An Out-of-Focus Image Calibration Method Based on Accurate Positioning of Concentric Circle Projection Center

During camera calibration, the plane target image is required to occupy most of the whole image, and the focused image is also needed. It is difficult to satisfy these conditions in large-scale structural vision measurement. Therefore, it is very important to explore a camera calibration method with a small calibration board for a large field of view. This paper proposed an out-of-focus image calibration method based on the accurate positioning of concentric circular projection center, which is not limited by the camera’s field of view and can overcome the influence of out-of-focus image on camera calibration. The proposed method consists of three parts: adaptive blurred circle image enhancement, accurate positioning of concentric circle projection center and camera calibration of concentric circle center point array. The simulation results show that under different Gaussian blur kernel radius conditions, the proposed accurate positioning method of the concentric circle projection center has better robustness. When Gaussian kernel radius K = 40, the positioning accuracy of the proposed method is improved by 41.88%. In the real image experiment, the internal parameters of the camera in the heavy defocus image have similar results to the Zhang’s calibration method in the focused image. Both simulation and real image experiments show that the proposed method can realize the calibration of large targets with a small plane calibration board.