Method and device for acquiring height of building in dual-polarized SAR (Synthetic Aperture Radar) image

Abstract

The invention provides a dual-polarized SAR image building height obtaining method and device. The method comprises the steps that a dual-polarized SAR image containing a building is obtained through aerial remote sensing; determining the position of a near-end bright line of the building image according to the dual-polarized SAR image as a near-end horizontal first position of the building along the ground; determining a far-end horizontal second position of a boundary line of a shadow area of the building image according to the SAR image; and calculating the building height according to the flight height, the lower visual angle, the near-end horizontal first position and the far-end horizontal second position. According to the invention, the problems of complex calculation and poor precision in the prior art for obtaining the building height through a synthetic aperture radar wide area monitoring and measuring technology are solved.

Description

Technical Field

[0001] The present application relates to the field of aviation remote sensing technology, and in particular to a method and device for acquiring building heights from dual-polarization SAR images using synthetic aperture radar wide-area monitoring and measurement technology. Background Art

[0002] Building height measurement and estimation is one of the important contents of aerial remote sensing applications, which is very important for wide-area ground monitoring, regional development planning, resource distribution estimation, etc. At present, common methods for obtaining building heights include: using total stations, optical rangefinders, GPS and other instruments for on-site measurement; extracting specific building heights by integrating geographic information system databases and building design drawings; estimating ground building heights using three-dimensional aerial images and photogrammetry techniques. Although these technologies have high measurement accuracy, they are expensive and highly dependent on previous auxiliary data, making them unsuitable for estimating the height of ground buildings distributed over a large area. Accurately and reliably measuring the height distribution of ground buildings without the need for reference information is of great significance to remote sensing measurement technology.

[0003] Due to the drawbacks of optical remote sensing equipment in measuring the height of surface buildings, the reflection and scattering characteristics of electromagnetic waves can be used to extract the height of buildings from microwave remote sensing imaging data. A typical implementation method is to generate two physically coherent images of the observation area based on interferometric synthetic aperture radar, and then infer the height of the building through the phase difference of the corresponding pixel points. However, the interferometric synthetic aperture radar imaging method has strict requirements on the baseline length between antennas and the registration accuracy of the image, especially in areas with complex terrain, a large number of control points need to be set as geometric correction points. In view of the shortcomings of interferometric synthetic aperture radar, domestic and foreign scholars have used SAR image amplitude information and prior parameters to invert the backscattering characteristics of the objects, establish a geometric optics-physical optics (GO-PO) model, and then derive the formula for calculating the height of the building.

[0004] At the current stage, there are still some problems that need to be solved in the building height estimation method based on a single SAR image: there are many constraints that need to be set manually, and the calculation process is complicated; it relies on prior knowledge and reference building heights, and the measurement independence is poor; the boundaries of scattering areas such as building overlap in SAR images are blurred, and the geometric distance of each area cannot be accurately judged; the building height is inverted by single-polarization images and prior knowledge, and the influence of other polarization information on height estimation is not comprehensively analyzed, resulting in poor measurement accuracy. Summary of the invention

[0005] The present application proposes a method and device for acquiring building heights from dual-polarization SAR images, which solve the problems of complex calculation and poor accuracy in the prior art.

[0006] In a first aspect, an embodiment of the present application provides a method for acquiring building height from a dual-polarization SAR image, comprising the following steps: Obtain dual-polarization SAR images containing buildings through aerial remote sensing; Determine the position of the near-end bright line of the building image according to the dual-polarization SAR image as the near-end first horizontal position of the building along the ground; Determine a far-end horizontal second position of a boundary line of a shadow area of ​​a building image according to the SAR image; The building height is calculated according to the flight altitude, the downward viewing angle, the near-end horizontal first position and the far-end horizontal second position. In one embodiment of the present application, determining a horizontal first position of a near-end bright line of a building image according to a dual-polarization SAR image specifically includes: In the bright line area of ​​the HH-SAR image along the proximal side of the secondary scattering, the first center line is fitted; Fit the second center line in the bright line area along the proximal side of the secondary scattering in the VV-SAR image; The average straight line of the first center line and the second center line is used as the proximal edge line of the building along the ground to determine the proximal horizontal first position.

[0007] In one embodiment of the present application, a distal boundary line of a shadow area is found in an HH-SAR image.

[0008] In one embodiment of the present application, the distance between the corner reflectors is determined based on the SAR images of multiple corner reflectors preset on the ground, and the distance between the near-end horizontal first position and the far-end horizontal second position is obtained. S 0.

[0009] In one embodiment of the present application, the bright line region uses the RANSAC algorithm to perform center line straight line fitting.

[0010] In one embodiment of the present application, the downward viewing angle of the radar antenna is set to θ The aircraft equipped with dual-polarization SAR radar takes off and enters the altitude H The flight route is greater than or equal to 3 synthetic aperture lengths and is parallel to the building to be measured. The radar generates HH and VV polarization SAR images under standard positive side-view conditions.

[0011] In a second aspect, an embodiment of the present application further proposes a device for acquiring building heights from dual-polarization SAR images, which is used to implement the method described in any embodiment of the first aspect of the present application, including: an acquisition module, a determination module, and a generation module.

[0012] The acquisition module is used to acquire dual-polarization SAR images containing buildings through aerial remote sensing. The determining module is used to obtain the proximal horizontal first position and the distal horizontal second position.

[0013] The generating module is used to calculate the building height according to the flight altitude, the downward viewing angle, the near-end horizontal first position and the far-end horizontal second position.

[0014] In a third aspect, an embodiment of the present application further proposes a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the method described in any embodiment of the first aspect of the present application.

[0015] In a fourth aspect, an embodiment of the present application further proposes an electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the computer program, the method described in any embodiment of the first aspect of the present application is implemented.

[0016] At least one of the above technical solutions adopted in the embodiments of the present application can achieve the following beneficial effects: In order to solve the above problem, the present invention proposes a building height estimation method based on dual-polarization SAR images. SAR images are generated for the same building by using synthetic aperture radar to generate vertically polarized (VV) electromagnetic waves and horizontally polarized (HH) electromagnetic waves, and then the difference between the two polarization images is used to further improve the measurement accuracy of the building height.

[0017] The building height is calculated by the distance between the center line of the SAR image and the shadow boundary of the dihedral angle formed by the building and the ground and the radar viewing angle, avoiding the need for prior parameters such as the dielectric constant of the wall and the ground, the roughness characteristic parameters, and the main length of the building in the traditional GO-PO model, making the implementation method simpler. The dihedral imaging centerline with the highest brightness of the building and the boundary of the darkest shadow area in the SAR image are used as the distance estimation boundaries, which are easy to obtain on the SAR image. This avoids the problem of large distance estimation errors caused by blurred boundaries between overlapping areas and other backscattering areas in previous SAR image height estimation based on geometric optics.

[0018] The relative distance is calculated using the corner reflector spacing in the SAR image, without the need for an optical map carrying geographic coordinate information. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 This is a flow chart of an embodiment of a method for acquiring building height from dual-polarization SAR images of the present application; Figure 2 (a) is a schematic diagram of the scattering intensity of each area of ​​ground buildings and the corresponding SAR image distribution under strict geometric optical conditions; Figure 2 (b) is a schematic diagram of the scattering intensity of each area of ​​ground buildings and the corresponding SAR image distribution when the radar transmission beam is approximately a parallel beam; Figure 3 This is a schematic diagram of ground building images in dual-polarization SAR images; Figure 4 This is a schematic diagram of the ground corner reflector image in the dual-polarization SAR image; Figure 5 This is an embodiment of the device for acquiring building height from dual-polarization SAR images of the present application; Figure 6 This is another embodiment of the device for acquiring building height from dual-polarization SAR images of the present application. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in combination with the specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

[0021] The building height estimation method based on dual-polarization SAR images can realize the height measurement of flat-topped buildings without prior information. In order to better illustrate the above technical solution, the technical solution provided by each embodiment of the present application is described in detail below in conjunction with the accompanying drawings.

[0022] Figure 1 The following is a flow chart of an embodiment of a method for acquiring building height from dual-polarization SAR images of the present application. A method for acquiring building height from dual-polarization SAR images comprises the following steps: Step 110: Acquire a dual-polarization SAR image containing buildings through aerial remote sensing.

[0023] In one embodiment of the present application, the downward viewing angle of the radar antenna is set to θ, and an aircraft equipped with a dual-polarization SAR radar takes off and enters a predetermined route at an altitude of H. The flight route is greater than or equal to 3 synthetic aperture lengths and remains parallel to the building to be measured in azimuth. The radar generates HH and VV polarized SAR images under standard positive side viewing conditions.

[0024] Step 120: Determine the position of the near-end bright line of the building image according to the dual-polarization SAR image as the near-end horizontal first position of the building along the ground.

[0025] In one embodiment of the present application, determining a horizontal first position of a near-end bright line of a building image according to a dual-polarization SAR image specifically includes: In the bright line area of ​​the HH-SAR image along the proximal side of the secondary scattering, the first center line is fitted; Fit the second center line in the bright line area along the proximal side of the secondary scattering in the VV-SAR image; The average straight line of the first center line and the second center line is used as the proximal edge line of the building along the ground to determine the proximal horizontal first position.

[0026] In one embodiment of the present application, the bright line area uses the Random Sample Consensus (RANSAC) algorithm to fit the center line. The RANSAC algorithm is used to fit the center line of the two bright line areas, with the confidence (p) set to 0.994 and the number of samples (m) set to 4. After obtaining two fitted straight lines, the average straight line of the two straight lines is used as the dihedral angle edge line of the building along the ground.

[0027] Step 130: Determine a far-end horizontal second position of a boundary line of a shadow area of ​​the building image according to the SAR image.

[0028] In one embodiment of the present application, a distal boundary line of a shadow area is found in an HH-SAR image.

[0029] Step 140: Calculate the building height according to the flight altitude, the downward viewing angle, the near-end horizontal first position and the far-end horizontal second position. In one embodiment of the present application, the distance between the corner reflectors is determined based on the SAR images of multiple corner reflectors preset on the ground, and the distance between the near-end horizontal first position and the far-end horizontal second position is obtained. S 0. The specific calculation process is: place two SAR radars at a distance perpendicular to the flight path. L cr After the SAR radar generates an image, the number of pixels corresponding to the center point spacing of the circular bright spots of the SAR image formed by the two corner reflectors is N cr , the number of pixels corresponding to the distance between the near-end horizontal first position and the far-end horizontal second position is N p ,but: (1) Therefore, before implementing steps 110 to 140 of the embodiment of the present application, corner reflectors with fixed spacing may be first arranged in an unobstructed open area near the building to be tested.

[0030] In one embodiment, the height estimate of the building should be h It is expressed as: (2) Among them, S 0 is the distance between the proximal horizontal first position and the distal horizontal second position.

[0031] Figure 2 (a) is a schematic diagram of the scattering intensity of each area of ​​the ground building. In the image after pixel-level registration of the two SAR images, the high-brightness bright line area along the secondary scattering side of the building is found in the HH-SAR image and the VV-SAR image. The bright line area at this time corresponds to point B on the ground in Figure 2 (a). Among them, θ1 represents the radar viewing angle corresponding to point B of the building, θ1+θ2 represents the radar viewing angle corresponding to point A of the building, θ1+θ2+θ3 represents the radar viewing angle corresponding to point C of the building, P represents the position of the aircraft, H represents the flight altitude, h represents the height of the building, ABCD represents the vertical interface contour of the building, R represents the distance between the radar antenna and point B of the building, E represents the intersection of the perpendicular line of point B and the extension line of PA, F represents the intersection of the arc with PB (length R) as the radius and the extension line of PA, G represents the intersection of the extension line of PC and the ground, and the color band A1-B1-D1-G1 represents the actual projection distance distribution image of each imaging point in the building on the ground. (both HH and VV images are consistent); A1 represents the intersection of the arc with P as the center and PA as the radius and the ground, B1 and D1 represent the intersection of the building and the ground (same as B and D points), G1 represents the intersection of the PC extension line and the ground (same as G point), and the color band A2-C1-B2-G2 represents the distance distribution of each imaging point in the building on the SAR image (both HH and VV images are consistent) image; A2 represents the projection of building point A on the SAR image, C1 represents the projection of building point C on the SAR image, B2 represents the projection of building point B on the SAR image, and G2 represents the projection of the intersection G of the PC extension line and the ground on the SAR image. The high-brightness bright line area along the secondary scattering side of the building in the HH-SAR image and VV-SAR image is located at B2 of the color band A2-C1-B2-G2.

[0032] Figure 2 (b) is a schematic diagram of the scattering intensity of each area of ​​the ground building and the corresponding SAR image distribution when the radar transmitting beam is approximately a parallel beam. Where D is secondary scattering, T is tertiary scattering, S is shadow, BG is ground scattering, BR is roof scattering, and L is wall scattering. In the figure, θ represents the lower viewing angle, h is the height of the building, W is the width of the building in the lower viewing angle plane, and the color band in the figure represents the projection distance distribution image of each imaging point of the building on the SAR image, where L+BR+BG, L+BG, D, T, and S respectively represent the SAR images formed by the corresponding adjacent parallel line areas on the lower left side (parallel lines are equidistant lines along the radar image). In the embodiment of the present application, the range of the lower viewing angle is determined to be .

[0033] Figure 3 The figure is a schematic diagram of the ground building image in the SAR image. The RANSAC algorithm is used to fit the center line of the two bright line areas. After obtaining two fitting lines, the average straight line of the two straight lines is used as the dihedral angle edge line of the building along the ground, as shown in Figure 3 As shown on the left; then in the HH-SAR image, find the boundary line of the shadow area along the distance to the far end, such as Figure 3 Shown on the right.

[0034] Figure 4 FIG. 1 is a schematic diagram of a ground corner reflector image in a SAR image. In one embodiment, four corner reflectors are used. Preferably, they are arranged in a straight line at a set interval. For example, near a ground building, the corner reflector interval is set to L0. And according to the distance of the corner reflectors (such as Figure 4 The distance between the dihedral angle edge and the boundary line of the shadow area along the distance to the far end can be measured and calculated. S 0.

[0035] It should be noted that the execution subject of each step of the method provided in the embodiment of the present application can be the same device, or the method can be executed by different devices. For example, the execution subject of step 110 and step 120 can be device 1, and the execution subject of step 130 can be device 2; for another example, the execution subject of step 110 can be device 1, and the execution subject of steps 120 and 130 can be device 2; and so on.

[0036] Figure 5 This is an embodiment of the dual-polarization SAR image building height acquisition device of the present application, used to implement the method described in any embodiment of the first aspect of the present application, including: an acquisition module 510, a determination module 520 and a generation module 530.

[0037] The acquisition module is used to acquire dual-polarization SAR images containing buildings through aerial remote sensing. For example, an aircraft equipped with a dual-polarization SAR radar flies along a specific straight line, the flight altitude is H, and the SAR radar's downward viewing angle is , generating two SAR images with HH and VV polarizations. Preferably, the track direction of the aircraft equipped with the dual-polarization synthetic aperture radar is parallel to the azimuth of the building to be measured.

[0038] The determination module is used to obtain the near-end horizontal first position and the far-end horizontal second position. For example, in the image after pixel-level registration, the high-brightness bright line area along the secondary scattering side of the building is found in the HH-SAR image and the VV-SAR image, and the center lines of the HH-SAR and VV-SAR high-brightness bright line areas are fitted with two straight lines respectively, and the average straight line of the two straight lines is used as the dihedral angle edge line of the building along the ground; in the HH-SAR image, the boundary line of the shadow area along the distance to the far end is found.

[0039] The generation module is used to calculate the building height according to the flight altitude, the downward viewing angle, the near-end horizontal first position and the far-end horizontal second position. The distance between the dihedral angle edge line and the shadow area along the distance to the far-end boundary line is calculated according to the distance measurement of the corner reflector. S 0, then calculate the estimated height of the corresponding building h , as shown in formula (2).

[0040] It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0041] Therefore, the present application also proposes a computer-readable storage medium on which a computer program is stored, and when the program is executed by a processor, the method described in any embodiment of the present application is implemented. And a computer program product, including a computer program or an instruction, when the computer program or the instruction is executed by a processor, the method described in any embodiment of the present application is implemented.

[0042] Furthermore, the present application also proposes an electronic device or computing device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the method described in any embodiment of the present application when executing the computer program.

[0043] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0044] These computer program instructions may also be stored in a computer readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0045] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0046] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory. The memory may include non-permanent storage in a computer-readable medium, in the form of random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory is an example of a computer-readable medium.

[0047] Figure 6It is a structural diagram of an electronic device provided in an embodiment of the present application, as another embodiment of the dual-polarization SAR image building height acquisition device of the present application. The electronic device 600 shown is only an example and should not bring any limitation to the functions and scope of use of the embodiment of the present application. It includes: one or more processors 620; a storage device 610, which is used to store one or more programs. When the one or more programs are run by the one or more processors 620, the one or more processors 620 implement the dual-polarization SAR image building height acquisition method provided in the embodiment of the present application. The method includes: Obtain dual-polarization SAR images containing buildings through aerial remote sensing; Determine the position of the near-end bright line of the building image according to the dual-polarization SAR image as the near-end first horizontal position of the building along the ground; Determine a far-end horizontal second position of a boundary line of a shadow area of ​​a building image according to the SAR image; The building height is calculated according to the flight altitude, the downward viewing angle, the near-end horizontal first position and the far-end horizontal second position.

[0048] The electronic device 600 further includes an input device 630 and an output device 640; the processor 620, storage device 610, input device 630 and output device 640 in the electronic device can be connected via a bus or other means, and the figure takes the connection via a bus 650 as an example.

[0049] The storage device 610, as a computer-readable storage medium, can be used to store software programs, computer executable programs, and module units, such as program instructions corresponding to the dual-polarization SAR image building height acquisition method in the embodiment of the present application. The storage device 610 may mainly include a program storage area and a data storage area, wherein the program storage area may store an operating system, an application required for at least one function; the data storage area may store data created according to the use of the terminal, etc. In addition, the storage device 610 may include a high-speed random access memory, and may also include a non-volatile memory, such as at least one disk storage device, a flash memory device, or other non-volatile solid-state storage device. In some instances, the storage device 610 may further include a memory remotely arranged relative to the processor 620, and these remote memories may be connected via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0050] The input device 630 may be used to receive input numbers, character information or voice information, and generate key signal input related to user settings and function control of the electronic device. The output device 640 may include electronic devices such as a display screen and a speaker.

[0051] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0052] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A method for acquiring building height from dual-polarization SAR images, characterized in that: The following steps are involved: Obtain dual-polarization SAR images containing buildings through aerial remote sensing; Determine the position of the near-end bright line of the building image according to the dual-polarization SAR image as the near-end first horizontal position of the building along the ground; Determine a far-end horizontal second position of a boundary line of a shadow area of ​​a building image according to the SAR image; According to the flight altitude H , bottom view θ , the first position of the near level and the second position of the far level to calculate the building height h : Among them, S 0 is the distance between the proximal horizontal first position and the distal horizontal second position.

2. A dual-polarization SAR image building height acquisition method as claimed in claim 1, characterized in that: Determining the horizontal first position of the near-end bright line of the building image according to the dual-polarization SAR image specifically includes: In the bright line area of ​​the HH-SAR image along the proximal side of the secondary scattering, the first center line is fitted; Fit the second center line in the bright line area along the proximal side of the secondary scattering in the VV-SAR image; The average straight line of the first center line and the second center line is used as the proximal edge line of the building along the ground to determine the proximal horizontal first position.

3. A method for acquiring building height from dual-polarization SAR images as claimed in claim 1, characterized in that: Find the far boundary line of the shadow area in the HH-SAR image.

4. A method for acquiring building height from dual-polarization SAR images as claimed in claim 1, characterized in that: According to the SAR images of two corner reflectors preset on the ground, the distance between the corner reflectors is determined, and the distance between the near-end horizontal first position and the far-end horizontal second position is obtained. S 0.

5. A method for acquiring building height from dual-polarization SAR images as claimed in claim 1, characterized in that: Place two SAR radars at a distance perpendicular to the flight path. L cr After the SAR radar generates an image, the number of pixels corresponding to the center point spacing of the circular bright spots of the SAR image formed by the two corner reflectors is N cr , the number of pixels corresponding to the distance between the near-end horizontal first position and the far-end horizontal second position is N p , then the distance between the first horizontal position at the proximal end and the second horizontal position at the distal end is: 。 6. A method for acquiring building height from dual-polarization SAR images as claimed in claim 1, characterized in that: The bright line area uses the RANSAC algorithm to perform center line straight line fitting.

7. A method for acquiring building height from dual-polarization SAR images as claimed in claim 1, characterized in that: Set the radar antenna downward viewing angle to θ , the aircraft equipped with dual-polarization SAR radar takes off and enters the predetermined route altitude H The flight route is greater than or equal to 3 synthetic aperture lengths and remains parallel to the building to be measured. The radar generates HH and VV polarization SAR images under standard positive side-view conditions.

8. A dual-polarization SAR image building height acquisition device, used to implement the method described in any one of claims 1 to 7, characterized in that: include: Get module, determine module and generate module; The acquisition module is used to acquire a dual-polarization SAR image containing buildings through aerial remote sensing; The determining module is used to obtain the first horizontal position of the proximal end and the second horizontal position of the distal end; The generating module is used to calculate the building height according to the flight altitude, the downward viewing angle, the near-end horizontal first position and the far-end horizontal second position.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the method according to any one of claims 1 to 7 is implemented.

10. An electronic device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the method according to any one of claims 1 to 7 is implemented.

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