in the vehicle compartment). Further, the temple  22  on the left eye side incorporates the image display controller  10 , which is explained below. 
     Referring back to  FIG.  1   , the image display controller  10  is described. As shown in  FIG.  1   , the image display controller  10  is composed of functional blocks including a feature extracting section  11 , a self-location estimating section  12 , a display position determining section  13 , an image displaying section  14 , an image data storing section  15 , an operation inputting section  16 , a self-location estimation frequency adjusting section  17 , and a position storing section  18 . The image display controller  10  is connected to the wearable terminal  20  through a wired or wireless channel. Further, the image display controller  10  is connected to the vehicle  30  through a wireless channel to receive vehicle information and course information from an ECU  32  and a navigation apparatus  34  of the vehicle  30 , respectively. 
     The feature extracting section  11  receives from the camera  28  of the wearable terminal  20  an image captured by the camera  28 . The feature extracting section  11  processes the received image to extract one or more features of the interior of the vehicle compartment. The feature, which is also referred to as a marker, is a part of the vehicle compartment that can function as an indicator of a particular position within the vehicle compartment. 
     The self-location estimating section  12  estimates, based on both the image captured by the camera  28  and the feature of the vehicle compartment extracted by the feature extracting section  11 , a relative location of the wearable terminal  20  that is defined with respect to the feature, and outputs the estimated relative location as the self-location of the wearable terminal  20 . 
     Based on both the self-location estimated by the self-location estimating section  12  and the feature extracted by the feature extracting section  11 , the display position determining section  13  calculates which position on the display  26  is intersected by a line connecting a pupil of the driver  200  and the feature. Then, the display position determining section  13  calculates a position on the display  26  at which the image is to be overlaid and displayed on the display  26 , and outputs the calculated position as a display position. Further, the display position determining section  13  sends the display position to the position storing section  18 . 
     The operation inputting section  16  is an operation part used by the driver  200  for operating the image display controller  10 . The driver  200  can operate the operation inputting section  16  to input an instruction as to whether or not to display an image on the wearable terminal  20  and, in a case of displaying the image, as to which image is displayed at which position. 
     The image data storing section  15  stores data of images of, for example, right and left arrow symbols for indicating directions of travel, the images being displayed on the display  26  of the wearable terminal  26  by the image display controller  10 . 
     The image displaying section  14  communicates with the vehicle  30 , based on operation inputs which are input through the operation inputting section  16  by the driver  200 , to receive vehicle information, such as speed information, from the ECU  32  of the vehicle  30  and/or course information including course change information, such as information about a right turn or a left turn, from the navigation apparatus  34 , and generates, based on the received vehicle information and/or course information, a content  70  which is an image to be displayed on the display  26  of the wearable terminal  20 . Then, the image displaying section  14  causes the display  26  to display, on the display  26 , the generated content  70  at the display position, which is input from the display position determining section  13 . 
     The position storing section  18  stores the display position output from the display position determining section  13 . Further, the position storing section  18  also stores a fixed position which is previously defined on the display  26 . 
     The self-location estimation frequency adjusting section  17  adjusts a frequency of outputting a self-location estimation command, based on the image that is input from the camera  28 , information about an actual location of the wearable terminal  20  that is input from the communication device  27  of the wearable terminal  20 , and the vehicle information received from the vehicle  30 . Further, the self-location estimation frequency adjusting section  17  reads out from the position storing section  18  a previously determined display position or the fixed position having been previously defined on the display  26 , and outputs the read position as the display position to the image displaying section  14 . The self-location estimated in the self-location estimating section  12  is input into the self-location estimation frequency adjusting section  17 . Still further, the self-location estimation frequency adjusting section  17  determines whether or not the self-location estimating section  12  successfully estimates the self-location, and, when determining that estimation of the self-location has failed, retrieves the fixed position previously defined on the display  26  from the position storing section  18  and outputs the retrieved position as the display position to the image displaying section  14 . 
     The above-described image display controller  10  is a computer, and the functional blocks are implemented by hardware as shown in  FIG.  3   . The image display controller  10  includes, as shown in  FIG.  3   , a CPU  151  which is a processor configured to perform information processing, a ROM  152 , a RAM  153  which temporarily stores data during information processing, an SSD  154  which stores programs, user data, and other data, a touch panel  157  installed as input and display means, and a data port  158  through which data are input and output. The CPU  151 , the ROM  152 , the RAM  153 , and the SSD  154  are connected to each other through a data bus  159 . The touch panel  157  and the data port  158  are connected via an input/output controller  156  to the data bus  159 . A communication interface  155  installed as communication means is also connected to the data bus  159 . 
     The feature extracting section  11 , the self-location estimating section  12 , the display position determining section  13 , the image displaying section  14 , and the self-location estimation frequency adjusting section  17  are implemented by the CPU  151  when the CPU  151  cooperatively operates with the ROM  152  and RAM  153  to execute the programs stored in the SSD  154  shown in  FIG.  3   . The image data storing section  15  and the position storing section  18  may be implemented by the SSD  154  shown in  FIG.  3    when predetermined image data and display position information are stored in the SSD  154 . The operation inputting section  16  is implemented by the touch panel  157 . In addition, communication with the vehicle  30  is established by the communication interface  155 . 
     Next, basic operations of the image display controller  10  according to an embodiment are explained with reference to  FIGS.  4  and  5   . The basic operations explained herein constitute an operation to display an image on the display  26  of the wearable terminal  20  using AR technology. In the following description, it is assumed that the driver  200  inputs through the operation inputting section  16  an instruction for displaying a content  70  including a course change indication  71 , a remaining battery power indication  72 , and a vehicle speed indication  73 , as shown in  FIG.  5   , on a lower right region of a front glass  40 , the lower right region being selected as a target display position. 
     Here,  FIG.  5    shows a visual field of the driver  200  wearing the wearable terminal  20  on which an image is displayed by the image display controller  10 . As shown in  FIG.  5   , a dashboard  41  in a front region of a vehicle compartment, a right front pillar  42 , a left front pillar  43 , a ceiling panel  44 , an air outlet port  49 , a handle  50 , a rearview mirror  51 , and other components are present in the visual field of the driver  200 . It should be noted that an arrow FR, an arrow UP, and an arrow RH illustrated in  FIG.  5    represent a forward direction (traveling direction), an upward direction, and a rightward direction of the vehicle  30 , respectively. The same is also applied to those illustrated in  FIGS.  6  and  9   . 
     The feature extracting section  11  of the image display controller  10  reads out the target display position input in step S 101  shown in  FIG.  4   , obtains an image captured by the camera  28  of the wearable terminal  20 , performs image recognition processing on the image in step S 102  shown in  FIG.  4   , and extracts a feature in step S 103  of  FIG.  4   . For example, as shown in  FIG.  5   , when the lower right region of the front glass  40  is the target display position of the content  70 , the feature extracting section  11  extracts as features a demarcation line  45  between the front glass  40  and the dashboard  41  and a demarcation line  46  between the front glass  40  and the right front pillar  42 . 
     In step S 104  of  FIG.  4    the self-location estimating section  12  estimates, based on an image of the interior of the vehicle compartment captured by the camera  28 , a relative location of the wearable terminal  20  defined with respect to the demarcation lines  45  and  46  which are extracted as the features by the feature extracting section  11 , and outputs the estimated relative location as a self-location. 
     The display position determining section  13  calculates, in step S 105  of  FIG.  4   , a plurality of positions on the display  26  of the wearable terminal  20 , the positions at which the display  26  is intersected by lines connecting the pupil of the driver  200  and particular points on the demarcation lines  45  and  46 , which are extracted as the features in the vehicle compartment. Then, the display position determining section  13  identifies a portion of the display  26  composed of the plurality of positions calculated as being intersected by the lines connecting the pupil and the particular points on the demarcation lines  45  and  46 , and outputs the identified portion of the display  26  as a display position on the display  26 . 
     The image displaying section  14  communicates with the vehicle  30  in step S 106  of  FIG.  4    to receive a vehicle speed and a remaining battery power of the vehicle information from the ECU  32  and also receive the course information from the navigation apparatus  34 . Then, the image displaying section  14  generates the content  70  shown in  FIG.  5    including the course change indication  71 , the remaining battery power indication  72 , and the vehicle speed indication  73  with reference to image data stored in the image data storing section  15 . Further, the image displaying section  14  displays the generated content  70  at the display position on the display  26  that is input from the display position determining section  13 . 
     When the image is displayed on the display  26  of the wearable terminal  20 , the driver  200  views the content  70  including the course change indication  71 , the remaining battery power indication  72 , and the vehicle speed indication  73 , as shown in  FIG.  5   , in the lower right region of the front glass  40  as if the content  70  were actually displayed in the lower right region of the front glass  40  of the vehicle  30 . 
     The image display controller  10  repeats operations in steps S 101  to S 106  of  FIG.  4    at high speed in order to adjust the display position of the content  70  in real time to a position which is stationary with respect to the demarcation lines  45  and  46  extracted as the features of the vehicle compartment. Accordingly, even when the relative location of the wearable terminal  20  with respect to the demarcation lines  45 ,  46  of the features of the vehicle compartment is changed due to movement of the face of the driver  200 , the content  70  is always viewed by the driver  200  as being displayed stationarily with respect to the demarcation lines  45  and  46  of the features. 
     For example, even when the wearable terminal  20  is shifted leftward as indicated by an arrow  91  in  FIG.  6    in response to movement of the face of the driver  200  toward the left, the driver  200  still views the content  70  displayed in the lower right region of the front glass  40  as if the content  70  were always stationarily displayed therein. However, as shown in  FIG.  6   , on the rightward outside of the rim  24  of the wearable terminal  20  where the display  26  is not present, the driver  200  cannot view a right part of the content  70 , and views the front glass  40 , the dashboard  41 , and the right front pillar  42  of the vehicle  30  on the rightward outside of the rim  24 . 
     Meanwhile, in a case where the camera  28  captures the image in the nighttime or against the sun, due to insufficient luminance, a demarcation line  48  between the front glass  40  and the ceiling panel  44  and a demarcation line  47  between the front glass  40  and the left front pillar  43 , for example, may be erroneously extracted as the features in place of the above-described demarcation lines  45  and  46  between the front glass  40  and the dashboard  41  and between the front glass  40  and the right front pillar  42 . In this case, the content  70  is suddenly moved from the position in the lower right region of the front glass  40  to a position in an upper left region of the front glass  40 , or may be moved depending on situations to a position which cannot be represented on the display  26  of the wearable terminal  20 . There may be another case where no features are extracted. In this case, because it is impossible to determine the display position of the content  70 , the content  70  cannot be displayed on the display  26 . 
     Given these circumstances, the image display controller  10  according to this embodiment is equipped with the self-location estimation frequency adjusting section  17  for adjusting the frequency of estimating the self-location, in order to prevent the occurrence of misalignment or non-display of the content  70  that is unexpected by the driver  200 . Hereinafter, an operation of the image display controller  10  according to an embodiment is explained with reference to  FIGS.  7  to  9   . It should be noted that the same steps as those of the basic operation previously described with reference to  FIG.  4    are designated by the same step numbers, and these steps will be only briefly explained. 
     The self-location estimation frequency adjusting section  17  determines in step S 201  of  FIG.  7    whether the actual location of the wearable terminal  20  is in the vehicle compartment. 
     The communication device  27  in the wearable terminal  20  outputs, when communication with the communication device  36  in the vehicle  30  is established, actual location information indicating that the actual location of the wearable terminal  20  is in the vehicle compartment. The self-location estimation frequency adjusting section  17  determines YES in step S 201  of  FIG.  7    when the actual location information is received from the wearable terminal  20 , and moves to step S 202  of  FIG.  7   . 
     In step S 202  of  FIG.  7   , the self-location estimation frequency adjusting section  17  determines whether or not the vehicle  30  is traveling or the wearer of the wearable terminal  20  is driving the vehicle  30 . 
     The determination may be performed using various parameters. For example, the self-location estimation frequency adjusting section  17  may receive information about an ON or OFF status of a starting switch of the vehicle  30  as the vehicle information from the ECU  32  of the vehicle  30  and may determine that the vehicle  30  is traveling when the ON status of the starting switch is received. Alternatively, the self-location estimation frequency adjusting section  17  may receive vehicle speed information as the vehicle information from the ECU  32  of the vehicle  30  and may determine that the vehicle  30  is traveling when the vehicle speed is equal to or greater than a predetermined threshold value, such as, for example, several kilometers per hour. Further alternatively, the self-location estimation frequency adjusting section  17  may perform an image analysis on the image input from the wearable terminal  20  and may determine that the wearer of the wearable terminal  20  is the driver  200  and is presently driving the vehicle  30  when the image represents a front view from a driver seat as illustrated in  FIG.  5   . 
     When determining NO in step S 202  of  FIG.  7   , the self-location estimation frequency adjusting section  17  moves to step S 203  of  FIG.  7    and sets a predetermined time period to a first predetermined length of time. On the other hand, when determining YES in step S 202  of  FIG.  7   , the self-location estimation frequency adjusting section  17  moves to step S 204  of  FIG.  7    and sets the predetermined time period to a second predetermined length of time which is longer than the first predetermined length of time. Here, a case where NO is determined in step S 202  of  FIG.  7    corresponds a state in which the driver  200  wearing the wearable terminal  20  has just gotten in the vehicle  30  and has not yet started the vehicle  30 . 
     As described above, the image display controller  10  rapidly repeats the operations in steps S 101  to S 106  of  FIG.  4    in the basic operations to adjust in real time the display position of the content  70  to the position which is stationary with respect to the features of the vehicle compartment. The first and second predetermined lengths of time are longer than a time interval between repetitions of a series of the operations in steps S 101  to S 106  of  FIG.  4    and may be set to 0.5˜1 second, 1˜3 seconds, or several seconds. 
     After setting the predetermined time period in step S 203  or S 204  of  FIG.  7   , the self-location estimation frequency adjusting section  17  moves to step S 205  of  FIG.  7    and repeats operations in step S 206  to S 209  of  FIG.  7    without estimating the self-location until the predetermined time period has elapsed. 
     In step S 206  of  FIG.  7   , the self-location estimation frequency adjusting section  17  determines whether or not the display position, which was previously determined in step S 206  of  FIG.  7   , is stored in the position storing section  16 . Immediately after the driver  200  wearing the wearing terminal  20  gets in the vehicle  30  and initiates the wearable terminal  20 , none of the operations in steps S 101  to S 106  of  FIG.  8    is performed; i.e., no self-location is estimated, and the display position is accordingly undetermined. Therefore, because the position storing section  18  does not store any previously determined display position, the self-location estimation frequency adjusting section  17  determines NO in step S 206  of  FIG.  7    and moves to step S 209  of  FIG.  7   . Then, in step S 209 , the self-location estimation frequency adjusting section  17  retrieves the fixed position stored in the position storing section  18 , the fixed position being previously defined on the display  26 , specifies the retrieved fixed position as the display position, and outputs the specified result to the image displaying section  14 . The image displaying section  14  displays in step S 208  of  FIG.  7    the content  70  on the display position input from the self-location estimation frequency adjusting section  17 . 
     After displaying the content  70  at the previously defined fixed position on the display  26  in step S 208  of  FIG.  7   , the self-location estimation frequency adjusting section  17  returns to step S 205  of  FIG.  7   , and causes the image displaying section  14  to continuously display the content  70  at the previously defined fixed position on the display  26  until the predetermined time period has elapsed. 
     An arbitrary position may be previously defined as the fixed position, and the fixed position may be, for example, a position slightly shifted to the right from the center of the display  26  as shown in  FIG.  9   . In this case, the driver  200  views the image in a region that is slightly shifted to the right from the center of the rim  24 . Because the position in this case is fixedly defined on the display  26  irrespective of the self-location which is the relative location of the wearable terminal  20  defined with respect to the features in the vehicle compartment, the content  70  is fixedly displayed on the same position of the wearable terminal  20 . Accordingly, during the predetermined time period in which operations to estimate the self-location and determine the display position are not performed, as the driver  200  moves his/her face to the left as indicated by an arrow  92  in  FIG.  9   , the content  70  is shifted along with the wearable terminal  20  leftward with respect to the demarcation lines  45  and  46  which are the features in the vehicle compartment. 
     When the predetermined time period has elapsed, the self-location estimation frequency adjusting section  17  determines YES in step S 205  of  FIG.  7   , and outputs a self-location estimation command to the feature extracting section  11 . Then, the feature extracting section  11 , the self-location estimating section  12 , the display position determining section  13 , and the image displaying section  14  perform, as in the case of the basic operations previously explained with reference to  FIG.  4   , the operations to extract the features, estimate the self-location, determine the display position on the display  26 , and display the content  70  at the determined display position on the display  26  as indicated in steps S 101  to S 106  of  FIG.  8   . The operations in steps S 101  to S 106  of  FIG.  8    are rapidly performed to enable real-time adjustment of the display position of the content  70  to the position that is maintained stationary with respect to the features in the vehicle compartment. 
     During the operations, the self-location estimated by the self-location estimating section  12  is input into the self-location estimation frequency adjusting section  17 , and the display position determined by the display position determining section  13  is stored in the position storing section  18 . 
     The self-location estimation frequency adjusting section  17  determines in step S 210  of  FIG.  8    whether or not the self-location estimating section  12  successfully completes estimation of the self-location. When the self-location estimating section  12  fails to estimate the self-location and thus no self-location is input from the self-location estimating section  12 , or when the estimated self-location is situated, for example, outside the vehicle compartment, the self-location estimation frequency adjusting section  17  determines NO in step S 210  of  FIG.  8   , skips the operation to determine the display position in step S 105  of  FIG.  8   , and jumps to step S 211  of  FIG.  8   . In step S 211 , the self-location estimation frequency adjusting section  17  retrieves the fixed position previously defined on the display  26  from the position storing section  18 , and outputs the retrieved fixed position as the display position to the image displaying section  14 . This can prevent displacement of the position of the content  70 , upon occurrence of a failure in estimation of the self-location, to an undesirable position which is not intended by the driver  200  wearing the wearable terminal  20 . 
     After displaying the content  70  in step S 106  of  FIG.  8   , the self-location estimation frequency adjusting section  17  returns to step S 201  of  FIG.  7   , and performs the operations in steps S 201  to S 205  of  FIG.  7   . Following this, when it is found that the display position is determined during the previously performed operations in steps S 101  to S 106  of  FIG.  8    and is accordingly stored in the position storing section  18 , the self-location estimation frequency adjusting section  17  determines YES in step S 206  of  FIG.  7    and moves to step S 207  of  FIG.  7    to read out the previously determined display position stored in the position storing section  18  and output the previously determined display position as the display position to the image displaying section  14 . 
     The image displaying section  14  displays the content  70  on the previously determined display position which is input from the self-location estimation frequency adjusting section  17 . Because the previously determined display position is the position defined on the display  26  irrespective of a present self-location which is a present relative location of the wearable terminal  20  within the vehicle compartment, the content  70  is displayed on a position fixed to the wearable terminal  20  as in the case of setting the display position to the fixed position having been previously defined on the display  26  as described above. Therefore, as the driver  200  moves his/her face, the content  70  moves together with the wearable terminal  20  relative to the demarcation lines  45  and  46  which are the features in the vehicle compartment. 
     After the predetermined time period has elapsed, the self-location estimation frequency adjusting section  17  determines YES in step S 205  of  FIG.  7    and outputs the self-location estimation command to the feature extracting section  11 . Then, the feature extracting section  11 , the self-location estimating section  12 , the display position determining section  13 , and the image displaying section  14  perform, as indicated in steps S 101  to S 106  of  FIG.  8   , the operations to extract the features, estimate the self-location, determine the display position on the display  26 , and display the content  70  at the determined display position on the display  26 . 
     During the operations, because the display position is adjusted based on the estimated self-location, the driver  200  can view the content  70  as if the content  70  is actually fixed to the lower right region of the front glass  40 . 
     As described above, when the wearable terminal  20  is present in the vehicle compartment, the image display controller  10  suspends both estimation of the self-location and determination of the display position to fix the display position to the previously-determined display position of the content  70  during the predetermined time period, and performs, every time the predetermined time period has elapsed, the operations to estimate the self-location and determine the display position for adjusting the display position of the content  70  to the position which is stationary with respect to the demarcation lines  45  and  46  which are the features in the vehicle compartment. 
     In this way, frequent occurrence of misalignment of the content  70  on the display  26  can be prevented when the wearable terminal  20  is used within the vehicle compartment. 
     On the other hand, the driver  200  who slightly moves his/her face within the vehicle compartment views the content  70  at a position slightly shifted from the lower right region of the front glass  40  during the predetermined time period in which estimation of the self-location and updating of the display position are suspended. Then, when the estimation of the self-location and the updating of the display position are performed after the predetermined time period, the driver  200  views the content  70  being shifted to a position in the lower right region of the front glass  40 . After that, when the driver  200  slightly moves his/her face again, the content  70  appears, for the driver  200 , to move to a position slightly shifted from the lower right region of the front glass  40  during the predetermined time period in which the estimation of the self-location and the updating of the display position are suspended, and subsequently move to the lower right region when the predetermined time period has elapsed. In this regard, it appears to the driver  200  that the display position of the content  70  is adjusted to the lower right region of the front glass  40  at an interval of the predetermined time period, so long as the driver  200  does not greatly move his/her face within the vehicle compartment. Accordingly, for the driver  200 , the content  70  appears to rest substantially in the lower right region of the front glass  40 . 
     Meanwhile, when NO is determined in step S 201  of  FIG.  7   ; i.e., when the actual location of the wearable terminal  20  is not found in the vehicle compartment but is found outside the vehicle compartment, the self-location estimation frequency adjusting section  17  skips operations in step S 202  to S 209  of  FIG.  7   , and outputs the self-location estimation command to the feature extracting section  11 . In response to the instruction, the feature extracting section  11 , the self-location estimating section  12 , the display position determining section  11 , and the image displaying section  14  rapidly repeat the operations to extract the features, estimate the self-location, determine the display position on the display  26 , and display the content  70  at the determined display position on the display  26  as indicated in steps S 101  to S 106  in  FIG.  8   . In this way, the display position of the content  70  is adjusted in real time to a position fixed to a feature extracted from objects located outside the vehicle compartment. 
     When the actual location of the wearable terminal  20  is outside of the vehicle compartment, the image display controller  10  may communicate with a mobile phone, for example, to acquire position information and course information adapted to a walker, and may cause the display  26  of the wearable terminal  20  to display an image including an arrow pointing toward a direction of travel in such a manner that the arrow can be viewed above a sidewalk. 
     Therefore, when the actual location of the wearable terminal  20  is outside of the vehicle compartment, the self-location estimation frequency adjusting section  17  does not perform the operations that cause a decrease in frequency of estimating the self-location and determining the display position, indicated in steps S 202  to S 209  of  FIG.  7   . Conversely, when the actual location of the wearable terminal  20  is in the vehicle compartment, the self-location estimation frequency adjusting section  17  decreases the frequency of estimating the self-location to a frequency lower than that of estimating the self-location when the actual location of the wearable terminal  20  is outside of the vehicle compartment. 
     As described above, the self-location estimation frequency adjusting section  17  performs, at the interval of the predetermined time period, estimation of the self-location and determination of the display position when the actual location of the wearable terminal  20  is in the vehicle compartment, to thereby decrease the frequency of performing the estimation and determination to the frequency lower than that of continuously performing the estimation and determination when the actual location of the wearable terminal  20  is outside of the vehicle compartment. Because of the decreased frequency of performing the estimation and determination, there can be hindered erroneous detection due to an undesirable change in environment within the vehicle compartment, as a result of which the display position of the content  70  is moved to an unexpected position for the driver  200 . 
     Meanwhile, the driver  200  who is driving the vehicle  30  keeps looking ahead, and therefore rarely moves his/her face. For this reason, the content  70  appears, for the driver  200 , to rest substantially in the lower right region of the front glass  40  even though the time interval between operations to determine the display position of the content  70  is extended. From this regard, in a case where it is only determined that the actual location of the wearable terminal  20  is in the vehicle compartment, the self-location estimation frequency adjusting section  17  determines YES in step S 202  of  FIG.  7    when the driver  200  wearing the wearable terminal  20  is driving the vehicle  30  or when the vehicle  30  is traveling, and sets, in step S 204  of  FIG.  7   , the predetermined time period to the second predetermined length of time longer than the first predetermined length of time to thereby decrease the frequency of performing the estimation of the self-location and the determination of the display position to a frequency lower than that of performing the estimation and the determination. In this way, there is further suppressed movement of the display position of the content  70  to an unexpected position for the driver  200  when the feature is erroneously detected due to an undesirable change in environments within the vehicle compartment. 
     In addition, prior to determining whether the predetermined time period has elapsed in step S 205  of  FIG.  7   , the self-location estimation frequency adjusting section  17  may analyze the captured image input from the camera  28  of the wearable terminal  20  to determine whether the camera  28  has captured the image during the nighttime or against the sum. In a normal state where the camera  28  has captured the image neither during the nighttime nor against the sun, the self-location estimation frequency adjusting section  17  may skip the operations in steps S 205  to S 209  shown in  FIG.  7    and may cause the feature extracting section  11 , the self-location estimating section  12 , the display position determining section  13 , and the image displaying section  14  to rapidly repeat the operations in steps S 101  to S 106  shown in  FIG.  8    for extracting the feature, estimating the self-location, determining the display position on the display  26 , and displaying the content  70  at the determined display position on the display  26 . 
     In this way, the content  70  is adjusted in real time to the position which is stationary with respect to the feature of the vehicle compartment when the camera  28  has captured the image neither during the nighttime nor against the sun, and thus a possibility of erroneously detecting the feature is low. Then, the frequency of estimating the self-location and determining the display position may be decreased only when the camera  28  has captured the image during the nighttime or against the sun, and thus the possibility of erroneously detecting the feature is high. In this way, it can be further prevented that the display position of the content  70  is moved to an unexpected position for the driver  200  due to erroneous detection of the feature when the camera  28  captures the image during the nighttime or against the sun. 
     The image display controller  10  which is attached to the wearable terminal  20  in the above description is not limited to such arrangement and may be mounted on the vehicle  30 . 
     Further, the operations of the image display controller  10  explained with reference to  FIGS.  4 ,  7 , and  8    may also constitute an image display controlling method for displaying an image on the display  26  of the wearable terminal  20 .