Abstract:
A video generation device includes a processor configured to detect a second vehicle present in front of a first vehicle or behind the first vehicle in a first direction in which the first vehicle travels. The first vehicle is mounted with the video generation device. The processor is configured to detect a first distance between the first vehicle and the second vehicle. The processor is configured to compare the first distance to a predetermined threshold value to acquire a first comparison result. The processor is configured to determine a first speed of a first image on basis of the first comparison result. The first image is included in a video and to be moved within the video in a direction determined on basis of the first direction. The processor is configured to generate the video on basis of the first speed and display the video via a display device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-013828 filed on Jan. 27, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a video generation device, and a video generation method. 
       BACKGROUND 
       [0003]    A traffic accident by a vehicle such as, for example, a four-wheeled vehicle is mainly caused by an excessive speed or an insufficient inter-vehicular distance. Further, when a driver does not notice that the driving road surface is uphill, the driver continues to drive the vehicle without changing a stepping amount of the accelerator and the vehicular speed is reduced, which is known as one of factors causing a traffic jam. 
         [0004]    As one of techniques of allowing a vehicle driver to maintain a proper vehicular speed, there is known a technology of guiding the driver to slow down when the traveling speed (vehicular speed) of the vehicle exceeds a predetermined speed. 
         [0005]    As one of techniques of guiding a driver to a deceleration operation or an acceleration operation, there is known a technology of guiding the driver by controlling the light emission time of light-emitting objects arranged along a road so as to impart a visually induced self-motion illusion (vection) to the driver. 
         [0006]    A related technique is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2013-159915. 
         [0007]    When the light emission time of light-emitting objects arranged along a road is controlled, it is very difficult to control the light emission time in response to a traveling state (a vehicular speed, an inter-vehicular distance, or the like) of respective vehicles. Therefore, it is difficult to impart a proper vection to respective vehicle drivers to guide the driver to a deceleration operation or an acceleration operation. 
       SUMMARY 
       [0008]    According to an aspect of the present invention, provided is a video generation device including a memory and a processor coupled to the memory. The processor is configured to detect a second vehicle present in front of a first vehicle or behind the first vehicle in a first direction in which the first vehicle travels. The first vehicle is mounted with the video generation device. The processor is configured to detect a first distance between the first vehicle and the second vehicle upon detecting the second vehicle. The processor is configured to compare the first distance to a predetermined threshold value to acquire a first comparison result. The processor is configured to determine a first speed of a first image on basis of the first comparison result. The first image is included in a video and to be moved within the video in a direction determined on basis of the first direction. The processor is configured to generate the video on basis of the first speed. The processor is configured to display the video via a display device. 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a diagram illustrating an exemplary configuration of a guide system according to a first embodiment; 
           [0011]      FIG. 2  is a diagram illustrating an exemplary functional configuration of a video generation device according to the first embodiment; 
           [0012]      FIG. 3A  is a flowchart illustrating a video display process according to the first embodiment; 
           [0013]      FIG. 3B  is a flowchart illustrating the video display process according to the first embodiment; 
           [0014]      FIG. 3C  is a flowchart illustrating the video display process according to the first embodiment; 
           [0015]      FIG. 4A  is a diagram illustrating an example of a displayed video; 
           [0016]      FIG. 4B  is a diagram illustrating an example of a displayed video; 
           [0017]      FIG. 5  is a diagram illustrating an exemplary configuration of a guide system according to a second embodiment; 
           [0018]      FIG. 6  is a diagram illustrating an exemplary functional configuration of a video generation device according to the second embodiment; 
           [0019]      FIG. 7A  is a flowchart illustrating a video display process according to the second embodiment; 
           [0020]      FIG. 7B  is a flowchart illustrating the video display process according to the second embodiment; 
           [0021]      FIG. 7C  is a flowchart illustrating the video display process according to the second embodiment; and 
           [0022]      FIG. 8  is a diagram illustrating an exemplary hardware configuration of a computer. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0023]      FIG. 1  is a diagram illustrating an exemplary configuration of a guide system according to a first embodiment. 
         [0024]    As illustrated in  FIG. 1 , the guide system according to the first embodiment includes a distance sensor  1 , a vehicular speed sensor  2 , a tilt sensor  3 , a video generation device  4 , a display device  5 , a radio communication device  6 , a position information provision device  7 , and a speed limit database (DB)  8 . The distance sensor  1 , the vehicular speed sensor  2 , the tilt sensor  3 , the video generation device  4 , the display device  5 , and the radio communication device  6  are mounted in a vehicle  9 . 
         [0025]    The distance sensor  1  is a sensor that detects a distance between the vehicle  9  and an object such as another vehicle present in front (in a traveling direction) of the vehicle  9 . The vehicular speed sensor  2  is a speed sensor that detects a traveling speed of the vehicle  9 . The tilt sensor  3  is an angle sensor that detects an inclination angle of a vehicle body in the front-back direction of the vehicle  9 . 
         [0026]    The video generation device  4  generates a video that guides a driver  10  of the vehicle  9  in relation to an operation of adjusting the traveling speed of the vehicle  9 . Videos generated by the video generation device  4  are generally classified into a video that guides the driver  10  to an operation of maintaining the current vehicular speed, a video that guides the driver  10  to a deceleration operation, and a video that guides the driver  10  to an acceleration operation. The video generation device  4  generates a video that guides the driver  10  on the basis of a speed limit (a maximum speed at which a vehicle is allowed to travel on a road) of a road  11  where the vehicle  9  is traveling, a traveling speed of the vehicle  9 , a distance from an object present in front of the vehicle  9 , an inclination angle of the vehicle  9 , and the like. The video generation device  4  acquires position information of the vehicle  9  from the position information provision device  7  such as a global positioning system (GPS) satellite through the radio communication device  6 . The video generation device  4  accesses a communication network  12  such as the Internet through the radio communication device  6  to acquire the speed limit of the road  11  where the vehicle  9  is currently traveling from the speed limit DB  8  over the communication network  12 . 
         [0027]    The display device  5  displays a video generated by the video generation device  4 . The display device  5  is provided to display the video generated by the video generation device  4  within the visual field of the driver  10  who is driving the vehicle  9 . As the display device  5 , for example, a head-up display (HUD) that projects and displays a video  13  on a windshield  901  or the like of the vehicle  9  may be used. 
         [0028]      FIG. 2  is a diagram illustrating an exemplary functional configuration of the video generation device according to the first embodiment. 
         [0029]    As illustrated in  FIG. 2 , the video generation device  4  includes a position information acquisition unit  401 , a speed limit acquisition unit  402 , an inter-vehicular distance detection unit  403 , a vehicular speed identification unit  404 , a gradient detection unit  405 , an in-video speed determination unit  406 , a video generation unit  407 , and a display control unit  408 . The video generation device  4  also includes a storage unit  410 . 
         [0030]    The position information acquisition unit  401  acquires position information of the vehicle  9  from the position information provision device  7  using a GPS or the like. 
         [0031]    The speed limit acquisition unit  402  acquires the speed limit of the road  11  where the vehicle  9  is traveling from the speed limit DB  8  on the basis of the position information of the vehicle  9 . 
         [0032]    The inter-vehicular distance detection unit  403  detects an inter-vehicular distance between an own vehicle and another vehicle present in front of the own vehicle on the basis of the output of the distance sensor  1 . The own vehicle is the vehicle  9  mounted with the distance sensor  1  used for detecting the inter-vehicular distance in the video generation device  4  (the inter-vehicular distance detection unit  403 ). That is, the own vehicle refers to the vehicle  9  driven by the driver  10  to be guided using a video generated by the video generation device  4 . 
         [0033]    The vehicular speed identification unit  404  identifies a current vehicular speed of the own vehicle  9  on the basis of the output of the vehicular speed sensor  2 . 
         [0034]    The gradient detection unit  405  detects an inclination angle of a vehicle body of the own vehicle  9  in the front-back direction, that is, a gradient of the road  11  (road surface) where the own vehicle is traveling, on the basis of the output of the tilt sensor  3 . 
         [0035]    The in-video speed determination unit  406  determines a movement speed of a guide image within the video that guides the driver  10 , on the basis of information such as the speed limit of the road  11  where the vehicle  9  is traveling, the traveling speed of the vehicle  9 , the inter-vehicular distance ahead of the vehicle  9 , the gradient of the road  11 , and information such as threshold values stored in the storage unit  410 . 
         [0036]    The video generation unit  407  generates a video including a guide image on the basis of the movement speed determined by the in-video speed determination unit  406 . The video generation unit  407  reads data serving as materials for the video, including data of the guide image, from the storage unit  410  to generate the video. 
         [0037]    The display control unit  408  causes the display device  5  to display the video generated by the video generation unit  407 . 
         [0038]    In the storage unit  410 , various threshold values used for determining the movement speed of the guide image, data serving as materials for the video, including data of the guide image, and the like are stored. 
         [0039]    The video generation device  4  in the guide system according to the present embodiment repeatedly executes a video display process illustrated in  FIGS. 3A to 3C  at predetermined time intervals while the driver  10  drives the vehicle  9 . 
         [0040]      FIGS. 3A to 3C  are flowcharts illustrating the video display process according to the first embodiment. 
         [0041]    As illustrated in  FIG. 3A , the video generation device  4  of the present embodiment acquires an inter-vehicular distance from another vehicle in front (S 1 ). For example, the in-video speed determination unit  406  causes the inter-vehicular distance detection unit  403  to perform the processing in S 1 . The inter-vehicular distance detection unit  403  acquires the output of the distance sensor  1  to detect (calculate) an inter-vehicular distance between the own vehicle and another vehicle present in front of the own vehicle. The inter-vehicular distance detection unit  403  notifies the in-video speed determination unit  406  of the detected inter-vehicular distance. 
         [0042]    Next, the video generation device  4  causes the in-video speed determination unit  406  to determine whether the inter-vehicular distance is equal to or less than a first threshold value TH 1  (S 2 ). 
         [0043]    When it is determined that the inter-vehicular distance is equal to or less than the threshold value TH 1  (S 2 ; Yes), the in-video speed determination unit  406  sets, on the basis of the inter-vehicular distance, the movement speed of a guide image within a video to be faster than a speed corresponding to a vertical component of a movement velocity of the circumference environment of the vehicle within the video (S 3 ). Hereinafter, for sake of simplicity, the speed corresponding to the vertical component of the movement velocity of the circumference environment of the vehicle within the video is referred to as a vertical movement speed of the circumference environment. In S 3 , the in-video speed determination unit  406  determines the movement speed of the guide image as a speed faster than a reference speed. The reference speed is, for example, the vertical movement speed of the circumference environment, which is determined on the basis of a current vehicular speed, a video size, or the like. The in-video speed determination unit  406  notifies the video generation unit  407  of the determined movement speed. 
         [0044]    After the movement speed of the guide image is determined in S 3 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 3B . The video generation unit  407  reads data serving as materials for the video to be generated, including data of the guide image, from the storage unit  410  to generate the video. The video generation unit  407  transmits the generated video to the display control unit  408 . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0045]    When it is determined that the inter-vehicular distance acquired in S 1  exceeds the threshold value TH 1  or when the inter-vehicular distance is not acquired in S 1  (S 2 ; No), the in-video speed determination unit  406  acquires a speed limit and a current vehicular speed (S 11 ). In the processing of acquiring the speed limit in S 11 , the in-video speed determination unit  406  causes the position information acquisition unit  401  to acquire current position information of the vehicle  9 . The position information acquisition unit  401  acquires the current position information of the vehicle  9  from the position information provision device  7  through the radio communication device  6 , and notifies the in-video speed determination unit  406  of the acquired position information. Then, the in-video speed determination unit  406  notifies the speed limit acquisition unit  402  of the current position information of the vehicle  9 , and causes the speed limit acquisition unit  402  to acquire the speed limit of the road  11  where the vehicle  9  is traveling. The speed limit acquisition unit  402  acquires the speed limit of the road  11  where the vehicle  9  is traveling from the speed limit DB  8  over the communication network  12  through the radio communication device  6 , and notifies the in-video speed determination unit  406  of the acquired speed limit. In the processing of acquiring the current vehicular speed in S 11 , the in-video speed determination unit  406  causes the vehicular speed identification unit  404  to identify the current vehicular speed. The vehicular speed identification unit  404  acquires the output of the vehicular speed sensor  2  to identify the vehicular speed, that is, the current traveling speed of the vehicle  9 . The vehicular speed identification unit  404  notifies the in-video speed determination unit  406  of the identified vehicular speed. 
         [0046]    When the processing in S 11  is ended, the in-video speed determination unit  406  subsequently determines whether the vehicular speed falls within an assumed range (S 12 ). The in-video speed determination unit  406  assumes the range of the vehicular speed on the basis of the speed limit acquired in S 11  to determine whether the current vehicular speed falls within the assumed range. The assumed range of the vehicular speed when the speed limit is X (km/h) may be appropriately set. The vehicular speed is set to be, for example, X-5 (km/h) or more, and X+5 (km/h) or less. When it is determined that the current vehicular speed falls within the assumed range (S 12 ; Yes), the in-video speed determination unit  406  subsequently performs processing in S 21  as illustrated in  FIG. 3C . 
         [0047]    When it is determined that the current vehicular speed is out of the assumed range (S 12 ; No), the in-video speed determination unit  406  subsequently determines whether the current vehicular speed is slower than the assumed range (S 13 ). When it is determined that the vehicular speed is slower than the assumed range (S 13 ; Yes), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be slower than the vertical movement speed of the circumference environment on the basis of the vehicular speed (S 14 ). In S 14 , the in-video speed determination unit  406  determines the movement speed of the guide image within the video as a speed slower than the reference speed. The in-video speed determination unit  406  notifies the video generation unit  407  of the determined movement speed. When it is determined that the vehicular speed is faster than the assumed range (S 13 ; No), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be faster than the vertical movement speed of the circumference environment on the basis of the vehicular speed (S 15 ). In S 15 , the in-video speed determination unit  406  determines the movement speed of the guide image within the video as a speed faster than the reference speed. The in-video speed determination unit  406  notifies the video generation unit  407  of the determined movement speed. 
         [0048]    After the movement speed of the guide image is determined in S 14  or S 15 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 3B . The video generation unit  407  reads data serving as materials for the video to be generated, including data of the guide image, from the storage unit  410  to generate the video. The video generation unit  407  transmits the generated video to the display control unit  408 . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0049]    When it is determined that the current vehicular speed falls within the assumed range (S 12 ; Yes), as described above, the in-video speed determination unit  406  subsequently acquires a gradient of the road surface (the road  11 ) (S 21 ) as illustrated in  FIG. 3C . In S 21 , the in-video speed determination unit  406  causes the gradient detection unit  405  to detect the gradient of the road surface. The gradient detection unit  405  detects the gradient of the road surface (road  11 ) where the vehicle  9  is traveling on the basis of the output of the tilt sensor  3 . The gradient (inclination angle) of the road surface is set to 0 degrees when the road surface is horizontal, and is set to be positive when the road surface is uphill. The gradient detection unit  405  notifies the in-video speed determination unit  406  of the detected gradient of the road surface. 
         [0050]    When the processing in S 21  is ended, the in-video speed determination unit  406  subsequently determines whether the gradient is a first angle threshold value THa or more, and a second angle threshold value THb or less (S 22 ). The first angle threshold value THa is a negative value, that is, a gradient threshold value in the case of a downhill road. The second angle threshold value THb is a positive value, that is, a gradient threshold value in the case of an uphill road. The angle threshold values THa and THb are any values that may be appropriately set. The angle threshold value THa is set to, for example, −5 degrees and the angle threshold value THb is set to, for example, +5 degrees. 
         [0051]    When it is determined that the gradient of the road surface is within the range from the first angle threshold value THa to the second angle threshold value THb (S 22 ; Yes), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be equal to the vertical movement speed of the circumference environment (S 23 ). 
         [0052]    When it is determined that the gradient of the road surface is out of the range from the first angle threshold value THa to the second angle threshold value THb (S 22 ; No), the in-video speed determination unit  406  subsequently determines whether the gradient is less than the first angle threshold value THa (S 24 ). When it is determined that the gradient is less than the angle threshold value THa (S 24 ; Yes), that is, when the road surface is downhill at an inclination steeper than the first angle threshold value THa, the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be faster than the vertical movement speed of the circumference environment (S 25 ). When it is determined that the gradient is not less than the first angle threshold value THa (S 24 ; No), the gradient is larger than the second angle threshold value THb, that is, the road surface is uphill at an inclination steeper than the second angle threshold value THb. In this case, the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be slower than the vertical movement speed of the circumference environment (S 26 ). 
         [0053]    After the movement speed of the guide image is determined in any one of S 23 , S 25 , and S 26 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 3B . The video generation unit  407  reads data serving as materials for the video to be generated, including data of the guide image, from the storage unit  410  to generate the video. The video generation unit  407  transmits the generated video to the display control unit  408 . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0054]      FIGS. 4A and 4B  are diagrams illustrating examples of a displayed video. In the video, it is assumed that the own vehicle  9  is traveling in upward direction of the screen. 
         [0055]    In the video display process according to the present embodiment, when the inter-vehicular distance between the own vehicle  9  and a vehicle in front is larger than a threshold value, the vehicular speed falls within an assumed range, and the road has a horizontal surface or a gentle slope, the movement speed of the guide image within the video is set to be equal to the vertical movement speed of the circumference environment (S 23 ). In this case, the display device  5  displays, for example, a video  13  as illustrated in the upper part of  FIG. 4A . A horizon  1301  is present near the center in the vertical direction of the video  13 , and a lane  1302  where the vehicle  9  is traveling and road shoulders  1303  are displayed below the horizon  1301  in the video  13 . On the road shoulders  1303  in the video  13 , objects (for example, columnar objects  1304  and  1305 ) present around the vehicle are displayed. The columnar objects  1304  and  1305  within the video  13  move in the direction approaching the vehicle (the direction approaching the left or right edge of the screen) along the road surface edges (boundaries between the lane  1302  and the road shoulders  1303 ). The columnar objects  1304  and  1305  within the video  13  move at a movement velocity V corresponding to the traveling speed (vehicular speed) of the vehicle  9 . 
         [0056]    Further, in the video display process according to the present embodiment, for example, as illustrated in the upper part of  FIG. 4A , a guide image  1306  is displayed on the lane  1302  within the video  13 . When the inter-vehicular distance between the vehicle  9  and the vehicle in front is larger than the threshold value TH 1 , the guide image  1306  is caused to move at a movement speed V 1  which is equal to a vertical component of the movement velocity V of the objects (the columnar objects  1304  and  1305 ) around the vehicle within the video. 
         [0057]    At this time, the columnar objects  1304  and  1305 , and the guide image  1306  are the same in the movement amount in the vehicle traveling direction (the vertical direction of the video  13 ), as illustrated in the upper part of  FIG. 4A . The driver  10  viewing the video  13  tends to recognize that the guide image  1306  moves at the same speed as the vertical movement speed of the objects around the vehicle such as the columnar objects  1304  and  1305 , and the guide image  1306  is one of the objects around the vehicle. Thus, when the video  13  is displayed in which the guide image  1306  is moving at the same speed as the vertical movement speed of the objects around the vehicle, such as the columnar objects  1304  and  1305 , as illustrated in the upper part of  FIG. 4A , the driver  10  is hardly guided to an acceleration or deceleration operation by the guide image  1306 . Accordingly, when the video  13  is displayed in which the guide image  1306  is moving at the same speed as the vertical movement speed of the objects around the vehicle, the driver  10  continues to drive the vehicle while maintaining the current vehicular speed. 
         [0058]    Meanwhile, in the video display process according to the present embodiment, in a case where the inter-vehicular distance from a vehicle in front is short, in a case where the vehicular speed is faster than the assumed range, or in a case where the road has a steep downhill surface, the movement speed of the guide image  1306  within the video  13  is set to be faster than the vertical movement speed of the circumference environment (S 3 , S 15 , and S 25 ). That is, in the video  13  displayed in these cases, as illustrated in the lower part of  FIG. 4A , a movement speed V 2  of the guide image  1306  is larger than a velocity component, in downward direction of the screen, of the movement velocity V of the objects(the columnar objects  1304  and  1305 ) around the vehicle. Thus, in the video  13  in the lower part of  FIG. 4A , the movement amount of the guide image  1306  is larger than the movement amount of the columnar objects  1304  and  1305  around the vehicle in a certain display period. The driver  10  viewing the video  13  feels that the guide image  1306  is approaching the vehicle  9  at a speed exceeding an assumed range. At this time, a visually induced self-motion illusion (vection) is imparted to the driver  10 , in which the traveling speed of the own vehicle  9  is recognized as a speed faster than an actual speed. Therefore, when the video  13  is displayed in which the movement speed of the guide image  1306  is faster than the vertical movement speed of the circumference environment, the driver  10  tends to naturally perform an operation of decelerating the vehicle  9  so as to increase the distance from the guide image  1306 . Accordingly, in a case where the inter-vehicular distance from the vehicle in front is short, the video  13  in which the movement speed of the guide image  1306  is faster than the vertical movement speed of the circumference environment is generated and displayed so as to guide the driver  10  to a deceleration operation to widen the inter-vehicular distance. Similarly, in a case where the vehicular speed is faster than the assumed range, or in a case where the road has a steep downhill surface, the video  13  in which the movement speed of the guide image  1306  is faster than the vertical movement speed of the circumference environment is generated and displayed so as to guide the driver  10  to a deceleration operation. 
         [0059]    Also, in the video display process according to the present embodiment, in a case where the vehicular speed is slower than the assumed range, or in a case where the road has a steep uphill surface, the movement speed of the guide image  1306  within the video  13  is set to be slower than the vertical movement speed of the circumference environment (S 14  and S 26 ). That is, in the video  13  displayed in these cases, as illustrated in  FIG. 4B , a movement speed V 3  of the guide image  1306  is less than a velocity component, in downward direction of the screen, of the movement velocity V of the objects (the columnar objects  1304  and  1305 ) around the vehicle. Thus, in the video  13  in  FIG. 4B , the movement amount of the guide image  1306  is less than the movement amount of the columnar objects  1304  and  1305  around the vehicle in a certain display period. The driver  10  viewing the video  13  feels that a distance between the guide image  1306  and the own vehicle  9  is widened. At this time, a vection is imparted to the driver  10 , in which the traveling speed of the own vehicle  9  is recognized as a speed slower than an actual speed. Therefore, when the video  13  is displayed in which the movement speed of the guide image  1306  is slower than the vertical movement speed of the circumference environment, the driver  10  tends to naturally perform an operation of accelerating the vehicle  9  so as to decrease the distance from the guide image  1306 . Accordingly, in a case where the vehicular speed is slower than the assumed range, or in a case where the road has a steep uphill surface, the video  13  in which the movement speed of the guide image  1306  is slower than the vertical movement speed of the circumference environment is generated and displayed so as to guide the driver  10  to an acceleration operation. 
         [0060]    In the case where the traveling speed of the vehicle  9  is slower than the assumed range, for example, the inter-vehicular distance between the own vehicle  9  and another vehicle in the rear may be decreased, which may probably lead to a collision accident or traffic jam. In the case where the driver  10  does not notice that the own vehicle  9  is traveling on a steep uphill road, and thus continues to drive without changing a stepping amount of the accelerator, the vehicle  9  may be decelerated, which may probably lead to a traffic jam. Therefore, in a traveling state where the inter-vehicular distance from another vehicle is sufficiently secured, in the case where the traveling speed of the vehicle  9  is slower than the assumed range, or in the case where the road has a steep uphill surface, the occurrence of a rear-end collision accident or traffic jam may be prevented by guiding the driver  10  to an acceleration operation. 
         [0061]    The video display process illustrated in  FIGS. 3A to 3C  is merely an example, and the order or contents of the process may be changed without departing from the gist of the present embodiment. 
         [0062]    The videos  13  illustrated in  FIGS. 4A and 4B  are merely examples, and the shape or display method of the guide image  1306  may be appropriately changed. For example, when the inter-vehicular distance between the own vehicle  9  and a vehicle in front is larger than a threshold value, the vehicular speed falls within an assumed range, and the road has a horizontal surface or a gentle slope, the guide image  1306  within the video  13  may be fixed at a predetermined location on the lane  1302 . In this case, when the driver  10  is to be guided to, for example, a deceleration operation, the video  13  is switched such that the guide image  1306  moves in the same direction (downwards on the screen) as that of the objects around the vehicle. Also, in this case, when the driver  10  is to be guided to an acceleration operation, the video  13  is switched such that the guide image  1306  moves in the opposite direction (upwards on the screen) to that of the objects around the vehicle. 
       Second Embodiment 
       [0063]      FIG. 5  is a diagram illustrating an exemplary configuration of a guide system according to a second embodiment. 
         [0064]    As illustrated in  FIG. 5 , a guide system according to the second embodiment includes a first distance sensor  1 A, a second distance sensor  1 B, a vehicular speed sensor  2 , a tilt sensor  3 , a video generation device  4 , a display device  5 , and a radio communication device  6 . Further, the guide system includes a position information provision device  7  and a speed limit DB  8 . The first distance sensor  1 A, the second distance sensor  1 B, the vehicular speed sensor  2 , the tilt sensor  3 , the video generation device  4 , the display device  5 , and the radio communication device  6  are mounted in a vehicle  9 . 
         [0065]    The first distance sensor  1 A is used to detect a distance between the vehicle  9  and an object such as another vehicle present in front (in a traveling direction) of the vehicle  9 . The second distance sensor  1 B is used to detect a distance between the vehicle  9  and an object such as another vehicle present behind the vehicle  9 . The vehicular speed sensor  2  is used to detect a traveling speed of the vehicle  9 . The tilt sensor  3  is used to detect an inclination angle of the vehicle  9  (the road surface where the vehicle  9  is traveling). 
         [0066]    The video generation device  4  generates a video that guides a driver  10  of the vehicle  9  in relation to an operation of adjusting the traveling speed of the vehicle  9 . Video generated by the video generation device  4  are generally classified into a video that guides the driver  10  to an operation of maintaining the current vehicular speed, a video that guides the driver  10  to a deceleration operation, and a video that guides the driver  10  to an acceleration operation. The video generation device  4  generates a video that guides the driver  10  on the basis of a speed limit of a road  11  where the vehicle  9  is traveling, a traveling speed of the vehicle  9 , a distance from an object present in front of or behind the vehicle  9 , an inclination angle of the vehicle  9 , and the like. The video generation device  4  acquires position information of the vehicle  9  from the position information provision device  7  using a GPS or the like through the radio communication device  6 . The video generation device  4  acquires the speed limit of the road  11  where the vehicle  9  is currently traveling from the speed limit DB  8  over a communication network  12  through the radio communication device  6 . 
         [0067]    The display device  5  displays a video generated by the video generation device  4 . The display device  5  is provided to display the video generated by the video generation device  4  within the visual field of the driver  10  who is driving the vehicle  9 . As the display device  5 , for example, a head-up display (HUD) that projects and displays a video  13  on a windshield  901  or the like of the vehicle  9  may be used. 
         [0068]      FIG. 6  is a diagram illustrating a functional configuration of the video generation device according to the second embodiment. 
         [0069]    As illustrated in  FIG. 6 , the video generation device  4  includes a position information acquisition unit  401 , a speed limit acquisition unit  402 , an inter-vehicular distance detection unit  403 , a vehicular speed identification unit  404 , a gradient detection unit  405 , an in-video speed determination unit  406 , a video generation unit  407 , and a display control unit  408 . The video generation device  4  also includes a storage unit  410 . 
         [0070]    The position information acquisition unit  401  and the speed limit acquisition unit  402  are the same as the position information acquisition unit  401  and the speed limit acquisition unit  402 , respectively, in the video generation device  4  according to the first embodiment. 
         [0071]    The inter-vehicular distance detection unit  403  detects an inter-vehicular distance between an own vehicle and another vehicle present in front of the own vehicle on the basis of the output of the first distance sensor  1 A. The inter-vehicular distance detection unit  403  also detects an inter-vehicular distance between the own vehicle and another vehicle present behind the own vehicle on the basis of the output of the second distance sensor  18 . 
         [0072]    The vehicular speed identification unit  404  and the gradient detection unit  405  are the same as the vehicular speed identification unit  404  and the gradient detection unit  405 , respectively, in the video generation device  4  according to the first embodiment. 
         [0073]    The in-video speed determination unit  406  determines a movement speed of a guide image within a video that guides the driver  10 , on the basis of information such as the speed limit of the road  11  where the vehicle  9  is traveling, the traveling speed of the vehicle  9 , the inter-vehicular distance ahead of or behind the vehicle, the gradient of the road  11 , and information such as threshold values stored in the storage unit  410 . 
         [0074]    The video generation unit  407  and the display control unit  408  are the same as the video generation unit  407  and the display control unit  408 , respectively, in the video generation device  4  according to the first embodiment. 
         [0075]    In the storage unit  410 , various threshold values used for determining the movement speed of the guide image, data serving as materials for the video, including data of the guide image, and the like are stored. 
         [0076]    The video generation device  4  in the guide system according to the present embodiment repeatedly executes a video display process illustrated in  FIGS. 7A to 7C  at predetermined time intervals while the driver  10  drives the vehicle  9 . 
         [0077]      FIGS. 7A to 7C  are flowcharts illustrating the video display process according to the second embodiment. 
         [0078]    As illustrated in  FIG. 7A , the video generation device  4  of the present embodiment acquires an inter-vehicular distance from another vehicle in front (S 1 ). For example, the in-video speed determination unit  406  causes the inter-vehicular distance detection unit  403  to perform the processing in S 1 . The inter-vehicular distance detection unit  403  acquires the output of the first distance sensor  1 A to detect (calculate) an inter-vehicular distance between the own vehicle and another vehicle present in front of the own vehicle. The inter-vehicular distance detection unit  403  notifies the in-video speed determination unit  406  of the detected inter-vehicular distance. 
         [0079]    Next, the video generation device  4  causes the in-video speed determination unit  406  to determine whether the inter-vehicular distance is equal to or less than a first threshold value TH 1  (S 2 ). 
         [0080]    When it is determined that the inter-vehicular distance is equal to or less than the threshold value TH 1  (S 2 ; Yes), the in-video speed determination unit  406  sets the movement speed of a guide image within a video to be faster than the vertical movement speed of the circumference environment on the basis of the distance from the vehicle in front (S 3 ). In S 3 , the in-video speed determination unit  406  determines the movement speed of the guide image as a speed faster than a reference speed. The reference speed is, for example, the vertical movement speed of the circumference environment, which is determined on the basis of a current vehicular speed, a video size, or the like. The in-video speed determination unit  406  notifies the video generation unit  407  of the determined movement speed. 
         [0081]    After the movement speed of the guide image is determined in S 3 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 7B . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0082]    When it is determined that the distance from the vehicle in front acquired in S 1  exceeds the threshold value TH 1  or when the inter-vehicular distance is not acquired in S 1  (S 2 ; No), the in-video speed determination unit  406  acquires a speed limit and a current vehicular speed (S 11 ). In S 11 , the in-video speed determination unit  406  performs, for example, the same processing as that in S 11  in the video display process according to the first embodiment. 
         [0083]    When the processing in S 11  is ended, the in-video speed determination unit  406  subsequently determines whether the vehicular speed falls within an assumed range (S 12 ). In S 12 , the in-video speed determination unit  406  performs, for example, a determination under the same condition as that in S 12  in the video display process according to the first embodiment. When it is determined that the current vehicular speed falls within the assumed range (S 12 ; Yes), the in-video speed determination unit  406  subsequently performs processing in S 21  as illustrated in  FIG. 7C . 
         [0084]    When it is determined that the current vehicular speed is out of the assumed range (S 12 ; No), the in-video speed determination unit  406  subsequently determines whether the current vehicular speed is slower than the assumed range (S 13 ). When it is determined that the vehicular speed is slower than the assumed range (S 13 ; Yes), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be slower than the vertical movement speed of the circumference environment on the basis of the vehicular speed (S 14 ). When it is determined that the vehicular speed is faster than the assumed range (S 13 ; No), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be faster than the vertical movement speed of the circumference environment on the basis of the vehicular speed (S 15 ). In S 13 , S 14 , and S 15 , the in-video speed determination unit  406  performs, for example, the same processing as those in S 13 , S 14 , and S 15 , respectively, in the video display process according to the first embodiment. 
         [0085]    After the movement speed of the guide image is determined in S 14  or S 15 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 7B . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0086]    When it is determined that the current vehicular speed falls within the assumed range (S 12 ; Yes), as described above, the in-video speed determination unit  406  subsequently acquires a gradient of the road surface (the road  11 ) (S 21 ) as illustrated in  FIG. 7C . In S 21 , the in-video speed determination unit  406  performs, for example, the same processing as that in S 21  in the video display process according to the first embodiment. 
         [0087]    When the processing in S 21  is ended, the in-video speed determination unit  406  subsequently determines whether the gradient is a first angle threshold value THa or more, and a second angle threshold value THb or less (S 22 ). In S 22 , the in-video speed determination unit  406  performs, for example, a determination under the same condition as that in S 22  in the video display process according to the first embodiment. 
         [0088]    When it is determined that the gradient of the road surface is out of the range from the first angle threshold value THa to the second angle threshold value THb (S 22 ; No), the in-video speed determination unit  406  subsequently determines whether the gradient is less than the first angle threshold value THa (S 24 ). When it is determined that the gradient is less than the first angle threshold value THa (S 24 ; Yes), that is, when the road surface is downhill at an inclination steeper than the first angle threshold value THa, the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be faster than the vertical movement speed of the circumference environment (S 25 ). When it is determined that the gradient is not less than the first angle threshold value THa (S 24 ; No), the gradient is larger than the second angle threshold value THb, that is, the road surface is uphill at an inclination steeper than the second angle threshold value THb. In this case, the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be slower than the vertical movement speed of the circumference environment (S 26 ). 
         [0089]    After the movement speed of the guide image is determined in any one of S 25  and S 26 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 7B . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0090]    When it is determined that the gradient of the road surface is within the range from the first angle threshold value THa to the second angle threshold value THb (S 22 ; Yes), the in-video speed determination unit  406  acquires an inter-vehicular distance from another vehicle in the rear (S 31 ). For example, the in-video speed determination unit  406  causes the inter-vehicular distance detection unit  403  to perform the processing in S 31 . The inter-vehicular distance detection unit  403  acquires the output of the second distance sensor  18  to detect (calculate) an inter-vehicular distance between the own vehicle and another vehicle present behind the own vehicle. The inter-vehicular distance detection unit  403  notifies the in-video speed determination unit  406  of the detected inter-vehicular distance. 
         [0091]    Next, the video generation device  4  causes the in-video speed determination unit  406  to determine whether the inter-vehicular distance from another vehicle in the rear is equal to or less than a second threshold value TH 2  (S 32 ). 
         [0092]    When it is determined that the distance from another vehicle in the rear is equal to or less than the threshold value TH 2  (S 32 ; Yes), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be slower than the vertical movement speed of the circumference environment on the basis of the distance from another vehicle in the rear (S 33 ). When the distance from another vehicle in the rear is larger than the threshold value TH 2  (S 32 ; No), the in-video speed determination unit  406  sets the movement speed of the guide image within the video to be equal to the vertical movement speed of the circumference environment (S 23 ). 
         [0093]    After the movement speed of the guide image is determined in any one of S 23  and S 33 , the video generation device  4  causes the video generation unit  407  to generate a video on the basis of the determined movement speed (S 4 ), as illustrated in  FIG. 7B . Next, the video generation device  4  causes the display control unit  408  to display the video generated by the video generation unit  407  on the display device  5  (S 5 ). When the processing in S 5  is ended, the video generation device  4  starts a next video display process. 
         [0094]    In the video display process according to the present embodiment, in a case where the inter-vehicular distances from the other vehicles are sufficiently long, the vehicular speed falls within an assumed range, and the road has a horizontal surface or a gentle slope, the movement speed of the guide image within the video is set to be equal to the vertical movement speed of the circumference environment (S 23 ). In this case, the display device  5  displays, for example, a video  13  as illustrated in the upper part of  FIG. 4A . Thus, in the case where the inter-vehicular distances from another vehicle in front and another vehicle in the rear are sufficiently long, the vehicular speed falls within the assumed range, and the road has a horizontal surface or a gentle slope, the driver  10  viewing the video  13  is guided to an operation of maintaining the current vehicular speed. 
         [0095]    Meanwhile, in the video display process according to the present embodiment, in a case where the inter-vehicular distance from a vehicle in front is short, in a case where the vehicular speed exceeds the assumed range, or in a case where the road has a steep downhill surface, the movement speed of the guide image within the video is set to be faster than the vertical movement speed of the circumference environment (S 3 , S 15 , and S 25 ). In these cases, the display device  5  displays, for example, the video  13  as illustrated in the lower part of  FIG. 4A . Thus, in the case where the inter-vehicular distance from another vehicle in front is short, in the case where the vehicular speed exceeds the assumed range, or in the case where the road has a steep downhill surface, the driver  10  viewing the video  13  tends to be guided to a deceleration operation due to the vection. 
         [0096]    Further, in the video display process according to the present embodiment, in a case where the vehicular speed is slower than an assumed range, in a case where the road has a steep uphill surface, or in a case where the inter-vehicular distance from another vehicle in the rear is short, the movement speed of the guide image within the video is set to be slower than the vertical movement speed of the circumference environment (S 14 , S 26 , and S 33 ). In these cases, the display device  5  displays, for example, the video  13  as illustrated in  FIG. 4B . Thus, in the case where the vehicular speed is slower than the assumed range, in the case where the road has a steep uphill surface, or in the case where the inter-vehicular distance from another vehicle in the rear is short, the driver  10  viewing the video  13  tends to be guided to an acceleration operation due to the vection. 
         [0097]    As described above, in the video display process according to the present embodiment, in addition to the video display process according to the first embodiment, a video that guides the driver  10  to an acceleration operation is generated and displayed when the inter-vehicular distance from another vehicle in the rear is less than the threshold value TH 2 . Thus, when, for example, another vehicle joins from a frontage road and is approaching from the rear side of the own vehicle  9 , the driver  10  may be temporarily guided to an acceleration operation so as to increase the inter-vehicular distance from the other vehicle in the rear. 
         [0098]    The video display process illustrated in  FIGS. 7A to 7C  is merely an example, and the order or contents of the process may be changed without departing from the gist of the present embodiment. 
         [0099]    The video generation device  4  according to the first and second embodiments may be implemented using a computer and a program executed in the computer. Hereinafter, descriptions will be made on the video generation device  4  implemented using the computer and the program with reference to  FIG. 8 . 
         [0100]      FIG. 8  is a diagram illustrating a hardware configuration of a computer. 
         [0101]    As illustrated in the drawing, a computer  15  includes a central processing unit (CPU)  1501 , a main memory  1502 , an auxiliary memory  1503 , an input device  1504 , and an output device  1505 . The computer  15  includes an interface device  1506 , a medium drive  1507 , and a communication control device  1508 . These components  1501  to  1508  are coupled to each other via a bus  1510  in the computer  15  such that data is exchanged between the components. 
         [0102]    The CPU  1501  is an arithmetic processing device that executes various programs including an operating system to control the overall operation of the computer  15 . 
         [0103]    The main memory  1502  includes a read-only memory (ROM) (not illustrated) and a random access memory (RAM) (not illustrated). In the ROM of the main memory  1502 , for example, a predetermined basic control program or the like which is read by the CPU  1501  when the computer  15  starts up is recorded in advance. The RAM of the main memory  1502  is used as a working storage area as necessary when the CPU  1501  executes various programs. The RAM of the main memory  1502  may be used to store, for example, the position of the vehicle  9 , the speed limit of the road  11 , the inter-vehicular distance, the vehicular speed, the gradient of the road surface, various threshold values, and the like. 
         [0104]    The auxiliary memory  1503  is a storage device such as a solid state drive (SSD), which has a larger capacity than the main memory  1502 . In the auxiliary memory  1503 , various programs to be executed by the CPU  1501 , various data, and the like may be stored. The auxiliary memory  1503  may be used to store, for example, a program including any of video display processes exemplified in the first and second embodiments. The auxiliary memory  1503  may be used to store, for example, information such as the position of the vehicle  9 , the speed limit of the road  11 , and the inter-vehicular distance, and data serving as materials for the video, including a guide image. Further, the auxiliary memory  1503  may be used to store, for example, a display history of the video  13  including the guide image. When the computer  15  is mounted with a hard disk drive (HDD) connected to the bus  1510 , the HDD may be used as the auxiliary memory  1503 . 
         [0105]    The input device  1504  is, for example, a keyboard device or a button switch. When an operator (driver or the like) of the computer  15  performs an operation such as pressing the input device  1504 , the input device  1504  transmits input information associated with the operation content to the CPU  1501 . 
         [0106]    The output device  1505  is, for example, a liquid crystal display, a pilot lamp, a speaker, or the like. The output device  1505  is used to display a video including a guide image, to check an operating state of the computer  15 , or the like. The output device  1505  may be a head-up display. 
         [0107]    The interface device  1506  is a device that connects the computer  15  to another electronic device or the like, and is provided with a connector compliant with universal serial bus (USB) standards, or connector standards of a vehicle wiring harness. The device to be coupled to the computer  15  via the interface device  1506  may be, for example, the distance sensors  1 A and  1 B, the vehicular speed sensor  2 , the tilt sensor  3 , the display device  5  such as a head-up display (HUD), and various electronic control units (ECUs) mounted in the vehicle  9 . A GPS receiver may also be an example of the device to be coupled to the computer  15  via the interface device  1506 . 
         [0108]    The medium drive  1507  reads a program or data recorded in a portable recording medium  16 , or writes data or the like stored in the auxiliary memory  1503  to the portable recording medium  16 . As the portable recording medium  16 , for example, a flash memory provided with a USB-standard connector, a SD-standard memory card, or the like may be used. Also, in a case of the computer  15  mounted with an optical disk drive as the medium drive  1507 , optical disks such as a compact disk (CD), a digital versatile disc (DVD), and a Blu-ray disc (Blu-ray is a registered trademark) may also be used as the portable recording medium  16 . The portable recording medium  16  may be used to provide a program including any of video display processes exemplified in the first and second embodiments. 
         [0109]    The communication control device  1508  is a device that communicably couples the computer  15  to the communication network  12  such as the Internet to control various communications between the computer  15  and another communication terminal (not illustrated) or the like through the communication network  12 . The communication control device  1508  may be used to acquire the speed limit of the road where the vehicle  9  is traveling from the speed limit DB  8  over the communication network  12 . By operating the computer  15  provided with the communication control device  1508  as the video generation device  4 , for example, the display history (guide history) of the video including the guide image, which has been stored in the auxiliary memory  1503 , may be transmitted to a predetermined server. When the guide histories accumulated in a plurality of computers  15  may be collectively managed by a server, for example, a transport service provider or the like is allowed to perform a safe driving evaluation, a driving guidance, or the like for each driver using the guide histories. 
         [0110]    The CPU  1501  of the computer  15  reads a program including any of video display processes exemplified in respective embodiments from the auxiliary memory  1503  or the like to execute the read program, so that a video that guides the driver to an acceleration operation or a deceleration operation is generated and displayed on a display device. At this time, the CPU  1501  of the computer  15  operates as the inter-vehicular distance detection unit  403 , the vehicular speed identification unit  404 , the gradient detection unit  405 , the in-video speed determination unit  406 , the video generation unit  407 , the display control unit  408 , and the like in the video generation device  4 . The RAM of the main memory  1502  or the auxiliary memory  1503  in the computer  15  serves as the storage unit  410  in the video generation device  4 . 
         [0111]    The computer  15  operating as the video generation device  4  does not need to include all of the components illustrated in  FIG. 8 , and some of the components may be omitted according to the applications or conditions. For example, when the computer  15  is an in-vehicle ECU, and is provided at a portion where the driver  10  is unable to operate the computer  15  while driving, the medium drive  1507  may be omitted from the computer  15 . When the radio communication device  6  (see, e.g.,  FIG. 1 ) is provided in the vehicle  9  in addition to the computer  15  operating as the video generation device  4 , the communication control device  1508  may be omitted from the computer  15 . 
         [0112]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.