Patent Publication Number: US-2023162390-A1

Title: Image display system and image controller

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-189236 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 to be superimposed on a field of vision of a user or an occupant of a vehicle and to an image controller. 
     BACKGROUND 
     A technique for displaying a predetermined image to be superimposed on a user&#39;s field of vision in such a manner as to allow the user to see a virtual object represented by the image as if the virtual object actually exists has been known. Patent Document 1, for example, discloses a technique of displaying, on smart glasses or a glasses-type display worn by a driver, an image of a leading vehicle that the driver&#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. 
     CITATION LIST 
     Patent Literature 
     [Patent Document 1] JP 2017-129406 A 
     SUMMARY 
     To enable a user to recognize a virtual object as if the virtual object were actually present, it is necessary to determine the position of an image of the virtual object to be displayed on the display (hereinafter referred to as a “subject image”) based on the position of the virtual object represented by the subject image in the real space and the position of the display in the real space. 
     In Patent Document 1, to identify the position of the display in the real space, a special marker is disposed on a dashboard, and a camera is attached to the display. The position of the display in the actual space is identified based on an image of a scene including the marker that is captured by the camera. The use of a marker as in the technique in Patent Document 1 enables detection of the position of the display in the real space with less computation. 
     In Patent Document 1, however, the marker is a physical marker, an object that actually exists. How such a physical marker is viewed significantly depends on the surrounding light environmental condition, such as illuminance or color temperatures. This may result in failure to detect the position of the physical marker from the image of the physical marker captured by the camera on the display. For example, an image of the physical marker captured in a dark environment such as at nighttime may be blacked out, or, in contrast, an image of the physical marker captured under strong sunlight may be whited out. In either case, detection of the position of the physical marker may be unsuccessful. Failure to detect the positon of the physical marker inevitably results in failure to detect the position of the display in the real space, further resulting in failure to determine the appropriate display position of the subject image. 
     An aspect of the disclosure is therefore aimed toward an image display system that enables more appropriate determination of the display position of a subject image, and an image controller. 
     In accordance with an aspect of the disclosure, an image display system includes a display configured to be attached to the head of a user, an occupant of a vehicle, to display a subject image to be superimposed on a field of vision of the user; a SLAM camera fixed to the display to capture a SLAM image of surroundings of the display; one or more light emitters disposed within a vehicle cabin, to emit light that serves as a marker; and an image controller configured to determine a display position of the subject image based on the SLAM image including the marker. 
     A marker created by emitted light can be detected in dark environments, such as at nigh time. This configuration enables reliable identification of the position of the display in real space and thus enables more appropriate identification of the display position of the subject image. 
     In this configuration, the light emitter may be an on-vehicle display disposed within the vehicle cabin to display an image, and the marker may be an image displayed in a display area of the on-vehicle display. 
     This configuration enables changing the shape, position, or brightness, for example, of the marker as desired. This enables providing a marker suitable for the environment within the vehicle to thereby enable further reliable identification of the display in real space. 
     The marker has a marker display condition including at least one of a luminance, a color, or a brightness, and the marker display condition may be variable. 
     Changing the brightness of the marker, for example, increases detectability of the marker, which enhances appropriate determination of the display position of the subject image. 
     In the above configuration, the marker display condition may be changed in accordance with environmental light conditions in the vicinity of the light emitter. 
     Changing the display condition of the marker in accordance with an environmental light condition increases detectability of the marker, which again enhances appropriate determination of the display position of the subject image. 
     The image display system may further include a light environment sensor configured to detect the environmental light condition in the vicinity of the light emitter, and the image controller may be configured to specify the environmental light condition in the vicinity of the light emitter based on a detection result of the light environment sensor. 
     The light environment sensor enables accurate detection of the environmental light condition surrounding the light emitter. This still further enhances appropriate setting of the display condition of the marker. 
     The image controller may be configured to specify the environmental light condition in the vicinity of the light emitter based on at least one of date and time, an illuminating state of a light of the vehicle, or the SLAM image. 
     This configuration enables detection of the environmental light condition surrounding the light emitter without employing the light environment sensor. 
     The image controller may be configured to change the marker display condition in accordance with the SLAM image. 
     This configuration further increases detectability of the marker. 
     The image controller may be configured to determine, in response to successful detection of the marker from the SLAM image, the display position of the subject image within a display area of the display, based on a virtual position of an object represented by the subject image in a real space and a position of the display in the real space that is obtained from the SLAM image, and to display the subject image at the determined position. 
     This configuration imparts the user with the illusion that the object represented by the subject image actually exists, thus allowing recognizable augmentation of the user&#39;s real environment. 
     The image controller may be configured to determine, in response to failure to detect the marker from the SLAM image, the display position of the subject image within a display area of the display independently of a position of the subject image in a real space, and to display the subject image at the determined position. 
     This configuration can provide the subject image to the user even after failure in detection of the marker. 
     The image display system may further include an on-vehicle display disposed within the vehicle cabin, and the image controller may be configured to display, in response to failure to detect the marker from the SLAM image, an image corresponding to the subject image on the on-vehicle display. 
     This configuration can provide an image having information that is equivalent to that of the subject image to the user even in to the event of a failure to detect the marker. 
     The marker may be disposed at a position within the field of view of the user who is sitting on a driver&#39;s seat and driving a vehicle. 
     This configuration enables the user to detect the marker and can provide the subject image while the user is driving. 
     The one or more light emitters may include two or more light emitters spaced apart from each other, and each of the two or more light emitters may emit light that serves as the marker. 
     A plurality of markers increase the accuracy in position detection of the display. 
     In accordance with another aspect of the disclosure, an image controller is configured to control driving of a display configured to be attached to the head of a user occupant of a vehicle. The display is configured to display a subject image to be superimposed on a field of vision of the user. The image controller is configured to cause one or more light emitters disposed within a vehicle cabin to emit light that serves as a marker; cause a SLAM camera fixed to the display to capture a SLAM image of surroundings of the display; and determine a display position of the subject image based on the SLAM image including the marker. 
     The marker created by emitted light can be detected in dark environments, such as at nighttime. This configuration enables reliable identification of the position of the display in real space, and thus enables more appropriate identification of the display position of the subject image. 
     The technique of the disclosure enables more appropriate determination of the display position of the subject image. 
    
    
     
       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    is an image view for explaining a space-fixed display mode and device-fixed display mode; 
         FIG.  5    schematically illustrates a field of view of a user with a subject image being displayed; 
         FIG.  6    schematically illustrates a field of view of a driver during night time; 
         FIG.  7    illustrates another example AR marker; 
         FIG.  8    illustrates an example brightness profile; 
         FIG.  9    illustrates example image display in response to failure to detect an AR marker; 
         FIG.  10    is a flowchart illustrating a flow of image display processing in the image display system; and 
         FIG.  11    is a flowchart illustrating a flow of visual SLAM processing. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The configuration of an image display system  10  will be described by reference to the drawings. While in the following specific embodiments are described for the ease of understanding, these 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 an on-vehicle system  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 eyeglass or goggle shaped device. The wearable device  12  includes a display  14 , a SLAM camera  16 , a pupil position sensor  18 , 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  occupant of the vehicle wearing the wearable device  12 . The wearable device  12  is in the form 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 scenery in front through the display area  22  with no image being 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 forward scenery in the field of view and the displayed image simultaneously. This image may at this time 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, specifies the position and attitude of the display  14  in real space based on an AR marker in the SLAM image, as will be described below. 
     The pupil position sensor  18  detects the positions of pupils in the 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, 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  to the image controller  30  and displays the subject image on the display  14  in accordance with an instruction from the image controller  30 . 
     Referring again to  FIG.  1   , the on-vehicle system  28  will be described. The on-vehicle system  28  is installed in a vehicle, and includes the image controller  30 , a meter display  40   a,  a multi display  40   b,  an electronic inner mirror  40   c,  and a light environment sensor  42 . The meter display  40   a,  the multi display  40   b,  and the electronic inner mirror  40   c  are installed in a vehicle and can be visually recognized by the driver during driving. In the following description, these displays will be referred to as “on-vehicle displays  40 ” unless discrimination among these displays is necessary. The on-vehicle displays  40  function as light emitters that emit light forming the AR marker, as will be described below. 
     The arrangement of the on-vehicle displays  40  will be described by reference to  FIG.  3   .  FIG.  3    schematically illustrates the field of view of the driver or the user  100 . The meter display  40   a  indicates information regarding the state of the vehicle, such as the speed and mileage. As illustrated in  FIG.  3   , the meter display  40   a  is disposed on the opposite side of the steering wheel  56  from the driver, which enables the driver to visually recognize the display area of the meter display  40   a  through the steering wheel  56 . 
     The multi display  40   b  indicates information regarding on-vehicle electronic instruments, such as a navigation device or an audio device. As illustrated in  FIG.  3   , the multi display  40   b  is disposed at the center of the instrument panel in the vehicle width direction, that is, on what is commonly called a center console. 
     The electronic inner mirror  40   c  displays an image from the rear of the vehicle as imaged by a rearview camera (not shown). The electronic inner mirror  40   c  is used in place of a rearview mirror which shows the rear of the vehicle by optical reflection. The electronic inner mirror  40   c  may be switchable between a digital mode showing an image and a mirror mode for showing the rear by optical reflection. As illustrated in  FIG.  3   , the electronic inner mirror  40   c  is disposed at a position where a rearview mirror is typically disposed, that is, in the vicinity of the upper end of the windshield glass. 
     Referring again to  FIG.  1   , the light environment sensor  42  detects an environmental light condition around the on-vehicle displays  40 . An environmental light condition refers to conditions including at least one of luminance or color temperature of light. The light environment sensor  42  may include at least one of an illuminance sensor that detects lightness of light or a color temperature sensor that detects color of light. A single light environment sensor  42  or two or more light environment sensors  42  may be disposed. For example, the light environment sensor  42  may be disposed on the wearable device  12 . In another embodiment, the light environment sensors  42  may be disposed near the respective on-vehicle displays  40 . The light environment sensor  42  may be disposed specifically for the image display system  10 , or an existing sensor installed in a vehicle may be used for the light environment sensor  42 . For example, some vehicles include an auto-lighting function to automatically turn on lights such as a headlight in response to darkness around the vehicle, and an illuminance sensor that is an auto-lighting sensor disposed for such an auto-lighting function may be used as the light environment sensor  42 . Further, some multi displays  40   b  contain an illuminance sensor to automatically adjust the emission brightness in accordance with the peripheral lightness, and such an illuminance sensor included in the multi displays  40   b  may be used as the light environment sensor  42 . 
     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 a computer. The memory  34  includes at least one of a main memory connected with the processor  32  via a memory bus and 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 disk, such as a hard disk drive. 
     The communication I/F  35  transmits and receives data, through wire or wirelessly, to and from other electronic devices or specifically the wearable device  12 , the on-vehicle displays  40 , and the light environment sensor  42 . For example, the communication I/F  35  may transmit and receive data to and from the on-vehicle displays  40  and the light environment sensor  42  through Controller Area Network (CAN) communication. The communication I/F  35  may further 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 implemented by a computer installed in the wearable device  12  or the on-vehicle displays  40 . While in the present embodiment, the image controller  30  is mounted in the vehicle, the image controller  30  may be incorporated in the wearable device  12  or in a mobile computer, such as a smartphone, which is separate from the wearable device  12 . 
     The image controller  30  generates data of the subject image to be displayed on the display  14 , as described above. The display  14  displays the subject image either in a “space-fixed display” mode or a “device-fixed display” mode. These two display modes will be described with reference to  FIG.  4   . 
     In the space-fixed display mode, a subject image that represents a predetermined object is displayed as if the object exists in real space. In the device-fixed display mode, a subject image that represents a predetermined object is displayed at a specific position in the display area  22  irrespective of the position of the object in real space. 
     Assuming, for example, that the user  100  is viewing a real space that actually includes a table  80  through the display area  22  of the display  14 , as illustrated in  FIG.  4   . In this case, displaying a subject image  50  that represents a “ball” in the display area  22  would naturally result in the field of vision of the user  100  simultaneously showing the real space including the table  80  and the subject image  50  of the “ball”, as indicated in a state S 1  in  FIG.  4   . 
     In the device-fixed display mode, the display position of a subject  72  indicated by the subject image  50 , which is a “ball” in the example of  FIG.  4   , is determined independently of its position in the real space. Therefore, in the device-fixed display mode, moving the eyepoint of the user  100  would not require changes in the display position, size, and shape of the subject image  50  within the display area  22  as indicted in a state S 2  in  FIG.  4   . 
     In the space-fixed display mode, the place in the real space where the subject  72  indicated by the subject image  50 , which is a “ball” in the example of  FIG.  4   , is located is specified, and the subject image  50  is displayed as if the subject  72  is actually present at the specified position. For example, assuming that, in the space-fixed display mode, the subject  72  or a “ball” is located on the table  80  in the real space. In this case, the display position, size, and shape of the “ball” within the display area  22  are changed such that the user  100  can view the “ball” on the table  80  even if the eyepoint of the user  100  moves as indicated in a state S 3 . 
     As described above, displaying the subject image  50  in the space-fixed display mode gives the user  100  the illusion that the subject  72  represented by the subject image  50  is actually present. In other words, displaying the subject image  50  in the space-fixed display mode would enable addition, deletion, emphasis, and attenuation of information with respect to the real-world environment, thereby augmenting the real world viewed by humans. This technique is typically referred to as “augmented reality” or “AR”. 
     An example display of the subject image  50  in the present embodiment will be now described.  FIG.  5    schematically illustrates the field of vision of the user  100  (or a driver in this example) with subject images  50   a,    50   b,  and  50   c  being displayed. In the example illustrated in  FIG.  5   , the subject image  50   a  for emphasizing a pedestrian, the subject image  50   b  indicating the travelling direction of the vehicle, and the subject image  50   c  indicating message that attracts driver&#39;s attention, are displayed in the space-fixed 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 an object moving with the pedestrian assuming that the 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 pedestrian and the user  100 . The subject image  50   a  may also change its shape in accordance with the position or posture of the pedestrian in the real world. 
     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 a position and in a size similar to those of a text box represented by the subject image  50   c  assuming that the object actually resides on the upper right portion of the steering wheel  56  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 eyepoint of the user  100 . 
     As described above, the space-fixed 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 space-fixed display mode further enables the subject image  50  to be displayed at a position correlated with that of the actual object, such as a pedestrian, thus effectively directing attention of the user  100  toward the object. 
     To achieve the space-fixed display, it is necessary to accurately detect the position of the pupils with respect to the display  14 , and the position and attitude of the display  14  in the real space. Based on the position and attitude of the subject in the real space, the position and attitude of the display  14  in the real space, and the positions of the pupils in the display  14 , the image controller  30  determines the position of, for example, the subject image  50  within the display area  22 . As described above, the positions of the pupils with respect to the display  14  are detected by the pupil position sensor  18 . 
     The position and attitude of the display  14  in the real space is calculated by the image controller  30  performing Visual Simultaneous Localization and Mapping (visual SLAM) based on a SLAM image captured by the SLAM camera  16 . Visual SLAM refers to a technique for estimating three-dimensional information of the environment and the position and attitude of the camera simultaneously based on images captured by the camera. To facilitate performing of visual SLAM, an AR marker  60  (see  FIG.  3   ) is disposed within the vehicle. To calculate the position and attitude of the display  14  in the real space based on the coordinates, size, and distortion, for example, of the image of the AR marker  60  within the SLAM image, the image controller  30  extracts an image of the AR marker  60  from the SLAM image captured by the SLAM camera  16 . 
     While an actual object has been used for such an AR marker  60 , in some cases, the image controller  30  is unable to detect an actual object because of the environmental light conditions within the vehicle. For example, in a dark environment such as during nighttime, an area around the AR marker  60  may be clipped in the SLAM image and may appear as a black area. In contrast, the AR marker  60  in strong sunlight may appear as a white area when shown in the SLAM image, which is referred to as blown-out highlights. In the case of such clipping, the image controller  30  is unable to detect the AR marker  60  or the position and attitude of the display  14  in the real space, failing to perform space-fixed display of the subject image  50 . 
     In this embodiment, the AR marker  60  for use in visual SLAM is composed of light emitted from the on-vehicle displays  40  (or light emitters). Specifically, in this embodiment, the on-vehicle displays  40  are caused to display an image that functions as the AR marker  60 . In the example illustrated in  FIG.  3   , cross-shape images displayed on the meter display  40   a,  the multi display  40   b,  and the electronic inner mirror  40   c,  respectively, function as the AR marker  60  for use in visual SLAM. 
     The AR markers  60  formed of light emitted from the on-vehicle displays  40  as described above appropriately appear in the SLAM image in a dark environment such as at nighttime, as illustrated in  FIG.  6   . This enables the image controller  30  to perform visual SLAM properly based on the SLAM image, and to thereby display the subject image  50  appropriately. 
     The image controller  30  is able to obtain the shape and display position of the AR marker  60 , which may be fixed or change as appropriate, at a required timing. For example, the display position and shape of the AR marker  60  may be predetermined and fixed. In this configuration, the image controller  30  prestores the determined display position and shape of the AR marker  60 . In another embodiment, the display position and shape of the AR marker  60  may be changed as appropriate. For example, the AR marker  60  may be displayed on the upper right corner of the multi display  40   b  which is showing map information and may be displayed on the lower right corner of the multi display  40   b  which is showing audio information. Further, the shape of the AR marker  60  may be changed between these two cases. In this configuration, one of the image controller  30  or the on-vehicle display  40  determines the display position and shape of the AR marker  60 , and transmits the determined information to the other through data communication. 
     The AR markers  60  displayed on the different on-vehicle displays  40  may have an identical shape or different shapes. Specifically, as illustrated in  FIG.  7   , the three on-vehicle displays  40   a,    40   b,  and  40   c  may respectively display the AR markers  60   a,    60   b,  and  60   c  having different shapes. The AR marker  60  may be provided specifically for use in visual SLAM, or an existing image may be used for the AR marker  60 . For example, the meter display  40   a  displays an image representing the speed unit, which is Km/h, in the example illustrated in  FIG.  7   , irrespective of execution of visual SLAM, and this image may be used as the AR marker  60   a.  Not just visible light but also invisible light, which can be detected by the SLAM camera  16 , may form the AR marker  60 . For example, an image of the AR marker  60  may be formed based on infrared light. 
     The image controller  30  may determine the marker display condition including at least one of luminance, color, or brightness in accordance with the environmental light conditions in the vicinity of the on-vehicle display  40  and instruct the on-vehicle display  40  to display the AR marker  60  under the determined marker display condition. For example, the image controller  30  may change the display luminance of the AR marker  60  in accordance with the illuminance in the vicinity of the on-vehicle display  40  (hereinafter referred to as “environmental illuminance”). In this configuration, the image controller  30  may prestore the luminance profile as illustrated in  FIG.  8    and determine the display luminance of the AR marker  60  based on this luminance profile. In the luminance profile illustrated in  FIG.  8   , the horizontal axis indicates the environmental illuminance and the vertical axis indicates the display luminance of the AR marker  60 . In the example illustrated in  FIG.  8   , the higher the environmental illuminance, the higher the display luminance of the AR marker  60 . This configuration allows the AR marker  60  to be clearly displayed under the environment with high environmental illuminance. Meanwhile, in the environment with low environmental illuminance, it is possible to prevent the AR marker  60  from being excessively bright, avoiding the user  100  from being dazzled. 
     In another embodiment, the image controller  30  may change at least one of color or brightness of the AR marker  60  in accordance with the color temperature in the vicinity of the on-vehicle display  40  (hereinafter referred to as “environmental color temperature”). For example, when the environmental color temperature is a color temperature with sunset-like strong red, the AR marker  60  may be changed to a color closer to blue. 
     The environmental light conditions in the vicinity of the on-vehicle display  40  may also be specified based on the detection result of the light environment sensor  42 . In another embodiment, the image controller  30  may estimate the environmental light conditions in the vicinity of the on-vehicle display  40  based on the date and time or the illuminating state of vehicle lights. For example, the image controller  30  may calculate the solar altitude based on the date and time and estimate the environmental light conditions in the vicinity of the on-vehicle display  40 , such as illuminance, based on the altitude. The image controller  30  may further modify the environmental light conditions estimated from the date and time, based on at least one of the weather, the vehicle position, or the vehicle orientation. For example, the image controller  30  may estimate the intensity of the sunlight based on the weather and modify the environmental light conditions estimated from the date and time, based on the estimation result. The image controller  30  may further estimate the degree of sunlight shielding, such as whether the vehicle is located indoors, based on the vehicle position, and modify the environmental light conditions estimated from the date and time based on the estimation result. The image controller  30  may also estimate whether the vehicle cabin is illuminated by direct sunlight based on the solar altitude and the vehicle orientation, and modify the environmental light conditions estimated from the date and time based on the estimation result. In another embodiment, the image controller  30  may estimate the environmental light conditions in the vicinity of the on-vehicle display  40  based on the illuminating state of lights that are obliged to be turned on in the nighttime, such as headlights. 
     In another embodiment, the image controller  30  may estimate the environmental light conditions in the vicinity of the on-vehicle display  40  based on the SLAM image captured by the SLAM camera  16 . As the wearable device  12  is attached to the head of the vehicle occupant, the SLAM image can be assumed to be an image of the vehicle interior. It is highly likely that the luminance and color of the entire SLAM image reflects the environmental light conditions of the vehicle interior, and thus, in the vicinity of the on-vehicle display  40 . The image controller  30  may therefore estimate the environmental light conditions in the vicinity of the on-vehicle display  40  based on the trend of luminance and color of the entire SLAM image. 
     While in the above description, the display condition of the AR marker  60  is changed based on the environmental light conditions in the vicinity of the on-vehicle display  40 , the image controller  30  may change the display condition of the AR marker  60  based on the SLAM image captured by the SLAM camera  16 . For example, it is highly likely that failure to extract the AR marker  60  from the SLAM image occurs from shortage of display luminance of the AR marker  60 . Therefore, when it is not possible to extract the AR marker  60  from the SLAM image, the image controller  30  may instruct the on-vehicle display  40  to increase the display luminance of the AR marker  60 . Further, in response to failure to extract the AR marker  60  from the SLAM image, the image controller  30  may instruct the on-vehicle display  40  to gradually change the display luminance and the color of the AR marker  60  in one direction and specify the display luminance, for example, upon proper detection of the AR marker  60 , as the display condition of the AR marker  60 . When the AR marker  60  extracted from the SLAM image include an edge that is not sufficiently clear, the image controller  30  may instruct the on-vehicle display  40  to change the display condition of the AR marker  60  so as to display the edge clearly. 
     As described above, in the present embodiment, the image displayed on the on-vehicle display  40  is used as the AR marker  60 . This configuration enables the image controller  30  to detect the AR marker  60  more reliably to thereby execute visual SLAM more properly and thus display the subject image  50  more properly. Under certain environmental light conditions within the vehicle, the image controller  30  fails to detect the AR marker  60  even after the luminance, for example, of the AR marker  60  has been changed. In this case, the image controller  30  may display the subject image  50  in the device-fixed display mode, rather than the space-fixed display mode. In another embodiment, in response to failure to detect the AR marker  60 , the image controller  30  may display an image corresponding to the subject image  50  on the on-vehicle display  40 . 
     Assuming, for example, a case wherein it is attempted to display a subject image  50   a  to direct the attention to a pedestrian, but the AR marker  60  cannot be detected, the image controller  30  may display, on the multi display  40   b,  an image that promotes attention to the pedestrian, as illustrated in  FIG.  9   . The image controller  30  may further display, on a specific location in the display area  22 , an image that promotes attention to the pedestrian (the subject image  50   a  in  FIG.  9   ). In displaying the subject image  50   a  in the device-fixed display mode, consideration should be made to prevent the image  50   a  from obstructing the driving operation. For example, as humans tend to acquire various information necessary for driving operation from the center of their field of vision, there is significant risk that an image displayed in the center of the field of vision may obstruct the driving operation. Therefore, in displaying the subject image  50   a  in the device-fixed display mode, the image  50   a  may be displayed at a corner of the display area  22 . Further, in displaying the subject image  50   a  in the device-fixed display mode, the image  50   a  may be translucent so as to allow the driver to visually recognize the surroundings through the image  50   a.    
     Referring now to  FIG.  10   , the flow of image displaying processing in the image display system  10  will be described. In response to a determination that display of the subject image  50  is necessary (Yes in step S 10 ), the image controller  30  performs visual SLAM processing to specify the position and attitude of the display  14  in the real space (S 12 ). The visual SLAM processing will be described in detail below. 
     In response to success of the visual SLAM processing (Yes in step S 14 ), the image controller  30 , based on the specified position and attitude of the display  14  in the real space, specifies the display position, for example, of the subject image  50  in the display area  22  (step S 16 ) and displays the subject image  50  on the display  14  in the space-fixed display mode (step S 18 ). Thereafter, the process returns to step S 10 , and similar processing is repeated. 
     In response to failure of the visual SLAM processing (No in step S 14 ), the image controller  30  displays an image corresponding to the subject image  50  on the display  14  in the device-fixed display mode or displays the image on the on-vehicle display  40  (S 20 ). Thereafter, the process returns to step S 10  to repeat similar processing. 
     Referring now to  FIG.  11   , the flow of the visual SLAM processing will be described. To perform the visual SLAM processing, the image controller  30  specifies the environmental light conditions in the vicinity of the on-vehicle display  40  (S 30 ). The environmental lighting condition may be specified based on the detection result of the light environment sensor  42  or based on the date and time, the illuminating state of lights, and the SLAM image, for example. 
     After the environmental light condition is specified, the image controller  30 , based on the environmental light conditions, determines the display condition, such as display luminance or color, of the AR marker  60  (step S 32 ), and instructs the on-vehicle display  40  to display the AR marker  60  under the determined display condition (step S 34 ). In response to this instruction, the on-vehicle display  40  displays the AR marker  60 . 
     Subsequently, the image controller  30  acquires the SLAM image captured by the SLAM camera  16  (step S 36 ). The image controller  30  further determines whether the AR marker  60  can be detected from the SLAM image (step S 38 ). In response to the determination that the AR marker  60  can be detected (Yes in step S 38 ), the image controller  30  specifies the position, size, and distortion, for example, of the AR marker  60  within the SLAM image (step S 40 ), and further calculates, based on the specified information of the AR marker  60 , the position and attitude of the display  14  in the real space (step S 42 ). 
     In response to the determination that the AR marker  60  cannot be detected from the SLAM image in step S 38  (No in step S 38 ), the image controller  30  proceeds to step S 20  (see  FIG.  10   ) without calculating the position or attitude of the display  14 . 
     As is clear from the above description, in the present embodiment, the AR marker  60  that is necessary for visual SLAM is formed of light emitted from the light emitter, which is specifically the on-vehicle display  40 . This configuration enables detection of the AR marker  60  and thus appropriate performing of visual SLAM under the dark environment such as in the nighttime. Further, in the present embodiment, to enable proper detection of the AR marker  60 , the display condition of the AR marker  60  is changed based on the environmental light conditions in the vicinity of the on-vehicle display  40  or the SLAM image that is captured. This configuration enables more reliable detection of the AR marker  60  and thus proper performing of visual SLAM with the change of the environmental light conditions in the vicinity of the on-vehicle display  40 . 
     The above description describes only examples, and, while the image display system should at a minimum be configured to use an image formed of light emitted from a light emitter as the AR marker  60 , the configuration of other elements may be modified. For example, in the above description, the on-vehicle display  40  that displays an image is used as the light emitter, other devices, such as an indicator lamp and illumination devices disposed within the vehicle cabin to emit light of a predetermined mode, may be used as the light emitter. 
     Further, while in the above description, the display  14  shows an image on the display area  22 , the display  14  may be a projector that projects an image onto the retina of the user  100 . Further, while in the above description, the user  100  visually recognizes the real space through the transparent display area  22 , the display area  22  may be configured to be opaque to prevent the user  100  from visually recognizing the real space through the display area  22 . In this configuration, the image controller  30  displays, on the display area  22 , a synthesis image including an image of the real space and a subject image representing a virtual object. 
     REFERENCE SIGNS LIST 
       10  image display system,  12  wearable device,  14  display,  16  SLAM camera,  18  pupil position sensor,  20  device controller,  22  display area,  24  rim,  26  temple,  28  on-vehicle system,  30  image controller,  32  processor,  34  memory,  35  communication I/F,  40  on-vehicle display,  40   a  meter display,  40   b  multi display,  40   c  electronic inner mirror,  42  light environment sensor,  50  subject image,  56  steering wheel,  60  AR marker,  72  subject,  80  table,  100  user.