A method for calibrating key parameters of underwater cameras

Abstract

The invention discloses a key parameter calibration method for an underwater camera, and belongs to the field of underwater imaging technology. An underwater camera is mainly composed of a camera, an underwater sealed cabin, and a sealed flat glass. Due to the influence of multi-medium refraction, the underwater camera imaging model is different from that in the air. The distance between the focus of the camera lens and the "glass-air" interface is a key parameter of the underwater camera imaging model, and this parameter cannot be directly measured. The calibration method of the invention realizes the calibration of the distance parameter between the focus of the camera lens and the "glass-air" interface by means of two spatial points with known distances. The method is simple and effective, and is conducive to eliminating the distortion of underwater images.

Description

Technical Field

[0001] The invention belongs to the field of underwater visual measurement, and in particular relates to a key parameter calibration method for an underwater camera. Background Art

[0002] Underwater platforms, underwater archaeology, underwater biological monitoring, underwater pipeline inspection, underwater monitoring of nuclear power cores, etc. all require underwater video inspection through underwater cameras. Underwater video observation is an important method for underwater detection. Underwater camera imaging is different from that in the air. In addition to the degradation of image quality caused by water absorption and scattering, more importantly, the light corresponding to the underwater target passes through multiple media with different refractive indices such as "water-glass window-air" during propagation. Therefore, the image obtained by the underwater camera will be severely deformed, thereby reducing the accuracy of two-dimensional size measurement based on the image. In order to reduce the imaging distortion introduced by underwater cameras when working underwater, there are currently two commonly used methods. The first method is an approximate method, which assumes that the focal length of the camera when working underwater is about 1.33 times that of the air, and then establishes an imaging model; the other method is to accurately establish the underwater camera imaging model by ray tracing, but this type of method usually ignores the influence of the thickness of the glass window on the imaging model, and requires the calibration of relevant key parameters in water.

[0003] The underwater camera imaging model is the glass refractive index n g , refractive index of water medium n w , the thickness of the glass t, the focal length of the camera lens f, and the distance d between the glass-air interface and the focal point of the camera lens. The refractive index of the glass n g , refractive index of water medium n w is a constant, the glass thickness can be measured by a micrometer, and the focal length f of the camera lens can be obtained by calibration in air using the Zhang Zhengyou calibration method. However, the distance d between the focus of the camera lens and the "glass-air" interface is an unknown number as a key parameter, which is difficult to measure directly and requires further calibration. Summary of the invention

[0004] The present invention aims to solve the above problems and provide a key parameter calibration method for an underwater camera, which aims to calibrate the distance parameters between the camera lens focus of the underwater camera and the "glass-air" interface with high precision.

[0005] A method for calibrating key parameters of an underwater camera comprises the following steps:

[0006] Step A. Obtain the internal parameters of the camera without sealing glass by using the Zhang Zhengyou calibration method in air x ,f y ,c x ,c y ; f x ,fy are the number of pixels on the imaging plane at focal length f, c x ,c y are the coordinate values ​​of the camera principal point in the pixel coordinate system.

[0007] Step B. Install the sealing glass to the front end of the camera and establish the world coordinate system at the interface between the camera optical axis and the sealing glass;

[0008] Step C. Place the checkerboard calibration plate perpendicular to the camera optical axis and measure the coordinate value Z of the checkerboard in the world coordinate system Z axis. W ; The coordinate values ​​of any two corner points on the chessboard in the world coordinate system are P1 (R w1 ,Z w ) and P2 (R w2 ,Z w ), the distance between them is a known value L 12 ;

[0009] Step D. Start the camera and collect the chessboard image. The coordinate values ​​of the two corner points P1 and P2 in the camera image coordinate system are (x1, y1) and (x2, y2), respectively. The distance between the two corner points is calculated as S. 12 ;

[0010] Step E. From L 12 With S 12 Equal, calculate the distance d between the camera lens focus and the "glass-air interface" interface;

[0011] Step F. Select multiple groups of corner points on the chessboard as calculation inputs, and use the least squares method to solve the optimal solution of the distance d between the camera lens focus and the "glass-air interface".

[0012] Furthermore, the underwater camera imaging model in step D is as follows:

[0013] ;

[0014] ;

[0015] In the above formula, t is the glass thickness, n w is the refractive index of water, f is the focal length of the camera, n g is the refractive index of glass, x i and i are the coordinate values ​​of point P in the image coordinate system, Rw is the distance between point P and the Z axis of the world coordinate system, and R i is the distance between point P and the camera principal point in the image coordinate system.

[0016] Furthermore, the specific calculation method in step E is as follows:

[0017] ;

[0018] ;

[0019] ;

[0020] ;

[0021] ;

[0022] ;

[0023] ;

[0024] In the above formula, t is the glass thickness, n w is the refractive index of water, f is the focal length of the camera, n g is the refractive index of glass, x1 and y1 are the coordinates of point P1 in the image coordinate system, R1 is the distance between point P1 and the camera principal point in the image coordinate system, x2 and y2 are the coordinates of point P2 in the image coordinate system, R2 is the distance between point P2 and the camera principal point in the image coordinate system.

[0025] The beneficial effect of the present invention is that when the underwater three-dimensional measurement data correction method based on structured light three-dimensional measurement is used to measure the underwater object to be measured, the method has the advantage of not requiring underwater calibration, that is, the distance between the focus of the underwater camera lens and the "glass-air interface" interface can be calibrated in the air, and the calibration value can be applied to the underwater camera model. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of a device used in the key parameter calibration method of an underwater camera of the present invention;

[0027] Figure 2 It is a flowchart of a key parameter calibration method of an underwater camera of the present invention;

[0028] Figure 3 It is a schematic diagram of the underwater camera imaging model of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is further described below with reference to specific examples and drawings.

[0030] like Figure 1 As shown in the figure, the underwater camera is mainly composed of a camera and sealed glass. Due to the influence of multi-medium refraction, the underwater camera imaging model is different from that in the air. The distance d between the camera lens focus and the "glass-air" interface is the key parameter of the underwater camera imaging model.

[0031] The present invention provides a key parameter calibration method for an underwater camera, which is used to calibrate the distance d. The specific process is as follows: Figure 2 As shown, the specific steps are as follows:

[0032] Step A. Obtain the internal parameters of the camera without sealing glass by using the Zhang Zhengyou calibration method in air x ,f y ,c x ,c y ;f x ,f y are the number of pixels on the imaging plane at focal length f, c x ,c y are the coordinate values ​​of the camera principal point in the pixel coordinate system.

[0033] Step B. Figure 1 As shown, the sealing glass is installed to the front end of the camera, and the world coordinate system is established at the interface between the camera optical axis and the sealing glass;

[0034] Step C. Place the checkerboard-shaped calibration plate perpendicular to the camera optical axis and measure the coordinate value Z of the checkerboard on the Z axis of the world coordinate system. w The coordinate values ​​of any two corner points on the chessboard in the world coordinate system are P1 (R w1 ,Z w ) and P2 (R w2 ,Z w ), the distance between them is a known value L 12 ,R w1 Two R's w2 They are the distances between points P1 and P2 and the Z axis of the world coordinate system.

[0035] Step D. Start the camera and collect the chessboard image. The coordinate values ​​of the two corner points P1 and P2 in the camera image coordinate system are (x1, y1) and (x2, y2) respectively. The distance between the two corner points can be calculated as S according to the underwater camera imaging model. 12 ;

[0036] like Figure 3 As shown, the underwater camera imaging model is as follows, where t is the glass thickness, n w is the refractive index of water, f is the focal length of the camera, n g is the refractive index of glass, n a is the refractive index of glass, (R w ,Z w ) is the coordinate value of point P in the world coordinate system, (R g2 ,Z g2 ) is the intersection of the imaging light path at point P and the front end of the sealing glass, (R g1 ,Zg1 ) is the point (R w ,Z w ) The intersection of the imaging light path and the rear end of the sealing glass, R i is the distance between point P and the camera principal point in the image coordinate system.

[0037] ;

[0038] ;

[0039] The distance between the two corner points is S 12 The calculation method is as follows:

[0040] ; ;

[0041] ;

[0042] ;

[0043] ;

[0044] ;

[0045] ;

[0046] In the above formula, t is the glass thickness, n w is the refractive index of water, f is the focal length of the camera, n g is the refractive index of glass, x i and i are the coordinate values ​​of point P in the image coordinate system, Rw is the distance between point P and the Z axis of the world coordinate system, and R i is the distance between point P and the camera principal point in the image coordinate system; d is the distance between the camera lens focus and the “glass-air interface” interface.

[0047] Step E. From L 12 With S 12 Equal, calculate the distance d between the camera lens focus and the "glass-air interface" interface;

[0048] ;

[0049] Step F. Select multiple groups of corner points on the chessboard as calculation inputs, and use the least squares method to solve the optimal solution of the distance d between the camera lens focus and the "glass-air interface".

[0050] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A key parameter calibration method for an underwater camera, characterized in that: The following steps are involved: Step A. Obtain the internal parameters of the camera without sealing glass by using the Zhang Zhengyou calibration method in air x ,f y ,c x ,c y ; where f x ,f y are the number of pixels on the imaging plane at focal length f, c x ,c y are the coordinate values ​​of the camera principal point in the pixel coordinate system; Step B. Install the sealing glass to the front end of the camera and establish the world coordinate system at the interface between the camera optical axis and the sealing glass; Step C. Place the checkerboard-shaped calibration plate perpendicular to the camera optical axis and measure the coordinate value Z of the checkerboard on the Z axis of the world coordinate system. w ; The coordinate values ​​of any two corner points on the chessboard in the world coordinate system are P1 (R w1 ,Z w ) and P2 (R w2 ,Z w ), the distance between the two points is a known value L 12 , R w1 and R w2 They are the distances between points P1 and P2 and the Z axis of the world coordinate system; Step D. Start the camera and collect the chessboard image. The coordinate values ​​of the two corner points P1 and P2 in the camera image coordinate system are (x1, y1) and (x2, y2) respectively. The distance between the two corner points can be calculated as S according to the underwater camera imaging model. 12 ; Step E. From L 12 With S 12 Equal, calculate the distance d between the camera lens focus and the "glass-air interface" interface; The specific calculation method in step E is as follows: ； ； ； ； ； ； ； In the above formula, t is the glass thickness, n w is the refractive index of water, f is the focal length of the camera, n g is the refractive index of glass, x1 and y1 are the coordinates of point P1 in the image coordinate system, R1 is the distance between point P1 and the camera principal point in the image coordinate system, x2 and y2 are the coordinates of point P2 in the image coordinate system, R2 is the distance between point P2 and the camera principal point in the image coordinate system; Step F. Select multiple groups of corner points on the chessboard as calculation inputs, and use the least squares method to solve the optimal solution of the distance d between the camera lens focus and the "glass-air interface".

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