Patent Publication Number: US-11377824-B2

Title: Display system for work vehicle and generation method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2019/003013, filed on Jan. 29, 2019. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2018-027202, flied in Japan on Feb. 19, 2018, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a display system for a work vehicle and a generation method. 
     BACKGROUND INFORMATION 
     There is a technology in which the surroundings of a work vehicle are captured by a camera and a bird&#39;s-eye view image looking down from above the surroundings of the work vehicle is displayed on a display. For example, in the display system described in International Publication WO 2016-031009, a plurality of cameras mounted on a work vehicle acquire image data of the surrounding environment of the work vehicle. A controller of the display system generates a bird&#39;s-eye view image by mapping the acquired images onto a projection model in a hemispherical shape. 
     SUMMARY 
     In the aforementioned display system, the shape of the projection model is fixed as a hemispherical shape. As a result, it is difficult to understand the actual shape of the surrounding environment of the work vehicle from the bird&#39;s-eye view image. For example, the bottom surface of the projection model is always a flat plane. As a result, even if the ground surface surrounding the work vehicle has inclination or unevenness, an image capturing the inclination or unevenness is projected onto a flat projection plane. Consequently, it is not easy to see that the ground is inclined or uneven from the bird&#39;s-eye view image. 
     An object of the present invention is to generate a display image with which the shape of the surrounding environment of a work vehicle can be understood easily. 
     A display system for a work vehicle according to a first embodiment includes a camera, a shape sensor, and a controller. The camera captures an image of the surrounding environment of a work vehicle and outputs image data indicative of the image. The shape sensor measures a three-dimensional shape of the surrounding environment and outputs 3D shape data indicative of the three-dimensional shape. The controller acquires the image data and the 3D shape data. The controller generates a three-dimensional projection model based on the 3D shape data. The three-dimensional projection model portrays the three-dimensional shape of the surrounding environment. By projecting the image onto the three-dimensional projection model based on the image data, display image data is generated indicative of a display image of the surrounding environment of the work vehicle. 
     In the display system for the work vehicle according to the present embodiment, the three-dimensional shape of the surrounding environment of the work vehicle is measured by the shape sensor and the three-dimensional projection model is generated based on the measured three-dimensional shape. As a result, the three-dimensional projection model has a shape that is the same as or is similar to the actual shape of the surrounding environment of the work vehicle. Therefore, by projecting the image captured by the camera onto the three-dimensional projection model, a display image is generated in which the shape of the surrounding environment of the work vehicle can be understood easily. 
     A generation method according to another embodiment is a generation method executed by a controller for generating display image data indicative of a display image of a surrounding environment of a work vehicle, the method including the following processes. A first process involves acquiring image data indicative of an image of the surrounding environment of the work vehicle. A second process involves acquiring 3D shape data indicative of a three-dimensional shape of the surrounding environment. A third process involves generating a three-dimensional projection model which portrays the three-dimensional shape of the surrounding environment based on the 3D shape data. A fourth process involves generating display image data by projecting an image onto a three-dimensional projection model based on the image data. 
     In the generation method according to the present embodiment, the three-dimensional shape of the surrounding environment of the work vehicle is measured by a shape sensor and a three-dimensional projection model is generated based on the measured three-dimensional shape. As a result, the three-dimensional projection model has a shape that is the same as or similar to the actual shape of the surrounding environment of the work vehicle. Therefore, by projecting the image captured by the camera onto the three-dimensional projection model, a display image is generated in which the shape of the surrounding environment of the work vehicle can be understood easily. 
     Effect of the Invention 
     According to the present invention, a display image can be generated in which the shape of the surrounding environment of a work vehicle can be understood easily. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a side view of a work vehicle according to an embodiment. 
         FIG. 2  illustrates a configuration of a display system according to a first embodiment. 
         FIG. 3  is a view for explaining 3D shape data acquired by a shape sensor. 
         FIG. 4  illustrates an example of a display image according to the first embodiment. 
         FIG. 5  is a flow chart illustrating processing executed by a controller of the display system according to a second embodiment 
         FIG. 6  is a view for explaining a warning condition of a point group density. 
         FIG. 7  is a view for explaining a warning condition of inclination. 
         FIG. 8  is a view for explaining a warning condition of undulation. 
         FIG. 9  illustrates an example of a display image according to the second embodiment. 
         FIGS. 10A-10C  are views of examples of topographies evaluated by the display system. 
         FIG. 11  is a side view of a work vehicle according to another embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following is a description of a display system for a work vehicle according to an embodiment with reference to the drawings. The display system according to the present embodiment is a system for displaying the work vehicle and the surrounding environment of the work vehicle.  FIG. 1  is a side view of a work vehicle  1  according to an embodiment. The work vehicle  1  is a bulldozer according to the present embodiment. The work vehicle  1  includes a vehicle body  3 , a work implement  4 , and a travel device  5 . 
     The vehicle body  3  includes the engine room  6 . An engine  7  and a driving device such as a hydraulic pump and the like are disposed inside the engine room  6 . A ripper device  9  is attached to a rear portion of the vehicle body  3 . 
     The travel device  5  is a device for causing the work vehicle  1  to travel. The travel device  5  includes a pair of crawler belts  11  which are disposed on one side and the other side in the transverse direction of the work vehicle  1 . The crawler belts  11  are each formed by a loop-shaped chain that extends in the longitudinal direction of the work vehicle  1 . The work vehicle  1  travels due to the crawler belts  11  being driven. 
     The work implement  4  is disposed in front of the vehicle body  3 . The work implement  4  is used for work, such as excavating, earth moving, or ground leveling. The work implement  4  includes a blade  12 , tilt cylinders  13 , lift cylinders  14 , and arms  15 . The blade  12  is supported on the vehicle body  3  via the arms  15 . The blade  12  is provided in a manner that allows for pivoting in the up-down direction. The tilt cylinders  13  and the lift cylinders  14  are driven by hydraulic fluid from a hydraulic pump  8  and change the attitude of the blade  12 . 
       FIG. 2  is a block diagram illustrating a configuration of a display system  2  and a processing flow of the display system  2  according to a first embodiment. As illustrated in  FIG. 2 , the display system  20  includes a plurality of cameras C1 to C4. The plurality of cameras C1 to C4 are attached to the vehicle body  3 . The plurality of cameras C1 to C4 are fish-eye lens cameras. The angle of view of each of the plurality of cameras C1 to C4 is 180 degrees. However, the angle of view of each of the plurality of cameras C1 to C4 may be less than 180 degrees. Alternatively, the angle of view of each of the plurality of cameras C1 to C4 may be more than 180 degrees. The plurality of cameras C1 to C4 includes a front camera C1, a first side camera C2, rear camera C3, and a second side camera C4. 
     As Illustrated in  FIG. 1 , the front camera C1 is attached to a front portion of the vehicle body  3 . Specifically, the vehicle body  3  includes a front camera support portion  16  as illustrated in  FIG. 1 . The front camera support portion  16  extends upward and forward from the front portion of the vehicle body  3 . The front camera C1 is attached to the front camera support portion  16 . The rear camera C3 is attached to a rear portion of the vehicle body  3 . 
     The first side camera C2 is attached to one side of the vehicle body  3 . The second side camera C4 is attached to the other side of the vehicle body  3 . In the present embodiment, the first side camera C2 is attached to the left side of the vehicle body  3  and the second side camera C4 is attached to the right side of the vehicle body  3 . However, the first side camera C2 may be attached to the right side of the vehicle body  3  and the second side camera C4 may be attached to the left side of the vehicle body  3 . 
     The front camera C1 captures images in front of the vehicle body  3  and acquires images Including the surrounding environment of the work vehicle  1 . The rear camera C3 captures images to the rear of the vehicle body  3  and acquires images including the surrounding environment of the work vehicle  1 . The first side camera C2 captures images to the left of the vehicle body  3  and acquires images including the surrounding environment of the work vehicle  1 . The second side camera C4 captures images to the right of the vehicle body  3  and acquires images including the surrounding environment of the work vehicle  1 . The cameras C1 to C4 output image data indicative of the acquired images. 
     As illustrated in  FIG. 2 , the display system  2  includes a controller  20 , a shape sensor  21 , an attitude sensor  22 , a position sensor  23 , and a display  24 . The shape sensor  21  measures a three-dimensional shape of the surrounding environment of the work vehicle  1  and outputs 3D shape data D1 indicative of the three-dimensional shape. The shape sensor  21  measures positions at a plurality of points on the surrounding environment of the work vehicle  1 . The 3D shape data D1 represents the positions of the plurality of points on the surrounding environment of the work vehicle  1 . The surrounding environment of the work vehicle  1  includes, for example, the ground surface around the work vehicle  1 . That is, the 3D shape data D1 Includes the positions of a plurality of points on the ground surface around the work vehicle  1 . In particular, the 3D shape data D1 Includes the positions of a plurality of points on the ground surface in front of the work vehicle  1 . 
     Specifically, the shape sensor  21  measures the distances from the work vehicle  1  of the positions of the plurality of points on the surrounding environment. The positions of the plurality of points are derived from the distances of the plurality of points from the work vehicle  1 . In the present embodiment, the shape sensor  21  is a laser imaging detection and ranging (LIDAR) device. The shape sensor  21  measures the distance to a measurement point by emitting a laser and measuring the reflected light thereof. 
     The shape sensor  21  includes, for example, a plurality of laser distance measuring elements aligned in the vertical direction. The shape sensor  21  measures the positions of the plurality of points at a predetermined cycle while rotating the plurality of laser distance measuring elements in the transverse direction around an axis that extends in the vertical direction. Therefore, the shape sensor  21  measures the distances to the points on the surrounding environment at fixed rotation angles and acquires the position of a three-dimensional point group. 
     The shape data includes, for each point, information about which element was used for the measurement, information about which rotation angle was used in the measurement, and information about the positional relationships between each element. In addition, the controller  20  has information indicative of the positional relationships between each element and the work vehicle  1 . Therefore, the controller  20  can acquire the positional relationships between the points on the surrounding environment and the work vehicle from the shape data. 
     The attitude sensor  22  detects the attitude of the work vehicle  1  and outputs attitude data D2 indicative of the attitude. The attitude sensor  22  is, for example, an inertial measurement unit (IMU). The attitude data D2 includes the angle (pitch angle) relative to horizontal in the vehicle front-back direction and the angle (roll angle) relative to horizontal in the vehicle transverse direction. The IMU outputs the attitude data D2. 
     The position sensor  23  is, for example, a global navigation satellite system (GNSS) receiver. The GNSS receiver s, for example, a reception device for a global positioning system (GPS). The GNSS receiver receives a positioning signal from a satellite and acquires position data D3, indicative of the positional coordinates of the work vehicle  1 , from the positioning signal. The GNSS receiver outputs the position data D3. 
     The shape sensor  21  is, for example, attached to the front camera support portion  16 . Alternatively, the shape sensor  21  may be attached to another portion of the vehicle body  3 . The attitude sensor  22  and the position sensor  23  are attached to the vehicle body  3 . Alternatively, the positional sensor  23  may be attached to the work implement  4 . 
     The controller  20  is connected to the cameras C1 to C4 so as to enable wired or wireless communication. The controller  20  receives the image data from the cameras C1 to C4. The controller  20  is connected to the shape sensor  21 , the attitude sensor  22 , and the position sensor  23  so as to enable wired or wireless communication. The controller  20  receives the 3D shape data D1 from the shape sensor  21 . The controller  20  receives the attitude data D2 from the attitude sensor  22 . The controller  20  receives the position data D3 from the position sensor  23 . 
     The controller  20  is programmed so as to generate a display image Is for displaying the surrounding environment of the work vehicle  1 , based on the aforementioned image data, the 3D shape data D1, the attitude data D2, and the position data D3. The controller  20  may be disposed outside of the work vehicle  1 . Alternatively, the controller  20  may be disposed inside the work vehicle  1 . The controller  20  includes a computation device  25  and a storage device  26 . 
     The computation device  25  is configured by a processor, such as a CPU. The computation device  25  performs processing for generating the display image Is. The storage device  26  is configured by a memory, such as a RAM or a ROM, or by an auxiliary storage device  26 , such as a hard disk. The storage device  26  stores data and programs used for generating the display image Is. 
     The display  24  is a device, such as a CRT, and LCD, or an OELD. However, the display  24  is not limited to the aforementioned displays and may be another type of display. The display  24  displays the display image Is based on an output signal from the controller  20 . 
     The generation of the display image Is will be explained in greater detail next. First, imaging is performed by the cameras C1 to C4. The controller  20  acquires a forward image Im1, a left side image Im2, a rearward image Im3, and a right side image Im4 from the respective cameras C1 to C4. The forward image Im1 is an image in the forward direction of the vehicle body  3 . The left side image Im2 is an image to the left of the vehicle body  3 . The rearward image Im3 is an image in the rearward direction of the vehicle body  3 . The right side image Im4 is an image to the right of the vehicle body  3 . 
     The controller  20  generates a three-dimensional projection model M1 based on the 3D shape data D1 acquired from the shape sensor  21 . As illustrated in  FIG. 3 , the controller  20  generates, based on the positions of the plurality of points on the surrounding environment of the work vehicle  1 , a polygon mesh which portrays the shape of the surrounding environment. The three-dimensional projection model M1 Includes polygons formed by linking adjacent points among the plurality of points. 
     Specifically, as illustrated in  FIG. 3 , the controller  20  generates a mesh that is formed by linking adjacent points among the plurality of points P(1,1), P(2,1), . . . , P(i,j), . . . measured in one scan by the shape sensor  21 . In this case, P(i,j) represents a point measured by the respective ith laser distance measuring element in the vertical direction and obtained at the jth rotation angle in the transverse direction. The controller  20  generates the triangle (P(i,j), P(i+1,j), P(i,j+1)) and the triangle (P(i+1,j), P(i,j+1), P(i+1,j+)) for the points P(i,j), P(i+1,j), P(i,j+1), P(i+1,j+). As a result, the controller  20  generates the three-dimensional projection model M1 represented by triangular polygons. 
     The shape sensor  21  periodically measures the three-dimensional shape of the surrounding environment. The controller  20  updates the 3D shape data D1 and generates the three-dimensional projection model M1 based on the updated 3D shape data D1. 
     The controller  20  generates a surroundings composite image Is1 from the images Im1 to Im4 acquired by the respective cameras C1 to C4. The surroundings composite image Isi is an image which shows the surroundings of the work vehicle  1  in a bird&#39;s-eye view manner. The controller  20  generates the surroundings composite image Is1 by projecting the images Im1 to Im4 acquired by the respective cameras C1 to C4 on the three-dimensional projection model M1 by texture mapping. 
     In addition, the controller  20  combines a vehicle image Is2 indicative of the work vehicle  1  with the display image. The vehicle image Is2 is an image representing the work vehicle  1  itself in a three-dimensional manner. The controller  20  determines the attitude of the vehicle image Is2 on the display image Is from the attitude data D2. The controller  20  determines the orientation of the vehicle image Is2 on the display image Is from the position data D3. The controller  20  combines the vehicle image Is2 with the display image Is so that the attitude and orientation of the vehicle image Is2 on the display image Is coincides with the actual attitude and orientation of the work vehicle  1 . 
     The controller  20  may generate the vehicle image Is2 from the images Im1 to Im4 acquired from the respective cameras C1 to C4. For example, portions of the work vehicle  1  are included in each of the images acquired from the cameras C1 to C4, and the controller  20  may generate the vehicle image Is2 by projecting the portions in the images onto a vehicle model M2. The vehicle model M2 is a projection model that has the shape of the work vehicle  1  and is stored in the storage device  26 . Alternatively, the vehicle image Is2 may be an existing image that was captured in advance, or a three-dimensional computer graphics image created in advance. 
     The display  24  displays the display image Is.  FIG. 4  illustrates an example of the display image Is. As illustrated in  FIG. 4 , the display image Is is an image that represents the work vehicle  1  and the surroundings thereof in a three-dimensional manner. As illustrated in  FIG. 4 , the display image Is is displayed by using the three-dimensional projection model M1 having an inclined shape that matches the actual inclined topography around the work vehicle  1 . In addition, the display image Is is displayed while the vehicle image Is2 is inclined so as to match the actual inclined attitude of the work vehicle  1 . 
     The display image Is is updated in real time and displayed as a moving image. Therefore, when the work vehicle  1  is traveling, the surroundings composite image Is1, the attitudes, orientations, and positions of the vehicle image Is2 in the display image Is are changed in real time and displayed in response to changes in the surrounding environment, the attitudes, orientations, and positions of the work vehicle. 
     In order to portray the changes in the attitude, orientation and position of the work vehicle  1 , the three-dimensional projection model M1 and the vehicle model M2 are rotated in accordance with a rotating matrix that represents changes from the attitude, orientation, and position when the work vehicle  1  began to travel, and are translated in accordance with a translation vector. The rotation vector and the translation vector are acquired from the aforementioned attitude data D2 and the position data D3. 
     With regard to the specific method for combining the images, a method represented, for example, in “Spatio-temporal bird&#39;s-eye view images using multiple fish-eye cameras,” (Proceedings of the 2013 IEEE/SICE international Symposium on System Integration, pp. 753-758, 2013) may be used, or a method represented in “Visualization of the surrounding environment and operational portion in a 3DCG model for the teleoperation of construction machines,” (Proceedings of the 2015 IEEE/SICE International Symposium on System Integration, pp. 81-87, 2015) may be used. 
     In  FIG. 4 , the display image Is is an image viewing the work vehicle  1  and the surroundings thereof from the left side. However, the controller  20  is able to switch the display image Is to images of the work vehicle  1  and the surroundings thereof from the front, the rear, the right side, from above, or from an angle in any of the directions. 
     In the display system  2  according to the present embodiment as explained above, the three-dimensional shape of the surrounding environment of the work vehicle  1  is measured by the shape sensor  21  and the three-dimensional projection model M1 is generated based on the measured three-dimensional shape. As a result, the three-dimensional projection model M1 has a shape that is the same as or similar to the actual topography around the work vehicle  1 . Therefore, the image of the surrounding environment can be presented in the display image Is in a shape that reflects the actual topography around the work vehicle  1 . Therefore, in the display system  2  according to the present embodiment, the display image Is can be generated in which the shape of the surrounding environment of the work vehicle  1  can be understood easily. 
     In addition, the actual attitude of the work vehicle  1  is measured by the attitude sensor  22  and the vehicle image Is2 is displayed in the display image Is so as to match the measured attitude. As a result, the vehicle image Is2 can be presented in the display image Is in the attitude that reflects the actual attitude of the work vehicle  1 . Consequently, a change in the attitude of the work vehicle  1 , such as a situation in which the work vehicle  1  has advanced into an inclined surface or performed a turn, can be presented accurately to an operator. 
     Next, the display system  2  according to the second embodiment will be explained. In the display system  2  according to the second embodiment, the controller  20  evaluates a plurality of regions included in the surrounding environment based on the 3D shape data D1. In the present embodiment, the controller  20  defines each triangular polygon of the aforementioned three-dimensional projection model M1 as one region. The configuration of the display system  2  and the generation method of the display image Is are the same as those of the first embodiment and explanations thereof are omitted. 
     The controller  20  sorts the regions into a plurality of levels and evaluates the regions. In the present embodiment, the controller  20  sorts the regions into a first level and a second level. The first level indicates that the regions are ones in which the entry of the work vehicle  1  is permitted. The second level indicates that the regions are ones which the entry of the work vehicle  1  is prohibited. 
       FIG. 5  is a flow chart illustrating processing performed by the controller  20  for evaluating a region. In step S 101 , the controller  20  determines whether a warning condition of a point group density is satisfied in each region. The warning condition of the point group density is represented by formula (1) below.
 
max( L 1( i ), L 2( i ), L 3( n )&gt; k×Lc   (1)
 
     L1(i), L2(i), and L3(i) are the lengths of the line portions that link the points which define each region. As illustrated in  FIG. 6 , the controller  20  calculates the lengths L1(i), L2(i), and L3(i) on each side of the triangle (Pi, Pi+1, Pi+2) indicative of each region, as the lengths of the line portions in each region. 
     That is, the controller  20  compares the lengths of the line portions of each region (Pi, Pi+1, Pi+2) with a predetermined threshold k×Lc and determines whether each region (Pi, Pi+1, Pi+2) includes any line portion greater than the threshold k×Lc. When a given region (Pi, Pi+1, Pi+2) satisfies the warning condition of the point group density, that is, a given region (Pi, Pi+1, Pi+2) includes a line portion greater than the threshold k×Lc, the controller  20  determines the applicable region (Pi, Pi+1, Pi+2) as a second level region in step S 102 . 
     As illustrated in  FIG. 1 , “Lc” is the length of the crawler belt  11 . The length of the crawler belt  11  is the length that the crawler belt placed on flat ground touches the flat ground and is referred to as the contact length. “k” is a predetermined coefficient that is greater than zero and smaller than one. Therefore, the threshold k×Lc is defined based on the length of the crawler belt  11 . For example, the coefficient “k” may be ½. However, the coefficient “k” may be a value different from ½. The coefficient “k” may be a fixed value or may be a value that can be set arbitrarily by the operator. The length Lc of the crawler belt  11  may be a length associated with the contact length. For example, the length Lc may be the entire length of the crawler belt  11  in the front-back direction. In the above case, the value of the coefficient k is modified as appropriate. The warning condition of the point group density may further include the condition represented by the following formula (2).
 
max( L 1( i ), L 2( i ), L 3( i ))&gt; k′×Lc′   (2)
 
     In this case, Lc′ is the center-to-center distance of the left and right crawler belts  11 , and is referred to as the crawler belt gauge width. The coefficient k′ is approximately 1. The controller  20  may determine that the warning condition is satisfied when both formula (1) and formula (2) are satisfied. 
     When a given region (Pi, Pi+1, Pi+2) does not satisfy the warning condition of the point group density, that is, when a given region (Pi, Pi+1, Pi+2) does not include a line portion greater than the threshold k×Lc, the processing advances to step S 103 . 
     In step S 103 , the controller  20  determines whether a warning condition of inclination is satisfied in a region in which the warning condition of the point group density is not satisfied. The warning condition of inclination is represented by the following formula (3).
 
cos −1 ( Nav·e   z )&gt;θ max   (3)
 
     In this case, as illustrated in  FIG. 7 , the controller  20  derives normal vectors Ni included in the subject region (Pi, Pi+1, Pi+2) and in a prescribed range A1(i) around the subject region, and calculates an average Nav of the normal vectors of the regions. The controller  20  determined angles formed by the average Nav of the normal vectors and gravitational force directions as inclination angles of the subject region (Pi, Pi+1, Pi+2). The aforementioned warning condition of inclination signifies that the inclination angle of the subject region (Pi, Pi+1, Pi+2) exceeds a threshold θmax. In formula (3), e z  is a unit vector in the gravitational force direction. 
     The threshold θmax is, for example, an upper limit inclination angle for which entry of the work vehicle  1  is permitted. However, the threshold θmax may be another value. The threshold θmax may be a fixed value or may be set arbitrarily by the operator. The predetermined range A1(i) is represented by a circle with the radius R centered on the centroid of the subject region (Pi, Pi+1, Pi+2). The radius R may be a fixed value. Alternatively, the radius R may be arbitrarily set by the operator. 
     When a given region (Pi, Pi+1, Pi+2) satisfies the warning condition of inclination, that is, when the inclination angle of the given region (Pi, Pi+1, Pi+2) is greater than the threshold θmax, the controller  20  determines the applicable region (Pi, Pi+1, Pi+2) as the second level region in step S 102 . When the given region (Pi, Pi+1, Pi+2) does not satisfy the warning condition of inclination, that is, when the inclination angle of the given region (Pi, Pi+1, Pi+2) is equal to or less than the threshold θmax, the processing advances to step S 104 . 
     In step S 104 , the controller  20  determines whether a warning condition of undulation is satisfied in a region in which the warning condition of the point group density is not satisfied. The warning condition of undulation is represented by the following formula (4). 
     
       
         
           
             
               
                 
                   
                     σ 
                     z 
                     2 
                   
                   = 
                   
                     
                       
                         1 
                         n 
                       
                       ⁢ 
                       
                         ∑ 
                         
                           
                             ( 
                             
                               
                                 Z 
                                 i 
                               
                               - 
                               Zav 
                             
                             ) 
                           
                           2 
                         
                       
                     
                     &gt; 
                     
                       σ 
                       max 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   4 
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     n is the number of points included within the subject determination range A2(i) as illustrated in  FIG. 8 . The determination range A2(i) in this case may be the same as the predetermined range A1(l) in step S 103 , or may be different. Zi is the height of a point Pi in the gravitational force direction. Zav is the average of the heights of the points included in the determination range A2(i). σ 2 z represents the dispersion of the points within the determination range A2(i). The aforementioned warning condition of undulation signifies that the dispersion σ 2 z of the heights of the points within the subject determination range A2(i) exceeds a threshold σ 2 max. That is, the warning condition of undulation signifies that changes in the undulation of the determination range A2(l) are large. 
     The threshold σ 2 max is, for example, an upper limit of the changes in undulation for which entry of the work vehicle  1  is permitted. However, the threshold σ 2 max may be another value. The threshold σ 2 max may be a fixed value or may be a value set arbitrarily by the operator. 
     When the warning condition of undulation of a given determination range A2(i) is satisfied, the controller  20  determines that the region included in the applicable determination range A2(i) is a second level region in step S 102 . When a given determination range A2(i) does not satisfy the warning condition of undulation, the processing advances to step S 105 . 
     In step S 105 , the controller  20  determines that the region in which none of the warning condition of the point group density, the warning condition of inclination, and the warning condition of undulation are satisfied is a first level region. 
     Next, the controller  20  displays the display image Is on the display  24 . The controller  20  displays each of a plurality of regions in a mode in accordance with the evaluation in the display image Is. Specifically, the controller  20  displays the second level regions and the first level regions is different colors. 
       FIG. 9  illustrates an example of the display image Is according to the second embodiment. In the display image Is illustrated in  FIG. 9 , a sharp downward slope Sp2 is present to the right of the work vehicle  1 . A sharp upward slope Sp3 is present to the left of the work vehicle  1 . 
     The controller  20  determines that the region Sp1 in front of the work vehicle  1  is a first level region. In addition, the controller  20  determines that the sharp downward slope Sp2 to the right and the sharp upward slope Sp3 to the left are second level regions. The controller  20  portrays the sharp downward slope Sp2 to the right and the sharp upward slope Sp3 to the left with a color different from the front region Sp1 in the display image Is. 
     In the display system  2  according to the second embodiment explained above, the controller  20  evaluates a plurality of regions included in the surrounding environment based on the 3D shape data D1, and displays the second level regions and the first level regions in different modes in the display image Is. As a result, the operator is able to easily notice the presence of the second level regions with the display image Is. In addition, the display image Is is projected onto the three-dimensional projection model M1 that reflects the actual topography around the work vehicle  1 . As a result, the regions evaluated as second level regions can be portrayed in the display image Is in shapes approximating the actual topography. 
     The controller  20  determines a region in which the warning condition of the point group density is satisfied as a second level region and displays the second level region in the display image Is in a mode that is different from the first level region. The ranges between each point are portions that are not measured by the shape sensor  21 . This signifies that as the lengths of the line portions L1(i), L2(i), and L3(l) in each region grow longer, the ranges not measured by the shape sensor  21  become larger. As a result, as illustrated in  FIG. 10A , there may be a region that cannot be measured with the shape sensor  21  if a sharp inclination is present between points Pi and Pi+1. 
     In the display system  2  according to the present embodiment, when at least one of the lengths among the lengths L1(i), L2(i), and L3(i) of the line portions is greater than the threshold k×Lc in a given region, the region is determined as a second level region. As a result, a region in which a sufficient density of a point group is not obtained can be determined as a second level region. Therefore, a region in which a sufficient density of the point group is not obtained because the shape sensor  231  is spaced far away from the region, can be determined as a second level region. Alternatively, a region in which an accurate topography cannot be measured because the lasers are blocked by the topography, can be determined as a second level region. 
     The threshold k×Lc is prescribed from the length of the crawler belt  11 . If a region that cannot be measured is longer than the threshold k×Lc prescribed from the length of the crawler belt  11 , there is a possibility that the inclination of the work vehicle  1  could exceed the upper limit inclination angle θmax when a depression is present in the region. In the display system  2  according to the present embodiment, such a region can be determined as a second level region, and can be displayed on the display image Is in a mode that is different from the first level regions. 
     The controller  20  determines that a region in which the warning condition of inclination is satisfied as a second level region, and displays the region in the display image Is in a mode different from the first level regions. As a result, as illustrated in  FIG. 10B  for example, a region including a sharp inclination that exceeds the upper limit inclination angle θmax permitted for the work vehicle  1  is determined as a second level region, and can be displayed on the display image Is in a mode different from the first level regions. 
     The controller  20  evaluates a subject region not only with the inclination angle of the region to be determined, but also with an average of the combined inclination angles of other regions included in the predetermined range A1(i) that surrounds the area. Consequently, the effect of changes in the point group density due to the distance from the shape sensor  21  or the topography can be mitigated and the evaluation can be performed with precision. 
     The controller  20  determines the determination region A2(i) in which the warning condition of undulation is satisfied as a second level region, and displays the region in the display image Is in a mode different from a region determined as a first level region. In a topography with large undulation, changes in the heights of the points included in the topography are severe. As a result, the controller  20  evaluates the severity of the undulations in a given determination range A2(i) based on the dispersion of the heights of the points in said determination range A2(i). Consequently, as illustrated in  FIG. 10C  for example, the controller  20  determines a region in which the undulation is large as a second level region, and displays the region on the display image Is in a mode different from the first level regions. 
     The display image Is illustrated in  FIG. 9  is a video image generated from a point of view seen from the forward right of the work vehicle  1 . However, the display image Is may be generated by changing the point of view as desired. For example, the controller  20  may switch the point of view in response to an operation by the operator. Consequently, the display image Is can be generated so that a portion that the operator particularly desires to see within the surrounding environment of the work vehicle  1  can be seen. 
     While embodiments of the present invention have been described above, the present invention is not limited to the embodiments and the following modifications may be made within the scope of the present invention. 
     The work vehicle  1 I is not limited to a bulldozer, and may be another type of work vehicle, such as a wheel loader, a hydraulic excavator, and a dump truck and the like. The work vehicle  1  may be a vehicle operated remotely by the controller  20  disposed outside of the work vehicle  1 . In this case, an operating cabin may be omitted from the vehicle body  3  as in a work vehicle  100  illustrated in  FIG. 11 . In  FIG. 11 , the same reference symbols are applied to the portions that correspond to the work vehicle  1  illustrated in  FIG. 1 . Alternatively, the work vehicle  1  may be a vehicle operated directly by an operator inside an operating cabin mounted on the work vehicle  1 . 
     The number of the cameras is not limited to four and may be three or less or five or more. The cameras are not limited to fish-eye lens cameras and may be a different type of camera. The dispositions of the cameras are not limited to the dispositions indicated in the above embodiments and may be disposed differently. 
     The attitude sensor  22  is not limited to an IMU and may be another type of sensor. The positional sensor  23  is not limited to a GNSS receiver and may be another sensor. The shape sensor  21  is not limited to a LIDAR device and may be another measuring device such as a radar. 
     A portion of the warning conditions may be omitted or changed in the second embodiment. Alternatively, another warning condition may be added. The contents of the warning conditions may be changed. The evaluation of the regions is not limited to the two levels including the first level and the second level, but an evaluation with more levels may be performed. 
     According to the present invention, a display image can be generated in which the shape of the surrounding environment of a work vehicle can be understood easily.