Calibration board and calibration method for joint calibration of visible light and infrared cameras

Abstract

The present invention relates to the field of calibration board technology, and in particular to a calibration board and calibration method for joint calibration of visible light and infrared cameras, comprising a substrate, the upper surface of the substrate is covered with a calibration object, the upper surface of the calibration object is provided with a plurality of circular black blocks, the lower surface of the substrate is installed with a foam plastic board, the upper surface of the foam plastic board is provided with a plurality of mounting holes at positions perpendicular to the center of the circular black blocks, a small tungsten filament bulb is installed inside the mounting hole, a DC power supply is installed inside the foam plastic board, and the plurality of small tungsten filament bulbs are connected to the DC power supply through wires. The present invention has the characteristics of simple structure, easy manufacture, low cost, high precision, etc., and can be directly applied to the calibration and calibration fields of infrared cameras such as autonomous driving, battlefield environment detection and aviation, which increases the practicality of the calibration board and improves the work efficiency of calibration.

Description

Technical Field

[0001] The present invention relates to the technical field of calibration plates, and in particular to a calibration plate and a calibration method for joint calibration of visible light and infrared cameras. Background Art

[0002] In applications such as machine vision, image measurement, photogrammetry, and 3D reconstruction, it is necessary to establish a geometric model of camera imaging in order to correct lens distortion, determine the conversion relationship between physical size and pixels, and determine the relationship between the 3D geometric position of a point on the surface of a spatial object and its corresponding point in the image.

[0003] Since the sensing spectra of visible light cameras and thermal infrared cameras are different, thermal infrared cameras cannot obtain the dot images on the calibration plate. Therefore, the traditional method for calibrating binocular visible light cameras is not suitable for the calibration of visible light and thermal infrared cameras. The existing design schemes for heating and cooling the calibration plates for visible light and infrared cameras have complex structures, cumbersome operations, low precision, high costs, etc., and cannot meet the needs of accurate calibration. To this end, we propose a calibration plate and calibration method for joint calibration of visible light and infrared cameras. Summary of the invention

[0004] The purpose of the present invention is to solve the shortcomings existing in the prior art, and to propose a calibration plate and a calibration method for the joint calibration of visible light and infrared cameras, which have the characteristics of simple structure, easy production, low cost and high precision. It can be directly applied to the calibration fields of infrared cameras such as autonomous driving, battlefield environment detection and aviation, has good market competitiveness and is worth recommending.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A calibration board for joint calibration of visible light and infrared cameras is designed, comprising a substrate, the upper surface of the substrate is covered with a calibration object, the upper surface of the calibration object is provided with a plurality of circular black blocks, a foam plastic board is installed on the lower surface of the substrate, a plurality of mounting holes are opened on the upper surface of the foam plastic board at positions perpendicular to the centers of the circular black blocks, small tungsten filament bulbs are installed inside the mounting holes, a DC power supply is installed inside the foam plastic board, and the plurality of small tungsten filament bulbs are connected to the DC power supply through wires.

[0007] Preferably, the substrate is made of a black opaque acrylic plate, and the ratio between the length and width of the substrate is 1:1.

[0008] Preferably, the length of the calibration object is shorter than the length of the substrate, and the width of the calibration object is shorter than the length of the calibration object.

[0009] Preferably, there are twenty circular black blocks in total.

[0010] Preferably, the ratio between the length and width of the foam plastic board is 1:1, and the length and width of the foam plastic board are the same as the length and width of the base plate.

[0011] A method for joint calibration of visible light and infrared cameras includes four parts: S1: turning on the power supply, adjusting the direction of the calibration plate, and simultaneously acquiring visible light images and thermal infrared images of the calibration plate in different directions.

[0012] S2: Obtain the coordinates of the center point of the circular black block in the visible light image and use it as an input parameter to calibrate the visible light camera.

[0013] S3: Repeat the steps of S2 using the coordinates of the center point of the white circle in the thermal infrared image to calibrate the thermal infrared camera.

[0014] S4: Input the intrinsic parameters of the two cameras, calculate the extrinsic parameters, and perform binocular joint calibration.

[0015] A method for joint calibration of visible light and infrared cameras, the S4 includes: S41: inputting the internal parameter matrix and distortion coefficient matrix of the two cameras respectively.

[0016] S42: Calculate the rotation matrix and translation vector between the two cameras through the stereoCalibrate() function.

[0017] S43: Correct the images of the two cameras through the stereoRectify() function.

[0018] The present invention proposes a calibration plate and a calibration method for joint calibration of visible light and infrared cameras, and has the beneficial effect that: in order to realize the calibration of thermal infrared cameras, the present invention adopts a method of opening a mounting hole on the surface of a plastic foam board and installing a small tungsten filament bulb in the mounting hole to simulate an infrared heat source, the position of each small tungsten filament bulb coincides with the center of a circular black block corresponding thereto, the picture taken by the visible light camera only has a circular black block, the light emitted by the bulb cannot pass through the circular black block, and will not affect the calibration of the visible light camera, and the white point of the picture taken by the thermal infrared camera is at the same position as the center of the circular black block taken by the visible light camera, and the calibration of the visible light and thermal infrared cameras can be completed by obtaining several pictures at different angles at the same time, the operation is simple, the calibration is quick and convenient, the time required for calibration is greatly saved, the efficiency of the calibration work is improved, and the calibration plate is very easy to manufacture and has a low cost, and can be directly applied to the calibration and calibration fields of infrared cameras such as autonomous driving, battlefield environment detection and aviation, thereby increasing the practicality of the calibration plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of the stereoscopic structure of a calibration plate for joint calibration of visible light and infrared cameras proposed in the present invention;

[0020] Figure 2 A schematic diagram of the three-dimensional structure of a foam plastic plate of a calibration plate for joint calibration of visible light and infrared cameras proposed by the present invention;

[0021] Figure 3 The present invention provides a calibration board, a small tungsten filament bulb and a DC power supply circuit diagram for joint calibration of visible light and infrared cameras.

[0022] In the figure: substrate 1, calibration object 2, round black block 3, foam plastic board 4, mounting hole 5, small tungsten filament bulb 6, DC power supply 7. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0024] Reference Figure 1-3 A calibration plate and calibration method for joint calibration of visible light and infrared cameras include a substrate 1, which is made of a black opaque acrylic plate. The ratio between the length and the width of the substrate 1 is 1:1, which can isolate the light emitted by a small tungsten filament bulb 6 when it is powered on. The picture taken by the visible light camera only has a circular black block, and the light emitted by the bulb cannot pass through the circular black block.

[0025] The upper surface of the substrate 1 is covered with a calibration object 2 , the length of the calibration object 2 is shorter than the length of the substrate 1 , and the width of the calibration object 2 is shorter than the length of the calibration object 2 .

[0026] A plurality of circular black blocks 3 are provided on the upper surface of the calibration object 2, with a total of twenty circular black blocks 3. A foam plastic board 4 is installed on the lower surface of the substrate 1. The ratio between the length and the width of the foam plastic board 4 is 1:1. The length and width of the foam plastic board 4 are the same as the length and width of the substrate 1, so that the position of the small tungsten filament bulb 6 can correspond one-to-one with the center of the circular black block 3. It is made of practical plastic material, has a low cost, and isolates heat transfer as much as possible.

[0027] A plurality of mounting holes 5 are provided on the upper surface of the foam plastic board 4 at positions perpendicular to the center of the circular black block 3, and a small tungsten filament bulb 6 is installed inside the mounting hole 5. A DC power supply 7 is installed inside the foam plastic board 4, and the DC power supply can be a 6V dry battery. The plurality of small tungsten filament bulbs 6 are connected to the DC power supply 7 through wires.

[0028] A method for joint calibration of visible light and infrared cameras includes four parts: S1: turning on the power supply, adjusting the direction of the calibration plate, and simultaneously acquiring visible light images and thermal infrared images of the calibration plate in different directions. The multiple visible light and thermal infrared images of the calibration plate in different directions can be 20 images each or other numbers taken simultaneously.

[0029] S2: Get the coordinates of the center point of the circular black block 3 in the visible light image and use it as the input parameter to calibrate the visible light camera. Use opencv according to Zhang Zhengyou's calibration method to calculate the intrinsic parameter matrix of the visible light camera. First, get the coordinates of the center of the circular black block 3 in the calibration plate, then get the three-dimensional coordinates of each center point in the environment, that is, the world coordinates, and then get the camera coordinates by rotation and translation. Finally, call the opencv function to calculate the intrinsic parameter matrix. The transformation relationship between the known image coordinates and pixel coordinates is:

[0030]

[0031] Among them, u0, v0 is generally half of the image pixel size, f is the focal length, dx and dy are the physical width and height of a pixel respectively, and f x =f / dx,f y =f / dy, the relationship between the world coordinates and the camera coordinates is known to be:

[0032]

[0033] R 3×3 is the rotation matrix, t 3×1 is the translation matrix, which can be simplified to:

[0034]

[0035] K is the camera intrinsic matrix, [R t] is the extrinsic matrix. In the calibration calculation process, the camera distortion problem also needs to be considered. Since the pinhole model is an approximate model of the camera and the camera lens may not be placed parallel to the photosensitive film, the image will be distorted, that is, the pixel coordinates actually corresponding to the world coordinates deviate from the theoretical position. The camera distortion is expressed as:

[0036]

[0037] The findCirclesGrid() function is used to determine whether the search image contains a circular calibration plate. If so, the coordinates of the center of each circular black block 3 are calculated. The drawChessboardCorners() function is used to draw the coordinates of the corner points in the original image. The find4QuadCornerSupix() function is used to refine the coordinates of the corner points of the circular calibration plate, and then determine the world coordinate system. Since the world coordinate system is arbitrarily specified by humans, a simpler coordinate system can be selected as the world coordinate system in the actual operation process. Considering that the circular black block 3 is a plane and each circle The spacing of the shape sizes is the same, so the first inner corner point in the upper left corner is taken as the coordinate origin, the plane where the calibration plate is located is taken as the z=0 plane, and the rows and columns of the inner corner points are taken as the x-axis and y-axis. In this way, the three-dimensional coordinates of the world coordinate system of the center of each circle can be directly given according to the size of the circular black block 3. For example, if the spacing between the centers of the circles is 10 cm, then the coordinates of the first inner corner point are (0,0,0), the coordinates of the second inner corner point in the same row are (10,0,0), and the coordinates of the second inner corner point in the same column are (0,10,0). Finally, the calibrateCamera() function is called to calculate the visible light camera intrinsic parameter matrix and distortion coefficient matrix.

[0038] S3: Repeat step S2 to calibrate the thermal infrared camera using the coordinates of the center point of the white circle in the thermal infrared image. Obtain the coordinates of the center point of the white circle in the infrared image and use them as input parameters for the calibration of the thermal infrared camera. Calibrate the thermal infrared camera through opencv according to the principle of Zhang Zhengyou calibration method, and calculate the intrinsic parameter matrix and distortion coefficient matrix of the thermal infrared camera. Since the infrared image has a low resolution and less detail information, the traditional corner point matching method will cause a large matching error. Therefore, the present invention effectively reduces the matching error by finding the center of the white circle.

[0039] S4: Input the intrinsic parameters of the two cameras, calculate the extrinsic parameters, and perform binocular joint calibration. S4 also includes: S41: Input the intrinsic parameter matrix and distortion coefficient matrix of each of the two cameras.

[0040] S42: The rotation matrix and translation vector between the two cameras are calculated by the stereoCalibrate() function. According to the transformation relationship, the imaging planes of the two cameras can be transformed to the same plane. At the same time, the x-axes of the images are collinear, so that the projection points of the coordinate points in space on the two images have the same height, that is, the y-coordinates are the same.

[0041] S43: Correct the images of the two cameras through the stereoRectify() function.

[0042] In summary, the present invention is based on Zhang Zhengyou's single-target calibration and dual-target calibration methods, and uses OpenCV to calibrate by finding the center of a circle. It has the characteristics of simple calibration method and high precision. Moreover, since a small tungsten filament bulb 6 is used to generate heat to simulate an infrared heat source, the calibration board has the characteristics of low cost and easy production. Moreover, the calibration board can simultaneously perform high-precision calibration on visible light and thermal infrared cameras.

[0043] Working principle: After power on, you can use the visible light camera and thermal infrared camera to simultaneously capture multiple images of the calibration plate at different angles. In the visible light image, the visible light camera is calibrated according to the position of the circular black block 3. The thermal infrared camera is calibrated according to the position of the center of the white dot in the captured image as the input parameter. The internal parameter matrix of the visible light camera can be calculated using opencv according to the Zhang Zhengyou calibration method. The camera is calibrated using the Zhang Zhengyou calibration method. First, the center coordinates of the circular black block 3 in the visible light image are obtained, and then the three-dimensional coordinates of each center point in the environment, that is, the world coordinates, are obtained. Then the camera coordinates are obtained by rotation and translation. Then, the opencv function is called to calculate the intrinsic parameter matrix and distortion coefficient matrix of the visible light camera. Similarly, the center coordinates of the white circle in the thermal infrared image are obtained, and then the camera coordinates are obtained according to the artificially specified world coordinate system transformation. The opencv function is called to calculate the intrinsic parameter matrix and distortion coefficient matrix of the thermal infrared camera. After the two cameras are calibrated separately, the binocular camera calibration function is called to calculate the rotation and translation between the two cameras, that is, the extrinsic parameter matrix. Finally, the images of the two cameras are corrected so that the two images have the same y coordinate. The present invention has a reasonable design, a clever structure, good market competitiveness, and is worthy of recommendation.

[0044] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A calibration plate for joint calibration of visible light and infrared cameras, comprising a substrate (1), characterized in that: The upper surface of the substrate (1) is covered with a calibration object (2), and the upper surface of the calibration object (2) is provided with a plurality of circular black blocks (3). The lower surface of the substrate (1) is provided with a foam plastic plate (4), and the upper surface of the foam plastic plate (4) is provided with a plurality of mounting holes (5) at positions perpendicular to the center of the circular black block (3). Small tungsten filament bulbs (6) are installed inside the mounting holes (5), and a direct current power supply (7) is installed inside the foam plastic plate (4). The plurality of small tungsten filament bulbs (6) are connected to the direct current power supply (7) via wires. The substrate (1) is made of a black opaque acrylic plate, and the ratio between the length and the width of the substrate (1) is 1:1; The length of the calibration object (2) is shorter than the length of the substrate (1), and the width of the calibration object (2) is shorter than the length of the calibration object (2); There are a total of twenty circular black blocks (3); The ratio between the length and width of the foam plastic plate (4) is 1:1, and the length and width of the foam plastic plate (4) are the same as the length and width of the base plate (1).

2. A calibration method using the calibration plate according to claim 1, characterized in that: It consists of four parts: S1: Turn on the power, adjust the direction of the calibration plate, and obtain the visible light of the calibration plate in different directions at the same time images and thermal infrared images; S2: Get the coordinates of the center point of the circular black block (3) in the visible light image and use it as the input parameter for the visible light camera Carry out calibration; S3: Repeat the steps of S2 using the coordinates of the center point of the white circle in the thermal infrared image to calibrate the thermal infrared camera; S4: Input the intrinsic parameters of the two cameras, calculate the extrinsic parameters, and perform binocular joint calibration.

3. A method for joint calibration of visible light and infrared cameras according to claim 2, characterized in that: The S4 further includes: S41: inputting the internal parameter matrix and distortion coefficient matrix of the two cameras respectively; S42: Calculate the rotation matrix and translation vector between the two cameras through the stereoCalibrate() function; S43: Correct the images of the two cameras through the stereoRectify() function.

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