A method for measuring the field of view angle of an AR light machine based on camera calibration

Abstract

The application relates to a kind of AR light machine field angle measurement methods based on camera calibration, comprising the following steps: preparing a black and white grid calibration board;Change the distance between calibration board and camera, so that the distance between them is greater than 10 times the focal length of camera;Transform the position of calibration board, repeat the direct imaging of calibration board by camera;Remove the calibration board, do not change the position and parameters of camera, control the direct projection of black and white grid image by AR micro display system;Adopt darkroom shooting mode, camera directly images light machine target;Through Zhang Zhengyou calibration method, the image of calibration board and the projected black and white grid image are calibrated and corrected, according to the calibration result, the focal length of camera at this time is recorded as f0;Image processing is carried out on the corrected image, the diagonal line size h of the image is obtained, and the diagonal line size is brought into the curve formula h=2f0·tanω of field angle-image height, and the measured field angle 2ω is directly solved.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a method for measuring the field of view angle of an AR optical machine based on camera calibration. Background Art

[0002] The field of view angle is a very important parameter for AR optical machines. Currently, there are two commonly used methods for measuring the field of view angle. The first is to use a luminance meter to measure the brightness at different angles. The second is to use a camera with a known focal length. According to the size of the image formed after passing through the camera, the field of view angle can be calculated using a formula. The relationship between the field of view angle (2ω) and the diagonal size (L) is: h = 2f·tanω, where f is the focal length of the camera. However, for the first method, it is easily affected by the external environment, which reduces the accuracy of the measurement results; for the second method, the imaging errors (such as distortion) of the optical system of the optical machine and the camera itself will also affect the measurement accuracy. Summary of the Invention

[0003] The present invention aims to provide a method for measuring the field of view angle of an AR optical machine based on camera calibration. Before measuring the field of view angle, this method uses AR projection black and white grid image imaging + calibration plate direct imaging to calculate and correct the distortion of the generated image. The AR projected black and white grid image contains the distortion of the optical machine optical system and the camera used for shooting. This process can simultaneously eliminate the measurement error caused by the distortion of the two optical systems, thereby improving the measurement accuracy. Note that the calibration plate is similar to the black and white grid image, and the four corners are white grids, such as Figure 1 As shown in the figure. During the distortion calculation, the current focal length of the camera is accurately measured. The AR projected black and white grid image after distortion correction is recorded as J n+1 ,like Figure 2 shown.

[0004] Then use image processing to obtain image J n+1 The diagonal size h of the image is directly solved by substituting this diagonal size into the field of view angle-image height curve formula h = 2f0·tanω to obtain the measured field of view angle 2ω. The method for obtaining h is to use darkroom imaging (i.e., minimize external light entering the optical system). At this time, the light intensity contrast between the target image edge and the background is very obvious. The image edge can be determined by using a light intensity threshold (i.e., pixels with light intensity greater than T are the target image, and pixels with light intensity less than or equal to T are the background). The number of pixels N occupied by the image diagonal can be determined based on the edge points. The pixel size of the camera is a fixed value (set as A), so h = N×A.

[0005] The purpose of the present invention is to address the deficiencies of the prior art and provide a method for measuring the field of view angle of an AR optical machine based on camera calibration, comprising the following steps:

[0006] (1) Prepare a black and white grid calibration plate with white grids at the four corners. The total length of the black and white grids in the horizontal and vertical directions is L1 and L2, and L1:L2 = the aspect ratio of the AR microdisplay.

[0007] (2) Change the distance between the calibration plate and the camera so that the distance between the two is greater than 10 times the focal length of the camera, and the calibration plate can be fully imaged and fill the camera's field of view as much as possible;

[0008] (3) Change the position of the calibration plate and repeat the steps of directly imaging the calibration plate through the camera to obtain a series of images of the calibration plate, which are recorded as T1, T2, ..., T n ;

[0009] (4) Remove the calibration plate, without changing the camera position and parameters, and control the AR micro-display system to directly project the black and white grid image (the number and pattern of black and white grids in the image are similar to the calibration plate). Use the darkroom shooting method (i.e., minimize the external light entering the optical system to ensure that the background of the image is relatively dark), and the camera directly images the optical target, which is recorded as T n+1 ;

[0010] (5) Using Zhang Zhengyou’s calibration method, T1, T2, ..., T n , T n+1 Perform distortion calibration and correction, where T n+1 The corrected image is denoted as J n+1 ,According to the calibration results, the focal length of the camera at this time is recorded as f0;

[0011] (6) Image J n+1 Perform image processing to obtain the diagonal size h of the image, and then substitute this diagonal size into the field angle-image height curve formula h=2f0·tanω, and directly solve to obtain the measured field angle 2ω. The method to obtain h is: Since the image J n+1 The four edge corners are relatively bright, while the background is very dark, so the light intensity contrast between the target image edge and the background is very obvious at this time. The image edge can be obtained by using the light intensity threshold (that is, pixels with light intensity greater than T are the target image, and pixels with light intensity less than or equal to T are the background); the number of pixels N occupied by the image diagonal can be known based on the edge point. The pixel size of the camera is a fixed value (set as A), then h = N×A.

[0012] Preferably, in the step of directly imaging the calibration plate through the camera, the calibration plate is placed in front of the camera and at a distance of more than 10 times the focal length of the camera to obtain an image of the calibration plate.

[0013] Preferably, in the step of repeatedly imaging the calibration plate directly through the camera, more than 10 images of the calibration plate in different postures are obtained by changing the posture of the calibration plate.

[0014] Further preferably, in the step of directly imaging the optical machine target using a camera, the optical machine is lit while the calibration plate is removed, and an image similar to the calibration plate is displayed in the optical machine as a test image.

[0015] Preferably, in the step of directly imaging the optical machine target using a camera, the camera does not change parameters and position, and is aligned with the output of the optical machine to capture images.

[0016] Beneficial effects

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The method for measuring the field of view angle of an AR optical machine based on camera calibration described in the present invention utilizes a camera and a calibration plate. The calibration plate adopts a grid pattern. Combined with the camera imaging the grid light emitted by the AR that is similar to the calibration plate, the total distortion of the optical machine and the camera can be corrected at one time, and the current focal length of the camera can be obtained at the same time; then, based on the corrected distorted AR grid image and the current focal length of the camera, high-precision measurement of the field of view angle is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings are used to provide a further understanding of the technical solution of the present invention and constitute a part of the specification. Together with the specific embodiments of the present application, they are used to explain the technical solution of the present invention and do not constitute a limitation on the technical solution of the present invention.

[0020] Figure 1 Schematic diagram of the calibration plate of the present invention.

[0021] Figure 2 3 is a schematic diagram of an image after correction of an AR projected image captured by the camera according to the present invention. DETAILED DESCRIPTION

[0022] The present invention is described in more detail below to facilitate understanding of the present invention.

[0023] The method for measuring the field of view angle of an AR optical machine based on camera calibration described in the present invention comprises the following steps:

[0024] (1) Prepare a calibration plate with white grids at the four corners. The total length of the black and white grids in the horizontal and vertical directions is L1 and L2, and L1:L2 = the aspect ratio of the AR microdisplay. The schematic diagram of the calibration plate is as follows: Figure 1 shown.

[0025] (2) Change the distance between the calibration plate and the camera so that the distance between the two is greater than 10 times the focal length of the camera, and the calibration plate can be fully imaged and fill the camera's field of view as much as possible;

[0026] (3) Change the position of the calibration plate and repeat the steps of directly imaging the calibration plate through the camera to obtain a series of images of the calibration plate, which are recorded as T1, T2, ..., T n ;

[0027] (4) Remove the calibration plate, without changing the camera position and parameters, and control the AR micro-display system to directly project the black and white grid image (the number and pattern of black and white grids in the image are similar to the calibration plate). Use the darkroom shooting method (i.e., minimize the external light entering the optical system to ensure that the background of the image is relatively dark), and the camera directly images the optical target, which is recorded as T n+1 ;

[0028] (5) Using Zhang Zhengyou’s calibration method, T1, T2, ..., T n , T n+1 Perform distortion calibration and correction, where T n+1 The corrected image is denoted as J n+1 ,According to the calibration results, the focal length of the camera at this time is recorded as f0;

[0029] (6) Image J n+1 Perform image processing to obtain the diagonal size h of the image, and then substitute this diagonal size into the field angle-image height curve formula h=2f0·tanω, and directly solve to obtain the measured field angle 2ω. The method to obtain h is: Since the image J n+1 The four edge corners are relatively bright, while the background is very dark, so the light intensity contrast between the target image edge and the background is very obvious at this time. The image edge can be obtained by using the light intensity threshold (that is, pixels with light intensity greater than T are the target image, and pixels with light intensity less than or equal to T are the background); the number of pixels N occupied by the image diagonal can be known based on the edge point. The pixel size of the camera is a fixed value (set as A), then h = N×A.

[0030] In a preferred embodiment, the method for measuring the field of view angle of an AR optical machine based on camera calibration includes the following steps:

[0031] (1) Prepare a black and white grid calibration plate with white grids at the four corners. The total length of the black and white grids in the horizontal and vertical directions is L1 and L2, and L1:L2 = the aspect ratio of the AR microdisplay.

[0032] (2) Change the distance between the calibration plate and the camera so that the distance between the two is greater than 10 times the focal length of the camera, and the calibration plate can be fully imaged and fill the camera's field of view as much as possible;

[0033] (3) Change the position of the calibration plate and repeat the steps of directly imaging the calibration plate through the camera to obtain a series of images of the calibration plate, which are recorded as T1, T2, ..., T n ;

[0034] (4) Remove the calibration plate, without changing the camera position and parameters, and control the AR micro-display system to directly project the black and white grid image (the number and pattern of black and white grids in the image are similar to the calibration plate). Use the darkroom shooting method (i.e., minimize the external light entering the optical system to ensure that the background of the image is relatively dark), and the camera directly images the optical target, which is recorded as T n+1 ;

[0035] (5) Using Zhang Zhengyou’s calibration method, T1, T2, ..., T n , T n+1 Perform distortion calibration and correction, where T n+1 The corrected image is denoted as J n+1 ,According to the calibration results, the focal length of the camera at this time is recorded as f0;

[0036] (6) Image J n+1 Perform image processing to obtain the diagonal size h of the image, and then substitute this diagonal size into the field angle-image height curve formula h=2f0·tanω, and directly solve to obtain the measured field angle 2ω. The method to obtain h is: Since the image J n+1 The four edge corners are relatively bright, while the background is very dark, so the light intensity contrast between the target image edge and the background is very obvious at this time. The image edge can be obtained by using the light intensity threshold (that is, pixels with light intensity greater than T are the target image, and pixels with light intensity less than or equal to T are the background); the number of pixels N occupied by the image diagonal can be known based on the edge point. The pixel size of the camera is a fixed value (set as A), then h = N×A.

[0037] Figure 2 An example of an image captured by a camera is shown. Figure 2 Here, |AB| represents the diagonal distance h.

[0038] exist Figure 2 In the example, the grayscale values ​​of the black background and the target image are very different. According to the threshold method, the pixel size of the effective range (non-black frame area) in the figure can be determined to be 1834*1032 pixels. The size of each pixel is 6.3μm, so the length of the effective range is 1834*6.3μm=11.55mm, and the width is 1032*6.3μm=6.50mm; the diagonal length is

[0039] Assuming that the focal length of the camera after calibration is f0 = 16 mm, according to the relationship between 2ω and h (i.e. h = 2f0·tanω), 2ω = 2*arctan(13.25 / 32) = 44.9852°≈45°, so the measured field of view angle is: 45°.

[0040] The preferred embodiments of the present invention are described above, but they are not intended to limit the present invention. Those skilled in the art may make improvements and changes to the embodiments disclosed herein without departing from the scope and spirit of the present invention.

Claims

1. A method for measuring the field of view angle of an AR optical machine based on camera calibration, characterized in that: The method for measuring the field of view angle of an AR optical machine based on camera calibration includes the following steps: (1) Prepare a black and white grid calibration plate with white grids at the four corners. The total length of the black and white grids in the horizontal direction is L1, and the total length in the vertical direction is L2. L1:L2 = the aspect ratio of the AR microdisplay. (2) Change the distance between the calibration plate and the camera so that the distance between the calibration plate and the camera is greater than 10 times the focal length of the camera, and the calibration plate can be fully imaged and fill the camera's field of view as much as possible; (3) Change the position of the calibration plate and repeat the steps of directly imaging the calibration plate through the camera to obtain a series of images of the calibration plate, which are recorded as T1, T2, ..., T n ; (4) Remove the calibration plate, do not change the camera position and parameters, and control the AR micro-display system to directly project the black and white grid image; use the darkroom shooting method, and the camera directly images the light machine target, which is recorded as T n+1 ; (5) Using Zhang Zhengyou’s calibration method, T1, T2, ..., T n , T n+1 Perform distortion calibration and correction, where T n+1 The corrected image is denoted as J n+1 ,According to the calibration results, the focal length of the camera at this time is recorded as f0; (6) Image J n+1 Perform image processing to obtain the diagonal size h of the image, and substitute this diagonal size into the field of view angle-image height curve formula h=2f0·tanω to directly solve for the measured field of view angle 2ω.

2. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 1, characterized in that: In the step of directly imaging the calibration plate through the camera, the calibration plate is placed in front of the camera and at a distance of more than 10 times the focal length of the camera to obtain an image of the calibration plate.

3. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 1, characterized in that: In the step of repeatedly imaging the calibration plate directly through the camera, more than 10 images of the calibration plate in different postures are obtained by changing the posture of the calibration plate.

4. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 1, characterized in that: In the step of using the camera to directly image the optical machine target, the optical machine is lit while the calibration plate is removed, and an image similar to the calibration plate is displayed in the optical machine as a test image.

5. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 1, characterized in that: In the step of directly imaging the optical machine target using a camera, the camera does not change parameters and position, and is aligned with the output of the optical machine to capture images.

6. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 1, characterized in that: In the black and white grid image directly projected by the microdisplay system that controls AR, the number and style of black and white grids are similar to those on the calibration plate.

7. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 1, characterized in that: The method for obtaining h is: using the light intensity threshold to obtain the edge of the image; according to the edge point, the number of pixels N occupied by the image diagonal is known. The pixel size of the camera is a fixed value, set as A, then h = N×A.

8. The method for measuring the field of view angle of an AR optical machine based on camera calibration according to claim 7, characterized in that: The light intensity threshold specifically means that pixels with light intensity greater than T are target images, and pixels with light intensity less than or equal to T are backgrounds, where T is a preset value.

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