1. Basic principles
Camera extrinsic parameter calibration, camera extrinsic parameters are calibrated by shooting a multi-angle checkerboard.
2. External reference calibration board design
The calibration board is divided into a vertical calibration board and a horizontal calibration board. Since the horizontal calibration board on the ground is not easy to be detected, this paper uses a vertical calibration board for camera calibration.
During the calibration process, the calibration board needs to be orthogonal to the coordinates of the vehicle body, that is, the vertical side of the calibration board needs to be perpendicular to the ground plane, the horizontal side needs to be perpendicular to the horizontal plane, and remain horizontal to the central axis of the car body. Here, it is recommended to design the calibration board bracket. Use the bracket to adjust the position of the calibration board. The specific design of the calibration board is shown in the figure below. The bottom bracket ensures that the upper and lower edges of the calibration board are level with the ground plane.
Calibration board and bracket diagram
3. Vertical calibration board placement
The placement of the vertical calibration board is shown in the figure below. It should be noted here that the calibration board needs to be perpendicular to the central axis of the vehicle body facing itself when calibrating the camera. The calibration plate can move back and forth, left and right under the condition of ensuring verticality so that the camera can capture a clearer and complete image of the calibration plate.
Calibration board placement diagram
In order to facilitate fixing the positions of the calibration plate and the body, we first need to confirm that the body is on a horizontal plane, and the ground where the vehicle is located cannot have uneven or obvious slopes. In a horizontal scene, we take the following steps to draw the body coordinate system on the ground, specifically The operation is as follows:
A In order to draw the front and rear horizontal lines, find the front and rear symmetrical points of the vehicle, as shown in the red point in A below.
B draw marks on the ground with a plumb weight, and connect the front and rear points respectively to obtain two lines parallel to the front and rear bumpers of the car, as shown in the red line of B in the figure below
C Since the front and rear points are symmetrical, the intersection point of the central axis of the body and the front and rear horizontal lines (that is, the midpoint of the front and rear symmetric points) can be obtained through two points, as shown in the figure below C blue point, the blue point is connected to the body central axis.
D respectively extend an equal distance from the midpoint of the front and rear horizontal lines to the left and right (for example, by 1.5m) to obtain the symmetrical points on both sides, as shown in the yellow point of D in the figure below. The distance between the yellow point and the vertical axis of the car body is equal.
After E, use a horizontal laser to draw a point perpendicular to the front and rear horizontal lines at the extended symmetrical point, as shown in the light blue line on the left side of Figure E.
F here can be regarded as drawing a vertical line from the front of the car to the rear of the car body through a laser square, and check the intersection point of the vertical line and the rear horizontal line, as shown in Figure F, the yellow point on the lower line is the point obtained by extending the lower horizontal line. The purple point below is the intersection of the vertical line and the rear horizontal line drawn with a square using the front line and point. We need to ensure that the difference between these two points is less than 4cm to ensure that the outer frame of the body conforms to the body coordinate system.
Through the above operations, we have obtained the circumscribed rectangle related to the body. We only need to ensure that the calibration plate is parallel to the line on the ground to ensure that the calibration plate and the coordinate system of the body are horizontal, and because the calibration plate ensures its horizontal relationship with the ground through the bracket, Therefore, the calibration plate can be orthogonal to the body coordinate system.
4. The external reference image shooting of the calibration board
After placing the calibration board according to the body coordinate system, the relative external reference image can be taken. The requirements for external reference image shooting are as follows:
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The illumination of the calibration board should be uniform to prevent shadows or spot reflections on the calibration board from affecting the algorithm's recognition of the calibration board;
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Due to the distortion of the camera and the installation at a certain angle, when the calibration board faces the camera at a certain angle, the checkerboard of the calibration board will have a large deformation, which cannot be recognized in the recognition algorithm, so it is necessary to ensure that the image distortion of the calibration board is not too large , that is, the angle between the optical axis of the camera and the normal of the calibration plate cannot be too large during the shooting process;
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The complete calibration board should be photographed for each calibration board picture taken;
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The combined range of multiple pictures of the calibration board needs to cover the field of view of the camera as much as possible;
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The calibration plate needs to be as flat as possible without obvious wrinkles;
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The black and white squares are square, the size is the same, the size of the square is not limited, as long as it can be clearly distinguished in the image, it is recommended to be around 30mm;
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Checkerboard calibration needs to take multiple checkerboard pictures, usually 20-30;
The specific shooting effect is as shown in the figure below. The top two pictures in the picture below meet the requirements. The left side of the bottom two pictures has certain reflections, and the right side has shadows, which will affect the calibration effect.
5. Calibration image post-processing
The images collected in the above process are used as input to the perception group for internal and external reference algorithms, and finally the camera images are spliced & aligned to the calibration files through the domain control. The process is shown in the following figure:
6. Summary
The above is a basic method of camera calibration, so what kind of scene needs camera calibration? In summary, camera calibration is required for all applications involving image coordinate transformation, such as 360-degree surround view systems for cars, 3D target detection, etc.
at last:
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