A sub-pixel displacement measurement method with dual variable adjustment

Abstract

The present invention discloses a sub-pixel displacement measurement method with dual variable adjustment. For any speckle image, the sub-pixel displacement measurement error can be greatly reduced by adjusting the dual adjustment variables α and β. Since the dual adjustment variables α and β become constant values ​​after the speckle image is calibrated, the present invention can also take into account the sub-pixel displacement measurement accuracy and measurement speed of any speckle image at the same time. Through the displacement measurement verification of speckle images with speckle diameters of 2 pixels to 20 pixels, the present invention can make the sub-pixel displacement measurement error <0.001 pixel.

Description

Technical Field

[0001] The present invention relates to the technical field of digital image correlation measurement, and in particular to a sub-pixel displacement measurement method with dual variable adjustment. Background Art

[0002] The sub-pixel displacement measurement accuracy and speed in digital image correlation measurement are related to the measurement method used. Commonly used sub-pixel displacement measurement methods include: gradient method, surface method, Newton iteration method, etc. Among them, the gradient method and surface method have the characteristics of fast measurement speed, but there is a problem that the sub-pixel displacement measurement accuracy is difficult to improve; the Newton iteration method has the characteristics of higher measurement accuracy than the gradient method and surface method, but there is a problem of slow sub-pixel displacement measurement speed.

[0003] In the displacement range of 0.1pixel to 0.9pixel, the error curve of sub-pixel displacement measurement is basically a superposition curve of linear error and S-type error. The linear error can be greatly reduced by improving the measurement method, but the reduction degree of S-type error is very limited. Moreover, the S-type error is also related to the image speckle diameter. The S-type error varies greatly for different speckle diameters. Therefore, how to reduce the S-type error under any speckle diameter and improve the sub-pixel displacement measurement accuracy is a key technical problem to be solved by the present invention. Summary of the invention

[0004] The purpose of the present invention is to remedy the defects of the prior art and provide a sub-pixel displacement measurement method with dual variable adjustment to solve the problem of sub-pixel measurement accuracy under arbitrary speckle diameter. By calibrating multiple images under different displacements, the dual adjustment variables are made constant. After calibration, the linear error and S-type error of the sub-pixel displacement measurement are significantly reduced.

[0005] The present invention is achieved through the following technical solutions:

[0006] A sub-pixel displacement measurement method with dual variable adjustment includes steps S0 to S8 and steps A0 to A3, specifically as follows:

[0007] S0: If the dual adjustment variables α and β of steps A0 to A3 have been calibrated, then go to step S6, otherwise go to step S1;

[0008] S1: Read N images with different displacements, record F 1 To F N , set the initial values ​​of the adjustment variables α and β, such as α=0.5, β=0;

[0009] S2: Calculate the adjacent image F through digital image correlation analysis, such as ZNCC method n+1 With F n The integer pixel displacement Dn+1,n and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C n+1,n (i, j) and calculate the Nth image F N With the first image F 1 The integer pixel displacement D N,1 and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C N,1 (i,j). n=1, 2, 3...N-1; i, j=-1, 0, 1;

[0010] S3: Enter steps A0 to A3, according to C n+1,n (i, j) and C N,1 (i, j) calculate the sub-pixel displacement d n+1,n and d N,1 ;

[0011] S4: Calculate adjacent images F n+1 With F n The accumulated displacement value of : Calculate the Nth image F N With the first image F 1 Displacement value: X 2 =D N,1 +d N,1 ;

[0012] S5: If |X 1 -X 2 |<0.001, the dual adjustment variables α and β in steps A0 to A3 become fixed values, the calibration is completed, and the process goes to S6; otherwise, α and β are modified and the process goes back to step S3;

[0013] S6: Read the two images F before and after the displacement 0 and F 1 , calculate the adjacent image F by the same digital image correlation analysis method as step S2 1 With F 0 The integer pixel displacement value D between 1,0 and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C 1,0 (i, j);

[0014] S7: Enter steps A0 to A3, according to C 1,0 (i, j) calculate the sub-pixel displacement d 1,0 ;

[0015] S8: Calculate 2 images F 1 and F 0 Displacement value between: X = D 1,0 +d 1,0 .

[0016] A0: Decompose the 3×3 correlation coefficient C(i,j) into the 3×1 correlation coefficient C in the x direction x (i) and the 1×3 correlation coefficient C in the y direction y (j), sign(·) is the sign function:

[0017]

[0018]

[0019]

[0020]

[0021]

[0022]

[0023] A1: Let C xmin =min{C x (-1),C x (0),C x (1)},C ymin =min{C y (-1),C y (0),C y (1)}, calculate

[0024] Sub-pixel displacement in the x direction Δx and sub-pixel displacement in the y direction Δy:

[0025]

[0026]

[0027] A2: If Δx==Δx 0 and Δy==Δy 0 , then proceed to A3; otherwise: let Δx 0 =Δx, Δy 0 =Δy, return to A0;

[0028] A3: If it is a displacement in the x direction, it returns Δx; if it is a displacement in the y direction, it returns Δy.

[0029] The 0.00001 in step A0 is any positive number smaller than 0.0001.

[0030] In step A1, the value of α in the Δx equation and the value of α in the Δy equation may be the same or different.

[0031] In step A3, for the in-plane displacement measurement, Δx and Δy are returned.

[0032] The advantages of the present invention are: (1) the present invention only needs to calibrate the dual adjustment variables to solve the problem of balancing the sub-pixel displacement measurement accuracy of different speckle diameters.

[0033] (2) The present invention significantly reduces the sub-pixel displacement measurement error. By calibrating the standard speckle image with a displacement of 0.1 pixel to 0.9 pixel, the sub-pixel displacement measurement error is less than 0.001 pixel when the speckle diameter is 2 pixel to 20 pixel. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Example speckle pattern with a speckle diameter of 10 pixels;

[0035] Figure 2 Different value pairs of double adjustment variables Figure 1 Impact diagram of sub-pixel displacement measurement error;

[0036] Figure 3 Example speckle pattern with a speckle diameter of 2 pixels;

[0037] Figure 4 Different value pairs of double adjustment variables Figure 3 Impact diagram of sub-pixel displacement measurement error. DETAILED DESCRIPTION

[0038] The following is combined with Figure 1-4 The technical solution of the present invention is further described in detail:

[0039] In order to verify the effectiveness of the present invention, a computer-generated standard speckle pattern is used for verification, such as Figure 1 As shown in FIG. 1 , the parameters of the speckle pattern are: 10 Gaussian speckle images with a height and width size of 200 pixels × 200 pixels, a speckle diameter of 10 pixels, and an x-direction displacement step of 0.1 pixels to 0.9 pixels. The present invention calibrates the 10 speckle patterns, so that the sub-pixel matching error is greatly reduced to less than 0.001 pixels, and is also effective for speckle patterns with a speckle diameter of 2 pixels to 20 pixels. Figure 3 is an example speckle pattern with a speckle diameter of 2 pixels. Figure 2 and Figure 4 As shown, a diagram showing the influence of different values ​​of the dual adjustment variables on the sub-pixel displacement measurement error, wherein the specific steps include steps S0 to S8 and steps A0 to A3.

[0040] Steps from S0 to S8 are as follows:

[0041] S0: If the dual adjustment variables α and β of steps A0 to A3 have been calibrated, for this example, α = 0.5079 and β = -0.06, then enter S6, otherwise enter S1;

[0042] S1: Generate 10 Gaussian speckle images with speckle image displacements of 0 pixel, 0.1 pixel, 0.2 pixel, ... 0.9 pixel, denoted by F 1 、F 2 、F 3 ......F 10 , set the initial values ​​of the adjustment variables α and β, such as α=0.5, β=0;

[0043] S2: Calculate the adjacent image F through digital image correlation analysis, such as ZNCC method n+1 With F n The integer pixel displacement D n+1,n and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C n+1,n (i, j) and calculate the 10th image F 10 With the first image F 1 The integer pixel displacement D 10,1 and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C 10,1 (i,j). n=1, 2, 3...9; i, j=-1, 0, 1;

[0044] S3: Enter steps A0 to A3, according to C n+1,n (i, j) and C 10,1 (i, j) calculate the sub-pixel displacement d n+1,n and d 10,1 ;

[0045] S4: Calculate adjacent images F n+1 With F n The accumulated displacement value of : Calculate the 10th image F 10 With the first image F 1 Displacement value: X 2 =D 10,1 +d 10,1 ;

[0046] S5: If |X 1 -X 2 |<0.001, then the dual adjustment variables α and β in steps A0 to A3 become fixed values. For this example, α=0.5079 and β=-0.06, the calibration is completed and the process goes to S6. Otherwise, α and β are modified and the process goes back to S3.

[0047] S6: Read any two images above, such as F 5 and F 8, calculate the adjacent image F by the same digital image correlation analysis method as S2 5 With F 8 The integer pixel displacement value D between 8,5 and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C 8,5 (i, j);

[0048] S7: Enter steps A0 to A3, according to C 8,5 (i, j) calculate the sub-pixel displacement d 8,5 ;

[0049] S8: Calculate 2 images F 8 and F 5 Displacement value between: X = D 8,5 +d 8,5 For this example, X = 0.3001 pixel, and F 8 and F 5 The displacement benchmark between them is 0.3pixel, and the error between the two is <0.001pixel.

[0050] The calculation results of speckle diameters of 10 pixels and 2 pixels are shown in Tables 1 and 2:

[0051] Table 1 Calculation results of speckle diameter of 10 pixels, α = 0.5079, β = -0.06

[0052] Displacement / pixel Calculated value / pixel Error / pixel 0.1 0.1000 0.0000 0.2 0.1999 -0.0001 0.3 0.3000 0.0000 0.4 0.4002 0.0002 0.5 0.4999 -0.0001 0.6 0.6000 0.0000 0.7 0.7002 0.0002 0.8 0.8003 0.0003 0.9 0.9002 0.0002

[0053] Table 2 Calculation results of speckle diameter of 2 pixels, α = 0.7215, β = -2.96

[0054] Displacement / pixel Calculated value / pixel Error / pixel 0.1 0.1002 0.0002 0.2 0.2004 0.0004 0.3 0.3003 0.0003 0.4 0.3995 -0.0005 0.5 0.4995 -0.0005 0.6 0.5998 -0.0002 0.7 0.6992 -0.0008 0.8 0.7993 -0.0007 0.9 0.8998 -0.0002

[0055] The steps from A0 to A3 are as follows:

[0056] A0: Decompose the 3×3 correlation coefficient C(i,j) into the 3×1 correlation coefficient C in the x direction x (i) and the 1×3 correlation coefficient C in the y direction y (j), sign(·) is the sign function:

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[0058]

[0059]

[0060]

[0061]

[0062]

[0063] A1: Let C xmin =min{C x (-1),C x (0),C x (1)},C ymin =min{C y (-1),C y (0),C y (1)}, calculate

[0064] Sub-pixel displacement in the x direction Δx and sub-pixel displacement in the y direction Δy:

[0065]

[0066]

[0067] A2: If Δx==Δx 0 and Δy==Δy 0 , then proceed to A3; otherwise: let Δx 0 =Δx, Δy 0 =Δy, return to A0;

[0068] A3: If it is a displacement in the x direction, it returns Δx; if it is a displacement in the y direction, it returns Δy.

[0069] The embodiments of the present invention are described in detail above, but the present invention is not limited to the above specific embodiments. Any modifications or equivalent changes made by those skilled in the art within the scope of the claims should be included in the protection scope of the present invention.

Claims

1. A sub-pixel displacement measurement method with dual variable adjustment, characterized in that: The specific steps include: S0: If the dual adjustment variables α and β have been calibrated, go to step S6, otherwise go to step S1; S1: Read N images with different displacements, record F1 to F N , set the initial values ​​of adjustment variables α and β; S2: Calculate adjacent image F through digital image correlation analysis n+1 With F n The integer pixel displacement D n+1,n and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C n+1,n (i, j), calculate the Nth image F N The integer pixel displacement D between the first image F1 N,1 and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C N,1 (i, j), n=1, 2, 3...N-1; i, j=-1, 0, 1; S3: According to C n+1,n (i, j) and C N,1 (i, j), calculate the sub-pixel displacement d n+1,n and d N,1 ; S4: Calculate adjacent images F n+1 With F n The accumulated displacement value of : Calculate the Nth image F N Displacement value from the first image F1: X2 = D N,1 +d N,1 ; S5: If |X1-X2|<0.001, the calibration is completed, the dual adjustment variables α and β become fixed values, and the process goes to S6; otherwise, α and β are modified and the process goes back to step S3; S6: Read the two images F0 and F1 before and after the displacement, and calculate the integer pixel displacement value D between the adjacent images F1 and F0 by the same digital image correlation analysis method as in step S2. 1,0 and the 3×3 correlation coefficient of the integer pixel displacement point, denoted by C 1,0 (i, j); S7: According to C 1,0 (i, j), calculate the sub-pixel displacement d 1,0 ; S8: Calculate the displacement value between the two images F1 and F0: X = D 1,0 +d 1,0 ; The steps of calculating sub-pixel displacement described in step S3 and step S7 are specifically as follows: A0: Decompose the 3×3 correlation coefficient C(i,j) into the 3×1 correlation coefficient C in the x direction x (i) and the 1×3 correlation coefficient C in the y direction y (j), sign(·) is the sign function: A1: Let C xmin =min{C x (-1),C x (0),C x (1)},C ymin =min{C y (-1),C y (0),C y (1)}, calculate Sub-pixel displacement in the x direction Δx and sub-pixel displacement in the y direction Δy: A2: If Δx=Δx0 and Δy=Δy0, proceed to step A3; otherwise : Let Δx0 = Δx, Δy0 = Δy, and return to step A0; A3: If it is a displacement in the x direction, it returns Δx; if it is a displacement in the y direction, it returns Δy.

2. The sub-pixel displacement measurement method with dual variable adjustment according to claim 1, characterized in that: In step A1, the value of α in the Δx equation and the value of α in the Δy equation may be the same or different.

3. The method for measuring sub-pixel displacement with dual variable adjustment according to claim 1, characterized in that: In step A3, for the in-plane displacement measurement, Δx and Δy are returned.

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