Patent Publication Number: US-2023154065-A1

Title: Imaging apparatus, image processing apparatus, display system, and vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This present application is a Continuation of U.S. patent application Ser. No. 17/231,908 filed on Apr. 15 2021, which is a Divisional of U.S. patent application Ser. No. 16/467,715 filed on Jun. 7, 2019, which is the U.S. National Phase of International Application No. PCT/JP2017/042292 filed on Nov. 24, 2017, which claims priority to and the benefit of Japanese Patent Applications No. 2016-239449 filed on Dec. 9, 2016 and No. 2016-245775 filed on Dec. 19, 2016, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an imaging apparatus, an image processing apparatus, a display system, and a vehicle. 
     BACKGROUND 
     Conventionally, technologies for displaying video images of an outside area of a moving body such as a vehicle are known. For example, PTL 1 set forth below discloses a technology for controlling power supply to a monitor that displays video images captured by a camera provided to a vehicle. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2009-40113 
     SUMMARY 
     A display control device comprising a controller configured to simultaneously displays a bird&#39;s-eye view image of a vehicle viewed from above, a video image behind the vehicle captured by an imaging means, and guide images extending rearward at intervals in the vehicle width direction of the vehicle on one screen of a display means. The controller is configured to display on the display means the first image in the lower right or lower left of the bird&#39;s-eye view image according to the position of an obstacle located in the rear right or rear left of the vehicle and to display a second image outside the guide image at the right end or left end of the video image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG.  1    is a block diagram schematically illustrating a configuration of a display system according to an embodiment of the present disclosure; 
         FIG.  2    is a diagram illustrating a vehicle having the display system mounted thereon viewed from a left side; 
         FIG.  3 A  is a diagram schematically illustrating an example of the exterior of the display apparatus of  FIG.  1   ; 
         FIG.  3 B  is a diagram schematically illustrating an example of the exterior of the display apparatus of  FIG.  1   ; 
         FIG.  3 C  is a diagram schematically illustrating an example of the exterior of the display apparatus of  FIG.  1   ; 
         FIG.  3 D  is a diagram schematically illustrating an example of the exterior of the display apparatus of  FIG.  1   ; 
         FIG.  3 E  is a diagram schematically illustrating an example of the exterior of the display apparatus of  FIG.  1   ; 
         FIG.  4    is a diagram illustrating a first example of a first video image; 
         FIG.  5    is a diagram illustrating a first example of a second video image corresponding to a display region of the first video image of  FIG.  4   ; 
         FIG.  6    is a diagram illustrating an example of a third marker to be superimposed on a detection object; 
         FIG.  7    is a diagram illustrating a first example of a first marker and a second marker displayed in the vicinity of the detection object; 
         FIG.  8    is a diagram illustrating a second example of the first marker and the second marker displayed in the vicinity of the detection object; 
         FIG.  9    is a diagram illustrating a third example of the first marker and the second marker displayed in the vicinity of the detection object; 
         FIG.  10    is a diagram illustrating a second example of the first video image; 
         FIG.  11    is a diagram illustrating a second example of the second video image corresponding to a display region of the first video image of  FIG.  10   ; 
         FIG.  12    is a diagram illustrating a third example of the first video image; 
         FIG.  13    is a diagram illustrating a third example of the second video image corresponding to a display region of the first video image of  FIG.  12   ; 
         FIG.  14    is a diagram illustrating a fourth example of the first image; 
         FIG.  15    is a diagram illustrating a fourth example of the second video image corresponding to a display region of the first video image of  FIG.  14   ; 
         FIG.  16    is a diagram illustrating a fifth example of the first video image; 
         FIG.  17    is a diagram illustrating a fifth example of the second video image corresponding to a display region of the first video image of  FIG.  16   ; 
         FIG.  18    is a diagram illustrating another example of the second video image corresponding to the display region of the first image of  FIG.  16   ; 
         FIG.  19    is a diagram illustrating a sixth example of the first video image; 
         FIG.  20    is a diagram illustrating a sixth example of the second video image corresponding to a display region of the first video image of  FIG.  19   ; 
         FIG.  21    is a diagram illustrating another example of the second video image; 
         FIG.  22    is a diagram illustrating a first example variation of the sixth example of the second video image; 
         FIG.  23    is a diagram illustrating a second example variation of the sixth example of the second video image; 
         FIG.  24    is a diagram illustrating a third example variation of the sixth example of the second video image; 
         FIG.  25    is a diagram illustrating a seventh example of the first video image; 
         FIG.  26    is a diagram illustrating a seventh example of the second video image corresponding to a display region of the first video image of  FIG.  25   ; 
         FIG.  27    is a diagram illustrating an eighth example of the first video image; 
         FIG.  28    is a diagram illustrating a positional relationship between a vehicle and a detection object; 
         FIG.  29    is a diagram illustrating an eighth example of the second video image corresponding to a display region of a first video image of  FIG.  24   ; 
         FIG.  30    is a diagram illustrating an example variation of the eighth example of the second video image; 
         FIG.  31    is a diagram illustrating an example in which a display region of  FIG.  30    is changed into a Wide View Display; and 
         FIG.  32    is a diagram illustrating another example of the second video image. 
     
    
    
     DETAILED DESCRIPTION 
     It is desirable to improve the convenience of technologies for displaying a video image of an outside area of a moving body in an image processing apparatus, an imaging apparatus, a display system, and a vehicle. Hereinafter, an embodiment of the present disclosure will be described with reference to the accompanying drawings. 
     Display System 
     A display system  10  according to the embodiment of the present disclosure will be described with reference to  FIG.  1   . 
     As illustrated in  FIG.  1   , the display system  10  includes an imaging apparatus  20 , an image processing apparatus  30 , and a display apparatus  40 . Each constituent element of the imaging apparatus  20  and the display system  10  can transmit and receive information via, for example, a network  51 . The network  51  may include, for example, wireless communication, wired communication, or a CAN (Controller Area Network). 
     In some embodiments, some or all of the constituent elements of the display system  10  may be integrally formed as one apparatus. For example, the image processing apparatus  30  may be incorporated in the imaging apparatus  20  or the display apparatus  40 . 
     As illustrated in  FIG.  2   , the imaging apparatus  20 , the image processing apparatus  30 , and the display apparatus  40  may be provided to a moving body  50 . The term “moving body” as used herein encompasses, for example, vehicles, watercraft, and aircraft. Vehicles include, for example, automobiles, industrial vehicles, rail vehicles, domestic vehicles, and fixed-wing aircraft that travel on a runway. Automobiles may include, for example, cars, trucks, buses, motorcycles, and trolley buses. Industrial vehicles include, for example, agricultural vehicles and construction vehicles. Industrial vehicles for industrial purposes include, for example, forklifts and golf carts. Industrial vehicles for agricultural purpose include, for example, tractors, tillers, transplanters, binders, combine harvesters, and lawn mowers. Industrial vehicles for construction purposes include, for example, bulldozers, scrapers, excavators, crane trucks, dump trucks, and load rollers. Vehicles may include those that travel on human power. Vehicle classifications are not limited to the above examples. For example, vehicles may include industrial vehicles authorized to travel on the road. The same type of vehicle may be included in a plurality of categories. Watercraft may include, for example, jetboats, motorboats, and tankers. Aircraft may include, for example, fixed-wing aircraft and rotorcraft. 
     The imaging apparatus  20  can capture an outside area of the moving body  50 . The imaging apparatus  20  may be disposed at any position inside or outside of the moving body  50 . For example, the imaging apparatus  20  is positioned in a rear portion of the moving body  50  as illustrated in  FIG.  2    to be able to capture an outside area behind the moving body  50 . The image processing apparatus  30  may be disposed at any position in the moving body  50 . The display apparatus  40  can be viewed by a subject  60 . The display apparatus  40  may be disposed in any position in the moving body  50 . For example, the display apparatus  40  is disposed in a dashboard of the moving body  50  as illustrated in  FIG.  2   . 
     Imaging Apparatus 
     The imaging apparatus  20  will be described in detail. The imaging apparatus  20  includes an imaging optical system  21 , an image sensor  22 , a communication interface  23 , and a controller  24 , as illustrated in  FIG.  1    by way of example. 
     The imaging optical system  21  forms a subject image. For example, the imaging optical system  21  may include an aperture and one or more lenses. 
     The image sensor  22  includes a plurality of pixels arranged in a two-dimensional manner. The image sensor  22  may include, for example, a CCD (Charge Coupled Device) image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor. The image sensor  22  can capture a subject image formed by the imaging optical system  21  and generate a captured image. 
     The communication interface  23  may include a communication interface capable of communicating with an external apparatus. The communication interface  23  may be capable of transmitting and receiving information via the network  51 . The external apparatus may include, for example, the image processing apparatus  30 . The term “communication interface” as used herein may include, for example, a physical connector or a wireless communication device. The physical connector may include an electrical connector which supports transmission of electrical signals, an optical connector which supports transmission of optical signals, or an electromagnetic connector which supports transmission of electromagnetic waves. The electrical connector may be a connector conforming to IEC60603, a connector conforming to the USB standard, a connector compatible with an RCA connector, a connector compatible with an S-terminal defined by EIAJ CP-1211A, a connector compatible with a D-terminal defined in EIAJ RC-5237, a connector conforming to the HDMI® (HDMI is a registered trademark in Japan, other countries, or both) standard, or a connector compatible with a coaxial cable including BNC (British Naval connector, Baby-series N connector, or the like). The optical connector may include a variety of connectors conforming to IEC 61754. The wireless communication device may include those conforming to Bluetooth® (Bluetooth is a registered trademark in Japan, other countries, or both) or standards including IEEE802.11. The wireless communication device includes at least one antenna. 
     The controller  24  includes at least one processor. The term “processor” as used herein may include a specialized processor dedicated to specific processing or a general-purpose processor configured to perform a specific function by reading a specific program. The specialized processor may include a DSP (Digital Signal Processor) or an ASIC (Application Specific Integrated Circuit). The processor may include a PLD (Programmable Logic Device). The PLD may include an FPGA (Field-Programmable Gate Array). The controller  24  may be one of a SoC (System-on-a-Chip) and a SiP (System In a Package) in which one or more processors cooperate with one another. 
     The controller  24  is configured to control overall operation of the imaging apparatus  20 . The controller  24  may cause the image sensor  22  to generate captured images at any frame rate. The frame rate may approximately correspond to, for example, a frame rate at which images can be displayed on the display apparatus  40 . The controller  24  may perform predetermined image processing on the captured image. The image processing may include, for example, exposure adjustment, white balancing, distortion correction, and the like. The controller  24  outputs the captured image to the image processing apparatus  30  via the communication interface  23 . For example, the controller  24  may sequentially output captured images at the frame rate described above. Hereinafter, each captured image output at the frame rate described above will also be simply referred to as a frame. A plurality of captured images output from the imaging apparatus  20  will also be referred to as first video images. For example, when the frame rate is 60 fps (Flame per Seconds), sixty images captured each second are output as the first video images. 
     Image Processing Apparatus 
     The image processing apparatus  30  will be described in detail. The image processing apparatus  30  includes a communication interface  31 , a memory  32 , and a controller  33 . 
     The communication interface  31  may include a communication interface capable of communicating with various external apparatuses. The external apparatuses may include, for example, the imaging apparatus  20 , the display apparatus  40 , an ECU (Electronic Control Unit or Engine Control Unit) provided to the moving body  50 , a speed sensor, an acceleration sensor, a rotation angle sensor, a steering angle sensor, an engine speed sensor, an accelerator sensor, a brake sensor, an illumination sensor, a raindrop sensor, a travel distance sensor, an obstacle detection device using a millimeter wave radar, an ultrasonic sonar or the like, an ETC (Electronic Toll Collection system) receiving apparatus, a GPS (Global Positioning System) apparatus, a navigation apparatus, an Internet server, a cellular phone, and the like. 
     The communication interface  31  may include a communication interface for pedestrian-vehicle communication, road-vehicle communication, and inter-vehicle communication. The communication interface  31  may include a receiver which is compatible with a light beacon for DSRC (Dedicated Short-Range Communication: a narrowband communication system) or VICS® (Vehicle Information and Communication System: VICS is a registered trademark in Japan, other countries, or both) provided in Japan. The communication interface  31  may include a receiver which is compatible with the road traffic information providing systems of other countries. 
     The communication interface  31  may be capable of acquiring various information from the external apparatuses. For example, the communication interface  31  may be capable of acquiring moving body information and environmental information. 
     The moving body information may include any information associated with the moving body  50 . For example, the moving body information may include, with respect to the moving body  50 , a speed, an acceleration, a turning gravity, an inclination, a direction, a turning status, a steering angle of the steering wheel, the temperature of the cooling water, a fuel remaining amount, a battery remaining amount, a battery voltage, an engine speed, a gear position, presence or absence of a reverse signal, the presence or absence of accelerator operation, an accelerator position, the presence or absence of brake operation, the brake pedal pressing degree, the presence or absence of parking brake operation, a rotational speed difference between the front and rear wheels or 4 wheels, tire pressure, an expansion and contraction amount of the damper, the spatial position of the eyes of the driver, the number of passengers and seat positions, seat belt fastening information, opening and closing of the doors, opening and closing of the windows, interior temperature, the presence or absence of operation of the air conditioning, air-conditioning setting temperature, an air-conditioning air volume, outside air circulation setting, an operating condition of the wiper, a driving mode, information about connection to an external device, current time, average fuel consumption, current fuel consumption, lighting states of various lamps, positional information, and route information to a destination. The various lamps may include, for example, headlamps, fog lamps, reversing lamps, position lamps, and indicator lamps. 
     The environmental information may include any information about the environment outside of the moving body  50 . The environmental information may include, for example, brightness around the moving body  50 , the weather, air pressure, outside air temperature, map information, traffic information, road construction information, temporary change of the speed limit of the road, objects detected by other vehicles, and lighting states of traffic signals. 
     The memory  32  may include a temporary storage device and a secondary storage device. The memory  32  may include, for example, a semiconductor memory, a magnetic memory, or an optical memory. The semiconductor memory may include a volatile memory or a nonvolatile memory. The magnetic memory may include, for example, a hard disk or a magnetic tape. The optical memory may include, for example, a CD (Compact Disc), a DVD (Digital Versatile Disc), and BD® (Blu-ray Disc®); BD and Blu-ray are registered trademarks in Japan, other countries, or both). The memory  32  stores various information and programs necessary for the operation of the image processing apparatus  30 . 
     The controller  33  includes one or more processors. The controller  33  controls overall operations of the image processing apparatus  30 . 
     The controller  33  may acquire the moving body information and the environmental information from an external apparatus via the communication interface  31 . The controller  33  may determine a predicted path of the moving body  50  on the basis of, for example, the moving body information. Hereinafter, the predicted path of the moving body  50  will also be referred to as a first predicted path. 
     The controller  33  may acquire the first video images from the imaging apparatus  20  via the communication interface  31 . The first video images include a detection region and a display region. 
     The controller  33  may detect at least a portion of a detection object in the detection region of the acquired first video images. The detection region of the first video images may be at least a portion of a captured image for each frame of the first video images. Each frame of the first video images may be referred to as a captured image. The detection region of the first video images may be larger than the display region. The detection region of the first video images may encompass the display region. The controller  33  can detect the detection object located inside of the display region. The controller  33  can detect the detection object located outside of the display region and inside of the detection region. A region inside of the detection region and the display region may be referred to as a first region. A region inside of the detection region and outside of the display region may be referred to as a second region. 
     The detection object may include a plurality of types of objects. The types of objects may include, for example, a person, another moving body, a road, a lane, a white line, a gutter, a sidewalk, a pedestrian crossing, a road sign, a traffic sign, a guardrail, a wall, and a traffic signal. The types of objects detectable by the controller  33  are not limited these examples. In a case where a portion of the detection object is concealed behind another body in the first video images, the detection object may include at least a portion that is not concealed behind the body. For example, when the lower body of a pedestrian is concealed behind an obstacle in the first video images, the controller  33  may detect the upper body of the pedestrian. Any appropriate object detection algorithms may be employed for the detection of at least a portion of the detection object. For example, the controller  33  may detect at least a portion of the detection object by employing algorithm such as pattern matching feature point extraction using the captured image for each frame of the first video images. 
     When the controller  33  detects at least a portion of the detection object in the first video images, the controller  33  may determine a predicted path of the detection object on the basis of the first video images. Hereinafter, the predicted path of the detection object will also be referred to as a second predicted path. Any appropriate algorithm may be employed for the determination of the second predicted path. For example, the controller  33  may determine the second predicted path on the basis of changes in the orientation and position of the detection object in the captured image for each frame of the first video images. 
     When the controller  33  detects at least a portion of the detection object in the first video images, the controller  33  may estimate a relative positional relationship between the moving body  50  and the detection object on the basis of the first video images. The relative positional relationship may include, for example, a distance between the moving body  50  and the detection object, or whether the first predicted path of the moving body  50  and the second predicted path of the detection object overlap with each other. Any appropriate algorithm may be employed for the estimation of the distance between the moving body  50  and the detection object. For example, the controller  33  may estimate the distance between the moving body  50  and the detection object by employing a motion stereo method using the captured image for each frame of the first video images signal. In some embodiments, the controller  33  may acquire information indicating the relative positional relationship between the moving body  50  and the detection object from an external apparatus via the communication interface  31 . 
     When the distance between the moving body  50  and the detection object decreases, the controller  33  may determine which one of the moving body  50  and the detection object contributes more to the decrease in distance. Any appropriate algorithm may be employed to determine the respective contributions of the moving body  50  and the detection object with respect to the decrease in distance. In one example, the controller  33  may detect a moving speed of the moving body  50  on the basis of the moving body information. The controller  33  may detect a moving speed of the detection object on the basis of, for example, a positional change of the detection object in the captured image for each frame of the first video images. The controller  33  may determine that one of the moving body  50  and the detection object that has a greater moving speed contributes more to the decrease in distance. In another example, when the moving speed of the moving body  50  is smaller than a reference value, the controller  33  may determine that the detection object contributes more to the decrease in distance. When the moving speed of the moving body  50  is equal to or larger than the reference value, the controller  33  may determine that the moving body  50  contributes more to the decrease in distance. The reference value may be set appropriately, and may for example be set to approximately zero. Operation of the image processing apparatus  30  on the basis of the respective contributions of the moving body  50  and the detection object with respect to the decrease in distance will be described in detail later. 
     When at least a portion of the detection object is detected in the first video images, the controller  33  may determine on the basis of the first video images whether there is a possibility of contact between the moving body  50  and the detection object. Any appropriate algorithm may be employed to determine the possibility of contact between the moving body  50  and the detection object. For example, when at least one of a condition specifying that the distance between the moving body  50  and the detection object is smaller than a predetermined threshold and a condition specifying that a rate at which the distance is decreasing is equal to or greater than a predetermined threshold is met, the controller  33  may determine that there is a possibility of contact between the moving body  50  and the detection object. Operation of the image processing apparatus  30  in accordance with the presence or absence of the possibility will be described in detail later. 
     The controller  33  may cause second video images corresponding to the display region of the first video images acquired from the imaging apparatus  20  to be displayed on the display apparatus  40 . In particular, the controller  33  may output the second video images to the display apparatus  40  via the communication interface  31 . For example, when the controller  33  detects a rearward movement of the moving body  50  on the basis of the moving body information, the controller  33  may cause the second video images to be displayed on the display apparatus  40 . For example, the controller  33  may detect the rearward movement on the basis of a shift position of the transmission gear. For example, the controller  33  may detect the rearward movement on the basis of a reverse signal output from the moving body at the time of a rearward movement. The second video images may include, for example, a video image extracted from the display region of the captured image for each frame of the first video images. The display region of the first video images may be at least a region of the captured image for each frame of the first video images. The display region may be smaller than the detection region. The display region may be encompassed within the detection region. A position, a shape, and a size of the display region may be determined appropriately. The controller  33  may change the position, the shape, and the size of the display region. The position, the shape, and the size of the display region may be changed to substantially match the detection region. 
     The controller  33  may cause various markers in combination with the second video images to be displayed on the display apparatus  40 . A method for such combining includes overwriting or mixing. The marker may include, for example, one or more images. The controller  33  may dynamically change a display mode of at least a portion of the marker superimposed on the second video images. The display mode may include, for example, a position, a size, a shape, a color, or a shading of at least a portion of a marker in the second video images. When the controller  33  displays a marker corresponding to the detection object detected in the first video images, the controller  33  may determine the display mode of the marker in accordance with the type of the marker. The operation of the image processing apparatus  30  for causing various markers to be displayed on the display apparatus  40  will be described in detail later. 
     Display Apparatus 
     The display apparatus  40  will be described in detail. The display apparatus  40  may include, for example, a liquid crystal display or an organic EL (Electro-Luminescence) display. The display apparatus  40  may display, for example, the second video images input from the image processing apparatus  30  via the network  51 . The display apparatus  40  may function as a touchscreen capable of receiving user operations. The display apparatus  40  may include a switch or a key capable of receiving a user operation. The switch may include a mechanical switch or an electronic switch. The key may include a mechanical key or an electronic key. When the display apparatus  40  receives a user operation, the display apparatus  40  may transmit a user input corresponding to the user operation to the image processing apparatus  30 . 
     The display apparatus  40  may be arranged at various locations of the moving body  50 .  FIG.  3 A  to  FIG.  3 E  illustrate example arrangement of the display apparatus  40 .  FIG.  3 A  illustrates a display apparatus  40   a  of an in-dashboard type arranged in the dashboard of the vehicle.  FIG.  3 B  illustrates a display apparatus  40   b  of an on-dashboard type arranged on the dashboard. In  FIG.  3 B , the display apparatus  40   b  is incorporated in the moving body  50 . The display apparatus  40   b  may be detachably attached on the dashboard.  FIG.  3 C  illustrates a display apparatus  40   c  that is built into a room mirror and capable of displaying a video image as necessary.  FIG.  3 D  illustrates a display apparatus  40   d  built in an instrumental panel. In  FIG.  3 D , the display apparatus  40   d  is arranged adjacent to the instruments such as a speedometer and a tachometer. In one embodiment, the display apparatus  40   d  may be configured as an LCD or the like and may constitute the entire instrumental panel that display a second video image together with images of the speedometer and the tachometer.  FIG.  3 E  illustrates a display apparatus  40   e  that is configured as a mobile information terminal such as a tablet computer. The display apparatus  40   e  may be configured as a mobile phone display. 
     The second video images and the various markers displayed on the display apparatus  40  under the control of the image processing apparatus  30  will be described in detail with reference to  FIG.  4    to  FIG.  18   . According to the present disclosure, the terms “up-down direction” and “left-right direction” in a video image or an image correspond to the two-dimensional directions in the video image or the image. According to the present disclosure, the terms “height direction”, “horizontal direction”, and “depth direction” in a video image or an image correspond to the three-dimensional directions in the space captured in the video image or the image. 
     First Example (Reference Example) 
       FIG.  4    illustrates a first example of a detection region  61  of a first video image acquired by the image processing apparatus  30  from the imaging apparatus  20 . In the example illustrated in  FIG.  4   , the detection region  61  is longer in the left-right direction than the up-down direction. The display region  62  is positioned in the center of the detection region  61  with respect to the left-right direction. The controller  33  may detect each of a pedestrian  63  and a vehicle  64  captured in the display region  62  of the first video image as a detection object. 
     The controller  33  determines whether one or more conditions are met on the basis of a relative positional relationship between a detection object detected within the display region  62  of the first video image and the moving body  50 . The one or more conditions may include, for example, a first condition specifying that the detection object is located on the first predicted path  65  of the moving body  50 . The one or more conditions may include, for example, a second condition specifying that at least a portion of the first predicted path  65  of the moving body  50  and at least a portion of the second predicted path of the detection object overlap with each other. When the controller  33  determines that one or more of the conditions are met, the controller  33  may cause a predetermined marker corresponding to the detection object to be superimposed on the second video images and displayed on the display apparatus  40 . The predetermined marker may include a first marker, a second marker, or a third marker. 
     In the first example, the controller  33  may determine that the one or more conditions are met with respect to the pedestrian  63 . In this case, the controller  33  may display a marker corresponding to the pedestrian  63 . The controller  33  may determine that the one or more conditions are not met with respect to the vehicle  64 . In this case, the controller  33  does not display the marker corresponding to the vehicle  64 . 
       FIG.  5    illustrates an example of a second video image corresponding to the display region  62  of the first video image illustrated in  FIG.  4   . When an aspect ratio of the display region  62  of the first video image differs from an aspect ratio of the screen of the display apparatus  40 , the controller  33  may output the second video image obtained by cutting out the display region  62  of the first video image and modifying the display region  62  in accordance with the aspect ratio of the screen of the display apparatus  40 . The second video image includes the pedestrian  63  and the vehicle  64  as illustrated in  FIG.  5   . 
     The controller  33  may cause guide lines  66  which indicate at least a portion of the first predicted path  65  of the moving body  50  illustrated in  FIG.  4    by way of example to be superimposed on the second video image and displayed on the display apparatus  40 . The controller  33  may dynamically change the guide lines  66  on the basis of, for example, a change of the steering angle of the steering wheel. 
     An area of the first video image is wider than the display region  62 . The controller  33  may change the area of the display region  62 . The controller  33  may cause an icon image  67  to be superimposed on the second video image and displayed on the display apparatus  40 . For example, an outline  67   a  of the icon image  67  illustrated in  FIG.  4    corresponds to a maximum area of the display region  62  when the area of the display region  62  is changed. A white rectangle  67   b  of the icon image  67  corresponds to the display region  62 . The icon image  67  illustrated in  FIG.  4    indicates a position and a size of the display region  62  relative to the maximum area of the display region  62 . 
       FIG.  6    illustrates an example of a marker to be superimposed on the pedestrian  63  in the second video image. Hereinafter, this marker will also be referred to as a third marker  68 . An outline  69  of the third marker  68  may substantially match the outline of the pedestrian  63  detected in the second video image. A region  70  within the outline  69  of the third marker  68  may be filled with a color or pattern corresponding to the type of the detection object, e.g., “person”. When the pedestrian  63  is detected in the first video image, the controller  33  may cause the third marker  68  to be superimposed on the pedestrian  63  in the second video image and displayed on the display apparatus  40 . This configuration enables the subject  60  to easily view the pedestrian  63  in the second video image. The controller  33  may hide the third marker  68  when a predetermined time has elapsed after the third marker  68  is displayed. 
       FIG.  7    illustrates examples of two types of markers superimposed in the vicinity of the pedestrian  63  in the second video image. Hereinafter, each of the two types of markers will also be referred to as a first marker  71  and a second marker  72 . For example, the controller  33  may cause the first marker  71  and the second marker  72  to be superimposed on the second video image and displayed on the display apparatus  40  after hiding the third marker  68 . 
     The controller  33  may move a position of the first marker  71  following the pedestrian  63  in the second video image. The subject  60  may easily recognize the pedestrian  63  because the first marker  71  follows the pedestrian  63 . The first marker  71  is displayed in the vicinity of the pedestrian  63  in a manner spaced apart therefrom. The subject  60  may easily recognize the behavior of the pedestrian  63  when the first marker  71  is displayed on the display apparatus  40 . The controller  33  may change a superimposing position of the second marker  72  relative to a superimposing position of the first marker  71  in the second video image. The controller  33  may move the second marker  72  relative to the position of the first marker  71  serving as a reference. 
     For example, when the distance between the moving body  50  and the pedestrian  63  decreases, the controller  33  may determine that the moving body  50  contributes more to the decrease in distance. In this case, the controller  33  may move the second marker  72  toward the first marker  71 . First, the controller  33  displays the second marker  72  in a position distanced from the first marker  71 . Subsequently, the controller  33  moves the second marker  72  toward the first marker  71  until the distance therebetween reaches a predetermined distance. Next, the controller  33  hides the second marker  72 . Then, the controller  33  displays the second marker  72  in a position distanced from the first marker  71  and repeats the operation described above. In this example, the second marker  72  approaches the first marker  71  displayed as an object. Thus, the subject  60  can recognize that the second marker  72  is approaching the first marker  71 . 
     For example, when the distance between the moving body  50  and the pedestrian  63  decreases, the controller  33  may determine that the pedestrian  63  contributes more to the decrease in the distance. In this case, the controller  33  may move the second marker  72  away from the first marker  71 . First, the controller  33  displays the second marker  72  in the vicinity of the first marker  71 . Subsequently, the controller  33  moves the second marker  72  away from the first marker  71  until the distance therebetween reaches a predetermined distance. Next, the controller  33  hides the second marker  72 . Then, the controller  33  displays the second marker  72  in the vicinity of the first marker  71  and repeats the operation described above. In this example, the second marker  72  is moving away from the first marker  71  displayed as the object. Thus, the subject  60  can recognize that the second marker  72  is moving away from the first marker  71 . 
     The controller  33  changes a moving direction of the second marker  72  relative to the first marker  71  on the basis of the respective contributions of the moving body  50  and the pedestrian  63  with respect to the decrease in distance between the moving body  50  and the pedestrian  63 . For example, the subject  60  can recognize whether the moving body  50  is approaching the pedestrian  63  or whether the pedestrian  63  is approaching the moving body  50 , on the basis of the moving direction of the second marker  72 . 
     The controller  33  may repeatedly expand and contract the second marker  72  centered on the first marker  71  in the second video image as illustrated in  FIG.  8    by way of example. The controller  33  may superimpose the first marker  71  or the second marker  72  having the same shape as the outline  69  of the pedestrian  63  in the second video image as illustrated in  FIG.  9    by way of example. The controller  33  may repeatedly expand or contract the second marker  72  centered on the first marker  71 . The controller  33  changes between expansion and contraction of the second marker  72  in accordance with the respective contributions of the moving body  50  and the pedestrian  63  with respect to the decrease in distance between the moving body  50  and the pedestrian  63 . 
     When the distance between the detection object and the moving body  50  represented by the first marker  71  and the second marker  72 , respectively, becomes smaller than a predetermined threshold, the controller  33  may superimpose another marker on the second video image. Hereinafter, the another marker will also be referred to as a fourth marker. The fourth maker may include any appropriate image. For example, the fourth marker may include an image showing an exclamation mark “!”. In this configuration, for example, when the pedestrian  63  and the moving body  50  represented by the first marker  71  and the second marker  72 , respectively, become close to each other and have a certain distance or less therebetween, the fourth marker is superimposed on the second video image and displayed. The fourth marker enables recognition that, for example, the pedestrian  63  is positioned in the vicinity of the moving body  50 . In some embodiments, the controller  33  may change display modes of the first marker  71  and the second marker  72  when the distance between the detection object and the moving body  50  represented by the first marker  71  and the second marker  72 , respectively, falls below the predetermined threshold. For example, the controller  33  may change the colors of the first marker  71  and the second marker  72 . This configuration for changing the colors of the markers enables the subject  60  to recognize that, for example, the pedestrian  63  is positioned in the vicinity of the moving body  50 . 
     The controller  33  can detect two detection objects located one behind the other in the depth direction. The controller  33  may display the first marker  71  and the second marker  72  representing the respective two detection objects located one behind the other. The controller  33  may display the first marker  71  and second marker  72  in different appearances together with the two detection objects. For example, the controller  33  may display the first marker  71  and the second marker  72  of a first detection object located on the rear side in an inconspicuous manner as compared to the first marker  71  and the second marker  72  of a second detection object located on the front side. For example, the controller  33  may change characteristics of the first marker  71  and the second marker  72  of the first detection object located on the rear side in the depth direction such that they are dark in color and have a high transmittances and thin lines as compared to the first marker  71  and the second marker  72  of the second detection object located on the front side. 
     Second to Fourth Examples (Reference Examples) 
       FIG.  10    illustrates a second example of the detection region  61  of the first video image acquired from the imaging apparatus  20  by the image processing apparatus  30 . In the example illustrated in  FIG.  10   , the detection region  61  is longer in the left-right direction than the up-down direction. The display region  62  is positioned at the center in the left-right direction of the detection region  61 . The controller  33  may detect each of the pedestrian  63   a  located inside of the display region  62  of the first video image and the pedestrians  63   b  and  63   c  located inside of the detection region  61  and outside of the display region  62  as a detection object. The controller  33  performs the same processing on the pedestrian  63   a  as the processing performed on the pedestrian  63  as illustrated in  FIG.  4    by way of example. 
     When a detection position of the detection object in the first video image is outside of the display region  62  and, simultaneously, inside of the detection region  61 , the controller  33  may cause a marker corresponding to the detection object to be superimposed on the second video image and displayed on the display apparatus  40 . Hereinafter, this marker will also be referred to as a fifth marker. When the controller  33  determines that there is a possibility of contact between the moving body  50  and the detection object, the controller  33  may display the fifth marker. When the detection position of the detection object in the first video image is located on the right side of the display region  62 , the controller  33  may cause the fifth marker to be superimposed on the right-side edge region of the second video images and displayed on the display apparatus  40 . When the detection position of the detection object in the first video image is located on the left side of the display region  62 , the controller  33  may cause the fifth marker to be superimposed on the left-side edge region of the second video image and displayed on the display apparatus  40 . 
     In the second example, a detection position of the pedestrian  63   b  is located on the right side of the display region  62 . The controller  33  may determine that there is a possibility of contact between the moving body  50  and the pedestrian  63   b.  In this case, the controller  33  may cause the fifth marker corresponding to the pedestrian  63   b  to be superimposed on the right-side edge region of the second video image and displayed in the display apparatus  40 . The fifth marker corresponding to the pedestrian  63   b  will be described in detail later. The detection position of the pedestrian  63   c  is on the left side of the display region  62 . The controller  33  may determine that there is no possibility of contact between the moving body  50  and the pedestrian  63   c.  In this case, the controller  33  does not need to display the fifth marker corresponding to the pedestrian  63   c.    
       FIG.  11    illustrates an example of the second video image corresponding to the display region  62  of the first video image illustrated in  FIG.  10   . As illustrated in  FIG.  11   , the pedestrian  63   a  is displayed in the second video image. The pedestrians  63   b  and  63   c  are not displayed in the second video image. 
     The controller  33  may cause, for example, an obstacle image  74  to be superimposed on the second video images and displayed on the display apparatus  40  as illustrated in  FIG.  11   . The obstacle image  74  indicates a detection result of an obstacle detection apparatus provided to the moving body  50  that uses ultrasonic sonar or the like. The obstacle image  74  may include an image  74   a,  an image  74   b,  and an image  74   c.  The image  74   a  is an image of the moving body  50  viewed from above. The image  74   b  is an image indicating that an obstacle is detected on the left rear side of the moving body  50 . The image  74   c  is an image indicating that an obstacle is detected on the right rear side of the moving body  50 . The detection result of the obstacle detection apparatus and a detection result of the detection object of the controller  33  may not necessarily coincide. In the example illustrated in  FIG.  11   , for example, the obstacle image  74  indicates that obstacles are detected on both the right rear side and the left rear side of the moving body  50 . On the other hand, the controller  33  may determine that there is no possibility of contact between the moving body  50  and the pedestrian  63   c  on the left rear side of the moving body  50 . In this case, the controller  33  does not need to display the fifth marker corresponding to the pedestrian  63   c.    
       FIG.  11    illustrates an example of a fifth marker  73  corresponding to the pedestrian  63   b.  The fifth marker  73  may include an icon image  73   a  and a band image  73   b.  The icon image  73   a  may be an image corresponding to a “person” as the type of the detection object. The icon image  73   a  enables the subject  60  to recognize that there is a person on the right side of the second video image. The band image  73   b  is, for example, an image of a band that extends in the up-down direction in the second video image. The band image  73   b  may be filled with color or a pattern corresponding to the “person” as the type of the detection object. The controller  33  may move the band image  73   b  within a right-side edge region  73   c  of the second video image. The controller  33  may change a moving speed and a width of the band image  73   b.    
     The fifth marker  73  will be described in detail. The controller  33  may determine the width of the band image  73   b  on the basis of the distance between the moving body  50  and the pedestrian  73   b.  For example, the controller  33   b  may increase the width of the band image  73   b  as the distance becomes smaller. The band image  73   b  enables the subject  60  to recognize the distance between the moving body  50  and the pedestrian  63   b  on the basis of the width of the band image  73   b.    
     The controller  33  may determine that the moving body  50  contributes more to the reduction of the distance between the moving body  50  and the pedestrian  63   b.  In this case, the controller  33  repeatedly moves the band image  73   b  in a first direction within the right-side edge region  73   c  of the second video image. The first direction may be, for example, a direction directing from the outside to the inside in the left-right direction of the second video image. The controller  33  may determine that the pedestrian  63   b  contributes more to the reduction of the distance between the moving body  50  and the pedestrian  63   b.  In this case, the controller  33  repeatedly moves the band image  73   b  in a second direction within the right-side edge region  73   c  of the second video image. The second direction may be, for example, a direction directing from the inside to the outside in the left-right direction of the second video image. The band image  73   b  enables the subject  60  to know whether the moving body  50  is approaching the pedestrian  63   b  or the pedestrian  63   b  is approaching the moving body  50 , on the basis of a moving direction of the band image  73   b.    
     The controller  33  may determine the moving speed of the band image  73   b  on the basis of the rate at which the distance between the moving body  50  and the pedestrian  63   b  is decreasing. For example, the moving speed of the band image  73   b  may be faster as the rate at which the distance is decreasing is faster. The band image  73   b  enables the subject  60  to recognize the rate of decrease of the distance between the moving body  50  and the pedestrian  63   b  on the basis of the moving speed of the band image  73   b.    
     In a state in which the fifth marker  73  is displayed, when the controller  33  detects, for example, a user input corresponding to a predetermined user operation, the controller  33  may change the display region  62  to encompass the detection position of the pedestrian  63   b  in the first video image. For example, the controller  33  may extend the display region  62  of the first video image in the left-right direction and shift the display region  62  to the right in the detection region  61  as illustrated in  FIG.  12   . This configuration includes the pedestrian  63   b  in the second video image as illustrated in  FIG.  13    by way of example. 
     The predetermined user operation described above may include any appropriate user operation. For example, the predetermined user operation described above may include a first user operation for changing a steering angle of the steering wheel of the moving body  50 . The fifth marker  73  may function as a GUI (Graphic User Interface) configured to receive a second user operation. Hereinafter, the GUI will also be referred to as an interface image. In this case, the predetermined user operation described above may include the second user operation. 
     The controller  33  may autonomously change the display region  62  such that the detection position of the pedestrian  63   b  in the first video image is included in the display region  62 . In this case, the controller  33  may maintain the autonomous change of the display region  62  until the pedestrian  63   b  stops being detected in the detection region  61  of the first video image. 
     The controller  33  may change the icon image  67  on the basis of the change of the display region  62  as illustrated in  FIG.  13    by way of example. 
     The controller  33  may change the display region  62  of the first video image on the basis of, for example, a pinch-in operation and a pinch-out operation of the display apparatus  40 . For example, the controller  33  may substantially match the display region  62  with the detection region  61  as illustrated in  FIG.  14   . In this case, all detection objects in the detection region  61  are displayed on the display apparatus  40  as illustrated in  FIG.  15    by way of example. 
     Fifth Example (Reference Example) 
       FIG.  16    illustrates a third example of the detection region  61  of the first video image acquired by the image processing apparatus  30  from the imaging apparatus  20 . In the example illustrated in  FIG.  16   , the detection region  61  is longer in the left-right direction than the up-down direction. The display region  62  is positioned at the center of the left-right direction of the detection region  61 . The controller  33  may detect each of a vehicle  64   a  indicated in the first predicted path  65  of the moving body  50  and a vehicle  64   b  and a pedestrian  63   d  indicated outside of the first predicted path  65 . 
     In the fifth example, a case in which the outside area of the moving body  50  is dark, such as at night or inside a tunnel will be described. When the outside area of the moving body  50  is dark, characteristic values of the first video image and the second video image may decrease. The characteristic values may include any parameters associated with the visibility of the video image. For example, the characteristic values may include at least one of a luminance value and a contrast ratio of the video image. The decrease of the characteristic value of the second video images may lead to a deterioration in the visibility of the second video image. 
     The controller  33  may perform predetermined image processing on a region corresponding to the detection object in the second video images. The predetermined image processing may include first processing to superimpose a marker corresponding to a detection object on the region. Hereinafter, this marker will also be referred to as a sixth marker. The sixth marker may include, for example, an image substantially corresponding to an outline of the detection object in the second video images. In this configuration, the sixth marker is superimposed on the detection object in the second video images. This enables the subject  60  to easily recognize the detection object in the second video images even when the characteristic value of the second video images is low. The specific image processing may include second processing to change the characteristic value of the region corresponding to the detection object in the second video images. For example, the controller  33  may change the characteristic value of the region in such a manner so as to improve the visibility of the region in the second video images. This configuration improves the visibility of the detection object in the second video images. Thus, the subject  60  can easily recognize the detection object in the second video images even when the characteristic value of the second video images is low. 
     The controller  33  may perform the specific image processing described above when one or more conditions are met. The one or more conditions may include a condition specifying that the detection object is located in the first predicted path  65  of the moving body  50 . The one or more conditions may include a condition specifying that the first predicted path  65  of the moving body  50  and the second predicted path of the detection object overlap with each other. The one or more conditions may include a condition specifying that the distance between the moving body  50  and the detection object is less than a predetermined threshold. The one or more conditions may include a condition specifying that the characteristic value of at least a portion of the second video images is less than a predetermined threshold. 
     In the fifth example, the controller  33  may determine that the one or more conditions described above are met with respect to the vehicles  64   a  and  64   b  and the pedestrian  63   d.  In this case, the controller  33  may cause sixth markers  75   a,    75   b,  and  75   c  corresponding to the vehicles  64   a  and  64   b  and the pedestrian  63   d,  respectively, to be superimposed on the second video image and displayed on the display apparatus  40  as illustrated in  FIG.  17    by way of example. The controller  33  may display the sixth markers on the detection objects in an overlapping manner in bright locations. 
     The controller  33  may change a shape of the guide lines  66 . The controller  33  may change the shape of the guide lines  66  in a region where the guide lines  66  and the detection object overlap with each other.  FIG.  18    illustrates an example of the shape of the guide lines  66 . The guide lines  66  are not displayed in a region in which the guide lines  66  overlap the sixth marker  75   a  in  FIG.  18   . The shape of the guide lines  66  is not limited to such omission and may accommodate other design changes. Design changes includes changes of color, changes of transmittance, changes of line type such as a change to a broken line, or changes of line thickness, and flashing. The controller  33  may change the shape of the guide lines  66  when the sixth marker is not displayed. The controller  33  may change the shape of the guide lines  66  when the first marker  71  and the second marker  72  are displayed as the detection objects. 
     According to the display system  10  of the first embodiment as described above, a variety of markers corresponding to the detection objects detected in the first video image are superimposed on the second video image and displayed on the display apparatus  40 . This configuration enables the subject  60  to recognize a relative positional relationship between the moving body  50  and the detection objects at a glance. Thus, the convenience of technologies for displaying video images of the outside area of the moving body  50  is improved. 
     Examples of various video images that are generated by the image processing apparatus  30  by synthesizing a guide wall image  80  and other images on the display region of the first video image and displayed on the display apparatus  40  will be described in detail below with reference to  FIG.  19    to  FIG.  31   . 
     Sixth Embodiment 
       FIG.  19    illustrates a sixth example of the detection region  61  of the first video image acquired from the imaging apparatus  20  by the image processing apparatus  30 . The imaging apparatus  20  may be arranged in such a manner as to capture a rear area behind a vehicle as the moving body  50 . In the example illustrated in  FIG.  19   , the detection region  61  is longer in the left-right direction than the up-down direction. The display region  62  is positioned at the center of the left-right direction of the detection region  61 . The detection region  61  is not limited to such a shape longer in the left-right direction than in the up-down direction and may have a variety of shapes including a square, a rectangle in portrait orientation, a round shape, and the like. The display region  62  may be arranged in a variety of positions and have a variety of sizes and shapes. For example, the display region is not limited to being positioned at the center of the detection region  61  and may be shifted to the left or right. The display region  62  is not limited to a portion of the detection region  61  and may cover the detection region  61  in its entirety. The controller  33  may change the display region  62  of the first video image in accordance with a pinch-in operation and a pinch-out operation in respect of the display apparatus  40 . 
     The controller  33  generates the second image by synthesizing a guide wall image  80  (see  FIG.  20   ) that indicates the first predicted path  65  of the moving body  50  in the display region of the first video images acquired from the imaging apparatus  20 . The controller  33  may output images obtained by superimposing the guide wall image  80  on the second video images and cause the display apparatus  40  to display the images. The guide wall image  80  may indicate a predicted path when the moving body  50  moves rearward. The guide wall image  80  is a marker that presents a three-dimensional impression to the subject  60 . The guide wall image  80  may be considered to be an image in which a virtual guide wall for guiding to a predicted path arranged in the real space of the first video image is mapped on the first video images captured by the imaging apparatus  20 . The guide wall image  80  may be visually recognized as a plurality of virtual translucent wall surfaces that extend from the road surface to a predetermined height in the height direction within the field of view of the subject  60 . The guide wall image  80  can be considered to be a three-dimensional display of guide lines formed as walls displayed on the display apparatus  40  when the vehicle moves rearward. The display of the walls may include other modes such as surfaces, films, plates, and the like. The three-dimensional display of the guide wall image  80  enables the subject  60  to easily recognize a location of a parking space on the road, a distance to an obstacle, and the like. 
     The controller  33  can recognize a detection object in the display region  62  of the first video images. The controller  33  can detect the detection object located in the first predicted path  56  in the display region  62  of the first video images. When the detection object is detected, the controller  33  can synthesize a first recognition wall image  83  as a virtual plane on a detection object side of the moving body  50 . The first recognition wall image  83  moves together with the detection object when the detection object moves. 
       FIG.  20    illustrates an example of a second video image corresponding to the display region  62  of the first video image illustrated in  FIG.  19   . The guide wall image  80  is arranged spaced apart upward from the bottom edge of the second video image. The guide wall image  80  may include two sidewall images  81   a  and  81   b  that extend in the upward direction and the depth direction. The sidewall image  81   a  is positioned on the left side in the horizontal direction, and the sidewall image  81   b  is positioned on the right side in the horizontal direction. A region between the sidewall image  81   a  and the sidewall image  81   b  indicates the first predicted path  65  which the vehicle as the moving body  50  is expected to follow. The space between the sidewall image  81   a  and the sidewall image  81   b  may be set to indicate the width of the moving body  50 . The sidewall images  81   a  and  81   b  are linear in  FIG.  20   . The controller  33  may acquire the moving body information from the communication interface  31  and curve the sidewall images  81   a  and  81   b  on the basis of the predicted path. In this case, the moving body information includes a steering angle of the steering wheel. 
     The guide wall image  80  may include a plurality of distance wall images  82   a,    82   b,  and  82   c  that extend in the height direction and the horizontal direction and indicate the respective depth direction distances from the moving body  50 . The distance wall images  82   a,    82   b,  and  82   c  may connect the sidewall images  81   a  and  81   b  together.  FIG.  20    illustrates the three distance wall images  82   a,    82   b,  and  82   c.  The number of distance wall images is not limited to three and may be two or more than three. A depth-direction space between the distance wall image  82   a  and the distance wall image  82   b  may be smaller than a depth-direction space between the distance wall image  82   b  and the distance wall image  82   c.  The height-direction length of the distance wall images  82   a,    82   b,  and  82   c  may be either longer or shorter than the height-direction lengths of the sidewall images  81   a  and  81   b.    
     A frame may be displayed on an outer periphery of each of the sidewall images  81   a  and  81   b  and each of the distance wall images  82   a,    82   b,  and  82   c  of the guide wall image  80 . The guide wall image  80  may include at least one of an auxiliary line  84   a  that extends in the depth direction, an auxiliary line  84   b  that extends in the horizontal direction, and an auxiliary line  84   c  that extends in the height direction. The guide wall image  80  may include all of the auxiliary line  84   a  that extends in the depth direction, the auxiliary line  84   b  that extends in the horizontal direction, and the auxiliary line  84   c  that extends in the height direction. The auxiliary line  84   a  that extends in the depth direction may be displayed on the sidewall images  81   a  and  81   b.  The auxiliary line  84   b  that extends in the horizontal direction may be displayed on the distance wall images  82   a,    82   b,  and  82   c.  The auxiliary line  84   c  that extends in the height direction may be displayed on the sidewall images  81   a  and  81   b  and the distance wall images  82   a,    82   b,  and  82   c.  Each of the auxiliary lines  84   a,    84   b,  and  84   c  may be in any appropriate number. When a plurality of lines of each of the auxiliary lines  84   a ,  84   b,  and  84   c  are provided, each of the auxiliary lines  84   a,    84   b,  and  84   c  have equal spaces therebetween. The space between each of the plurality of auxiliary lines  84   a,    84   b,  and  84   c  may be varied according to conditions such as a height from the road surface, a distance from the moving body  50 , and the like. The frame displayed on the outer periphery of each of the sidewall images  81   a  and  81   b  and the distance wall images  82   a,    82   b,  and  82   c  may be considered as a portion of each of the plurality of auxiliary lines  84   a,    84   b,  and  84   c.  When a plurality of lines of each of the auxiliary lines  84   a,    84   b,  and  84   c  are provided, only one of them is appropriately denoted by the corresponding reference sign in  FIG.  20    and other diagrams described below. 
     When the distance wall image  82   a  is mapped in the real space captured in the first video images, the distance wall image  82   a  serves as a first distance wall image that is located closer to the moving body  50  than the wall images  82   b  and  82   c.  The distance wall image  82   a  may be in different color from the colors of the distance wall images  82   b  and  82   c.  For example, the distance wall images  82   b  and  82   c  may be in translucent white, and the distance wall image  82   a  may be in translucent red. The sidewall images  81   a  and  81   b  may be in translucent white. The color described above are set by way of example, and the sidewall images  81   a  and  81   b  may be in color different from the colors of the distance wall images  82   b  and  82   c.    
     The transmittances of the sidewall images  81   a  and  81   b  may be varied in the depth direction. The transmittances of the sidewall images  81   a  and  81   b  may be set to increase from the front side to the rear side in the depth direction. The color saturation and brightness of the sidewall images  81   a  and  81   b  may be varied in the depth direction. For example, the color saturation may decrease from the front side in the depth direction. The color saturation may also be referred to as color density. The sequential transition of the transmittance, color saturation, brightness, and chromaticity can be said to be gradation. 
     The transmittance of each of the distance wall images  82   a,    82   b,  and  82   c  may be varied in accordance with the height from the road surface. The transmittance of each of the distance wall images  82   a,    82   b,  and  82   c  may be set to increase as the height from the road surface in the height direction increases. The color saturation or the brightness of each of the distance wall images  82   a ,  82   b,  and  82   c  may be varied in accordance with the height from the road surface. For example, the color saturation may decrease as the height from the road surface increases. 
     The auxiliary lines  84   b  and  84   c  on the distance wall image  82   a  serving as the first distance wall image may be in the same color as the auxiliary lines  84   b  and  84   c  of the distance wall images  82   b  and  82   c.  The auxiliary lines  84   a,    84   b,  and  84   c  may be in any color. For example, when the wall surfaces of the guide wall image  80  except for the distance wall image  82   a  are in white, the auxiliary lines  84   a,    84   b,  and  84   c  may be in white having the brightness higher than the wall surfaces or having the transmittance lower than the wall surfaces. 
     The controller  33  may detect the vehicle  64  displayed in the display region  62  of the first video images as the detection object. In the example of  FIG.  20   , the vehicle  64  is located within the first predicted path  65 . The detection object detected by the controller  33  is not limited to a vehicle and may include, for example, a building, a fence, an obstacle on the road, a person, an animal, and the like. When the controller  33  detects a detection object, the controller  33  can synthesize the first recognition wall image  83  on a detection object side of the moving body  50 . When the controller  33  detects a detection object, the controller  33  may change the height-direction length of the guide wall image  80 . When the controller  33  detects a detection object, the controller  33  may reduce the height-direction length of the guide wall image  80 . In this way, the controller  33  can clarify the display when the first recognition wall image  83  is displayed and draw attention of the subject  60  to the detection object. 
     The first recognition wall image  83  of the detection object located within the first predicted path  65  extends in the height direction and the horizontal direction. The first recognition wall image  83  may be displayed as an opaque or translucent surface in any color. In  FIG.  20   , the first recognition wall image  83  is displayed as a translucent rectangle surface positioned on the side of the vehicle  64  opposing to the moving body  50 . The horizontal direction width of the first recognition wall image  83  may match the width of the vehicle  64 . The horizontal direction width of the first recognition wall image  83  may substantially match the width of the vehicle  64 . The height of the first recognition wall image  83  may be set to a predetermined value. The transmittance of the first recognition wall image  83  may be varied in accordance with the height from the road surface. The transmittance of the first recognition wall image  83  may be set to increase as the height from the road surface increases. The color saturation or the brightness of the first recognition wall image  83  may be varied in accordance with the distance from the road surface in the height direction. For example, the color saturation may decrease as the distance from the road surface in the height direction increases. The first recognition wall image  83  may include auxiliary lines that extend in the horizontal direction and the height direction. 
     The controller  33  may shift the auxiliary lines of the first recognition wall image  83 , which extend in the horizontal direction, in the height direction within the frame of the first recognition wall image  83 . The auxiliary lines may be displayed in such a manner as to appear from the bottom edge region of the frame of the first recognition wall image  83  and vanish at the top edge region of the frame. The controller  33  may display both of or any one of a relative position of the moving body  50  and the vehicle  64  and a change of a relative distance therebetween by shifting, in the height direction, the auxiliary line of the first recognition wall image  83  that extends in the horizontal direction. For example, when the distance between the moving body  50  and the vehicle  64  is decreasing, the controller  33  may shift, in the height direction, the auxiliary line that extends in the left-right direction at a high speed. The controller  33  may shift the auxiliary line in the height direction faster as the distance between the moving body  50  and the vehicle  64  is shorter. The controller  33  may shift, in the height direction, the auxiliary line of the first recognition wall image  83  that extends in the horizontal direction regardless of the location of, and the distance from, the detection object. 
     The color of the first recognition wall image  83  may be varied in accordance with the depth direction distance between the moving body  50  and the vehicle  64 . For example, when the distance between the moving body  50  and the vehicle  64  is longer than a predetermined distance as illustrated in  FIG.  20   , the controller  33  may display the first recognition wall image  83  in blue. When the moving body  50  moves rearward reducing the distance to the vehicle  64 , the controller  33  may change the color of the first recognition wall image  83  from blue to yellow and then to red. 
     When the distance between the moving body  50  and the vehicle  64  is longer than the predetermined distance, the controller  33  does not need to display the first recognition wall image  83 . When the moving body  50  and the vehicle  64  relatively approach each other and the distance therebetween reaches the predetermined distance, the controller  33  displays the first recognition wall image  83  after, for example, highlighting the vehicle  64  in the second video image. Such highlighting includes displaying the outline of the vehicle  64  in bold lines, flashing the outline of the vehicle  64 , painting the image of the vehicle  64 , or the like. The controller  33  may change the display mode in accordance with a change in the relative position with respect to the vehicle  64 . For example, when the vehicle  64  relatively moves away from the moving body  50 , the first recognition wall image  83  does not need to be displayed. 
       FIG.  21    is a diagram illustrating an example of the second video image in a state in which the distance between the moving body  50  and the vehicle  64  decreases when the moving body  50  moves rearward. The first recognition wall image  83  is displayed in red. Thus, the subject  60  can be warned about the risk that further rearward movement may lead to collision with the vehicle  64 . 
     When the detection object and the guide wall image  80  overlap with each other, the controller  33  may change the display of the guide wall image  80 . For example, the controller  33  may hide at least a portion of the guide wall image  80  that overlaps with the vehicle  64  in the second video image as illustrated in  FIG.  21   . In particular, the controller  33  does not display, in the display apparatus  40 , the portion of the guide wall image  80  that overlaps with the vehicle  64  and positioned remote from the moving body  50  in the depth direction from the vehicle  64  or the first recognition wall image  83 . Thus, the display apparatus  40  can display a simple and intuitive image to the subject  60 . The manner in which the display of the guide wall image  80  is changed is not limited to hiding an overlapping portion. For example, the controller  33  may increase the transmittance of the portion of the guide wall image  80  which overlaps with the detection object. 
     EXAMPLE VARIATIONS 
     The guide wall image  80  is not limited to the examples illustrated in  FIG.  20    and  FIG.  21   .  FIG.  22    to  FIG.  24    illustrate example variations of the guide wall image  80 . 
     First Example Variation 
     In an example illustrated in  FIG.  22   , one of the auxiliary lines  84   b  of each of the distance wall images  82   a,    82   b,  and  82   c  that extends in the horizontal direction and is positioned lowest in the height direction may be displayed. The distance wall image  82   a  that includes the auxiliary line  84   b  in the lowest position alone can enable easy recognition of the height from the ground surface. The distance wall image  82   a  that includes the auxiliary line  84   b  in the lowest position alone can enable easy recognition of the distance from the recognition wall image. At each end of the distance wall images  82   a,    82   b,  and  82   c  in the horizontal direction, an auxiliary line  84   c  that extends in the height direction may be displayed. On each of the outer peripheries of the distance wall images  82   a,    82   b,  and  82   c,  the auxiliary line  84   b  positioned at the bottom and the auxiliary lines  84   c  that are positioned at the horizontal ends and extends in the height direction at the left-right direction ends may be displayed. Each of the distance wall images  82   a,    82   b,  and  82   c  may be displayed by the auxiliary line  84   b  that extends in the horizontal direction at the bottom of the outer periphery and the auxiliary lines  84   c  that extend in the height direction at the left-right direction ends. 
     Each of the distance wall images  82   a,    82   b,  and  82   c  may include a semi-transparent surface. Each of the distance wall images  82   a,    82   b,  and  82   c  may have the transmittance that increases, or a color saturation that decreases, moving away from the road surface in the height direction. The auxiliary lines  84   b  and  84   c  positioned on the outer periphery of the distance wall image  82   a  closest to the moving body  50  may be displayed in color different from the other auxiliary lines. For example, the auxiliary lines  84   b  and  84   c  may be red. The other auxiliary lines may be, but are not limited to, white. The first recognition wall image  83  displayed in front of the vehicle  64  may be displayed as a semitransparent surface in any color such that the transmittance increases or the color saturation decreases moving away from the road surface in the height direction. The top edge boundary of the first recognition wall image  83  may be hidden such that the first recognition wall image  83  is displayed to gradually vanish in the height direction. 
     In the example illustrated in  FIG.  22   , two auxiliary lines  84   a  extending in the depth direction of the side wall images  81   a  and  81   b  of the guide wall image  80  at the top and bottom thereof are displayed. Auxiliary lines  84   a   1  positioned at the bottom may be displayed to standout more than auxiliary lines  84   a   2  positioned at the top. To make the line stand out, a bold line, a line with lower transmittance, a brighter line and the like may be used. The auxiliary lines  84   a   2  positioned at the top and the auxiliary lines  84   a   1  positioned at the bottom may be displayed by the lines in the same display mode. The lines in the same display mode include lines having similar thicknesses, transmittances, brightnesses, and the like. 
     Each of the auxiliary line  84   a  extending in the depth direction may be displayed so as to become gradually paler and vanish at the farthest position in the depth direction. The side wall images  81   a  and  81   b  may be displayed in such a manner that the edge regions thereof at the farthest position in the depth direction are visible. 
     The auxiliary lines  84   c  of the side wall images  81   a  and  81   b  extending in the height direction may be displayed at least at the edge regions of the most front side and positions intersecting with the distance wall images  82   a,    82   b,  and  82   c.  These auxiliary lines  84   c  extending in the height direction may be displayed to standout more than other auxiliary lines  84   c  extending in the height direction. The auxiliary lines  84   c  extending in the height direction may have different thicknesses. Among the auxiliary lines  84   c,  the auxiliary lines  84   c  displayed at the intersections with the distance wall image  82   a  may be bolder than the auxiliary lines  84   c  displayed at the intersections with the distance wall images  82   b  and  82   c.    
     Second Example Variation 
     The guide wall image  80  illustrated in  FIG.  23    is different from the guide wall image  80  of  FIG.  22    with respect to the side wall images  81   a  and  81   b.  In  FIG.  23   , at least two auxiliary lines  84   a  positioned at the top and bottom extending in the depth directions of the side wall images  81   a  and  81   b  of the guide wall image  80  may be displayed. Among the auxiliary lines  84   a  extending in the depth direction, the one positioned lowermost in the height direction may be displayed to standout more than the others. A plurality of auxiliary lines  84   a  may be displayed by the lines in the same display mode. The guide wall image  80  of  FIG.  23    may employ the variations in transmittance, color, and color arrangement that may be employed by the guide wall image  80  of  FIG.  22   . 
     Third Example Variation 
     The guide wall image  80  illustrated in  FIG.  24    is different from the guide wall image  80  of  FIG.  22    with respect to the side wall images  81   a  and  81   b.  In  FIG.  24   , the auxiliary lines  84   a  of the side wall images  81   a  and  81   b  of the guide wall image  80  that extend in the depth direction and positioned on the bottom are displayed. The auxiliary lines  84   a  of the guide wall image  80  that extend in the depth direction and positioned on the bottom may correspond to lines on the road surface mapped to the real space captured in the first video images together with the auxiliary lines  84   b  extending in the horizontal direction. The guide wall image  80  of  FIG.  24    may employ the variations in transmittance, color, and color arrangement that may be employed by the guide wall image  80  of  FIG.  22   . The auxiliary lines  84   c  extending in the height direction may have different thicknesses. The thicknesses of the auxiliary lines  84   c  arranged from the front side in the depth direction may decrease in the depth direction. 
     In the guide wall image  80  illustrated in each of  FIG.  20    to  FIG.  24   , the display apparatus  40  may omit some or all of the auxiliary lines  84   a,    84   b,  and  84   c.    
     Seventh Example 
       FIG.  25    illustrates a seventh example of the detection region  61  of the first video image obtained by the image processing apparatus  30  from the imaging apparatus  20 . In the example illustrated in  FIG.  25   , the detection region  61  is longer in the left-right direction than the up-down direction. The display region  62  is positioned at the center of the left-right direction of the detection region  61 . The display region  62  is not limited to a portion of the detection region  61  and may cover the detection region  61  in its entirety. 
     In the seventh example, the moving body  50  is a vehicle moving rearward toward a parking space in a parking lot. The imaging apparatus  20  may be positioned on the rear side of the moving body  50  and capture a rear area behind the moving body  50 . The controller  33  may detect, from the detection region  61 , each of the vehicle  64   b  and the pedestrian  63  displayed at least partially in the display region  62  outside of the first predicted path  65  and inside of the display region  62  as detection objects. In the example illustrated in  FIG.  25   , the controller  33  determines that the vehicle  64   a  in the first predicted path  65  of the moving body  50  is located farther than a predetermined depth direction distance and thus does not treat the vehicle  64   a  as the detection object. 
       FIG.  26    illustrates an example of the second video image corresponding to the display region  62  of the first video image illustrated in  FIG.  25   . The controller  33  recognizes the vehicle  64   b  and the pedestrian  63  as the detection objects located inside of the display region  62  of the first video image and displays the first recognition wall images  83   a  and  83   b  configured as virtual flat planes on the portions of the vehicle  64   b  and the pedestrian  63  opposing to the moving body  50 . Each of the first recognition wall images  83   a  and  83   b  may be displayed as a translucent planes parallel to the side wall images  81   a  and  81   b  extending in the height direction and the depth direction. The controller  33  may acquire changes in the distances between the relative positions of the vehicle  64   b  and the moving body  50  and between the relative positions of the pedestrian  63  and the moving body  50  and display the side wall images  81   a  and  81   b  when the relative positions change in a direction reducing the distance. In a manner similar to the above, any appropriate algorithm may be employed for the estimation of the distance between the moving body  50  and the detection objects. 
     The controller  33  may estimate the horizontal direction distances between the vehicle  64   a  and the pedestrian  63  as the detection objects and the guide wall image  80  and determine the colors of the first recognition wall images  83   a  and  83   b  on the basis of the estimated distances. Here, the distances between the detection objects and the guide wall image  80  refers to distances between the detection objects and a virtual guide wall displaying the guide wall image  80  that is assumed to be present in the real space in the first video image. The controller  33  may estimate the first distance between positions of the detection objects and the position of the guide wall image  80  on the basis of a position in which the guide wall image  80  is mapped in the real space captured in the first video image and the positions of the detection objects. The controller  33  may provide a threshold of the first distance and cause the display apparatus  40  to display each of the first recognition wall images  83   a  and  83   b  in different colors such as blue, yellow, red, or the like in the stated order from one with a longer horizontal direction distance. Each of the first recognition wall images  83   a  and  83   b  may include an auxiliary line that extends in the depth direction. The controller  33  may shift the auxiliary lines of the first recognition wall images  83   a  and  83   b,  which extend in the depth direction, in the height direction within the frame of the first recognition wall image  83  in a manner similar to the auxiliary lines of the first recognition wall image  83  of  FIG.  20    that extend in the horizontal direction. 
     The controller  33  estimates first positions as the depth direction locations of the vehicle  64   b  and the pedestrian  63  as the detection objects. The controller  33  may at least partially change the display of the side wall images  81   a  and  81   b  opposing the vehicle  64   b  and the pedestrian  63  in correspondence with the first positions. The change in the display mode includes a change of color of the wall surface, brightness, color saturation, color or thickness of the frame and/or auxiliary line, or any combination thereof. For example, colors of a portion  81   a   1  of the side wall image  81   a  opposing the vehicle  64   b  and a portion  81   b   1  of the side wall image  81   b  opposing the pedestrian  63  are changed in  FIG.  26   . The portions of the side wall images  81   a  and  81   b  may correspond to regions defined by the auxiliary lines  84   c  of the side wall images  81   a  and  81   b  that extend in the height direction. The colors of the portions  81   a   1  and  81   b   1  of the side wall images  81   a  and  81   b  may be varied on the basis of the first distance. When the detection object is a large obstacle relative to the length indicated by the side wall images  81   a  and  81   b,  the controller  33  may change the colors of the side wall image  81   a  or the side wall image  81   b  in its entirety. Such a large obstacle may include a large vehicle such as a trailer, building, or the like. 
     When the detection object moves, the controller  33  may estimate the second predicted path of the detection object and estimate the first position as a position at which the second predicted path and the side wall images  81   a  and  81   b  intersect with one another. The controller  33  may estimate the first position as a position at which the second predicted path in the real space captured in the first video images and the side wall images  81   a  and  81   b  mapped to the real space intersect with one another. The controller  33  may change the display of the side wall images  81   a  and  81   b  at least partially on the basis of the first position. In this case also, the colors of the portions  81   a   1  and  81   b   1  of the side wall images  81   a  and  81   b  may be changed on the basis of the first distance. When the detection object is a very large obstacle, the controller  33  may change the colors of the side wall image  81   a  or the side wall image  81   b  in their entirety. 
     In another display mode, the controller  33  may change the display of portions of the side wall images  81   a  and  81   b  positioned farther in the depth direction from the first recognition wall images  83   a  and  83   b.  In yet another display mode, the controller  33  may change the display of portions of the side wall images  81   a  and  81   b  positioned farther in the depth direction from positions where the second predicted paths and the side wall images  81   a  and  81   b  intersect with one another. 
     The display of the first recognition wall images  83   a  and  83   b  may be changed on the basis of their heights from the road surface. The transmittance, color saturation, brightness and the like of the first recognition wall images  83   a  and  83   b  may be varied on the basis of the heights thereof from the road surface, in a manner similar to the first recognition wall image  83  of the sixth example. 
     As described above, the display system  10  can spatially display the distance to and/or location of a detection object that can be an obstruction to the rearward movement in a three-dimensional manner by using the guide wall image  80  and the first recognition wall images  83   a  and  83   b.  Also, from the predicated path of the detection object in motion, a position where there is a risk of collision may be displayed in a three-dimensional manner. As described above, the display system  10  can spatially display a detection object and warning content in a three-dimensional manner by displaying each of the guide wall image  80  and the first recognition wall images  83   a  and  83   b  as a wall (or a plane) having a height. Thus, the subject  60  can easily recognize the display and the warning content. 
     Eighth Example 
       FIG.  27    illustrates an eighth example of the detection region  61  of the first video image acquired from the imaging apparatus  20  by the image processing apparatus  30 . The imaging apparatus  20  may be arranged to capture a rear area behind the moving body  50  as the vehicle. In the example illustrated in  FIG.  27   , the detection region  61  is longer in the left-right direction than the up-down direction. The display region  62  is positioned at the center of the left-right direction of the detection region  61 . The controller  33  may detect each of the pedestrian  63  and the vehicle  64   b  as the detection objects captured in the second region inside of the detection region  61  and outside of the display region  62  of the first video image. In the eighth example, the controller  33  determines that a depth direction distance to the vehicle  64   a  captured in the display region  62  is longer than a predetermined distance and thus does not treat the vehicle  64   a  as a detection object. 
       FIG.  28    is a diagram schematically illustrating locations of the moving body  50  and the pedestrian  63  and the vehicle  64   b  as the detection objects viewed from above along the height direction in the real space captured in the first video image. The imaging apparatus  20  can capture subjects within regions F 1  and F 2  in the real space. The regions F 1  and F 2  correspond to the detection region  61  of the first video image. The region F 1  is the first region corresponding to the display region  62  of the first video image. The region F 2  corresponds to the second region inside of the detection region  61  of the first video image and outside of the display region  62 . 
       FIG.  28    illustrates a virtual guide wall  90  in broken lines obtained by mapping the guide wall image  80  of the second video image in the real space, for the purpose of illustration. Virtual side walls  91   a  and  91   b  correspond to the side wall images  81   a  and  81   b  displayed in the second video image, respectively. Virtual distance walls  92   a,    92   b,  and  92   c  correspond to the distance wall images  82   a,    82   b,  and  82   c  in the second video image, respectively. 
     In  FIG.  28   , also, each of virtual lines  95   l   1  to  95   l   5  and  95   r   1  to  95   r   5  shown as broken lines indicates a horizontal direction distance between the virtual side walls  91   a  and  91   b  of the moving body  50  and the detection object, for the purpose of illustration. The horizontal direction distance is also referred to as a second distance. The virtual lines  95   l   1  to  95   l   5  are positioned on the left side of the moving body  50 . The virtual lines  95   r   1  to  95   r   5  are positioned on the right side of the moving body  50 . Intervals between the virtual lines  95   l   1  to  95   l   5  and between the virtual lines  95   r   1  to  95   r   5  may be appropriately determined. Intervals between the virtual lines  95   l   1  to  95   l   5  and between the virtual lines  95   r   1  to  95   r   5  may be equal to one another. The controller  33  can determine which one of the virtual lines  95   r   1  to  95   r   5  and  95   r   1  to  95   r   5  is closest to the detection object, or which two of the virtual lines  95   l   1  to  95   l   5  and  95   r   1  to  95   r   5  include the detection object in a region therebetween, on the basis of the first video image. 
     The second video image corresponding to the first video image of  FIG.  27    is illustrated in  FIG.  29    by way of example. In the eighth example, the guide wall image  80  may be displayed in a display method similar to those described in the sixth example and the seventh example. When the detection objects are located in the second region of the first video image, the controller  33  synthesizes the second recognition wall images  89   a  and  89   b  in the edge region on the side where the detection objects are located in the second video image. The second recognition wall images  89   a  and  89   b  may include wall portions  96   a  and  96   b  extending in the height direction and the depth direction and floor portions  97   a  and  97   b  extending in the depth direction and the horizontal direction. The wall portions  96   a  and  96   b  extend from the bottom edge region to top edge region in the left-side and right-side edge regions of the second video image in  FIG.  29   . The wall portions  96   a  and  96   b  may extend partially in the up-down direction at the left-side and right-side edge regions of the second video image instead of extending throughout the up-down direction. Similarly, the floor portions  97   a  and  97   b  may extend partially or entirely in the left-right direction at the bottom edge of the second image. The floor portions  97   a  and  97   b  may be displayed between the guide wall image  80  and the bottom edge of the second video image. 
     The controller  33  may change the displays of the second recognition wall images  89   a  and  89   b  on the basis of the second distances of the detection objects. The change of display includes change of color, transmittance, color saturation, brightness, size, as well as dynamic display methods including flashing and moving. In one of a plurality of embodiments, the change of display may be, for example, a change of color. As for the display color, red, yellow, and blue may correspond to a high risk, a medium risk, and a low risk, respectively. The pedestrian  63  illustrated in  FIG.  28    is located on the virtual line  95   r   3  which is the third to the right side in the horizontal direction from the virtual side wall  91   b  of the moving body  50 . In this case, the controller  33  may determine that there is a medium risk and display the second recognition wall image  89   b  on the right side in yellow. Also, the vehicle  64   b  illustrated in  FIG.  28    is located remote from the virtual line  95   l   5  which is fifth to the left side in the horizontal direction from the virtual side wall  91   a  of the moving body  50 . In this case, the controller  33  may determine that there is a low risk and display the second recognition wall image  89   a  on the left side in blue. In one of the plurality of embodiments, the change of the displays may include a lateral movement of the auxiliary lines  96   a   1  and  96   b   1  that vertically extend in the wall portions  96   a  and  96   b.  The controller  33  may change the moving speed of the wall portions  96   a  and  96   b  in accordance with the length of the second distance. 
     The second recognition wall images  89   a  and  89   b  may be semitransparent. Display characteristics of each of the wall portion  96   a  of the second recognition wall image  89   a  and the wall portion  96   b  of the second recognition wall image  89   b  may be varied in the up-down direction. For example, each of the wall portion  96   a  of the second recognition wall image  89   a  and the wall portion  96   b  of the second recognition wall image  89   b  may have a transmittance that decreases in the downward direction. Also, each of the second recognition wall image  89   a  and the second recognition wall image  89   b  may have a color density that increases in the upward direction. Display characteristics of each of the floor portion  97   a  of the second recognition wall images  89   a  and the floor portion  97   b  of the second recognition wall images  89   b  may be varied in the left-right direction. For example, the floor portion  97   a  of the second recognition wall image  89   a  may have a transmittance that increases from the left side toward the center of the second video image, and the floor portion  97   b  of the second recognition wall image  89   b  may have a transmittance that increases from the right side toward the center of the second video image. Also, the floor portion  97   a  of the second recognition wall image  89   a  may have a color saturation that decreases from the left side toward the center of the second image, and the floor portion  97   b  of the second recognition wall image  89   b  may have a color saturation that decreases from the right side toward the center of the second image. The color saturation can also be referred to as a color density of the image. 
     The controller  33  may determine the types of the detection objects and display icon images  99   a  and  99   b  corresponding to the types on the floor portions  97   a  and  97   b  of the second recognition wall images  89   a  and  89   b.  A known technology may be employed for the determination of the type on the basis of the image of the detection object detected in the first video images. For example, the controller  33  may determine the type of an object by performing pattern matching of an outline shape of the detection object using a model pattern. For example, an icon of a person motif may be selected for the pedestrian  63 , and an icon of a vehicle motif may be selected for the vehicle  64   b . The controller  33  can estimate the second distance in the horizontal direction between the virtual side walls  91   a  and  91   b  obtained by mapping the side wall images  81   a  and  81   b  in the real space and the detection object in the real space. The second distance can be determined on the basis of a positional relationship between each line obtained by mapping the virtual lines  95   l   1  to  95   l   5  and  95   r   1  to  95   r   5  in the real space onto the detection region  61  of the first video image and the image of each of the detection objects. The controller  33  may change the display positions of the icon images  99   a  and  99   b  on the basis of the second distance. 
     In the display example of the second video image illustrated in  FIG.  29   , first distance recognition lines  98   l   1  to  98   l   5  and  98   r   1  to  98   r   5  are provided in a manner corresponding to the virtual lines  95   l   1  to  95   l   5  and  95   r   1  to  95   r   5  of the real space, respectively. The first distance recognition lines  98   l   1  to  98   l   5  are positioned on the left side of the side wall image  81   a  and arranged sequentially from the center to the left side. The first distance recognition lines  98   r   1  to  98   r   5  are positioned on the right side of the side wall image  81   b  and arranged sequentially from the center to the right side. Each of the first distance recognition lines  98   l   1  to  98   l   5  and  98   r   1  to  98   r   5  indicates a display position of the icon images  99   a  and  99   b  and does not need to be displayed in the second video image. 
     In  FIG.  28   , the pedestrian  63  is located on the virtual line  95   r   3 , which is the third on the right side in the horizontal direction from the virtual side wall  91   b  of the moving body  50 . Thus, the controller  33  may display the icon image  99   b  of the pedestrian  63  on the first distance recognition line  98   r   3 , which is the third on the right side, in the second video image of  FIG.  29   . As illustrated in  FIG.  29   , the icon image  99   b  may be displayed in the three-dimensional manner as a surface having a depth-direction length and a height-direction length on the first distance recognition line  98   r   3  extending in the depth direction in the second video image. When viewed stereoscopically, the icon image  99   b  may be displayed as a surface parallel to the sidewall images  81   a  and  81   b  and the second recognition wall images  89   a  and  89   b.  The display mode of the icon image  99   b  such as the color, color saturation, and brightness may be changed on the basis of the second distance, in a manner similar to the second recognition wall  89   b.  For example, the icon image  99   b  may be displayed in yellow. 
     In  FIG.  28   , the vehicle  64   b  is located remote from the virtual line  95   l   5  which is the fifth on the left side in the horizontal direction from the virtual side wall  91   a  of the moving body  50 . In this case, the controller  33  may position the icon image  99   a  of the vehicle  64   b  on the first distance recognition line  9815  positioned on the left side in the second video image of  FIG.  29   . Alternatively, the controller  33  may arrange the icon image  99   a  such that the icon image  99   a  seems to be positioned on the same surface as the wall portion  96   a  of the second recognition wall image  89   a  when the second video image is viewed stereoscopically. The display mode of the icon image  99   a  may be changed on the basis of the second distance, in a manner similar to the icon image  99   b.  For example, the icon image  99   a  may be displayed in blue. 
     When one or both of the moving body  50  and the detection objects move and the position of the detection object relative to the moving body  50  changes, the position and the display mode of each of the second recognition wall images  89   a  and  89   b  and each of the icon images  99   a  and  99   b  also change. When the moving body  50  enters the display region  62 , the detection objects may be displayed in the display mode as described in the seventh example. An example of a change in the display mode when the vehicle  64   b  moves from the region F 2  to the region F 1  in the real space illustrated in  FIG.  28    will be described. 
     First, it is assumed that the vehicle  64   b  is located on the left side of the virtual line  9515  in the real space in  FIG.  28   . At this time, as described above, the second recognition wall image  89   a  is displayed in the second video image of  FIG.  29    and, simultaneously, the icon image  99   a  is displayed on the leftmost side of the floor portion  98   a  of the second recognition wall image  89   a.  At this time, the second recognition wall image  89   a  and the icon image  99   a  may be displayed in blue. 
     In the real space of  FIG.  28   , as the vehicle  64   b  approaches closer to the virtual guide wall  90 , the display mode of the second recognition wall image  89   a  displayed in the second video image of  FIG.  29    is changed. For example, the color of the second recognition wall image  89   a  is sequentially changed from blue to yellow and then to red. Concurrently, the display position of the icon image  99   a  is moved from the first distance recognition line  98   l   5  to the first distance recognition line  98   l   1 . The color of the icon image  99   a  may be sequentially changed from blue to yellow and then red. Thus, the subject  60  can recognize that the vehicle  64   b  is approaching outside of the second video image. Also, because the three-dimensional display is employed, the subject  60  can spatially recognize the approach of the vehicle  64   b,  and the probability of oversight of information can be reduced. 
     When the vehicle  64   b  is approaching the moving body  50 , the icon image  99   a  may be moved toward the center of the image. The subject  60  can recognize the moving direction of the vehicle  64   b  by viewing the movement of the icon image  99   a.    
     Further, when the vehicle  64   b  enters the region F 1  from the region F 2  in the real space of  FIG.  28   , the vehicle  64   b  enters the display region of the first video images and thus is included in the second video images and displayed on the display apparatus  40 . Also, the second recognition wall image  89   a  is hidden, and the first recognition wall image  83   a  is displayed on the side of the vehicle  64   b  opposing the guide wall image  80  as illustrated in  FIG.  26   . When the second recognition wall image  89   a  is hidden, the controller  33  may flash the second recognition wall image  89   a  a plurality of times so as to alert the subject  60 . The controller  33  displays the first recognition wall image  83   a  on the side of the vehicle  64   b  opposing to the moving body  50 . Further, the controller  33  estimates the second predicted path of the vehicle  64   b  and estimates the first position where the second predicted path and the side wall images  81   a  and  81   b  intersect with one another. The controller  33  may change the display mode of the portion  81   a   1  of the side wall image  81   a  that includes the first position. The controller  33  may sequentially change the color of the portion  81   a   1  of the side wall image  81   a  from, for example, blue to yellow and then to red, as the vehicle  64   b  approaches closer. This enables the subject  60  to easily recognize that the vehicle  64   b  is approaching and also that the second predicted path of the vehicle  64   b  overlaps with the first predicted path  65  of the moving body  50 . 
     According to the one of the plurality of embodiments of the present disclosure, as described above, the three-dimensional displays of the guide wall image, the first recognition wall image, and the second recognition wall image facilitate the spatial recognition of a position of a detection object. According to the one of the plurality of embodiments of the present disclosure, also, the change of the display mode of each of the wall surfaces facilitates recognition of information such as a caution and warning. Further, the three-dimensional display can reduce the probability of oversight of the information. 
     Example Variation 
       FIG.  30    illustrates another example of the second video image corresponding to the first video image of  FIG.  27   . View display regions  101  and  102  that indicate a selection state between “Normal View Display” and “Wide View Display” are added to the lower left portion in  FIG.  30   . The “Normal View Display” and the “Wide View Display” will be described later. Some of the display modes of the guide wall image  80 , the second recognition wall images  89   a  and  89   b,  and the icon images  99   a  and  99   b  in  FIG.  30    may be different from those of  FIG.  29   . For example,  FIG.  30    illustrates the guide wall image  80  in the display mode similar to that of  FIG.  22   . 
     The wall portions  96   a  and  96   b  of the second recognition wall images  89   a  and  89   b  may include at least auxiliary lines  96   a   1  and  96   b   1 , respectively, that extend in the height direction. One of the plurality of auxiliary lines  96   a   1  of the wall portion  96   a  and one of the plurality of auxiliary lines  96   b   1  of the wall portion  96   b  are denoted with the respective reference signs in  FIG.  30   . The wall portions  96   a  and  96   b  of the second recognition wall images  89   a  and  89   b  may indicate the distance to the detection object by using a color and may have transmittance that varies from the front side to the rear side in the depth direction. The wall portions  96   a  and  96   b  may have transmittances that gradually increase in the height direction or may have color saturations that gradually decrease and vanish at a predetermined height position. 
     By changing the transmittance and the color saturation of the wall portions  96   a  and  96   b  and the display modes of the auxiliary lines  96   a   1  and  96   a   2 , the controller  33  can indicate that an object is present within the predetermined distance range and also indicate a distance to the detection object or a change of the distance to the detection object. The change of the transmittance or the color saturation of the wall portions  96   a  and  96   b  may be referred to as change in gradation. For example, the controller  33  may indicate that a detection object is present within the predetermined distance by moving the auxiliary lines  96   a   1  and  96   a   2  of the wall portions  96   a  and  96   b.  The controller  33  may indicate that a detection object is present within the predetermined distance by increasing the color saturations of the front portions of the wall portions  96   a  and  96   b  to a maximum and gradually reducing their color saturations in the depth direction. The auxiliary lines  96   a   1  and  96   b   1  of the wall portions  96   a  and  96   b  and the changes in their transmittances are not essential. The controller  33  may omit the auxiliary lines  96   a   1  and  96   b   1  from the wall portions  96   a  and  96   b.  The controller  33  does not need to display changes (the gradations) in the transmittances of the wall portions  96   a  and  96   b.    
     The floor portions  97   a  and  97   b  of the second recognition wall images  89   a  and  89   b  are provided with one or more second distance recognition lines  100   l  and  100   r,  respectively, in addition to the first distance recognition lines  98   l  and  98   r.  In  FIG.  30   , a plurality of the second distance recognition lines  1001 ,  100   r  exist but for each only one reference sign is denoted. The first distance recognition lines  98   l  and  98   r  and the second distance recognition lines  100   l  and  100   r  do not need to be displayed on the display. The first distance recognition lines  98   l  and  98   r  may correspond to the horizontal direction distances between the moving body  50  and the detection objects, and the second distance recognition lines  100   l  and  100   r  may correspond to the depth direction distances between the moving body  50  and the detection objects. Each of the plurality of the second distance recognition walls  100   l  and  100   r  may correspond to the depth direction distance when the auxiliary lines  84   c  of the side wall images  81   a  and  81   b  extending in the height direction are mapped in the real space. 
     The subject  60  can recognize the horizontal direction distance and the depth direction distance to the detection object on the basis of the positions of the icon images  99   a  and  99   b  on the floor portions  97   a  and  97   b  with respect to the horizontal direction and the depth direction. The icon images  99   a  and  99   b  are three-dimensional images corresponding to the types of the detection objects. The floor portions  97   a  and  97   b  may be positioned on the front side of the front ends of the side wall images  81   a  and  81   b  and within the lines extended from the side wall images  81   a  and  81   b . The positional relationship between the second recognition wall images  89   a  and  89   b  and the guide wall image  80  is not limited thereto and may be changed appropriately. 
     In  FIG.  30   , the display apparatus  40  displays the view display regions  101  and  102  that indicate the selection status between the “Normal View Display” and the “Wide View Display” in the lower right portion. The “Normal View Display” and the “Wide View Display” displays a video image in different angles. In one of a plurality of embodiments, the “Normal View Display” corresponds to the angle of the display region  62  of the second video image illustrated in  FIG.  4   . In one of a plurality of embodiments, the “Wide View Display” corresponds to the angle of the detection region  61  of the first video image illustrated in  FIG.  4   . The “Wide View Display” does not need to display the detection region  61  of the first video image in its entirety and may display a video image in an angle that includes the “second video image” extracted from the detection region  61 . The second video images illustrated in and before  FIG.  30    are displayed in the “Normal View Display”. Which one of the “Normal View Display” and the “Wide View Display” is selected for the video image being displayed is indicated by changing the display of the view display regions  101  and  102  indicating the selection status. For example, the change of the display of the view display regions indicating the selection status includes a change of the color, the color saturation, and the color of characters of the view display regions  101  and  102 . 
     The controller  33  may change between the “Normal View Display” and the “Wide View Display” according to an image processing result of the first video image, or upon receiving an input in respect of the display apparatus  40  from the subject  60  and the like. For example, when the controller  33  detects the approach of a detection object in the “Normal View Display”, the controller  33  may change the displayed video image to the “Wide View Display”. When the controller  33  does not detect a detection object within the predetermined region in the “Wide View Display”, the controller  33  may change the displayed video image to the “Normal View Display”. When the display apparatus  40  includes a touch panel screen, the controller  33  may change the display mode upon receiving a signal from the display apparatus  40  indicating that the subject  60  has performed a selection by touching the view display region  101  or  102  on the touch panel screen. 
       FIG.  31    is a diagram illustrating an example of a video image that has been changed from the “Normal View Display” illustrated in  FIG.  30    to the “Wide View Display”. The view angle of the video image in the “Wide View Display” is wider in the horizontal direction than that in the “Normal View Display” in  FIG.  30   . In  FIG.  31   , the pedestrian  63  and the vehicle  64   b  located outside of the display region in the “Normal View Display” illustrated in  FIG.  30    are displayed. In the video image illustrated in  FIG.  31   , because the view angle is wider in the horizontal direction than that in the “Normal View Display” of  FIG.  30   , the space between the side wall images  81   a  and  81   b  on the left right sides of the guide wall image  80  is relatively reduced. 
     In  FIG.  31   , because the pedestrian  63  and the vehicle  64   b  are within the display region, the second recognition wall images  89   a  and  89   b  are not displayed. The controller  33  may recognize the vehicle  64   b  and the pedestrian  63  as the detection objects within the detection region  61  of the first video image and display the first recognition wall images  83   a  and  83   b  as virtual flat planes on the sides of the vehicle  64   b  and the pedestrian  63  opposing to the moving body  50 . The controller  33  estimates the locations of the vehicle  64   b  and the pedestrian  63  as the detection objects in the depth direction. On the basis of the estimated locations, the controller  33  may change the display of the side wall images  81   a  and  81   b  opposing to the vehicle  64   b  and the pedestrian  63 . In  FIG.  31   , the display of the portion  81   b   1  of the side wall image  81   b  and the portion  81   a   1  of the side wall image  81   a  are changed in correspondence with the pedestrian  63  and the vehicle  64   b,  respectively. Changes of display includes a variety of aspects. For example, the change of display includes changes of color, changes of thickness of the auxiliary lines surrounding the outer periphery, changes of type of the lines, starting and stopping flashing, change to a flashing cycle, or the like. The display methods and the display modes of the first recognition wall images  83   a  and  83   b,  the portion  81   a   1  of the side wall image  81   a,  and the portion  81   b   1  of the side wall image  81   b  may be similar to those of the example of  FIG.  26   . 
     According to the one of the plurality of embodiments as illustrated in  FIG.  30    and  FIG.  31   , the video image captured by the imaging apparatus  20  may be displayed on the display apparatus  40  in the “Normal View Display” or the “Wide View Display” having different view angles in a switchable manner. The “Normal View Display” enables the subject  60  to easily recognize the presence and the location of the detection object located outside of the region of the video image being displayed by viewing the second recognition wall images  89   a  and  89   b  and the icon images  99   a  and  99   b.  The autonomous or manual switchover between the “Normal View Display” and the “Wide View Display” by the display apparatus  40  enables the subject  60  to recognize the detection object, detected outside of the image in the “Normal View Display”, in the video image. The “Wide View Display” enables stereoscopic recognition of the positional relationship with the detection object by displaying the guide wall image  80  and the first recognition wall image  83   b.  In this way, the display system  10  according to the one of the plurality of embodiments can enable the subject  60  to easily grasp the surrounding situation and be aware of risks. 
     Another Example 
       FIG.  32    illustrates another example of the second video image.  FIG.  32    corresponds to  FIG.  24   . In  FIG.  32   , the guide wall image  80  is displayed by white lines. In  FIG.  32   , the first recognition wall image  83  is displayed by white lines. In the example of  FIG.  32   , the second video image includes a first semitransparent image  80   a.  In the example of  FIG.  32   , the second video image includes a second semitransparent image  83   c.    
     The controller  33  may generate the second video image by sequentially superimposing a first image and a second image on the display region of the first video images obtained from the imaging apparatus  20 . The controller  33  may generate a synthesized image by sequentially superimposing the first image and the second image in the stated order on each frame image of the first video images. The controller  33  may output the second video images and cause the display apparatus  40  to display the second video images. The second images may include the guide wall image  80  that indicates the first predicted path  65  of the moving body  50 . The first images include the first semitransparent image  80   a  (a transmittance gradation image) that has a transmittance that gradually increases moving in the upward direction. The controller  33  may superimpose the guide wall image  80  on the first semitransparent image  80   a.  The controller  33  may superimpose the first semitransparent image  80   a  on the display region of the first video images. The controller  33  may synthesize the first semitransparent image  80   a  between the display region of the first video images and the guide wall image  80 . The first semitransparent image  80   a  has a transmittance that gradually that decreases moving in the downward direction. The first semitransparent image  80   a  changes its color to transparent toward the top edge region from the bottom edge region. The first semitransparent image  80   a  gradually changes its color to transparent from a second color different from a first color of the guide wall image  80 . The first color may include, for example, white or cyan. The second color may include, for example, black. 
     By virtue of the color difference between the first color and the second color, the guide wall image  80  can be easily viewed. By virtue of the color difference between the first color and the second color, the guide wall image  80  can be easily viewed in a situation where the first video image includes a large amount of color that is similar to the first color. For the guide wall image  80  illustrated in the example of  FIG.  32   , the first semitransparent image  80   a  facilitates recognition on a ground surface covered with snow. Because the guide wall image  80  has transmittance that increases in the upward direction, the second image becomes similar to the first image in an upper portion of the guide wall image  80 . The second video image does not include a large amount of added color in regions displaying remote areas. The second video images have less impact on the visibility of the regions on the rear side of the guide wall image  80 . Because the first semitransparent image  80   a  has transmittance that increases from the lower side to the upper side, it becomes easier to ascertain a sense of depth in the second video images. Because the first semitransparent image  80   a  has transmittance that increases from the lower side to the upper side, the second video image conveys a bright impression. 
     The controller  33  may generate the second video images by sequentially superimposing the third image and the fourth image on the display region of the detection object recognized in the first video image. The fourth image may include the first recognition wall image  83  that indicates the presence of the recognized detection object. The third image includes the second semitransparent image  83   c.  The controller  33  may superimpose the first recognition wall image  83  on the second semitransparent image  83   c.  The controller  33  may superimpose the second semitransparent image  83   c  on the display region of the detection object. The controller  33  may synthesize the second semitransparent image  83   c  between the display region of the detection object and the first recognition wall image  83 . The second semitransparent image  83   c  has a transmittance that gradually decreases in the downward direction. The second semitransparent image  83   c  changes its color to transparent toward its top edge region from its bottom edge region. The second semitransparent image  83   c  gradually changes its color to transparent from a fourth color different from a third color of the first recognition wall image  83 . The second semitransparent image  83   c  gradually changes from the fourth color different from the third color of the first recognition wall image  83  to a transparent color. The third color may include, for example, red, yellow, and white or cyan. The second color may include, for example, black. 
     By virtue of the color difference between the third color and the fourth color, the first recognition wall image  83  can be easily recognized and, also, the first recognition wall image  83  may be easily recognized when a detection object is of color similar to the third color. 
     The presence of the second semitransparent image  83   c  facilitates recognition of the first recognition wall image  83  in white as illustrated in the example of  FIG.  32    when the first recognition wall image  83  is displayed in front of a white track. 
     Although the present disclosure has been described based on various figures and embodiments, it should be appreciated that those who are skilled in the art may easily perform various changes or modifications on the basis of the present disclosure. Accordingly, such changes and modifications are included in the scope of the present disclosure. For example, a function included in each means or each step may be rearranged, avoiding a logical inconsistency, such that a plurality of means or steps are combined, or a means or a step is subdivided. 
     For example, each constituent element and function of the display system  10  in the above embodiment may be rearranged. For example, some or all of the configurations and functions of the image processing apparatus  30  may be included in at least one of the imaging apparatus  20  and the display apparatus  40 . 
     Some of the constituent elements of the display system  10  in the above embodiment may be positioned outside of the moving body  5 . For example, the image processing apparatus  30  and the like may be configured as a communication apparatus such as a mobile phone or an external server and connected to other constituent elements of the display system  10  in a wired or wireless manner. 
     REFERENCE SIGNS LIST 
       10  display system 
       20  imaging apparatus 
       21  imaging optical system 
       22  image sensor 
       23  communication interface 
       24  controller 
       30  image processing apparatus 
       31  communication interface 
       32  memory 
       33  controller 
       40  display apparatus 
       50  moving body 
       51  network 
       60  subject 
       61  detection region 
       62  display region 
       63 ,  63   a,    63   b,    63   c,    63   d  pedestrian 
       64 ,  64   a,    64   b  vehicle 
       65  first predicted path 
       66  guide line 
       67  icon image 
       68  third marker 
       69  outline 
       70  region 
       71  first marker 
       72  second marker 
       73  fifth marker 
       73   a  icon image 
       73   b  band image 
       73   c  right-side edge region 
       74  obstacle image 
       75   a,    75   b,    75   c  sixth marker 
       80  first semitransparent image 
       81   a,    81   b  side wall image 
       81   a   1 ,  81   b   1  portion of side wall image 
       82   a,    83   b,    82   c  distance wall image 
       83 ,  83   a,    83   b  first recognition wall image 
       83   c  second semitransparent image 
       84   a,    84   b,    84   c  auxiliary line 
       89   a,    89   b  second recognition wall image 
       90  virtual guide wall 
       91   a,    91   b  virtual side wall 
       92   a,    92   b,    92   c  virtual distance wall 
       95   l   1  to  95   l   5 ,  95   r   1  to  95   r   5  virtual line 
       96   a,    96   b  wall portion 
       97   a,    97   b  floor portion 
       98   l   1  to  98   l   5 ,  98   r   1  to  98   r   5  first distance recognition line 
       99   a,    99   b  icon image 
       1001 ,  100   r  second distance recognition line 
       101 ,  102  view display region