Patent Publication Number: US-2023162377-A1

Title: Device and method for measuring vehicle occupant moved distance, and non-transitory computer-readable storage medium

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-191422 filed on Nov. 25, 2021, which is incorporated herein by reference in its entirety including the specification, claims, drawings, and abstract. 
     TECHNICAL FIELD 
     The present disclosure relates to a device and a method for measuring a distance moved by a vehicle occupant (hereinafter “moved distance”), and to a non-transitory computer-readable storage medium. 
     BACKGROUND 
     Technologies have been proposed to sense movement of vehicle occupants based on image sequences (hereinafter referred to as “frame images”) captured by an image capturing device which images the interior of a vehicle. 
     For example, JP 2017-199123 A discloses a passenger movement sensor system installed inside a bus. This system determines whether or not a person (passenger) is moving around on the bus by locating the person from captured images periodically output from a camera which captures the interior of the bus. When a passenger is moving around on the bus, an alarm is output to alert the driver of a need to exercise caution. JP 2015-108941 A discloses an image recognition device which obtains a distance moved by the center of a driver&#39;s face. This device obtains the center of the driver&#39;s face from respective time-sequential frame images of the driver&#39;s face output from a camera and obtains the distance moved based on the center of the driver&#39;s face obtained from the first-frame image and the second-frame image subsequent to the first-frame image. 
     A distance moved by an occupant (moved distance) may be obtained by locating a target human image of the occupant whose moved distance is to be measured from respective time-sequential frame images obtained by an image capturing device which captures the interior of a vehicle, and by estimating the center of the occupant based on the target human image. The moved distance may be obtained based on the distance between the center estimated in the first-frame image, which is one of the time-sequential frame images, and the second-frame image subsequent to the first-frame image. 
     In calculating a moved distance of a vehicle occupant in order to sense that the occupant has moved around, an image capturing the occupant before moving around (occupant remaining at one spot) serves as the first-frame image, which is the starting image from which the calculation of the moved distance is orientated. In this case, the center of the target human image (image of the occupant) in the first-frame image may be used as the start point of the moved-distance calculation. 
     However, occupants of a vehicle may temporarily and significantly change their posture in a manner which would not appropriately be deemed as moving around. A significant change in posture of an occupant may cause a significant change in the center of the target human image in the first-frame image. If the image of the occupant in the temporarily and significantly changed posture is used as the first-frame image, the start point of the moved-distance calculation is set at the center of the occupant in the significantly changed posture, resulting in an erroneous start point. A moved distance of the occupant calculated based on this erroneous start point may result in an inaccurate calculation of the moved distance. 
     An object of a vehicle occupant moved-distance measuring device according to the present disclosure is to set a start point of a moved-distance calculation based on the center of an occupant obtained when the occupant has maintained a stable posture for a time period matching or exceeding a threshold time period to calculate the moved distance of the occupant based on the center of the occupant represented by the target human images located from time sequential frame images of the interior of the vehicle. 
     SUMMARY 
     A vehicle occupant moved-distance measuring device according to an embodiment of the present disclosure includes a center estimator, a stable area setter, a start-point setter, and a moved-distance calculator. The center estimator is configured to locate a target human image corresponding to an occupant whose moved distance is to be measured from each of the time-sequential frame images obtained by an image capturing device which captures the interior of a vehicle in a fixed image capturing area, and obtain an estimated center of the occupant based on the target human image. The stable area setter is configured to set a stable area in which an estimated center obtained from a temporary reference image is placed at the center. The temporary reference image is one of the time-sequential frame images. The start-point setter is configured to set, as a start point, a predefined point within the stable area when the start-point setter determines that the estimated center obtained from multiple subsequent images have remained within the stable area for a time period matching or exceeding a threshold time period. The multiple subsequent images are multiple time-sequential frame images following the temporary reference image. The moved-distance calculator is configured to calculate a moved distance of the occupant represented by the target human image based on a distance between the start point and an estimated center obtained from a measurement-target image which is the time-sequential frame image following the multiple subsequent images. 
     In this configuration, when the estimated center of the occupant has remained within the stable area for a time period matching or exceeding a threshold time period, it can be assumed that the occupant has maintained a stable posture without a significant change in their posture for the time period matching or exceeding the threshold time period after the temporary reference image was captured. This configuration can thus set, as the start point of the calculation of the occupant moved-distance, the estimated center of the occupant when the occupant has maintained a stable posture for a time period matching or exceeding a threshold time period. 
     When the estimated center obtained from the multiple subsequent images moves out from the stable area within the threshold time period, the stable area setter may reset the stable area using the subsequent image with the estimated center moved out from the stable area as the temporary reference image. 
     With this configuration, when the estimated center moves out from the stable area within the threshold time period (in other words, when the occupant has significantly changed their posture within the threshold time period after the temporary reference image was captured), the start point is not set within this stable area, but an appropriate start point is set based on a reset stable area. 
     The center estimator may deem the center of a circumscribed rectangle around the target human image as the estimated center. 
     With this configuration, the center of an occupant can be estimated using a simple process. 
     The vehicle may be an autonomously driven vehicle. The vehicle occupant moved-distance measuring device may further include a signal output unit configured to output a drive start disable signal which prevents the vehicle from starting through an autonomous driving when the moved distance calculated by the moved-distance calculator is equal to or longer than a threshold distance. 
     With this configuration, the vehicle can be prevented from starting through autonomous driving when an occupant is moving around. 
     The vehicle occupant moved-distance measuring device may further include a notification controller configured to control an output of a notification to the occupants of the vehicle when the moved distance calculated by the moved-distance calculator is equal to or longer than a threshold distance. 
     With this configuration, when an occupant is moving around, a notification can be output to the occupant and other occupants. 
     A non-transitory computer-readable storage medium according to an embodiment of the present disclosure stores commands executable by a computer. The commands cause the computer to execute setting a stable area, estimating a center, setting a start point, and calculating a moved distance of an occupant. The setting of a stable area is performed by locating, from a temporary reference image, a target human image corresponding to the occupant whose moved distance is to be measured. The temporary reference image is one of time-sequential frame images obtained by an image capturing device which captures the interior of a vehicle in a fixed image capturing area, and by obtaining an estimated center of the occupant based on the target human image of the temporary reference image, and further by setting the stable area in which the estimated center obtained from the temporary reference image is placed at the center. In estimating the center, the center is estimated in multiple subsequent images which are time-sequential frame images following the temporary reference image. Estimating the center is performed by locating the target human images from the multiple subsequent images, obtaining estimated centers of the occupant from the multiple subsequent images based on the target human images, and determining whether or not the estimated center has remained within the stable area for a time period matching or exceeding a threshold time period based on the estimated center obtained from the multiple subsequent images. In setting the start point, a predefined point within the stable area when the estimated center obtained from the multiple subsequent images determined to have remained within the stable area for the time period matching or exceeding the threshold time period is set as the start point. In calculating the moved distance, the moved distance of the occupant represented by the target human image is calculated by locating the target human image from a measurement-target image which is the time-sequential frame image following the multiple subsequent images, obtaining an estimated center of the occupant based on the target human image of the measurement-target image, and calculating the moved distance of the occupant represented by the target human image based on a distance between the start point and the estimated center obtained from the measurement-target image. 
     A vehicle occupant moved-distance measuring method according to an embodiment of the present disclosure includes setting a stable area, estimating a center, setting a start point, and calculating a moved distance. The setting of the stable area includes locating, from a temporary reference image, a target human image corresponding to an occupant whose moved distance is to be measured. The temporary reference image is one of time-sequential frame images obtained by an image capturing device which captures the interior of a vehicle in a fixed image capturing area. The setting of the stable area also includes obtaining an estimated center of the occupant based on the target human image of the temporary reference image, and setting the stable area in which the estimated center obtained from the temporary reference image is placed at the center. In estimating the center, the center in multiple subsequent images which are time-sequential frame images following the temporary reference image by locating the target human images in the multiple subsequent images is estimated, an estimated center of the occupant is obtained from the multiple subsequent images based on the target human images, and whether or not the estimated center has remained within the stable area for a time period equal to or longer than a threshold time period is determined based on the estimated center estimated from the multiple subsequent images. In setting a start point, a predefined point within the stable area is set as the start point when the estimated center obtained from the multiple subsequent images is determined to have remained within the stable area for the time period equal to or longer than the threshold time period. In calculating the moved distance, the moved distance of the occupant represented by the target human image is calculated by locating the target human image from a measurement-target image which is the time-sequential frame image following the multiple subsequent images, obtaining an estimated center of the occupant based on the target human image of the measurement-target image, and calculating the moved distance of the occupant represented by the target human image based on a distance between the start point and the estimated center obtained from the measurement-target image. 
     The vehicle occupant moved-distance measuring device according to the present disclosure can set, as the start point of the moved-distance calculation, a position based on the estimated center of the occupant when the occupant has maintained a stable posture for a time period matching or exceeding a threshold time period to calculate the moved distance of the occupant based on the estimated center of the occupant represented by the target human image located from each of the time-sequential frame images capturing the interior of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present disclosure will be described based on the following figures, wherein: 
         FIG.  1    is a configuration block diagram of a vehicle occupant moved-distance measurement system according to an embodiment of the present disclosure; 
         FIG.  2    is a configuration block diagram of a vehicle occupant moved-distance measuring device according to an embodiment of the present disclosure; 
         FIG.  3    is an example diagram of a temporary reference image; 
         FIG.  4    is a first example diagram of a subsequent image; 
         FIG.  5    is a second example diagram of another subsequent image; 
         FIG.  6    is an example diagram of a measurement-target image; and 
         FIG.  7    is a flowchart showing processes performed by the vehicle occupant moved-distance measuring device according to an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG.  1    is a configuration block diagram of a vehicle occupant moved-distance measurement system  10  according to an embodiment of the present disclosure. The vehicle occupant moved-distance measurement system  10  is a system to measure a moved distance of an occupant of a vehicle A. In the present embodiment, the vehicle A is a bus on which multiple passengers travel. Specifically, the vehicle A is an autonomously drivable vehicle. Autonomous driving means that almost all driving control is performed by an autonomous drive computer mounted on the vehicle A. An operator of the vehicle A may be present to perform some portion of drive control during autonomous driving. In the present specification, an occupant indicates at least one of a passenger or the operator of the vehicle A. 
     The moved-distance measurement system  10  includes an image capturing device  12 , a display  14 , a voice output device  16 , and a moved-distance measuring device  18 . 
     The image capturing device  12  may be a digital camera. That is, the image capturing device  12  may include a lens and a CCD image sensor or the like to convert the light received by the lens from an object to electric signals. The CCD image sensor may be configured with CCD elements arranged in a two-dimensional array. Each CCD element may correspond to each pixel of time-sequential frame images captured by the image capturing device  12 . 
     The image capturing device  12  is mounted inside the vehicle A and captures the interior of the vehicle A in a fixed image capturing area to obtain time-sequential frame images. In the present embodiment, the image capturing device  12  is mounted at the ceiling of the vehicle A and the image capturing area encompasses substantially the entirety of the interior of the vehicle. 
     In the present embodiment, the image capturing device  12  is a video camera that captures video. When the video frame rate is, for example, 30 FPS (frame per second), the captured video consists of 30 frames per second. The image capturing device  12  is not limited to a video camera as long as the device can capture time-sequential frame images of the interior of the vehicle A. For example, the image capturing device  12  may be a still camera which intermittently captures still images as the frame images. The frame images captured by the image capturing device  12  are sequentially sent to the moved-distance measuring device  18  in the captured order (in time-sequence). 
     The display  14  may include, for example, digital signage or an indicator. The display  14  is provided inside the vehicle A. As described in detail below, the display  14  is used to notify the occupants of the vehicle A in response to commands from the moved-distance measuring device  18 . 
     The voice output device  16  may be, for example, a speaker. The voice output device  16  is also provided inside the vehicle A. Similarly as the display  14 , the voice output device  16  is used to notify the occupants of the vehicle Ain response to commands from the moved-distance measuring device  18 . 
       FIG.  2    is a configuration block diagram of the moved-distance measuring device  18 . In the present embodiment, the moved-distance measuring device  18  is mounted on the vehicle A. 
     A data input-output interface  30  may include, for example, various connectors. The moved-distance measuring device  18  may be data-communicatively connected to the image capturing device  12 , the display  14 , the voice output device  16 , and other devices via the data input-output interface  30 . For example, the data input-output interface  30  may sequentially receive frame images from the image capturing device  12 . The data input-output interface  30  may send command signals to the display  14  or the voice output device  16 . 
     A memory  32  may include, for example, an embedded multimedia card (eMMC), a read only memory (ROM), or a random access memory (RAM). The memory  32  stores a moved-distance measurement program used to operate respective units of the moved-distance measuring device  18 . The moved-distance measurement program may be stored in a non-transitory computer-readable storage medium, such as a universal serial bus (USB) memory or a CD-ROM. The moved-distance measuring device  18  performs the moved-distance measurement program by reading the storage medium which stores the moved-distance measurement program. 
     As shown in  FIG.  2   , the memory  32  stores time-sequential frame images  34  received from the image capturing device  12 . In the present specification, the frame images  34  are distinguished in accordance with their usages. One of the frame images  34  is referred to as a temporary reference image  34   a . Multiple frame images  34  subsequent to the temporary reference image  34   a  are referred to as subsequent images  34   b . The frame image  34  following the subsequent images  34   b  is referred to as a measurement-target image  34   c.    
     A processor  36  may be configured with, for example, an electronic control unit (ECU), a central processing unit (CPU), and other devices. The processor  36  is not limited to a single processing device. Multiple processing devices at physically remote locations may serve together as the processor  36 . The processor  36  operates in accordance with the moved-distance measurement program stored in the memory  32  or other storage medium to provide functions as a center estimator  38 , a stable area setter  40 , a start-point setter  42 , a moved-distance calculator  44 , a signal output unit  46 , and a notification controller  48 . The center estimator  38  locates a target human image corresponding to the occupant whose moved distance is to be calculated from each of the frame images  34  including the temporary reference image  34   a , the multiple subsequent images  34   b , and the measurement-target image  34   c . This locating process of the target human image from the frame images may be performed using a well-known method. For example, a background image with no occupants may be captured in advance by the image capturing device  12 . The center estimator  38  may create difference images by comparing the respective frame images with the background image. The center estimator  38  may determine whether the difference images contain human images based on the sizes and shapes of objects in the difference images. The center estimator  38  may then determine whether or not the human image located in each frame image is the target occupant based on the features obtained from the human images. This process can locate the target human images corresponding to a single target occupant from the respective frame images. 
     Next, the center estimator  38  estimates the center of the occupant represented by the target human image based on the located target human image in each frame image. This estimation process of the center of the target human image is described below using the processes applied to the temporary reference image  34   a  as an example.  FIG.  3    is an example of the temporary reference image  34   a . It is assumed that the target human image P has been located from the temporary reference image  34   a  using the above described process. In the present embodiment, the center estimator  38  initially sets a circumscribed rectangle  60  around the target human image P. The center estimator  38  then sets the center (the intersection of the diagonals of the circumscribed rectangle  60 ) as the estimated center  62  of the occupant represented by the target human image P. In other words, the center estimator  38  recognizes a point (defined by coordinates) in the frame image as the estimated center  62  of the occupant represented by the target human image P in the present embodiment. The estimated center  62  obtained from the temporary reference image  34   a  is referred to as an estimated center  62   a.    
     Although the center estimator  38  may obtain the estimated center  62  of the occupant represented by the target human image P using another method, for example, based on the shape of the target human image P, the amount of calculation to obtain the estimated center  62  can be reduced by obtaining the estimated center  62  of the occupant in the above described simple manner. In particular, as described in detail below, in the present embodiment, because the movement of the occupant is sensed in real time by sequentially obtaining the estimated center  62  of the occupant represented by the target human image P from the frame images sequentially sent from the image capturing device  12 , the estimated center  62  can be obtained in a simple manner. 
     In response to receipt of sequential frame images from the image capturing device  12 , the center estimator  38  locates the target human image P from the received frame images and obtains the estimated center  62  of the occupant represented by the target human image P. 
     The stable area setter  40  sets a stable area  64  in which the estimated center  62   a  obtained from the temporary reference image  34   a  is placed at the center. In the present embodiment, as shown in  FIG.  3   , the stable area setter  40  sets, as the stable area  64 , a circular area covering pixels in a circle of a radius r with the estimated center  62   a  defined by coordinates located at the center in the coordinate system of the image. 
     As described below, the stable area  64  is used to determine whether or not the occupant represented by the target human image P has significantly changed their posture. Specifically, it is determined that the occupant has significantly changed their posture when the estimated center  62  of the occupant represented by the target human image P in the subsequent images  34   b  following the temporary reference image  34   a  moves out from the stable area  64 . The size of the stable area  64  (that is the radius r) should be determined as required by a manager or other personnel of the moved-distance measurement system  10  to allow a reasonable posture change of the occupant represented by the target human image P while enabling sensing of a significant posture change of the occupant that would result in an inaccurate calculation of the moved distance of the occupant. The stable area setter  40  may vary the radius r in accordance with the size of the target human image P in the temporary reference image  34   a  (for example, setting a longer distance r for a larger target human image P), because the size of the target human image P becomes larger in the temporary reference image  34   a  when the occupant represented by the target human image P is closer to the image capturing device  12 . 
     The start-point setter  42  determines whether or not the estimated center  62  of the target human image P has remained within the stable area  64  for a time period matching or exceeding a threshold time period predetermined by the manager or other personnel of the moved-distance measurement system  10  based on the estimated center  62  obtained from the multiple subsequent images  34   b  by the center estimator  38 . The multiple subsequent images  34   b  are multiple time-sequential frame images following the temporary reference image  34   a .  FIG.  4    shows a first example of the subsequent image  34   b . The estimated center  62  obtained from each of the multiple subsequent images  34   b  is referred to as an estimated center  62   b.    
     In the present embodiment, the start-point setter  42  checks the multiple subsequent images  34   b  in a time sequential order and determines whether or not the estimated center  62   b  represented by the coordinates is located within the stable area  64  defined by coordinates. The start-point setter  42  determines that the estimated center  62  of the occupant represented by the target human image P have remained within the stable area  64  for a time period matching or exceeding the threshold time period when all the subsequent images  34   b  captured in the threshold time period have the estimated centers  62   b  within the stable area  64  as shown in  FIG.  4   . For example, the number of the subsequent images  34   b  may be 30 when the frame rate of the video captured by the image capturing device  12  is 30 FPS and the threshold time period is one second. This indicates that the occupant represented by the target human image P has maintained a stable posture without a significant posture change for the threshold time period after the temporary reference image  34   a  was captured. 
     When the start-point setter  42  determines that the estimated center  62  of the occupant represented by the target human image P have remained within the stable area  64  for a time period matching or exceeding the threshold time period, the start-point setter  42  sets a predefined position within the stable area  64  as a start point  66  from which the moved distance of the occupant is originated. The predefined position (that is, the start point  66 ) may be any arbitrary point within the stable area  64 . For example, in the present embodiment, among the subsequent images  34   b  captured within the threshold time period, the estimated center  62  obtained from the last subsequent image  34   b  is used as the start point  66 . The start point  66  may also be the center of the estimated centers  62  obtained from the multiple subsequent images  34   b , or the center of the stable area  64  (that is, the estimated center  62   a  obtained from the temporary reference image  34   a ). 
       FIG.  5    shows a second example of a subsequent image  34   b . As shown in  FIG.  5   , when the estimated center  62   b  obtained from any one of the subsequent images  34   b  captured within the threshold time period moves out from the stable area  64 , in other words, when the estimated center  62   b  is located outside the stable area  64  within the threshold time period, it is indicated that the occupant represented by the target human image P has significantly changed their posture within the threshold time period. In such a case, the start-point setter  42  does not set the start point  66  in this stable area  64 . 
     In this case, the stable area setter  40  resets the stable area  64  to one based on a new temporary reference image  34   a  using the subsequent image  34   b  with the estimated center  62   b  outside the stable area  64  as the new temporary reference image  34   a . The start-point setter  42  then repeats the above described processes to the subsequent images  34   b  following the new temporary reference image  34   a.    
     The moved-distance calculator  44  calculates the moved distance of the occupant represented by the target human image P based on the distance between the start point  66  set by the start-point setter  42  and the estimated center  62  of the measurement-target image  34   c  which is the time-sequential frame image following the multiple subsequent images  34   b .  FIG.  6    is an example of the measurement-target image  34   c . The estimated center  62  obtained from the measurement-target image  34   c  is referred to as an estimated center  62   c.    
     Naturally, the distance between the start point  66  and the estimated center  62   c  in the coordinate system of the images differs from the moved distance of the occupant represented by the target human image P in the actual space. The moved-distance calculator  44  may calculate the moved distance of the occupant represented by the target human image P in the actual space by applying correction factors to the distance between the start point  66  and the estimated center  62   c  in the coordinate system of the images based on the distance between the image capturing device  12  and the start point  66 , the distance between the image capturing device  12  and the estimated center  62   c , the angle between the direction of incident light to a CCD element of the image capturing device  12  (in other words, light corresponding to each pixel of the frame images) and the optical axis of the camera lens of the image capturing device  12 , and other values. The moved-distance calculator  44  may calculate the moved distance of the occupant represented by the target human image P in the actual space by initially defining multiple specific spots on fixed objects (for example, seats and handrails) within the image capturing area of the image capturing device  12 , storing in advance distances between these specific spots in the actual space in the memory  32 , and referring to the distances between the specific spots in the actual space. 
     The distance from the image capturing device  12  to the start point  66  or to the estimated center  62   c  can be obtained by the processes described below. For example, as described on the webpage published on the Internet at &lt;https://www.global.toshiba/jp/technology/corporate/rdc/rd/topics/19/1910-03.html&gt;, an image showing distances between the image capturing device  12  and points of the image-captured objects represented by respective pixels (defined by coordinates) of the frame images can be obtained by inputting respective frame images to a distance measurement learning unit which has been already trained. The moved-distance calculator  44  can thus obtain the distance from the image capturing device  12  to the start point  66  by inputting the subsequent images  34   b  (or the temporary reference image  34   a ) to the distance measurement learning unit, or the distance from the image capturing device  12  to the estimated center  62   c  by inputting the measurement-target image  34   c  to the distance measurement learning unit. 
     The moved-distance calculator  44  calculates the moved distance of the occupant represented by the target human image Pin the actual space based on the measurement-target image  34   c  in response to receipt of the measurement-target image  34   c  from the image capturing device  12 . The moved-distance calculator  44  determines that the occupant has moved around when the calculated moved-distance of the occupant represented by the target human image P is equal to or longer than the threshold distance which has been set by the manager or other personnel of the moved-distance measurement system  10 . The threshold distance may be set to a value greater than the moved-distance of the estimated center  62  which would be caused when the occupant merely changed their posture. 
     When the moved-distance calculator  44  determines that the occupant represented by the target human image P has moved around, the signal output unit  46  outputs a drive start disable signal to the autonomous drive computer to prevent the vehicle A from starting through autonomous driving. Because the autonomous drive computer is prevented from starting autonomous driving of the vehicle A, starting of the vehicle A c through autonomous driving while the occupant is moving around in the vehicle can be prevented. The autonomous drive computer is able to later restart autonomous driving after the drive start disabled state is released, for example, by an operator on board the vehicle A. 
     When the moved-distance calculator  44  determines that the occupant represented by the target human image P has moved around, the notification controller  48  may output a notification to the occupants of the vehicle A. 
     Specifically, the notification controller  48  may send a notification command signal to control the display  14  to display a notification. For example, when the display  14  is a digital signage, the notification controller  48  commands to display, as a notification to the passengers, a text message “please do not move around” or the like on the digital signage, possibly with an illustration. When the display  14  is an indicator, the notification controller  48  notifies the operator that an occupant is moving around by lighting up or flashing the indicator. 
     The notification controller  48  may send a notification command signal to the voice output device  16  to perform a voice notification. For example, the notification controller  48  commands the voice output device  16  to output a voice message “please do not move around” as a notification to passengers. The notification controller  48  may command the voice output device  16  to output an voice message “a passenger is moving around” as a notification to the operator. 
     The moved-distance measurement system  10  according to embodiments of the present disclosure is outlined below. The flow of processes performed by the moved-distance measuring device  18  are described below based on the flowchart shown in  FIG.  7   . 
     In step S 10 , the moved-distance measuring device  18  receives the temporary reference image  34   a  from the image capturing device  12 . The center estimator  38  locates, from the temporary reference image  34   a , the target human image P corresponding to the occupant whose moved distance is to be calculated, and obtains the estimated center  62   a  of the occupant based on the located target human image P. 
     In step S 12 , the stable area setter  40  sets the stable area  64  in which the estimated center  62   a  obtained in step S 10  is placed at the center. Steps S 10  and S 12  are referred to as the step of “setting a stable area”. 
     In step S 14 , the moved-distance measuring device  18  receives the subsequent images  34   b  from the image capturing device  12 . The center estimator  38  locates the target human image P from the subsequent images  34   b  and obtains the estimated center  62   b  of the occupant represented by the target human image P. 
     In step S 16 , the start-point setter  42  determines whether or not the estimated center  62   b  represented by the coordinates obtained in step S 14  is within the stable area  64  which has been set in step S 12 . When the estimated center  62   b  is outside the stable area  64 , the process proceeds to step S 18 . 
     In step S 18 , the stable area setter  40  replaces the temporary reference image  34   a  with the subsequent image  34   b  with the estimated center  62  outside the stable area  64 . Then, again in step S 12 , the stable area setter  40  sets the stable area  64  in the new temporary reference image  34   a.    
     When the estimated center  62   b  is within the stable area  64  in step S 16 , the process proceeds to step S 20 , in which the start-point setter  42  determines whether or not the process to determine whether or not the estimated center  62   b  is within the stable area  64  has been performed for all the subsequent images  34   b  captured within the threshold time period after the temporary reference image  34   a  was captured. When this process has not been completed for all the subsequent images  34   b  captured within the threshold time period, the process returns to step S 14 . Again in step S 14 , the moved-distance measuring device  18  receives the next subsequent image  34   b  from the image capturing device  12 , and the center estimator  38  locates the target human image P from the subsequent image  34   b  and obtains the estimated center  62   b  of the occupant represented by the target human image P. When the process has been completed for all the subsequent images  34   b  captured within the threshold time period, the process proceeds to step S 22 . Steps S 14 , S 16 , and S 20  are collectively referred to as the step of “estimating the center”. 
     In step S 22 , the start-point setter  42  sets, as the start point  66 , a predefined position within the stable area  64  which has been set in step S 12 . Step S 22  is referred to as the step of “setting the start point”. 
     In step S 24 , the moved-distance measuring device  18  receives the measurement-target image  34   c  from the image capturing device  12 . The center estimator  38  locates the target human image P from the measurement-target image  34   c , and obtains the estimated center  62   c  of the occupant represented by the target human image P. 
     In step S 26 , the moved-distance calculator  44  calculates the moved distance of the occupant represented by the target human image P in the actual space based on the distance between the start point  66  and the estimated center  62   c . Step S 26  is referred to as the step of “calculating the moved distance”. When the moved distance of the occupant calculated by the moved-distance calculator  44  is equal to or longer than the threshold distance, the signal output unit  46  outputs, to the autonomous drive computer, the drive start disable signal to prevent the vehicle A from starting through autonomous driving. Further, when the moved distance of the occupant calculated by the moved-distance calculator  44  is equal to or longer than the threshold distance, the notification controller  48  controls the display  14  and/or the voice output device  16  to output a notification to the occupants of the vehicle A. 
     Although the vehicle occupant moved-distance measuring device according to embodiments of the present disclosure are described above, the device is not limited to the above embodiments. Various changes are applicable without deviating from the scope of the present disclosure. 
     For example, although in the embodiments described above the moved-distance measuring device  18  is mounted inside the vehicle A, the moved-distance measuring device  18  may be a computer external to the vehicle A (for example, a server computer). In such a case, the frame images captured by the image capturing device  12  are sequentially sent to the moved-distance measuring device  18  via communication lines, and the moved-distance measuring device  18  calculates the moved distance of the occupant of the vehicle A using the above described processes based on the image frames received via the communication lines.