Electro-hydraulic varifocal lens-based method for tracking three-dimensional trajectory of moving object

1. An electro-hydraulic varifocal lens-based method for tracking a three-dimensional (3D) trajectory of a moving object, comprising:

step 1, calibrating an electro-hydraulic varifocal lens under different focal distances to obtain a functional relation between a focusing control current and a camera's intrinsic parameters;

step 2, establishing an electro-hydraulic varifocal lens-based optical imaging system model to obtain a functional relation between a focusing control current of the electro-hydraulic varifocal lens and an optimal object distance;

step 3, initializing an object tracking algorithm, generating an object tracking box, and selecting a to-be-tracked object;

step 4, carrying out autofocusing, and recording a focusing control current after the autofocusing is completed, as well as a size of the object tracking box in an image and center point coordinates after undistortion; wherein the autofocusing in step 4 comprises first autofocusing and subsequent autofocusing, and the first autofocusing specifically comprises:

searching an initial focusing control current at a certain stride, calculating a sharpness evaluation value of an internal image region of the object tracking box, obtaining a maximum sharpness evaluation value and a focusing control current corresponding to the maximum sharpness evaluation value, and setting a sharpness evaluation threshold:

K=αD max   (3)

wherein a denotes a preset sharpness confidence level, and α<1; K denotes a sharpness evaluation threshold used in the subsequent autofocusing; and D max denotes a maximum sharpness evaluation value; and

after the first autofocusing is finished, recording a size of the object tracking box in an image and center point coordinates after undistortion;

wherein the subsequent autofocusing specifically comprises:

calculating a sharpness evaluation value D i of the internal image region of the object tracking box; and

if D i >K, directly recording the focusing control current I i at this moment, as well as a size size i of the object tracking box in an image and center point coordinates after undistortion; or

if D i <K, reading a size size i of the object tracking box in the image at this moment, comparing the size with a size size i−1 of the object tracking box at last successful focusing, and adjusting a focusing control current to complete focusing; and after the focusing is completed, recording the focusing control current and the size of the object tracking box in the image after focusing and center point coordinates after undistortion;

step 5, calculating and recording, by a camera projection model, coordinates of the object in 3D space; and

wherein the camera projection model in step 5 is:

wherein (x i , y i ) denote center point coordinates of an object tracking box in an undistorted image, cx and cy denote coordinates of a camera's optical center on a pixel plane, s denotes a slant parameter between horizontal and vertical edges of a camera's photosensitive element, f xi , f yi , denote equivalent focal distances of a camera in x and y directions corresponding to a focusing control current I i at this moment respectively, and (X i , Y i , Z i ) denote 3D coordinates of a center point of a tracked object; and Z i =u i , wherein u i denotes an optimal object distance corresponding to the focusing control current Ii at this moment;

step 6, repeating steps 4-5 for the same tracked object, and sequentially connecting the recorded coordinates of the object in 3D space into a trajectory.

2. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 1 , wherein step 1 specifically comprises calibrating the electro-hydraulic varifocal lens under multiple focusing control currents to obtain the functional relation between the focusing control current and the camera's intrinsic parameters by curve fitting:

( f x ,f y )= H ( I )  (1)

wherein f x and f y denote equivalent focal distances of a camera in x and y directions of the pixel plane respectively, in a unit of px; and I denotes a focusing control current of an electro-hydraulic varifocal lens; and

obtaining coordinates of a camera's optical center on the pixel plane, and a slant parameter between horizontal and vertical edges of a camera's photosensitive element.

3. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 2 , wherein step 2 specifically comprises recording an optimal object distance under multiple focusing control currents by using the electro-hydraulic varifocal lens-based optical imaging system model, and conducting curve fitting on the recorded data to obtain a functional relation between focusing control currents of the electro-hydraulic varifocal lens and the optimal object distance:

u=F ( I )  (2)

wherein u denotes an optimal object distance, and I denotes a focusing control current of the electro-hydraulic varifocal lens.

4. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 1 , wherein when D i <K, the focusing control current is adjusted based on a comparison result of size i and size i−1 , which specifically comprises:

if size i <size i−1 , searching the focusing control current at a certain stride in the direction where the optimal object distance becomes longer, calculating a sharpness evaluation value in the object tracking box, and completing focusing after the sharpness evaluation value is greater than or equal to the sharpness evaluation threshold; or

if size i >size i−1 , searching the focusing control current at a certain stride in the direction where the optimal object distance becomes shorter, calculating a sharpness evaluation value in the object tracking box, and completing focusing after the sharpness evaluation value is greater than or equal to the sharpness evaluation threshold.

5. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 4 , wherein the undistortion in step 4 specifically comprises: calculating an undistorted image of a current frame according to a distortion model, and reading and recording center point coordinates of the object tracking box in the undistorted image.

6. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 5 , wherein the sharpness evaluation value is calculated using a Laplacian function, and the Laplacian function is expressed as:

D ( f )=Σ y Σ x |G ( x,y )|  (5)

wherein G(x, y) denotes convolution of a Laplacian operator at a pixel point (x, y), and the Laplacian operator is expressed as:

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7. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 4 , wherein the sharpness evaluation value is calculated using a Laplacian function, and the Laplacian function is expressed as:

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8. The electro-hydraulic varifocal lens-based method for tracking a 3D trajectory of a moving object according to claim 1 , wherein the sharpness evaluation value is calculated using a Laplacian function, and the Laplacian function is expressed as:

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