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 |
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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.
Mathematic model of line-structured light sensor
Line-structured light three-dimension measurement system
The process for calibration of line-structured light system
Image processing. (a) Image of light strip captured by CCD; (b) Corner points extraction; (c) Characteristic points extraction
Image of characteristic point
Error distribution. (a) The proposed method; (b) The method of Ref. [20]
3D shape of the stepped object. (a) Workpiece of stepped object; (b) Restored result using the proposed method