Ping Yishan, Liu Yuankun. An easy line-structured light system calibration method based on homography matrix[J]. Opto-Electronic Engineering, 2019, 46(12): 180677. doi: 10.12086/oee.2019.180677
Citation: Ping Yishan, Liu Yuankun. An easy line-structured light system calibration method based on homography matrix[J]. Opto-Electronic Engineering, 2019, 46(12): 180677. doi: 10.12086/oee.2019.180677

An easy line-structured light system calibration method based on homography matrix

    Fund Project: Supported by National Major Scientific Instruments and Equipment Development Project (2013YQ490879)
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  • An easy line-structured light system calibration method is proposed, which is based on the constructed light plane and homography matrix. In this method, the sequential images of the light plane and calibration target are obtained at different positions by shifting a translating target plane within the depth of camera's field, then a series of feature points would be extracted from these images to form a light plane. Then, a homography matrix, which is the mapping relationship between the light plane and the image plane of camera, can be calculated. In the experiment, the 3D data can be obtained by using this homography matrix when image coordinates of the light plane are extracted in an arbitrary image. Then the entire object can be measured by using a translation device. For the real data of calibration, the maximum residual error is less than 0.05 mm, standard deviation is less than 0.02 mm, the relative error of the measured distance between the two planes is less than 1.3%. The proposed method can make the entire calibration process easy and flexible to use.
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  • Overview: Among many active vision measuring technologies, three-dimensional (3D) sensing technology based on laser triangulation measurement model has been fully carried out in various fields of applications. When measuring an object, a line-structured light is projected onto the surface of measured object, and camera captures those images which contain height information of the measured object from a certain angle. The process, which is called system calibration, is a key step in the whole 3D measurement and can directly affect the accuracy of measurement. However, the existing calibration methods of line-structured light system involve time-consuming and complicated procedures. To address this issue, this paper presents a practicable calibration method based on a homography matrix as shown in fig, which does not need the camera calibration as well as the calculation of the light plane equation. For system calibration, two corresponding images must be captured at each position, one is with the light stripe which called light plane, the other is without the light stripe e.g. a calibration plane and it will be moved by a translation stage. The light plane is preprocessed by filtering and threshold method, then to extract the pixel coordinates of light stripe center by gray weighted centroid algorithm. And, some error points are removed via maximum likelihood method, and to fit the remaining valid points into a linear equation. The intersection lines are extracted between each light plane and the calibration plane, and a series of intersection lines can be obtained after moving the calibration plane several times to forming a virtual plane, which is the actual light plane. Then the corner feature points are extracted from calibration planes by Harris corner detection algorithm, and fitted the corner feature points into a linear equation. Combining the two linear equations, the extracted image coordinates of feature points are the intersection points of two fitting lines. When the world coordinates of feature points are set, the corresponding relationship between light plane and image plane is represented by the mapping (Homography). To an end, this calibration method only needs to calibrate two or more light planes at different positions. And the maximum residual error is less than 0.05 mm, standard deviation is less than 0.02 mm. The relative error of the measured distance between the two planes is less than 1.3%. The experimental results have demonstrated the feasibility and validity of the proposed method in 3D measurement with simple system calibration procedures. Moreover, the entire calibration process is practicable to simplify the experimental procedures and easy to be applied in industrial inspection.

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